T&T: Engine loading at low RPM

[Robin Brueckner]

"...In other words, if a CPP can increaseload at low rpms, would we escape the
engine damage of low rpm use that accompanies fixed pitch propellers carrying
a low load?..."

In a word: Yes.

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. An
increase in diameter does increase prop efficiency; I'll leave it to others to
opine on how increased pitch alone may/may not affect prop efficiency. I'm not
sure.

This subject of low RPM engine operation has been discussed several ways on
the Hatteras Owners website: in a nutshell, if you have a turbo diesel and
measure the turbo exhaust temp to be sure you are in the normal operating
range, say 600 or 700 degrees for a two cycle Detroit, you are in peachy keen
shape. But if you get that result at moderate RPM, it's not possible to run at
normal high operating cruise RPMS...the engine will be overloaded, overheat,
or incur other unpleasant results.

On the other hand, if your prop is already about the maximum diameter because
of insufficient additional hull clearance, you may not gain much....that goes
back to whether increasing pitch improves efficiency. One way to accomplish
this would be to move from a three blade to four blade prop, or just buy a
three blade with greater pitch.

Whether a particular engine is itself more efficient at lower RPMs, with
proper increased loading, is an additional issue.

 Rob Brueckner
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