The Boating Forum - Inboard Prop Expert?
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05-27-2008, 06:34 AM
I run a 36' Stamas Express with Mercruiser 8.2L gas engines. It's a big heavy boat with a 13' 2" beam and fully wet (384 fuel and 90 water) it strains to get .6 MPG
With $4.25 marina gas, I'm looking to improve fuel efficiency if possible
I switched to 4-blade Nibral props (19") a year ago and she runs smooth with no vibration and WOT is right on.
Does anyone know if increasing the cup or pitch on the props would plane the boat sooner at lower RPM and increase fuel numbers? I realize that I would be sacrificing WOT number and WOT speed but I don't really care about those. I'm just trying to squeeze the most miles from each gallon.
If your boat is propped right, it will reach the lower end of its full throttle range in a loaded condition. I assume you mean 8.1's? If so, they are rated at 4200-4600 at wot. With full fuel & water and 6 people on board if you were at 4200-4300 that would be perfect. Simple fact is it is horsepower that burns fuel, not really rpms. A gas engine is most effcient at its torque peak, which on the 8.1 seems to be 3300 rpm; and most 34-38' boats run most effciently at 20-21 kts, so if you are getting 21+ kts at 3300 rpm, that is as good as it will get from an eficiency standpoint. Most important thing for eficiency - clean bottom.
05-27-2008, 08:07 AM
What LI said except I aim at the top of the WOT envelope. This prevents an engine overload situation.
When dealing with gas engines, they really can't be overloaded (like turbo diesels can). If they make rated rpm, in the 8.1's case 4200 under full load (meaning they are not overstressed (technically different than an overload)) conditions, it will allow for the increase in rev's that a light loaded condition will cause - some people will also check their boats light to make sure they don't exceed the maximum rpm of 4600 under use, which is just as bad for longevity. Unlike diesels, gas engines have lower (as a percentage of top rpm) maximum cruise rpms - in the 8.1's case it is 3600 rpm (compared to say a Cummins QSB 380 diesel which is 3000 rpm top and 2800 cruise), irrespective of whether wot is set at 4200 or 4600 - which is done to protect the valve train, which is the weak link in gas engines.
05-27-2008, 04:00 PM
Depending upon the year of boat 8.2's, or better known as the 502 were a common engine. Fully loaded at normal cruise those engines do get about .6 per mile. The specs on the 502 are 4600 to 5000 rpm.