Conference unifix::sailing

    Is your boat sail or power?  I ask because it looks like way too
    much horsepower for a 20 foot sailboat!  If you plan to drive the
    boat in displacement mode, then it is not too difficult to make
    a good guess at size and pitch.  If this is a planing boat then
    I recommend you contact the hull designer for a recommendation.
    
    For sailboats the most important data needed is how big (diameter)
    a "wheel" (prop) you can spin.  The rule of thumb is the bigger
    the better, but at least 15% of the diameter clearance between hull
    and blade tips.
    
    A good book on the topic is "Theory and Practice of Propellers for
    Auxiliary Sailboats" by J.R. Stanton.  It is 68 pages of practical
    data and available from good marine bookstores.
    

    Grant,
    
    Any experience that you can share re: your feathering Max-Prop?
    
    I had been expecting to put one on my Sabre but recent discussions
    with the factory have left me confused. They claim that there have
    been so many complaints from owners who have added this prop that
    they refuse to install one upon request. They'll only supply the
    standard fixed or a folding Martec.
    
    The complaints revolve around a loss of 1 knot of motoring speed
    which they cannot seem to recover by experimenting with different
    pitch settings. Supposedly there is not enough hull clearance to
    go beyond a 2" pitch change. This confuses me since my measurements
    indicate that there is almost 13" of distance from the shaft to
    the hull and an 18" diameter prop should have plenty of room. Pitch
    change shouldn't affect the diameter, right?
    
    Also, I just spent the last week at the Miami Boat Show (I know,
    tough duty, but someone......) and the Max-Prop people there insist
    that there should be no such loss of speed, plus they claim that
    the prop has been installed on a number of Sabre 36's and are going
    to supply me with references. Also the Westerbeke people reviewed
    the process for adjusting pitch to RPM for a proper match and they
    don't expect any problems.
    
    Sabre is generally very good with technical issues of this type.
    But this time I'm trying to figure out if they're over-reacting
    to one or two noisy, but mistaken, customers.

    In the mean time I have to buy a prop and get my IMS measurements
    completed soon! Hate to put a Martec on if I don't have to.
    
    John_R

    A note:
    
    To avoid vibration, power loass, and possible damage, minimum clearance
    between propellor tip and hull should be about 45-50% of propellor
    radius. This becomes more important as pitch increases.
    
    Ross Faneuf
    

    Ross,
    
    Where can you point me to get more information on these minimum
    clearance figures? Since my clearance would be 4"/9" (or 44%) I'd
    be right on the hairy edge if your 45-50% figures are accurate.
    
    On the other hand, if Grant's figures are correct then I'm still
    OK.
    
    A Maxprop is > $1100. I can't afford to experiment.

    John

>>>>>    A Maxprop is > $1100. I can't afford to experiment.

Hell, I can't afford a Maxprop! (The 'max' in Maxprop must refer to the 
price.)

    Yea. I think I will put a wing nut on my fixed prop first :^)

    The clearance figures come from a paper on prop clearances, the
    effects of shrouding, the effects of bodies in the prop stream (e.g.
    a prop strut, and resulting vibration, power loss, and
    cavitation/erosion effects. The paper was published in about 1980,
    and I will have to go searching. Some older texts give 20-30% figures,
    but the paper's reported bad effects made it clear that higher
    clearances were desirable. Note that you can go lower, but vibration
    and possible erosion become worse. I'll see if I can get more detail...
    
    Ross Faneuf
    

    re .2
    
    John, my first (and lengthy) reply got lost and in the meantime
    I've been away for a week, so here goes again.
    
    My MAXprop works well (22" D by 18" P).  It is a 3 blade version
    and set according to PYI's guess which agreed to what I computed
    out of the book mentioned in .1.  This pitch pushes the boat about
    one knot faster at an engine rpm of 2100.  RPM's are limited by
    clutch slip which should get corrected at launching this year. 
    The pitch is the same as the fixed 2 blade that I removed, so I
    may end up dropping a notch if a can't get RPM up to 2400.  By the
    way, 2100 seems like an honest 8 knots and more power may just dig
    a bigger hole in the water!
    
    Under sail I've gained at least 1/2 knot below 12 knot apparent
    wind.  Also the helm is more responsive without the prop turbulence.
    
    I can't say much about reliability after 1 year, but if their
    mechanical engineering was as well done as the machining, it should
    last a long time.  Burr Bros installed it and PYI got the taper
    right, but forgot the keyway.  This was done locally.
    
    ROLLICK didn't get much use last summer, so I had severe barnacle
    growth on the prop, which cost 1.5 knots.  I scraped them off and
    everything got well.  I've written to PYI about compatible antifouling
    paints (before 1 Jan), but don't have a response from them as yet.
    
    The three blade has less vibration etc than the two and gives me
    more thrust as well.  The boat STOPS if I gas it in reverse.  But,
    all the fuzziness of moving backwards are unchanged, somehow I'd
    convinced myself this was going to get better as well.
    
    Yes, they are expensive, but going faster under power and sail,
    less vibration, and easier/safer docking are significant benefits.
    I bought mine at Newport when ROLLICK was in the show and got a
    30% discount, which is quite a savings.  I'd guess that any boatyard
    would get the same, so you might shop around or tap a friend for
    a deal.
    
    Good luck!
    
    

    re .7
    
    I checked my reference (Stanton) and 15% of diameter is the MINIMUM
    suggested.  Note that this is not too far from 40% of radius.  A
    chart in the text shows 15% clearance yielded a hull force of 12%
    of maximum (not sure how "maximum" is detirmined).  It gets pretty
    asymtotic after 20% of diameter.
    
    My recollection from boatyard inspections of props when I was
    considering a new one for the Mariner is that:
    
    1. most sailboat props don't have enough diameter
    2. 15% of dia is about average clearance
    3. most aperature installations have terrible losses
    
    The 4/9 is 22% on diameter, and I would think should work well.
                                                      
    

    I tracked down the source of the complaints concerning the Max-Prop
    and found out what the 'real' problem was. Selecting the proper
    pitch may be a trial and error process. It appears that Sabre is
    concerned about possible warranty costs associated with hauling
    and/or divers, so they take the safe way out. The boats with problems
    weren't even Sabre's but were another manufacturer (C&C) by the
    same dealer.
    
    I've ordered a two blade, 18" which Max-prop says should be set to
    a 12" pitch according to their calculations for my engine.
    The standard solid was an 18"x10".
    
    Will let everyone know what happens.