25' pontoon engine question

[Pontune]

I have a 25 ft sun tracker with a 115 with 8 people in it I only get about 18 mph I'm wondering if going to a 150 will make a big difference

 

[ahicks]

What is your top rpm at wide open throttle (WOT)?

17' sounds like a lot of prop to me. I consider stainless to be bling at these performance levels. Fine if you have it, but I would never pay for a new one.

Our 90hp Honda (4 stroke) will run our fairly light 24' to 25mph on either a 13 or 15" prop.

 

[Scott Danforth]

I have a 25 ft sun tracker with a 115 with 8 people in it I only get about 18 mph I'm wondering if going to a 150 will make a big difference

Not in speed, however the noise level and fuel consumption would go up.

2 logs no strakes 5 adults average build & 3 kids under 10 has a 17 pitch s.s prop

you would need lifting strakes and maybe a 3rd log to get the 'toon up on plane

 

[HotTommy]

The Mercury Prop Selector tool suggests a pitch of about 13 for a standard gear case and about 14-15 for the Command Thrust gear case. It appears you have too much pitch. That can be confirmed by noting the RPM at WOT. If it is well less than specified for your engine (e.g., 5500 RPM), then you could improve performance with a different prop. .... My 24' two log pontoon boat with a Mercury 115 HP Command Thrust engine sees about 21-22 MPH with the same load you describe.

 

[Pontune]

WOT is at about 5800 rpm I have been told that putting a 150 on it would make a big difference on top speed roughly 25 mph just not sure if that is true

 

[wahlejim]

Something is not adding up here. Using gear ratios of 2.05 and 2.31, your prop slip is coming in at 55-60. Either your readings are inaccurate (not your fault, could be faulty Tach or speedometer), or you are way over propped. I am guessing a combination of the 2. I would verify readings, and try a 13 pitch prop as suggested. You should be able to see mid 20s. I would dial the prop in, much cheaper and easier than a new engine.

 

[Pontune]

Ok I'll try that thanks

 

[HotTommy]

Pontune,
Google Mercury Prop Selector and check the results for yourself. If your speed is coming from a water driven speedometer, load a GPS app onto your smart phone and check it again. I suppose its possible your prop is slipping on the hub, but that usually results in a sudden high RPM with little thrust. .... I agree with wahlejim - this is not the typical wrong prop story. Something is off.

 

[HotTommy]

Pontune,
Here's another data point for your consideration. I just took my boat out in calm conditions with a very light load (just me and a half tank of gas). I hit 26.5 MPH using GPS.