T&T: Engine loading at low RPM

[Arild Jensen]

Concerning the use of CPP on recreational boats.
North American engine builders are dead set against use of CPP on
recreational boats because they fear such operators ar not trained and
knowledgeable enough to use them corectly. They fear it will result in
engine abuse and warranty claims.
Ther is perhqaps some justification for this concern. .
Also if you use a CPP you must have an exhaust gas temperature gauge.
Without it you have noo feed back to tel you if you exceed loading limits.

I think it's a mistake to use low RPM efficiency as the principal criteria
to judge if a CPP should be used.
The original use was to adjust engine speed and hull speed under extremely
high loading such as when dragging or trawling a big net full of fish. In
previous posts we have already established nobody is going fishing in their
trawler. <VBG>
So where else might a CPP make sense? How about making fine adjustments to
hull speed while keeping the engine RPM within a requird limit?
Canal running, ditch crawling, and jilling about while waiting for a lock or
bridge to open are all situations where  extremely slow forward speed is
required. But if you are runnning anything else from the main engine that
does require more than dead slow idling speed, then a change in pitch is
useful. In fact it can give you finer control than adjusting the throttle.
Remember also that maximum torque is not developed at idle. There is a curve
versus RPM. Sometimes you are better of selecting a faster RPM closer to the
torque peak and then adjusting the pitch. Michael Kasten is one advocate of
CPP for recreational boats. He had an article on his website describing the
merits.

regards
Arild

> -----Original Message-----
Rob Brueckner wrote.
>
> But whether or not you gain any efficiency is another issue.
>
> A prop selected so load is increased at lower RPM, either larger
> diameter and or pitch, or a controllable pitch prop,  can potentially
cause the prop to
> absorb much the same HP that would otherwise only occur at high RPM. <<<
snip >>>>