[PUP] :PPM-propeller efficiency

[Bob Frenier]

I hope we can get back to the very lively and interesting topic of the
mythical perfect passage maker. It sure perked up this list when it started.

 

No matter whether the PPM costs $300,000 or $3,000,000, the efficiency of
the drive train is a key element of range and range is a key element of any
PPM. That brings me to the efficiency of the propeller. Whenever I look at
engine manufacturers' charts, there appears to be an extremely low level of
propeller efficiency at the engine's lowest rpms in a propeller that has
been selected to be perfectly pitched and sized at maximum engine rpms. And
since we have discussed at length the fairly extreme difference in fuel burn
at low rpms vs. high rpms, aren't we faced with a dilemma? In other words,
just at the rpm which brings out the best efficiency in the engine, the
efficiency of the propeller stinks. Leaving aside for a moment whether
buying a controllable pitch propeller ("CPP") will save enough fuel to pay
for itself, won't a CPP increase propeller efficiency dramatically at those
low rpms where the engine sips fuel? And won't the CPP put a perfect load on
the engine at any rpm, so running slow doesn't gum things up?

 

Most of the discussion I've read about CPPs dismisses them because they are
"designed for boats with varying loads," like fishing trawlers; but that
doesn't answer the question of range posed above. Any of you experience guys
care to weigh in on this? 

 

A note on steel: there seems to be pretty widespread agreement that modern
coatings on steel have handled the rust issue--assuming no standing water is
allowed by the hull design and there is modest maintenance. Having a lot of
detailed photos of the welding and coating processes plus a written report
from an engineer who inspected the steel and coating in the boat builder's
yard before it all got covered up might allay the fears of a subsequent
buyer. The interaction between steel and aluminum has apparently been
handled with modern techniques that bond the two metals together, so a steel
hull with aluminum wheel house is pretty common and straightforward.

 

I know zip about composites. Would its advocates prefer to be aground on
rocks in one of them versus a steel hull? I'm ready to learn.

 

 

Regards,

Bob Frenier

Advantage Apparatus, LLC

Chelsea, VT