Conference vicki::boats

    
    re .0
    
    I went from a 19 to a 17 on my 16' Glastron w 90 Merc.
    Although the difference is only about 12% as I recall, there was
    a big improvement in out of the hole ski performance, as it got
    the motor into a higher torque range.
                                               
    I only gave up a couple of MPH at the top end that I didn't care
    about, and some fuel economy that I don't care about.
    
    Is your boat doing somthing you don't like?
    
    Don

Mark,
The "pitch" of a prop is the distance it would move forward through the water
in one revolution, if there were no slippage.  Typically slippage is measured
at somewhere around 10-20%.  A 21" prop would theoretically move you 4" further
forward in one rev than a 17" prop.  The effect is that increasing the pitch
will hurt your acceleration, but give a higher top speed, assuming you have
enough power to turn the prop.  Decreasing the pitch will give added power for
acceleration or pulling up a skier, but at the expense of some top end speed.

The change from aluminum to stainless alone, without changing the pitch should
give a slight improvement in both acceleration and top speed.  The reason being
that the blades on a stainless prop are about half the thickness of those on
an aluminum prop.  Less blade thickness = less friction = better acceleration
and speed.

The best way to tell if your boat is propped correctly is by using a tachometer.
Let's assume you've got a 19" prop, and your engine is rated for 120 horsepower
at 4600 rpm.  If you can only pull 4200 rpm then your boat is over propped.
You should decrease the pitch from 19" to 17".  If that same engine is able to
reach let's say 5000 rpm, then you are under-propped, and should increase the
pitch to 21".

There are other factors to be considered, such as number of blades, diameter,
and how much the prop is "cupped".  A prop that is cupped has a different pitch
at the leading than at the trailing edge.  Literally entire books and charts
have been written on prop theory and selection.  It is not an exact science, so
some experimentation is often necessary if you're looking for that last bit
of performance out of your boat.

Good Luck.
Rick W.

    
Mark,
    There is a little more info needed before you can judge what effect
    a prop swap will have. Mercruiser has some handy charts in their
    Quicksilver Parts and Accessories catalog. I have also written
    a basic program which predicts boat speed. I havent run it on 
    a real large sample of boats, but it appears to be ballpark.
    
    The info necessary to judge the prop change would be:
    approx boat weight:
    Engine rpm's at WOT:		(wide open throttle)
    Gear Reduction     :		(Catalog for '84 says 1.98)
    					 for a Merc 140 I/O
    
    I would estimate a 2" change pitch will change engine RPM's 
    @WOT 200-300 RPMs approx.
    
    I.E. if 	RPM = 4600 w/ 19" prop
    Then new    RPM may = 4400 w/ 21" prop
    or   new    RPM may = 4800 w/ 17" prop
    
    There are also some other factors. If your engine has enough HP's
    to swing the new prop, going to a 21" will increase speed, but
    also increase time to plane. Conversly, going from a 19" to a 17" will
    increase WOT rpm's, decrease time to plane, and probably decrease
    top speed.
    
    I tried a 21" on my boat (211 4Winns Liberator, OMC King Cobra 335).
    It came with a 19". The boat went 2-4 MPH slower, and took forever
    to get on plane. I also tried the new Omc VIPER prop (19") and it
    appeared to add 2 MPH due to a different design. We don't know for
    sure as the mechanic from the dealer was also in the boat for the
    test ride so his weight may have effected the test.
    
    Also, a stainless prop will flex less. An aluminum 17" may actually
    drop in pitch to 15-16" when starting to get on plane. 
    
    If you really want to go Hi-Tech, get Land & Sea's new variable
    pitch prop. It shifts depending on torque load and is "programmable".
    It can go from (for example) 15" to 25" while under way. L&S is
    in Salen, NH. The prop goes for around $500. I'd love t try one
    myself. :^)

    If there is any interest I could post the "speed" program here.
    I would be interested to see if it is close for other boats.
    
    Scott Malcolm 
    (wondering in snowless NH if I wasted money on inside storage)
    

    The prop-world is even more complex than usually appreciated. Most
    things you read about proper prop selection center on top speed/WOT
    performance with some comments on acceleration thrown in too.
   
    I don't know if I'm that different from Mr Average Boater but I
    tend to run my boat (21'/200hp Mercury) mostly at something like
    50% of WOT. My prop selection is consequently based on optimum
    performance at cruising speed (around 25 knots). I really dont't
    care if top speed is 42 or 45 or 46.5 or whatever because I'm cruising
    with my family, not racing the boat. 
     
    And prop selection is even more complicated at that power level than at
    WOT. There is no "makers recommended rpm range"  to go for and who has 
    ever seen published a partial-throttle torque curve diagram of an
    outboard? (I havn't seen even a makers full-throttle one!)
     
    The boats behaviour at different loadings is critical too. The closer 
    you cruise to the planing threshold the more you appreciate a "high-torque"
    (to use car vocabulary, maybe "high-push" would be right for boats!)
    prop which saves you from suddenly dropping out of planing.
    
    It is all like a single-shift car, try to optimize gas milage and
    you loose acceleration, go for up-the-hill performance and you will
    be over-reving on level ground.
    
    
    What this all boils down to is:
    
    1. Decide WHAT you want to otimize
    
    2. THEN TRY DIFFERENT PROPS!
    
    
    The relationship between all the parameters are so complicated that 
    practical tests are the only viable methode if you really want to find 
    out what is best for you! Theories just give you a reasonable starting 
    point. 

    -jcl

    If, however, your engine is meant to cruise at 4,400 RPM, don't
    chose a prop that allows you 4,000 RMP maximum. You would be loading
    your engine and reduce its life.
    
    Ray

    Perhaps more important.....when switching to a stainless prop be
    sure our engine won't end up over reving. If your engine already
    reaches max RPMs with a 19" alum. and you switch to stainless, you'll
    likely need to move to a 21" pitch to ovoid exceeding the top end
    limits. My top RPMs still went up even though I choose 21" pitch.
    
    Jerry
    

    Further comments, seeing all the warnings about overreving etc.
    
    Flame on!
    I don't see anything wrong in chosing a prop which makes it possible
    to overrev at WOT, if that prop is the one that best satisfies your
    needs at normal speeds/throttle levels. (Ok, I by it if WOT is what
    you normally use!). There is nothing saying that use of common sense
    in connection of power setting is forbidden neither is there anything
    saying that use of all settings between idle and full should be avoided
    at all cost!!
    If the engine overrevs, throttle down! That's one reason in addition
    to snazzy looks to have a revcounter.
   
    Cars (except those with ignition cut-outs) can be overreved
    in all gears, except top, if you don't use your brains. 
    So why demand of the boat to be proof to that kind of abuse?
    
    Flame off.
    
    Remember the analogue to the single shift car: Many manual-shift
    cars have no use of top gear in town driving. If you had to pre-select
    the gear to use before starting a trip (forgetting minor problems
    like getting the car rolling etc!) a city trip choise would certainly
    not be top gear. Same for a boat, mostly cruising, choose a lower
    pitch prop to optimize accelaration and speed stability independent
    of wind and waves, sacrifacing economy a bit. If top speed or optimum
    economy a the most important things then go for a higher pitch one.
    If you take lower pitch then beware of overreving, if you take
    high-pitch then be prepared to pay the price in degraded acceleration
    and greater load sensitivity.
    
    Using an over-pitched prop, that is one where the engine cannot
    reach even the lower end of the rated power rpm is like using overdrive
    in a car, it has it uses in certain conditions with economy and
    sound level being optimized but posing dangers for the engine if
    used with too high throttle openings.
    
    Now, outboarders in particular should beware of making too much
    of this analogue with cars. Two-strokers like most outboards have
    very different power, torque and consumption characteristics from
    the typically four-stroke car engines.
     
    And boats ain't cars either!
    
    And thank heaven for that!!
    
    -jcl
    
     

    Re .7
    
    You missed the point! No one can dictate what prop you choose or
    how you use it...over reving included.
    
    However, someone may not be aware that they damage their engine
    because of the improved RPMs. This is an info and experience
    sharing conference and this is not worth a flame....unless you
    wanted to draw a crowd.
    
    Cheers,
    Jerry
    

RE: .7   
>>  Flame on!
>>  I don't see anything wrong in chosing a prop which makes it possible
>>  to overrev at WOT,

>>    Cars (except those with ignition cut-outs) can be overreved
>>  in all gears, except top, if you don't use your brains. 
 

The analogy to a car is not a good one.  If a car were capable of over-
revving in high gear, the speed at which you would be travelling would
prevent most people from doing it anyway.  Unlike a car, the top speed
of most boats is slow enough that most people *will* occasionally hold
the throttle wide open trying for that last mph.   If you want to gear
your car or prop your boat so that it can over-rev at top speed that's
certainly your right.  Unless you can guarantee that you will be the
only person to ever drive your boat I think you're asking for trouble.
I know that when I pick up my new boat this spring I'm going to want
to ride behind it on skis once in a while, meaning someone else will
have to drive it.

Rick W.

    
    
    	re: .7, .8
    
    		Yes, the "other driver" is the first thing
    	that comes to my mind also. My Nautique's manual says
    	redline is 4400, but not to cruise at that RPM for
    	any length of time. It recommends cruising at no more
    	than 3600 so I have to continually tell new drivers to
    	obey the rules.
    
    	Rick
    

    Nope, I don't want to draw a crowd. 
    I just get worried/upset/agitated  when I see a discussion on an
    important subject limiting iself by assuming too much and making
    constants out of a number of variables.
    Knowing how easy it is to be misunderstood in this kind of written 
    exchanges when expressing strong, non-conformant opinions I decided 
    to FLAME just to make sure no one got offended
    Seems I barely made it! Sorry for that!
    
    I fully agree that everyone has the right to select prop any way
    he wants and that this is a conference to share experiences and
    opinions and help us all to get the most out of our boating.
    
    
    As to .9 , I specificially said all gears except top!
    I still think my comparison was valid!
    In all gears except top cars can be overreved and drivers are expected
    to use the throttle accordingly. Why cannot boaters be assumed to
    have the same common sense?
    
    Agreed, the "other driver" poses a problem, how grave has to be
    determined case by case. If a was renting out boats I would certainly
    never dream of propping them so they could be overreved. But as to
    my own boat that never goes out without a skipper that I can rely
    on the matter is different.
    
    Let me get back to the real subject and what I have been trying
    to say: 
    Selecting a prop that allows overreving is one possibilty
    amongst many others. It has its advantages and disadvantages, even
    poses some dangers, but it should not automatically be excluded
    from the list of viable choices. 
    
    I felt a "mental barrier" against it was being raised by default 
    and felt it was to all our advantages if it was torn down or at 
    least questioned!
    
    Cheers, and may everyone be happily propped!
    Jan

   One point I haven't seen in this discussion is the OTHER end of the scale
where you may be under-propped.  What happens in this situation is you cause
your prop to cavitate and you go nowhere fast.   I had the misfortune this
past summer of trying to get away cheap and I tried a propeller that was
too small in an effort to get me up on skis.  What I was looking for was a
19 pitch for my 75hp Mariner.  What I got was 19 pitch, but it had also had
the diameter chopped some.  The effect was that instead of getting the quick
hole-shot I wanted and "watch the tach for over-rev", I got no more than 
no-wake speed because as soon as I gave it enough throttle to come up on plane,
the prop cavitated (something like spinning your wheels on an ice patch).  The
guy took the prop back 'no questions asked'.

  In this case my problem was the chopped diameter, but the same thing could
happen if you have too many horses for the prop you've chosen.

Al

  

RE:
>>  Knowing how easy it is to be misunderstood in this kind of written 
>>  exchanges when expressing strong, non-conformant opinions I decided 
>>  to FLAME just to make sure no one got offended

Apparently we have a difference of opinion on the meaning of a "FLAME".

RE:
>>  In all gears except top cars can be overreved and drivers are expected
>>  to use the throttle accordingly. Why cannot boaters be assumed to
>>  have the same common sense?
    
I restate my case:  A boat is entirely different from a car.  In a boat top 
speed is generally fairly slow, thus the temptation to push for that last bit
of speed.  I certainly don't want to risk blowing my motor bacause I looked
away from the tach for a few seconds.  And I would not want to put one of my
passengers in the position of feeling that he owed me a new motor because he
made the same mistake while driving my boat.

RE:
>> I felt a "mental barrier" against it was being raised by default and felt it
>> was to all our advantages if it was torn down or at least questioned!

You're right, there is a mental barrier against it, if for no other reason
than the fact that what you are advocating is not a generally accepted
practice.  You can prop your boat any way you like, as long as you understand
that your boat will different from most.  There will be an adjustment period
for any new drivers, and extra caution will be required.  If your method works
for you then by all means go with it.  The more common practice is to select
a prop that will allow your boat to just reach its maximum hp/rpm, and no
flames are required if someone gives that advice to an inexperienced boater.

Rick W.

    				(Thanks for the help)
    
    	First off I would like to thank all of you for your inputs .
    It appears that proper prop selection is not as cut and dry 
    as you would think . I have called several places and have found
    a used steal prop in 19" for $250.00 . Is this a fair price? The
    new props were $354.00 . That sounds like alot of BUCKS to me .
    	Reguarding .1 Yes the boat is doing a couple of things that
    I don't like . First off last year I damaged the aluminum prop and
    had it repaired . However they only ground it down . I have a buddy
    that goes 250LBS. and I can just barely get him up to ski . But
    a lady friend of mine that weighs about 130 lbs. pops up like a
    cork . Also the boat has NEVER planed out before 3000 rpm but with
    6 people in the boat it would do 42 mph now only about 38 mph .
    So I am sure it's do to the prop being smaller . When the boat was
    new the rpm's would top out at 4200 . I went in and adjusted the
    throttle cable to open up the butterfly on the carberator and the
    rpm's went up to 4600 . 
    	Someone stated in one reply,can't remember which one,that
    if I stay with the same pitch prop and go with the steel prop that
    I should have more hole power and top end . If this is true then
    maybe I should stay with the 19". Because I could pull anyone before
    the prop damage.
    		THANKS AGAIN
    			MARK
    	

    Anyone know what the "Factory Standard" prop is that is supplied
    with a Mercruiser 120? I don't think my owner's manual says anything
    about it. I think I'm over propped and would like to mess with it
    this summer. ie, takes too long to get on plane, low engine revs,
    good top end. What about a "Doel-Fin"????
    
    Don

    
    
    	I have a 17 ft bowrider with a merc 120 I/O that came with
    	a 17 inch pitch prop. For skiing I think to prop is pitched
    	to much for my boat.
    
    	Len DiPinto
    

    Interesting Len,
    	We have a 16 1/2' Larson bowrider with a Mercruiser 120 and
    a 17" pitch prop. With two adults and three kids on board it still
    will pull a 200 pounder up on a slalom ski. Granted it has to work
    at it but it does it.


         __
    * __|__|__  *    *    *     *
  *     (**)   V *  *  *   |* *   *
   *   (    )--| *  _______0_______  *  	   	
 _____(______)_|_________U___U______*___
	                
      "The Sno-man"			

    Mine is a 19' Chaparral bow rider that comes up quick with one or
    two people but, with a full crew it takes forever. Its a relatively
    heavy boat which makes me wonder what I really have for a prop.
    
    I think I'll go back to "Plan 1", get a Doel Fin for $80.00.
    
    Don

    
    
    	Maybe it's the motor??? It is very sluggish pulling a 150lb
    	person up on a slalom ski. I had Marine USA check it out but
    	they said it runs ok. Being new to all of this I  am not sure
    	whats causing the boat to hesitate on the start. Could novice
    	skiers put that much of a strain on the boat as to cause it
    	to hesitate and bog out?
    
    	Len DiPinto
    

    	There is a very good article in this months BOAT magazine that
    	talks about prop pitch . I thought it was a very interesting.
    	According to this article selecting proper prop pitch is NOT
    	a exact science . What works for one boat may or may not work
    	for another . There are just to many varibles . 
    	I have decided to go with a stainless steal since it does produce
    	less drag, the blades are thinner ,causing less friction and
    	more applied power . But the big question is with pitch ?
    	Right now I am looking for a dealer who will let me try a 19"
    	and a 17" . I did find a used 19" for 250.00 that's still
    	a chunk of change though . I'll let ya'll know in a couple of
    	weeks what happened .
    			CAPTIAN MARK
    

	Stanless Steel is the way to go.  I usually find that just changing from
Aluminum to Stell (same everything) will drop RPM ~100 (top end) and give me 
a little better all-around "look-and-feel".

	Also going from 3 to 4 blades helped my rig quite a bit (260hp, V8,
20'Sunbird).  This seems to help the bigger motors rather than the 4 banger's.
My partner-in-crime also tried a 4 blade, but it acutally made his rig run
worse (170hp, straight-4, 19' Larson).  You really have to get out the ole'
calculator, understand your hull, talk to LOTS of people with simular rigs,
then make your best educated guess!  Also find a marina that will allow you
to exchange the prop (assuming no damage) in case you guess WRONG...

	I usually find that if you have a sizeable circle of BOATING friends
that most of them will allow you to TRY their prop for a short spin (with
them present if the prop is Stanless!).  That worked for me an allowed me to
pick the RIGHT prop the 1st time, but only after trying 6 of my friend's
props over 3 months.

	BTW, I was flying ON (no joke) Winni this Satruday and the edges are
looking blue!  We landed out at my friend's island and the ice is still 2'.
There were several people with vehicles on the ice.  I guess its just the
first step off land that gets those people still intent on
DRIVING out to fish!!!

	Cheers

	garry

    As mentioned in an earlier note I was out last weekend testing
    different props. The results might be of some general interset judging
    from previous notes so I will put down my observations here:
        
    Boat:   21' moderate V, weight as tested about 2700 lbs
    Engine: Mercury 200hp V6, 1988.
    Propellers: 1. Aluminum 14 x 19 (dia" x pitch")
    		2. Aluminum 13 3/4 x 21
    		3. Steel 14 3/4 x 21 (Mirage)
    		4. Steel 14 3/8 x 21 (Laser II)
    All props where standard Mercury Quicksilver types .          
                                                                  
    Instruments: VDO Compact Logg (0..50 knots)                   
    		 Fuel flow meter (Cant remember the make just now!)
    		 Measuring glasses, stopwatch, map etc for instrument
    		 calibration.                                     
                                                                  
    
    
    Speed at WOT : 	1. 46 knots at  5900 rpm
    			2. 45	"    "  5400  "
    			3. 47   "    "  5400  "
    			4. 46   "    "  5500  "
        
    No surprises here, top speed differences almost non-existent and
    in fact far more sensensitive to external factors like trim angel,
    wind, boat loading etc as to prop choice. 19" prop allowing slight
    over-reving at wot with light load.
        
    RPM/Speed relationship: Between 3000 and 4500 rpm all the 21" pitch
    props were equal with the 19" one being just so much slower than
    one could expect from the difference in pitch. Above 4500 and below
    3000 the Mirage prop provided somwhat more speed for a given rpm
    value. (e.g 17 knots at 2500 rpm vs 14 knots for the others).
    Below 3000 rpm the 19" prop gave almost the same values as the 21"
    ones.
        
    Consumption/h vs rpm:
    Up to 3500 rpm there was no significant difference, after that the
    19" prop as expected had a lover consumption, the 21" aluminum one
    and the Mirage were higher and the Laser II was somwhere midway
    between those two and the 19" one.
        
    Consumption/mile vs speed:
    (This was really the measurement I was interested in!)
    Very similiar "bath-tub curves" for all four props with high
    consumption at slow and high speed and a fairly flat bottom in between.
    The "bottom" went from 20 to 38 knots for all props except for the
    Laser II where it went all the way to 40 knots and furthermore lay
    some 10% below the values for the others.
    Going above 40 knots or below 15 resulted in a steeply worsening
    economy with consumtion valus going up to 50% above those at 30
    knots.
        
    Acceleration:
    Not a very interesting measurement to me as I'm not doing any
    waterskiing but I decided to do some testing anyhow. It became a
    non-event when I found out how superior the Laser II was. With that
    prop I could slam the throttle from idle to wide open and just watch
    the boat take off with and acceleration that outpaced the speed
    indicator, with the others the throttle had to be carefully handled
    in order to not break loose the prop and have a "all rpm, no
    acceleration" situation. Due to getting tired I changed props before
    trying acceleration with the Mirage SS prop so it was never tested.
    I could kick myself now for that!
    I have no figures on accelaration, suffice it to say that the Laser
    II had acceleration, the others just allowed speed increase!
        
    Comments on the propellers: Both aluminium ones are standard propellers
    supplied with Mercury/Mariner outboards, the Mirage is a more or
    less standard type SS propeller intended for Mercruiser drives.
    The Laser II is a special SS propeller for Mercury outboards looking
    quite normal until you notice the slots in the hub in front of each
    blade. Those slots let out exhust gases in a controlled way into
    the blade area in high power/low speed situations and so let the
    prop slip in a controlled way. Like greasing the tires of a dragster
    I think! And it sure works beautifully!
        
    General comments: Testing props is hard work if properly done. Speed
    and consumption is affected by so many factors that much care has
    to be taken to get reliable measurements. Every measurement has
    to be done at least twice over and averages calculated etc etc.
    Instruments are not to be relied on. I found my speedometer to be
    non-linear above 35 knots altough it was spot on at least at 20
    and 30 knots. (At those speeds it was sensitive to boat attitude
    hovever, slight turns to port adding a knot or two!)
    The fuel consumption meter was accurate at medium flow when calibrated.
    How it is at WOT I don't know, I'm not that keen to run for a couple
    of ours at WOT just to calibrate the meter!
    The motor trim angle meter is useless. The system used by Mercury
    is built in such a way that all possible play in a number of linkages
    gets into the final measurement. I had to use a ruler to measure
    the distance from a point on the boat to the front of the engine
    in order to get the same trim angle for all props.
        
    Conclusion: There is not that much difference between the (standard)
    props. If acceleration is an issue there is just one choice: Mirage
    II. That prop gives the best fuel economy too. But it costs almost
    five times more than an aluminum prop....
        
    I hope all this will of some use to others trying to select the
    right prop for their boat, Mercury owners in  particular and others
    too!
        
    -jcl 	   	               
                                                      

 >>If acceleration is an issue there is just one choice: Mirage II.
 >>That prop gives the best fuel economy too. But it costs almost
 >> five times more than an aluminum prop....
        
  -jcl 	   	               
      
                                                
Great report, sounds like you really did your homework!  Just one 
comment; your final conclusion of the Mirage II as being the best
I assume is a typo (possibly a comination of the Mirage and the 
Laser II?).  From your testing results it appears that the Laser II
is the best?

Also, how were you able to get your hands on 4 props for testing?
Did you have to purchase all of them, were you able to borrow them
from a dealer or what?  It sounds like a custom stainless prop will
seriously improve performance, but at $3-500 a pop experimenting is
out of my price range!

Rick

    re .1  
                 
    Mirage II: Yes it was a typo, Laser II it should be.
                        
    Props: One (the 19") was of course my own, the others were borrowed
    from the dealer where I bought the motor. (In return they got copies 
    of all the graphs I produced!)
    I agree that bying that set of props would have been a financial
    disaster. In particular as finding the prop giving the best cruise
    economy was a primary goal. I would have had to use that prop for the
    next 90 summers to recover the hardware expenses through a 10% gain
    in fuel efficiency!                           
          
    -jcl                                          
                                  
    

	This is even more of a novice question than .0      I have a
	similar rig - 17' tri-hull, Merc 140 I/O.  My father distroyed
	my last prop with about 15 minutes on it.  I need to start
	looking for second hand props.  The number on the prop is
	
	48-65992A4-21P		What does that mean?  I assume the
	pitch may be 21"  (21P).  How do I find the diameter?  This
	is a 3-blade, so a ruler is eyeballing alum to air.  Radius
	doesn't work because too much alum missing and err is doubled.
	Is the diameter in the above number?

	This prop gives me more power for water skiing than I need and
	more top speed than I need, so I don't need to change.  However,
	I need a spare for when one is being rebuilt.
regards,
alan

    moved by moderator...
    
================================================================================
Note 945.0                ^^^ 2 Blades OR 3 Blades ^^^                No replies
MTWASH::GALLO                                        19 lines   3-FEB-1992 07:39
--------------------------------------------------------------------------------
                       QUESTION ON PROPS
    
               I have 14ft deep V aluminum with 15hp
               Sears/Force ingine. I bumped some rocks
               last year and I would like to get a new
               prop. My question is I have a 2 blade prop
               now, will a 3 blade give me more power.I
               notice smaller outboards have 3 blades i 
               feel the 2 blade is cutting the water.
               This boat is used for fishing but sometimes
               I like to see how fast it can go. One windy
               day I had it going 21mph. B^)   Mr Moderator
               please feel free to move this if you have to.
 
 Thanks in Advance
    Mike 
    
    
    

================================================================================
Note 945.1                ^^^ 2 Blades OR 3 Blades ^^^                    1 of 1
TOOK::SWIST "Jim Swist LKG2-2/T2 DTN 226-7102"        5 lines   3-FEB-1992 09:01
--------------------------------------------------------------------------------
    This is theory only:  The fewer number of blades on a prop the more
    efficient it is, but the more it vibrates.   So you won't get more
    power out of more blades (assuming pitch and all else is equal).
    
    

Mike,
There is so much to be considered when choosing a prop that entire books 
have been written about it.  It's not as simple as going from a 2 blade 
to a 3 blade prop to get extra power.  Your motor needs to have enough 
power to turn a bigger prop, since it takes more of a "bite" out of the
water.  In general, if you go to a prop with more blades or surface area,
you need to reduce the pitch to compensate.

If you're getting 21mph out of a 15hp outboard on a 14' boat, you're 
propped pretty close to perfect, in my opinion. Any attempts to experiment
with props could cost plenty and may actually slow you down.  You may be
better off to try shifting weight around, or changing the trim angle or
height of the motor.  If all these things and the prop are close to being
right, the only way to increase speed much further is to buy more HP.

Changing props may be a moot point anyway, since you mentioned your motor 
is a Sears/Force. I'm not sure what that means, I didn't think Force ever
sold motors to Sears.  Sears presently uses Eska motors, in which case
there may be little or no selection of optional props, leaving you with no
choice but the stock prop.

Rick

    
    
    Some of the letr seras water cooled motors with full gear shift were
    FORCE (Chrysler ) Motors. ESKA is also resold by Sears, but as far as I
    know, the 9 and 15 HP models with watercooling and full gearshift are
    force. 
    
    Dick
    

                            
                   The motor is a Sears Gamefisher But it
                   indentical to the Force 15 I bought it
                   new in 89. Motor runs fine, I paid 999$
                   I also bought the service plan. to be
                   on safe side, I never needed it. B^(
                   Anyway I need a new prop and just wanted
                   to gets some good advice from you Power Boaters.
                   Its true my only choice might be the standard Prop.    
                    
    Thanks Again 
       Mike G.
    

    
    
    	Has anyone tried a High Five prop out?  What I'm trying to find
    	out is if there is a significant difference between a High Five
    	and a good three blade stainless prop. There is a big difference
    	in price but is it worth it?
    
    	This would go on a 190 Sea Ray with the 260hp alpha. 
    
    	Craig
    

    
    	i was wondering if someone in the states would have any idea 
    of the price of a 
    
    spinelli open face cleaver style prop about 26-27 pitch to suit 
    mercury mod vp.
    
    their about $1500 oz,  and im wondering if its a big price difference,
    ill look for someone going there if it is.(i just cant afford that kind of
    bucks).
    
    thanks in advance.
    
    i wasnt sure if this should go in the wanted note or not 
    as im only after prices  and comparisons 
    etc. so if you feel its wrong pls move it.
    
    tks
    the other reg 
    
                         

    Moved by moderator.
    
================================================================================
Note 998.0                        PROP QUESTION                       No replies
USMFG::WGRABOWSKI                                     9 lines  22-JUN-1992 07:58
--------------------------------------------------------------------------------
    I was able to find a manual for the 1961 10 h.p. evinrude I just
    bought and noticed the prop on the motor is not what is called
    for in the manual. The prop on the motor is an 8" x 7" and should
    be an 8 1/4" x 8 1/2" pitch. Would the right prop give me a little
    more speed? Could I over rev the engine with the current prop?
    The motor seems fine but I want to set it up correctly.
    
                                                 thanks,
                                                 wayne  241-4405

    
      Prop selection is not a simple thing by any means. It all depends on
    your boat and the way you use it. I had a motor exactly like your ten
    for a long time. If you can tell me (us?) what size and type of boat
    you plan to use it on, I would be happy to make my recommendations.
    
      As for your other question... yes, you can overrev the motor with
    that prop, you could even lug it excessively. Neither is good for the
    motor, though overrevving it is what you wish to avoid the most.
     
    				Kenny

    The motor is being used on a 14' light duty [capacity 700 lbs.]
    aluminun Sea Nymph fishing boat. Of course right where it gives the
    max h.p. rating the sticker is ripped but I felt 10 h.p. was about
    right. The boat is normally used with two people and light equip.
    load. Since writing the note I spoke with two boat dealers. Dealer#
    one says the 8 x 7 prop I have was the "power prop" selection for
    that year. He said the 8 1/4 x 8 1/2 in the manual has been dropped
    and replaced by an 8 1/4 x 9. Dealer #2 says the 8 x 7 prop is for
    an 8 h.p. I didn't know evinrude made an 8 h.p. back then. Dealer#
    1 got his info off a chart so I tend to believe it but any inputs
    are welcome.
    
                                                   wayne
    
                                                         

    
      Indeed, OMC did not make an 8hp back then. Dealer 2 may just be
    quoting new info because they do make an 8hp now. It is true that the
    smaller pitch prop would be for more power. What that really means is
    the lesser pitch prop would be used on a heavier boat where you would
    need 'power' and could not expect speed. Now this can actually be
    confusing because it is not like putting on the 'speed' prop will make
    you go faster. In fact, on a good sized boat for a 10 (which you have)
    it is very likely that the smaller prop will make you go faster than
    the 'speed' prop. Since the smaller prop will allow the engine to run
    closer to its rated rpm (Which is where it makes its rated HP)
       In that year, OMC made 5 1/2, 7 1/2(I think), 10, 18 etc. The 18 is
    physically a lot bigger than the 5 1/2 thru 10. So I think you would know if
    you had that one. So I think the most hp you could have is 10. In that
    case and on the boat you have it on, you *do* have the right prop. 
    
    				Kenny

    Thanks for the response. As this was going to be a spare prop anyway,
    I went to the boat dealer to check out the information. He had a 
    chart from Evinrude that said for a 1961 10 h.p. had a choice of two
    prop applications. For 14' - 16' boats with heavy loads they
    recommend the 8" x 7" prop, the one I have. For 12' - 14' boats with
    any size load they recommend the 8.25" x 9". My boat is actually a 
    little under 14' so I bought the 8.25" x 9". Which ever works best
    I will use with the other becoming the "spare". It cost me $39.95
    which I felt was reasonable for a new weedless prop.
    
                                                    wayne

    Does anyone have the phone number for customer service at Mercury
    Marine. I would like to take with someone on prop selection.
    My 7.4l mercruiser ate another prop this weekend. I guess I have to 
    switch to a solid hub. That should fix the spun hub problems...
    
    Steve

    
    See note 983.1

    Well folks, I believe I'm to the end of the story with the Queen's
    Wheels!
    
    In the previous note about prop pitch,  I was chasing everything but
    the neighbor's dog looking for answers.
    
    Some of the things I found out as I got into the thing are:
    
    My current wheels are 23 inch, 22 pitch -- 3 Blade.
    
    The real xmission ratio is 2.9:1, not 2.0:1
    
    The Naval Museum in Norfolk Va has all the information on the Scurvy
    Queen -- All I need to do is get them the hull number -- Guess what I'm
    going to do.
    
    Anyway --- After all this running around, calling boating folks and
    stuff, I finally got an answer from Michigan Wheel (Prop folks)
    
    They say (from their print out):
    
    The Queen should have 22x19 wheels - 4 blade
    
    15% of the propellor is recommended clearance
    
    Propellors are highly loaded -- Sizing in this range is experimental
    and requires water testing for evaluation.
    
    Decreasing the diameter or pitch will increase Engine RPM.
    
    
    What I learned in my foray into this world was that props need to be
    "Square."  That means that pitch and diameters should be close (23x22)
    
    If the pitch goes less, handling at low speed sucks -- And as the
    Queen's captain, I ain't interested in bouncing off of things as I try
    to look cool motorin' out of the marina.
    
    If the pitch goes up, the stern squats and you don't get good
    operational performance.
    
    I also learned that proping a boat is akin to black magic -- Ergo after
    running a computer simulation, the Wheel company says -- Try it and see
    what happens -- Great anaysis!
    
    Anyway,  as I went through this drill, I had prop replacement costs go
    from a range of $600/wheel to $1000/wheel.
    
    I considered changing transmission ratios down to 2.57:1 and that
    ranged from $2,000-$3,000.
    
    With the latest information to decrease pitch and diameter and go to a
    4 blade prop, I wonder if I can cut down the three blade and get in the
    same range -- 
    
    According to a prop shop (Black magic artist), I was told that I can
    get 400 RPM by going down one inch in diameter.  I'm supposed to get
    150 RPM (average) for each degree decrease in pitch -- If I use the
    computer print out, it looks like 22X19 gives me:  400+(3*150)= 850 RPM
    increase. This puts me to the 4,250-4,350 WOT RPM range.
    
    What does 4 blade do?
    
    If I go with the prop adjustment, it's a $200 problem!
    
    Things are looking up on the Scurvy Princess -- The engine goes back
    into her tomorrow -- I'll be fishing on the Princess by April 1!!
    
    

    
    That's certainly what I'd do! Have one of the props' pitch
    adjusted to equal the other one and then use the technical
    advice that the place that wanted to sell you wheels for
    1000 clams each gave you... Try it and see what happens...
    
    Rick

Moved by moderator

           <<< FOUNDR::DISK$PAGE_SWAP1:[NOTES$LIBRARY]BOATS.NOTE;1 >>>
                                -< Powerboats >-
================================================================================
Note 1252.0                 3.0 litre prop selection                  No replies
SNMFS::BOWMAN                                        56 lines   1-APR-1996 00:42
--------------------------------------------------------------------------------
    
    
    Ive just bought a 3.0 litre 225 mariner efi motor and put it on the
    
    back of a 17 foot open cockpit runabout. 
    
    I used to have a 2.4 litre mod vp which had a top end of about 75-78 mph
    
    runing a 26 pitch straight six chopper at 6 and 1/2 inches raised.
    
    
    
    the new 3.0 is over 80 mph no problems im off the speedo now running a 
    
    full blde v6 chopper. 
    
    the problem is at take off im starting to take off but as the boat
    
    starts to plane I spin the prop and lose pull until the prop bites
    
    ive tried dropping the motor down to 5 1/4 inches and this has helped
    
    slightly.
    
    ive also tried a 23 pitch mirage 26p tubed cleaver 24p chopper 
    
    but the best one so far is still the 26p v6 chopper.
    
    
    
    I beleive what is happening is that ive got too much power for the 
    
    blade area of the props.
    
    
    I suppose what im asking is does anyone have any ideas.
    
    this does cause problems at the beginning of races when pulling out 
    
    skiers as they start to come up then get dropped back in until the 
    
    propellor starts to bite.
    
    
    also if social skiing its a real problem as well
    
    thanks in advance 
    
    
    the other reg
    
    
    
    
                                              
                                                         

moved by moderator

           <<< FOUNDR::DISK$PAGE_SWAP1:[NOTES$LIBRARY]BOATS.NOTE;1 >>>
                                -< Powerboats >-
================================================================================
Note 1252.1                                                               1 of 1
BIRDIE::WHYNOT "Malibu Skier"                         7 lines   1-APR-1996 10:15
                       -< Variable pitch prop perhaps? >-
--------------------------------------------------------------------------------
    reg,
    
       Would a Tourque Shift (tm) Prop be suitable for this application?
    I don't know if they (Land and Sea Inc.) make racing props...anyone
    know?
    
    Doug

 Alas, someone that might actually pass me! I think 75 in my new 19 1/2
 footer will be a tad more stable. 


 In that rhelm of high performance its trial and error. Perhaps Bob at 
 marine USA (Worcester, MA.) could give you some help. He races boats and has
 similar boats as you have. (508-791-7116) In addition to his help, they have
 many props there. If it is a prop issue they are usually willing to let you 
 test some while trying to find the one you like. I did that with my boat. I
 went through 4 props on my 225 before I found the right one for me.

 -Sully

    Sully...
    
      The situation that (the other) reg has is this:
    
    He races boats with a skier in tow on a river, so out-of-the-hole
    performance as well as top end speed are the requirements.
    
    Also, it seems like he has been experimenting with different props;
    He's now trying to optimize.
    
    One more thing...He's down in Australia, so a local dealer probably
    couldn't help him.
    
    Once again, I was wondering if a torque shift prop would be the ticket 
    for his application...Any ideas?
    
    Doug_Just a dumb inboard guy-what do I know about props  ;^)
    
    [reg-when you flush the toilet in Australia, does the water go
    clockwise or counter-clockwise??] 

 How was I suppose to know?...

 Now, there's a real nutty sport. I thought bass fishing was a bit stupid...

 Clutch props on high performance motors tend to spin and toss cups. Each one 
 of the 3 blades can be individually replaced. I'd hate to be skiing at
 75mph period but, add the thought of a cup spinning off. Don;t know if
 you could duck on that one. 
 Clutch props are not known for their top end. Most folks I know that have them
 get superior holes shots and great performance till they get to the top 
 end. Then I usually wave to them on the way bye...

 -Sully

    
    
    thanks for the replys.
    
    
    	Doug.
    
    	the water does spin the other way. i just tried it in the sink and
    
    it looked clockwise.
    
    im afraid a torque shift isnt the answer for high speed, people have
    
    tried them but racing is different.
    
    
    Sully 
    
    	although impractical at the moment thanks for the numbers as
    
    every now and then a friend will go across the big puddle
    
    to the u.s.a and these contacts can really excite them.
    
    the prices there are incredibly low compared to ours as we not only
    
    get import duy but about 3 stages of profit and tax.
    
    
    by the way my boat is  slow compared to the big boats out there
    
    skiers have been clocked at 116+ miles per hour on radar.
    
    
    
    theres even a tri rig with three 2.4 litre mod vp's on a 21 foot connelly
    
    its called mr walker.
    
    then theres the standard twin rigs and the great biiiiigggggg inboards
    
    the fastest of these is supposed to do approximately 130 mph
    
    im running a class where dont have to spend as much.
    
     in my class you have to run ski's no longer than 70 inches and 
    
    ropes no longer than 118 feet.
    
    these rules mean the skiers are the ones who win the race not the boat.
    
    the fastest they've let me run so far is 75 mph however im hopeful
    
    well get to sit on 80mph soon especialy in the sydney bridge 112
    
    kilometres long and grafton 108 kilometres long.
    
    
    we actualy pulled a third in our class at the last river race
    
     when we bolted on the 3.0 litre
    
    so the top end and acceleration are definitely there its just this 
    
    terrible takeoff.
    
    
    im going to try a few more engine heights. " oh for a hydraulic jacking
    
    plate" $$$$$
    
    
    one thing i forgot to mention is ive got a hundred or so litre ballast
    
    tank up the front which i can fill or dump whilst moving.
    
    so far i beleive the takeoff is better with the tank full.
    
    
    
    thanks for the info so far and any more you can think of.
    
    
    
    tks
     
    the other reg

    
    
    
    Late news; ive just heard from a mate who is partly sponsored by 
    
    mercury that there's a new style of propellor and he'll have a 25 and 23
    
    in the next few days.
    
    
    apparently this has been a problem with the 3.0 litres and mercury 
    
    has designed a new prop to suit this motor.
    
    soooo hopefuly ill be able to get one in the next week or two.
    
    after trying his to make sure.
    
    tks reg
    
    

    
    
    	Well i tried the 25 pitch trophy 4 blade prop small diameter
    
    	it pulled out just fine but the top end was a little lacking with
    
    	two skiers up i was only at 70 mph when the throttle stop came
    
    	into play. 
    
    	ive wound down the throttle stop and will try again with the motor
    
    	raised an inch or so from where it is.
    
    	
    
    	there has been a special tune done to the motor since the last time
    
    	so i should have tried the 26 chopper for top end again, ill do this 
    
    	next time as well.
    	
    
    
    	im also trying to get a 26 pitch large diameter trophy to try
    
    	 which could make a difference to top end.
     
    	ill probably wind up buying a 23 pitch large diameter for social 
    
    	skiing.
    
    
    	at 900 dollars a prop im not real keen to be buying just to try
    
    	
    	thanks for any feed back but i beleive im entering the realms of
    
    	the unknown.
    
    
    	tks the other reg
    
    
    
    	

    
    testing so far:
    
    well i tried a big blade 23" and  26" trophy.
    
    surprising results !!!
    
    
    the 23" big blade still cavitated at take off and did not have 
    
    much top end it pulled out ok but nothing special.
    
    "i was going to use this as a social prop and it probably would have 
    
    been adequate".
    
    
    the 26"big blade leapt out didnt have any cavitation and had great top end.
    
    
    wether taking off slowly or quickly the pull with the 26" was great.
    
    soooo im going to use the 26" big blade as a social prop as well 
    
    as a race prop.
    
    im now tring to source a 26" small blade for trial to see if that 
    
    makes a difference. 
    
    
    ive sent back the 23" 
    
    
    the other reg
    
    
    

    
    
    
    	well i tried the 26" small blade and it was hopeless
    
    	i beleive the problem is the diameter of the hub as it
    
    	allows exhaust to come round into the blades.
    
    	ive come to the conclusion that what i really need 
    
    	is a big diameter hub with the small diameter blades 
    
    	to give the revs.
    
    	the only way around this i can see is to buy a 
    
    	big blade 26 and thin the blades or take off the diameter.
    
    	however im not really keen to spend $900 to buy a prop to modify.
    
    	back to looking at props 
    
    	the other reg