Pontoon speed vs rpm

[Devlinreacher]

I recently purchased a 2015 Harris Grand Mariner 250 with a Merc. 250 verado Pro. With 3/4 tank of fuel and 4 passengers I am running 4000 rpm’s and only running 22-23 mpr… 3200rpm. Is 16 mpr. I am around 42 mph at WOT 6100rpm. Am I wrong in thinking that although, wot seems correct, that at 4000 rpm I should be going faster than 22? Low speeds seem to be too high rpm’s burning about 9-10 gph averaging 3600 rpm’s. Thoughts?

 

[Texasmark]

Apparently your "prop slip" is high at the lower speeds. Go-fast.com has a prop slip calculator. You input your gear ratio...like a 2:1 would only have 2 entered, propeller pitch and your RPMs. Answer comes up "theoretical max speed" which would be a screw turning in a piece of wood type thing. Enter your actual speed and click on the "slip". Do it for WOT first to get your lowest slip number and then work back through your lower RPMs for your slip numbers.

Different props, usually SS as they are higher performance, can make a big difference in your slip numbers. Engine height and trim position can too. You may need to go to a 4 blade "stern lifting" SS prop (Solace makes one that I use....different boat and engine). However, top end could suffer, usually suffers, and you can't have your cake and eat it too. Have to decide what is most important.

 

[ahicks]

Maybe consider, if you haven't already, that it takes X amount of fuel to make Y amount of horsepower, and your prop is using that Y amount of horsepower to do what it's doing.

I doubt you're going to change X or Y much, but the prop may be a different story.....

All sorts of variables, compromises, and priorities come into play here, so providing much in the way of help will be difficult.

Are you pulling anyone regularly? How big a priority is that?

 

[airshot]

For whatever reasons my experience with pontoons is that they need more hp than a onventional V bottom of the same size to go the same speed. When running at 10-15 mph they do pretty good on fuel but when real speed is wanted, it takes a lot of extra hp to get them moving. Extra hp means more fuel. Never saw a toon that was easy on fuel when speed is wanted. Your issues are comon with tooners, from my experiences

 

[jlh3rd]

I recently purchased a 2015 Harris Grand Mariner 250 with a Merc. 250 verado Pro. With 3/4 tank of fuel and 4 passengers I am running 4000 rpm’s and only running 22-23 mpr… 3200rpm. Is 16 mpr. I am around 42 mph at WOT 6100rpm. Am I wrong in thinking that although, wot seems correct, that at 4000 rpm I should be going faster than 22? Low speeds seem to be too high rpm’s burning about 9-10 gph averaging 3600 rpm’s. Thoughts?

these are my readings I took this summer on a lake in Pa.
The pontoon is an '07, twin 25" tubes, 24'. Two adults on board, full 24 gallon tank, no wind.
Tach is a mercury smartcraft, speed is GPS, motor is a 2021 Merc. 115 hp Command thrust, prop is a black max ,3 blade, 16x12.....
I was trying to see if there was a certain speed that gave the best mileage..
Mostly linear results as you can see....
Hope this gives a little perspective as to your question....

 

[Scott Danforth]

pontoons are they hydrodynamic equivalent of a brick

they take a whole lot more hp than you think

the speed to RPM is never linear, its a 4th order polynomial curve

youre fine, go boating

 

[jlh3rd]

pontoons are they hydrodynamic equivalent of a brick

they take a whole lot more hp than you think

the speed to RPM is never linear, its a 4th order polynomial curve

youre fine, go boating

although I was referring to mpg.......I looked at my rpm vs speed and except for the 6mph gain between 3000 to 4000 rpm....which I attribute to either a planing effect....or a mistake on my part.....the readings showed basically linear results for every 1000 rpm increase.......so, ?

however, the gph is more geometric.....is that the same as a 4th order polynomial curve...?

 

[QBhoy]

The prop slip will always be enormous at low planing speeds and lots of hull in the water. I’d imagine especially so, on a pontoon. Are they not notorious for venting at low planing speeds. More than a normal hull…at least. Seems that the engine height set up for these is a tricky thing to get right. Read about similar things with these often.
Doesn’t help that they aren’t really the most hydrodynamic of things perhaps

 

[jlh3rd]

tell me about it......Probably why the newer ones have the motor pod extended past the 'toons' ends....

 

[Kelley2403]

If you want to go fast and take it easy on the MPG's get a tri-toon.