PROP PITCH ON 32+2 CB

[Doug]

We have a 1997 32+2 CB with a Cummins 370 hp turbo (6BT6)

When we first got the boat the prop pitch was 22 X 25. Wide open the engine would only turn 2850 RPM. Our Cummins mechanic told us it should turn 3000 RPM under full load. He suggested we have the prop repitched, which we did. First to 22 X 24 and later to 22 X 23. Now we can get the engine to turn 3050 wide open, but our fuel milage has changed substantially. We now get almost 3 MPG at 8 knots, but less than 1 MPG at 13 knots, which is the area we like to travel. I want to repitch the prop, but do not know if I should go back to 25 or go to 24" pitch. I know this may be somewhat a strain on the engine not being able to turn 3000 RPM, but I doubt I will live long enough to see any damanges. Any thoughts on this?

Doug Fuller
RELAX'N
St. Petersburg, FL

[Veebyes2]

Hmmmm, I believe mine is a 19X19. It is 4 blade & cupped. The engine is only a 315hp Cummins. Max RPM is where it should be for that engine, about 2600. I'd be concerned about overpropping. Any savings made in the shorterm could be more than gobbled up in repairs longterm.

[ScubaPete]

The mechanic was right about pitching the prop to reach 3000+ RPM at WOT for the 370 hp 6BTA. For the 315/330 hp 6BTA, WOT should be at least 2800+ RPM. Overloading the engine is the primary long term concern here. If you never (or rarely) push the engine above hull speed, then that may not be too much of a concern. There is a ton of information on the boatdiesel web site about this, especially with these two Cummins engines.

[jruesr]

Something is very wrong here. Hopefully it is your data. I have a 32 sportsfish with a 370 Cummins. My mileage averages 1.6 kt miles per gallon while I run 15 kts all day. You cannot compare prop pitch between these different boats unless you know the gear ratio on your drive system. Even then you need advice of a good prop shop. I would suggest that you keep the boat so it will run 3000+ rpm and recheck all your data (tach calibration, where the tach is reading the engine, fuel usage and gps data). If you over-prop your boat you simply do not allow enough air intake to burn the fuel and the result is unburned carbon in the system (not good). Perhaps try cleaning your bottom. Hopefully there is something simple but if you get less than 1 kmpg then keep looking for the answer.

[bigdishdoctor]

I have a 32+2 with a 370BTA. When I purchased the boat I had the same 22x25 4-blade LH my WOT rpm was 2800. Long story short, my final pitch turned out to be 23-inches. After lowering the pitch (WOT 3050) the motor sounded less strained and mpg did decreace by 10% or so. My fuel burn at 2600 rpm (15-16 knts) is around 13 gal/hr. When conditions allow 2600 rpm is where the boat runs best. The above numbers are with the boat well loaded with fuel, gear and people. Note that sea, wind, trim and weight have a significant affect on mpg. That being said your numbers sound reasonable and I would not repitch for better mpg.