[C320-list] Maxprop RPM v. boat speed

[Jeff Church]

Pete,

What do you have the pitch set at? You appear to be over-pitched, but I 
don't have the engine specs here. Isn't the 1-hour rpm rating somewhere 
around 3700? Ideally, you should be able to reach that rpm. As Larry 
pointed out, the tachometer is about as accurate as the fuel gauge. 
Borrow a laser tach or buy one at a hobby shop for about $20. They use 
them to measure the prop speed on radio control airplanes and helicopters.

Before you change anything you should verify that the engine can reach 
the no-load rpm rating. If the engine can't reach the no-load rpm, it's 
time to call a diesel mechanic. If it can reach that rpm, it's time to 
play with prop pitch.

PYI will tell you that rpm will change 12% for each 2 degrees of pitch. 
I think the actual change that I experienced was less than that. 
Unfortunately, I don't have complete records any more (sold the boat), 
but I do know that when my 15" MaxProp Classic was pitched at 18 degrees 
(with a heavily loaded boat), engine rpm was just a bit too low, and 
maximum speed in calm seas with a clean bottom was about 7.1 knots. At 
16 degrees rpm was high enough, speed was down to about 6.8 knots, and 
the engine ran cooler. The downside was that the prop had a bit less 
bite and boat speed suffered in rough seas. 17 degrees would have been 
just about perfect, but the MaxProp Classic can only be set in 2 degree 
increments. If you have the MaxProp VP, 1 degree adjustments are possible.

In any case, the correct pitch for your boat should be somewhere in that 
range, but it will depend somewhat on how you use the boat. If you just 
day-sail in a lightly loaded boat you will probably want to go with a 
bit more pitch. If you cruise extensively with a heavily loaded boat 
you'll want a bit less pitch so that the engine isn't overloaded.

Good luck.

JeffC


Sea Chalet wrote:
> I've got a Maxprop on my 2001 C320 with the 3GM30 engine.  I'm 
> wondering if I have the pitch set properly to achieve maximum boat 
> speed and engine RPM.
> This is what I measured (avg. of opposite directions):
> 2000 rpm - 4.3 kn
> 2500 rpm - 5.3 kn
> 3000 rpm - 5.8 kn
> 3200 rpm (max.) - 6.1 kn.
> How does this look?   Should I be getting higher speed with higher 
> max. rpm?  Are there any other resources out there to check?
> Thanks, all.
> Pete Peterson
> Sea Chalet
> #815
>
>