prop for economy

I have a 2001 Carver Mariner 350 with twin Volvo (GM 350) 315 PS HP V-Drive turning 20/20 4 bladed prop. Gear ratio is 2.57. Displacement is around 18500lbs. I want to know if increasing pitch would give me more fuel economy at crusie. The engine WOT is 4600 to 5000. I presently get 26.5 knots at 5000 WOT. Someone said that increasing pitch would give me better economy at cruise. I cruise between 3500 and 3800 and that gives me 16 to 18 knots. I also having been spending more time at displacement speeds because of gas prices.

Thanks Steve

 

I may be out in left field on this one but I think you'd find your engines being overtaxed by adding more pitch. Performance out of the hole will suffer as you engines take up more load due to the increase in pitch. The reason I say that is we have a single I/O VP Duoprop. When we got the boat it had B6's on it. It was a dog and we could barely get it up on plane as the engine struggled against the pitch of the props. After doing everything, tune up, timing, rebuilt carb etc., we put on a set of B5's, it got better but still the engine strained against the pitch.

Just this weekend we installed a set of B4's and it was like a lightbulb going on. The boat just jumped out of the water and accelerated smoothly to plane in about 5 seconds. The engine was just humming along vibration and noise free and it just seemed to be running so very effortless. We lost some speed at cruise and some low speed manoeuverablility. I haven't had a chance to test fuel consumption yet (I have the garmin fuel flow sensor thingy but waiting for network components). From our usual evening trips up river and back I'd say we're looking at at least 10% fuel savings.

The other thing that'll happen with more pitch is cavitation. I believe this was the result of a lot of vibration and noise from our O/D. It may be different with inboards but I think they're still prone to cavitation. Cavitation will definitely degrade your performance.

If you're props are getting you to your spec WOT they're probably what's right for your boat. It's a fine balance of give and take. Now running at displacement speeds probably throws all that rationale out the window. More pitch at slow speeds may result in some slightly better mileage but at what cost? Fouling plugs etc. and forget about getting a good holeshot! On the other hand if the pitch increase is large your engines will have to produce more power at slower RPM to move them which I would think burn just as much or more fuel.

FWIW just a few ideas.