[Public-List] Standard Prop Size and Pitch

[George Dinwiddie]

Brian Palmer wrote:
> On L'Esprit, I've got an Atomic 4 with the 13x7 prop.  The engine runs
> strong and smooth, but when in gear, I can't rev higher than maybe
> 1900RPMs.  If I push the throttle higher, the engine doesn't go any faster.
> I think it's just that the engine is working hard enough that it simply
> can't spin the prop any faster under that load.
> 
> It's not a problem, since I can motor along at 6 kts or so on flat water.  I
> hear people mentioning 2300RPMs...  It would be nice to have this as reserve
> power if I was motoring against the wind and chop or something.  Do those of
> you who also have A4s with the standard 13x7 2 blade find the same
> limitation?

It's true that the A30 with an Atomic 4, a 1:1 transmission, and a 13x7 
prop is over-propped by all recommendations.  It seems to work pretty 
well in spite of that.  While you're not getting as many horses out of 
the engine at the lower RPM, I think you may be getting more torque.

It would be interesting to know the performance in various conditions 
and the fuel efficiency of various props, but I think gathering real 
data would be hard to do.  I also think that the benefits are marginal, 
particularly considering it's a sail boat rather than a power boat.

By all means, try another prop if you want, and let us know the results 
you notice.  I just wonder about the return on investment tweaking 
something that's worked for half a century.

  - George

-- 
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   When I remember bygone days                         George Dinwiddie
   I think how evening follows morn;            gdinwiddie at alberg30.org
   So many I loved were not yet dead,           http://www.Alberg30.org
   So many I love were not yet born.
                'The Middle' by Ogden Nash
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