Conference unifix::sailing

    I try to avoid the pleasure of anchoring in grass because the
    Danforth I use like to fill up with weeds and not bother to
    dig in.
    
    Having a smaller boat (34') my tackle is a bit more user friendly.
    I can "feel" the anchor take its bite thru the nylon rode. I too
    back under substantial power (about 3/4 up the power curve) after
    I think the anchor "in" to be sure it is set.
    
    One thing I do that I notice many do not, is give the anchor time
    to begin to dig in.  When I lower the anchor to the bottom, I ease
    the line out as the boat heads back down wind, at first I put nearly
    no tension on the line, then begin to pull on the line as it goes
    out of my hands (hand over hand) with increasing tension untill
    I reach the point where I want to cleat it off. During this time
    I am not driving the boat backwards, I let the wind blow me backwards.
    I can feel the anchor moving thru the vibrations in the line (which
    you cannot thru the chain), and it always seems settled by the time
    I am 70' or so out on the rode. I always anchor myself, so when
    the rode is all payed out and the boat is settled downwind, I stroll
    back to the cockpit and back down (again slowly at first). When
    I reach about 1800 RPM in reverse I start looking for landmarks
    to line up and use to tell if I am staying in one place. I the ones
    I have chosen stay put for a minute, I am satisfied that the anchor
    is going nowhere.
    
    What I have seen others do, range from dropping everything in one
    spot and going below, to backing under power too quickly from the
    spot where the anchor was dropped, which never gives it a chance
    to dig in (just plow little channels all over the anchorage).
    
    Walt 

    My routine on my previous 36' Mariner was similar to what Walt
    describes in -.1.  It worked well, but I don't think I was ever
    sure about how much throttle to test the set with.
    
    I looked in Chapmans and "a back down load in excess of any anticipated
    loads should be applied."  VERY helpful!  Elsewhere I found "the
    average cruising sailboat generates 20 lbs of thrust per horsepower."
    Chapmans also cites a test example using the same rule of thum
    
    So, my back of the envelope thrust calculation is in the ballpark.
    Walt's 3/4 power for enough time to make sure that you're not dragging
    looks like a good way to go.
    

Estimating the area of a boat exposed to the wind is difficult since the 
mast and rigging contribute significantly to the area, especially head 
on. Another way of estimating the wind force is 

	force = .000134 * loa * loa * wind speed * wind speed

(see note 373) where force is in pounds, loa in feet, and wind speed in 
knots.

re .2: 20 pounds of thrust per horsepower may well be a good estimate.
However, I would assume that the horsepower here is the horsepower
actually being delivered by the engine, not the rated horsepower.
Moreover, the thrust number is probably forward thrust, and reverse
thrust is usually less than forward. Based on fuel consumption, it
appears that our 25 hp diesel (max power) is actually delivering about 8
hp at cruising speed (1800 rpm). Backing down to set the anchor with our
engine at 1800 rpm would imply a load of about 160 lbs applied to the
anchor rode. This is (for a 32 foot boat) about the load imposed by a 34
knot wind. So it would appear that backing down as hard as possible for 
a minute or two is an excellent way to ensure that your anchor won't 
drag too easily. 

Alan

    RE: .1   
    
    My appologies for poor proof reading of my reply.  I managed
    to leave out a couple letters and punctuation marks which makes
    reading my input difficult.
    
    Walt

    re .3
    The equation I found for thrust (force) from a propeller:
    
       f (lbs)  = W/G*(VO-VI)
    
    where
       
       f = force in lbs
       W= weight of column of flowing water in lbs/sec
       g = 32.2 ft per sec*sec
       VO=output velocity of water in ft/sec
       VI=input velocity of water in ft/sec
    
    So, Alan your horsepower estimate at cruising speed is low for output
    when the boat isn't moving.  VI isn't zero since there is flow around
    the hull from the prop, but it probably isn't equal to boat speed
    at "cruising power" either.
    
    There is some loss in the drive train, engines tend to be over-rated,
    there is a big difference between 2 and 3 blade props, angles of
    prop shaft, aperature effects etc. so there are lots of "adjustments"
    to the calculation...  I started with the column having a radius
    of .7 the prop radius, used the pitch as the advance per revolution,
    and then multiplied by .5 (fudge factor) to get the thrust. I guessed
    at VI at 2 kts.
                                               
    The alternative formula for wind force yielded 310# versus my guess
    which did include the area of the mast and rigging.  If we split
    the difference that yields about 550+# at 35 mph (43.5 LOA).
                   
    So, 3/4 power looks very safe as a test for 35 mph, half power may
    be ok.
    

    
    	I would think that another *SUBSTANTIAL* source of error would
    be the fact that it isn't the steady pull that will uproot an anchor.
    Surely the surges caused by riding up onto a wave front, then being
    let go and having another one hit just as the rode gets tight again
    are going to add a large factor to this....., no ?   Or is this
    all accounted for by chosing the right scope ?, somehow I doubt
    it.
    
    	Reg
    

    Yes, I think that the surge from wave action or veering of the boat
    would increase the load above the steady load from the wind.  I
    think the only way to help in a big storm is with an adequate nylon
    rode that can act as a big shock absorber.  The boating mags told
    of all chain rode boats that broke loose in a surprise Cabo san
    Lucas storm whereas nylon rodes held.
    
    Now this raises another interesting engineering problem, ie What
    is the optimal rode for reducing load on the anchor?
    
    If, for a minute, we eliminate concern about chafe, what size (dia)
    nylon should be used?  Obviously, some fraction of the breaking
    strength should not be exceeded as this will permanently stretch
    the line.  It would seem the the goal is to store as much energy
    as possible in the rope "spring" or would this just increase the
    veering oscillations?  
    
    Another idea would be to engineer the line to have very high hysteresis
    or some other frictional losses to dissipate energy.
    
    I've never seen an analysis of these issues in a quantitative analysis
    or dynamic simulation.  Anybody know of a place to look?
    

    William Van Dorn's Oceanography and Seamanship explores dynamic
    loads on anchors and rodes.  It is not casual reading.  
    
    Steve

re .9:

Walt, I'm so glad you like my ideas. I just wish you'd give me credit 
for them! Please see see notes 69 and 373. My lawyer will be in touch 
regarding royalty payments. (This is said with a big grin.)

Seriously, SAILING notes contains a wealth of not always easily found 
information, information that is still as useful today as it was years 
ago. (I wrote note 69 in December 1984, for example.) People new to 
SAILING might find it useful to read the old notes. We should all make a 
greater effort to use keywords to make information more readily 
accessible. (At the moment, DIR/KEYWORD=ANCHORING lists two notes. I 
know that there are others.)

Alan

re .7:

As a first approximation to answering the questions asked in .7, 
consider the following:

Any anchor rode will elongate (stretch) when a load is applied to it. 
Elongation is usually specified as the percent change in length at a 
given applied load. 

Let E = the fractional change (the percent change divided by 100) in 
        length at applied load T
    T = the applied load in pounds
   L0 = the length of the rode with no applied load
   L1 = the length of the rode with an applied load F
    F = the load on the rode in pounds

Mathematically, 

  (L1-L0)/L0 = E*F/T			(1)

Or

  F = T*(L1-L0)/(E*L0)			(2)

This says that to increase the length of the rode from L0 to L1 a load F 
must be applied. Note that as E decreases (ie, the rode becomes less 
elastic or, conversely, as the rode becomes more rigid), the force 
required for a given length change increases rapidly. 

To give this some reality, for the three-strand nylon rope sold by West
Marine, E=.165 for T=.15*S where S is the breaking strength of the rope.
For 1/2 inch rope, S=7500 pounds. 

To stretch a 100 foot length of this rope to 105 feet, a load of 341 
pounds must be applied. 

Now assume a boat is anchored in calm water and assume that the anchor 
rode is straight (in reality, it will sag). Assume that the distance (in 
feet) from the bow roller to the bottom is V and that the length of the 
anchor rode is L0. Next assume that a wave of height W (in feet) passes 
so quickly that the boat rises vertically but does not move horizontally 
(not too far from reality). One of two things happens: either the rode 
must stretch to length L1 or the bow of the boat must be further 
submerged. Well, actually both of these happen, but mostly the rode 
stretches.

Some simple geometry yields the following equation:

   L1 - L0 = sqroot(L0*L0 + W*(W+2*V)) - L0		(3)

This result can be used in equation (2) to estimate the load on an 
anchor rode that results from a passing wave. 

A chain rode has essentially zero stretch (E approximately zero), so 
that once a chain rode is pulled straight, the loads caused be waves 
become extremely large (approach infinity). 

Now assume that the 1/2 inch nylon rope discussed above is used for the 
anchor rode. For V=20, W=10, and L0=100, F=168 pounds. This is a rather 
moderate load given the assumption of a wave 10 feet high. However, the 
elongation given in the West catalog is the elongation when the load is 
applied fairly slowly. When the load is applied abruptly (as by a 
passing wave), the elongation is less. How much less, I don't know.
Assume it is only one-fifth as much. Then the load due to the wave
becomes 842 pounds. This load is enough to cause serious problems (like
dragging the anchor). (It is interesting to note that this wave is 
stretching the anchor rode only 2.47 feet.) 842 pounds is 11% of the 
breaking strength of the rope.

Generally, the safe working load of a rope is taken to be 20% of the 
breaking strength (1500 pounds for the 1/2 rope). If 3/8 nylon rope is 
used for the rode, the load due to the passing wave is reduced (since 
the thinner rope stretches more for a given load). However, the load as 
a percentage of the strength of the rope remains the same.

This would imply that the smaller the rope rode, the better. Still,
hmmmmm. I can easily believe that a more exact analysis of the dynamic 
loads involved in anchoring would be extremely complex.

Remember that splices and knots reduce the strength of the rope 
significantly. A bowline has only about 60% of the rope strength and an 
eyesplice over a steel thimble only about 90%. And, of course, there is 
the minor problem of chafe. I'll still use at least 1/2 inch nylon rope 
for my primary rode.

Alan

    RE: .9 and .10
    
    I must have read those notes. Unfortunately, I have gotten out of the
    habit of searching back for references to KEYWORDS and TITLES,
    so I forgot that: 1) the mushroom idea probably did come from Alan's
    note in 69, and 2) I had already described my anchoring technique
    at length in 373.
    
    But isnt it nice to know someone found success with a technique
    even if they dont know where they picked it up ? I never did claim
    I invented it, but I do remember thinking thru the problem and
    arriving at the same solution last year - no sense of Deja Vu either
    so note 39 was buried deep in my subconsious, if not forgotten.
    

    re .11
    
    It's interesting that some simple geometry gives insight about how
    all chain rodes are a bummer in a storm!  Alan's approximation gives
    me hope that something other than a massive simulation might yield
    results.
    
    One comment:  I would believe that rate of application of force
    to a nylon line is analagous to force applied to a spring.  That
    is if the spring mass is small, the inertial dynamics don't matter.
    Or nylon (a polymer or plastic, as you like) has linear stress vs
    strain until one gets into the permanent stretch area (creep). 
    In fact the 20% of breaking strength rule of thumb may come to be
    because that is the max load for sustained periods of time without
    significant creep.
    
    

re .13:

I agree that an ideal spring has the same elongation (or compression) 
for a given applied force no matter how quickly or slowly the force is 
applied. Metal springs are close to ideal. However, I can think of at 
least three mechanisms responsible for the elongation of a nylon rope 
under load. First, of course, the nylon fibers stretch (until, as was 
mentioned, the elastic limit is passed). Second, in a twisted three 
strand rope the twisted fibers tend to straighten under load. Third, the 
fibers tend to pull together, reducing the space between them. These
last two mechanisms would occur even if the nylon fibers were inelastic.
It is interesting to note that West Marine also sells braided nylon
rope. This rope stretches only (as I recall) some 6% at a load of 15% of
the rope's breaking strength. Three strand rope stretches 16.5% at the
same load. The difference must be due to the construction of the ropes 
since I would assume that the same nylon fibers are used for both.

The reason I suspect that nylon rope elongates less when a load of short 
duration is applied quickly (wave action on a anchor rode) than when 
a long term load is applied slowly is because nylon is a plastic. One of 
the characteristics of many (most?) plastics (and other materials, too) 
is that they creep (or deform) very slowly under load. The rate of creep 
is dependent on the nature of the material and can be quite fast or 
slow.

My approximation indicates to me that me that if the loads due to waves
were small (as a result of the short term elongation being equal to the
long term elongation), then anchoring in big seas is no problem. We know
it is. 

Alan

    Just to confuse the issue, nylon can be heat treated to maximize
    or minimize the tendency to stretch.  
    
    I'm familiar with kernmantle (braided sheath around bunched fibers)
    ropes used for rock climbing and caving.  The climbing rope is
    optimized for stretch to absorb energy and break a fall, the caving
    rope is made so as to minimize stretch so as to make climbing the
    rope with ascenders easier.  
    
    Don't assume that all braided ropes will behave the same with regards
    to elongation and energy storage under tension.
    
    I almost wish there was a uniform labeling requirement for cordage
    that would spell out type of construction (laid ropes differ in
    quality according to how many times they're twisted which way),
    breaking strength, and elongation.  
    					J

We anchored off Fort Adams in Newport, R.I., USA to listen to the 
Newport Jazz.  It is a hard place to anchor, as there are many boats in 
a very small space, and the water is 40 foot (12M) deep almost right up 
to the shoreline.  The bottom is mud.

It was amusing to watch someone throw out a small anchor with 50 foot 
(14M) of scope out, cleat it down and turn off his motor.  Unless, 
of course,  he was on your anchor (or upwind/current of you).  I had this 
happen.  I moved backwards about 3 foot, and then held.  Nice discussion!

I have a Catalina 27, and was using a Danforth 18 with 15 foot of 3/8
chain, and had no problem holding on about 2.5 scope (90 foot of line 
+ 15 foot of chain).  Wind was blowing at about 15 knots peak gust and 
there was about 1 knot of current...

I learned last year that you need a big anchor to hold and as little 
scope as possible to limit your swing.

The Jazz was real nice.


Phil

    re .15	Interesting that you should bring this up at this time.
    As one of my other selves I have recently come across essentially
    the same data as it applies to arborists (or is it arborers ?  well,
    tree cutters).  It seems that the folks who specialize in trimming and
    felling big trees limb by limb (in crowded city locations) have
    recently switched from the climbing type to the caving type of safety
    line for exactly the stretch factor you mentioned.  I keep meaning to
    bring my Sampson ropes book in and entre something about these things here,
    but there's lots of math that I don't have symbols for and graphs that I
    couldn't represent anyway.  I'll post the publication name and
    Sampson's address (somewhere in Shirley, Mass) instead, soooonnnn...
    
    	Reg
    

    Having once had the unnerving slip problem I vowed it would never
    happen again and now tend to "over-anchor" if anything like heavy
    weather is predicted.
    
    Firstly, I always carry at least 2 types of anchor. My main workhorse
    is a 15lb CQR (what size boat I hear you say - 20' says I) but I
    also have on board a danforth and a small collapsible fishermans.
    The danforth is used for fore-and-aft anchoring in a tide-way, or
    for kedging but the fishermans really proves it's worth as a tandem
    anchor with the CQR, especially on weed where the CQR is poor.
    
    Usual tandem technique is fishermans out first with 15' of chain
    to the CQR and then 25' of chain attached to the warp up to the
    stemhead. This has never slipped - touch wood - and is a technique
    frequently used by the Mediterranean yachtsmen.
    
    As to digging in, I only have a 4hp outboard, but it pulls in hard
    enough, the boat coming to a dead stop at all times.
    
    The mathematics of anchor pull I leave to the experts, relying on
    the traditional size of anchor to length of chain to length of boat
    that have been used successfully for generations!
    
    Brian

    I had a discussion with a New England Ropes engineer at Newport
    boat show about how to configure a rode for maximum damping (or
    loss), the idea being the more energy dissipated in the rode, the
    better for the boat.  I learned rope engineers don't think this
    way - as they want the least energy loss in most applications.
    
    Some interesting anecdotal information:
    
    Rodes subject to cycled stress and or shock loads demonstrate a
    continuous decrease in breaking strength.  It was strongly recommended
    that a mooring pennant be thrown away after a hurricane exposure.
    
    Catenary(s) are the easiest way to damp out shock loads.  Large
    sea tows may have out a mile of wire that droops 100's of feet.
    Nylon hawsers (eg 12 inch !!) have been used for what are called
    "straight line tows" and yield significant gains in tow efficiency,
    but are rarely used because the high loads break things.
    
    As mentioned in -.n construction and twist play an important part
    in the lines performance.  Water trapped inside the rope has little
    effect.  Lubricants used during construction of nylon rope wash
    out over time, and the nylon absorbs a significant amount of water,
    which affect the strength.  Nylon does not have a yield stress point
    like most materials - it just keeps stretching, as much as 300%
    before it breaks.
    

    
    	Last week I put out the anchor in the usual way, backed down,
    and down, and down......, could see the bottom, still only felt
    like small fish were nibbling at the anchor once in a while.
    
    	Eventually I decided to haul it up and check that there was in
    fact a shackle between the chain and the anchor, just the kind of
    thing I'd be likely to forget.  Sure enough, the anchor (Danforth)
    was there, but I had snagged the perfect sized smooth rock right
    between the tines (blades, whatever the damned things are called).
    Which made the "Anchor" into a smooth tipped bottom skipping piece
    of junk.  This was a *VERY* fortunate experience, I had snagged
    the rock right on the surface before getting any bite at all, it
    could have happened a couple of inches down and fooled me into thinking
    that I was dug in.  I am currently looking for another style of
    anchor, i.e. plow, grapnel, Navy, whatever - but *NOT* another Danforth !
    
    	Reg
    

    I am a newcommer to this file so forgive my impertinance in putting
    my input to the subject of Anchoring. The Royal Yachting Association
    that looks after boating interests here in the UK also is the examining
    body for various boating qualifications. In summary their view on
    anchoring is thus : 
    			- Always carry as large an anchor as you can
    afford or stow. It should not be below that recommended by the builder.
                        
    			- The Kedge should be at least two thirds the
    size of the main anchor.  
                        	
    			- Each should have a minimum of 2 Meters of
    suitable chain attached
    
    			- In settled conditions veer 2.5 times the depth
    of chain or 3 times the warp. If conditions become unsettled veer
    more using the skippers judgement.....
    
    			- When anchoring drop in a controlled manner
    the depth of the water plues one/two meteres (To avoid pile up on
    the anchor) touch astern settle and then veer the rest.
    
    			- Once all the chain/warp has been veered take
    three compass bearings  and check again after 15 mins etc.
    
    All pretty sensible stuff. When I was in the Channel Islands this
    summer I anchored of the island of sark that is well known for its
    upredictable swell heights I ended up veering 5 times the depth
    of water. I was using chain and held in winds of force 6/7 with
    considerable tidal streams running ie greater than 5 knts. 
    
    I carry a CQR as a main anchor (30 Imperial pounds) and a Danforth
    as my kedge. My boat is a 28 ft bilge keeler. 
    
    I hope someone finds this interesting if not actually useful.......
    
    Pete
    

    re .20, jammed Danforth
    
    I've had a similar experience with a rock jam.  A credible friend
    also picked up a beer can on the tip of a CQR plow, so I don't think
    they are foolproof either
    
    A good reason to set carefully!
    

    Anyone ever try "downwind anchoring?"  I didn't see it
    mentioned here, and I haven't yet looked at the other
    notes mentioned.  
    
    Basically, the technique uses all the sage
    advice about lowering the anchor to the bottom and feeding the 
    line out hand over hand, but instead of doing so bow-to-wind,
    you do it while driving with the wind -- keep on a couple of knots
    of speed.  As the anchor touches down, steer the boat into a shallow
    turn to guide the rode down one side of the keel.  Play the anchor
    to get it to catch as you feed out the scope.  Ease out
    somewhat more than the appropriate scope, and snub off the line.
    At this point, keep your hands OUT OF THE WAY.
    Keep the direction of the boat's travel at a slight tangent to 
    the rode, thus when it springs taught, the majority of the boat's 
    momentum is channeled into setting the anchor, and the secondary
    effect spins the boat around -- at which time you should shorten rode 
    to the proper length, and double check feel once the boat settles back.
    
    This method takes practice, but it WORKS.
    If the anchor doesn't catch, you have to go around for a second
    try, but when it does set, it's in for the duration.
    I've only done this with a Danforth, and I've never had to
    "go around" more than twice -- regardless of bottom condiditions.
    The boat is 45 foot and heavy, and we normally drop a 22 pound
    Hi-tensile Danforth with 20 feet of chain on 3/4 dacron.  This is our 
    "lunch hook" configuration, but it's held us and two other similar
    boats in gale conditions -- that after another fellow's 
    35 pound CQR went belly up!

    Eventhough this is 8 months after the last entry I hope you are
    still reading this topic.
    
    re .20  
    
    While anchored in Mexico I once had a 65 ft power boat catch my
    bow anchor.  He method of getting free was to apply full power to
    his boat.  I had a 22s Danforth holding my stern and this anchor
    in sand was able to hold me even with him applying full power. 
    My bow line was streached tight as heck.
    
    The moral of the story is "An anchor should be used for the bottom
    conditions it is designed for"
    
    re .7
    
    I was in the storm at Cabo san Lucas the night of Dec. 8,1982 and
    I do not recall hearing nor reading afterwards about any chain that
    parted.  I was very supprised about the short length of chain being
    used.
    
    We had run into a sister ship ( we gross about 34,000 lbs.) that
    was using a 105# CQR. The reply to our joking was that when he put
    the anchor down he didn't move.  After the storm we upgraded our
    gear.  We carry a 75# and 65# CQR with 300 ft. of 3/8 high test
    chain and 200 ft. of 3/4 nylon on one side and 100 ft of 3/8 high
    test chain and 400 ft of 3/4 nylon on the other side (bow). On the
    stern we carry a danfoth 22s and 40s with 50 ft of 1/4 chain and
    250 ft of 1/2 nylon and 75 ft of 3/8 chain and 600 ft of 5/8 nylon.
    
    We use a MORITZ manual single stroke (no gear) windlass and it takes
    us about 40 minutes to pull up the 300 ft of chain.  Twice, once
    in Mexico in '83 we reanchored pulling up our chain twice because
    we were not happy the way we were sitting, and in Bora Bora in '86
    we pulled up our chain and reanchored twice because we were not
    happy the way we were sitting in relationship to the other boats
    as the wind shifted - my wife works the windlass and you should
    have heard her yell at me, but it was her idea to reanchor because
    of the other boats.
    
    We were enjoying a cove to ourselves in Taha when another sailboat
    came in and dropped their anchor right where ours was.  We made
    them move - and not 20 minutes afterwards another boat came in 
    and dropped their anchor in the same spot.  Since there was plenty
    of room we made the move also.
    
    Unfriendly?  Heck no,  we wanted to be able to sleep well at night.
    
    
    Remember, you and your crews' safety depends on your anchor and
    your anchoring ability.....as well as your boat.
    

    A yacht anchored in a bay near here renowned for the amount of debris
    on the bottom which fouls anchors so he set a trip line to his CQR
    only to have a power boat attempt to pick it up as a mooring!! They
    both drifted off merrily towards the rocks .....

re -.1: 

Unfortunately, I've heard similar (unfunny) stories before. We have our
boat's name painted on our trip line buoy in an attempt to prevent such
nonsense.