Complicated subject. Knee jerk comment -- I really worry about vibration.
Mea culpa. Sorry.

Dave Gerr (Propeller Handbook, International Marine, 1989) is not super
helpful here. "Assuming we settle on the acceptable minimum diameter ...
there are few disadvantages of [a smaller propeller] at cruising speed and
above." I read him (as para-quoted above and in other places) to say that
there is a minimum below which THOU SHALT NOT GO, but above that, diameter
is less important -- not Evan's second or third power to efficiency.

So, I should have said, "really worry about tip clearance" as insufficient
tip clearance will eliminate the advantage you get from the AquaDrive. Gerr
suggests 10% (of the diameter between the tips and the hull) absolute
minimum, 15% probably OK, 20% best.




--
Jim Woodward
www.mvFintry.com


..
"Evan Gatehouse" wrote in message
...

"Jim Woodward" jameslwoodward at attbi dot com wrote in message
...

I've made my thoughts on this very clear in a recent thread -- I'd buy
the
traditional MaxProp three blade in a second and never look back. I'd
also
consider going an inch or two smaller in diameter (and up in pitch)
since
you're not near square and a principal contributor to noise and
vibration
is
blade to hull distance -- every half inch counts.

I wouldn't go smaller if possible. Prop efficiency is related to diameter
to some power (^2 or ^3 I can never remember). So "an inch or two" would
be
huge difference in efficiency - and a 16" prop is too small for the engine
size Glenn is installing.

I think that you can just compare prop dimensions and see what tip
clearance you can get. If you can get 20% of prop diameter great.

No opinions on the props under considerationj other than I don't like Max
props flat blades with no twist.

I might have to consider a similar propblem in a few years. I'll let the
group know in a week or so what the next boat might be


--
Evan Gatehouse

you'll have to rewrite my email address to get to me
ceilydh AT 3web dot net
(fools the spammers)