Re: [Amel Yacht Owners] Re: Props and rpm



I completely agree with the engineering theory of your analysis.  

Where we might differ is I simply do not believe that a on a boat like an SM the drag difference between an Autoprop(worst) and a Maxprop (best) would be at all noticeable to anybody cruising. Especially since people who would chose the Autoprop for the promise of its improved motoring performance are most likely to be motoring at the times when the difference would be largest.

All of these options have drag an order of magnitude lower than a fixed three blade prop, and that difference is most certainly easily noticeable!  But the differences in drag between the various folding/feathering options simply isn't significant given all the other variables involved.

In the real world, an Autoprop costs about $1000 more than a classic Maxprop of similar size. If I accept on faith the maximum fuel economy improvement that Autoprop claims in their marketing (30%), to generate a positive payback would require more miles of motorsailing  than I will likely do in the rest of my cruising life.  Other people do motor sail a lot more than I do and might come to a different conclusion.  

Bill Kinney
Sm160, Harmonie
Rock Sound Harbour, Eleuthera, Bahamas

---In amelyachtowners@..., <jvenegas@...> wrote :

I did some studying when I was buying a prop for my previous Beneteau 361.  I found that selecting the type of folding prop depended on the objective.  If it is to reduce drag while sailing the Flexofold or Max prop are pretty good doing that, and better than the Auto-prop which, at low wind conditions tends to take strange configurations causing extra drag. In the category the Max prop because its blades are flat has the lowest drag.
If the objective is to minimize fuel consumption during motoring, the blade of the Flexofold and the Autoprop are better shaped with an angle of attack that is reduced from the center outwards (twist).  This is important because the speed of the blade relative to the water increases with the radius of a rotating object.  The Autoprop blades have a constant angle of attack (flat blades) which is less efficient and can cause flow separation, turbulence, and possibly cavitation reducing their efficiency. 

Finally, if optimizing fuel consumption while motor-sailing is the objective, the  Autoprop is best as it adjusts its angle of attack as the wind increases and keeps the prop working efficiently.  Fix pitched props like the Flex or Max as the wind increases they end up spinning adding little power.   

In summary,  If cost is not the issue, I find that the Autoprop is Ideal for those of us that tend to motor-sail when the wind is low or comes from the nose.

My 5 cents.

Jose Venegas
Ipanema SM2K 278

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