Re: Fuel Economy

[Don]

Well, most of those pontoon boats have a certain hull speed, pushing the throttle down farther after reaching that speed usually does nothing more than lug the engine and waste fuel. You mention the speed limit of 5mph, I would guess your friend is trying to exceed that limit.
Definitely have a look at the prop, you may need a different pitch for such a specific application. Some manufacturers make special "hi thrust" props for pontoons/no planning hulls. Has the pontoon boat been pulled out of the water lately? Perhaps the pontoons are fouled with marine growth or full of water. The prop may be beat up or have a spun hub as well.