Pontoon Boat & Deck Boat Forum

My boat is a few weeks old now. I've got 30 hours on it and want to purchase another prop for a main, and use the current 13.5"/15p for the spare. I'd like to go with a 4 blade prop. I currently get around 24 MPH at about 5200 RPM. I think I can do better than this but don't know which way to go with the new prop. The specs below show an RPM range of 5000 - 6000. At 5200, I think some power is being wasted.

What would you go with to get the max performance out of my F90?

My boat is the Bennington SLX20.



My motor specs.

F90 Specifications
Engine Type In-Line Four Cylinder
Displacement 97 ci (1596 cc)
Bore x Stroke 79 x 81.4mm (3.11 x 3.20in)
Prop Shaft Horsepower 90hp @ 5500rpm
Compression Ratio 9.6:1
Fuel Induction EFI/SOHC/4 Valves Per Cylinder
Exhaust Through Propeller
Intake Single Throttle Valve
Ignition TCI Micro Computer
Spark Plug LFR5A-11-00-000
Alternator Output 25AMP
Starting System Electric w/PTT
Lubrication Wet Sump
Engine Oil Capacity 3.7L/3.5L w/without filter
Full Throttle RPM Range 5000 ~ 6000 rpm
Cooling Water/Thermostatic Control
Recommended Engine Oil Yamalube® 4M (See owner's manual)
Recommended Fuel Regular Unleaded (Minimum Pump Octane 87)
Recommended Fuel Filtration Yamaha Mini 10-Micron Fuel/Water Separating Filter (external)
Ethanol Blend Limit 10% Maximum
Gear Ratio (25:11) 2.27:1
Gear Shift F-N-R
Shaft Length L = 20" X = 25"
Degree of Tilt 70°
Degree of Trim -4° through +16°
C.A.R.B. Rating 3-Star
Dry Weight 25" shaft / 169 kg (372 lbs) ; 20" shaft / 166 kg (366 lbs)
Steering Angle (maximum) 35° from center, either direction
Warranty 3 Year Limited Pleasure Boat - 1 Year Limited Commercial

This be general advise; selecting props for pontoons is nowhere near as simple as it sounds as we have the hull dynamics worse than that of a BARGE...

Your prop diameter is something you likely CAN'T go larger on but you can go a little smaller when switching to a 4-blade. Example my stock prop is a 14" but in the 4-blade Spitfire a 13.8" was called for....

If you reduce your PITCH by 1" your RPM should increase by 200 RPM +/- this may increase your speed if your engine is currently unable to increase RPM due to being at max load. But it may not as less pitch means less distance traveled per revolution...

If you increase your pitch by say 1" again you should go faster by whatever speed 200 RPM equals to but again if your engine is already at max load as is you may indeed end up slower as the engine can't swing that much pitch...

Oh and BTW 24 MPH out of a 90 HP on a 20 foot pontoon is pretty good in my book...

My 18 foot pontoon with a 60 HP is hard pressed to break 14/15 MPH with a boat load of riders and poking at 18/19 MPH with just me on board and whatever junk is stowed under the seats... Thus far I've tried three different props a 14x11 3-B stock, 13.8x11 4-B Spitfire and 14x10EC 3-B with very little change in speed as my motor just is at its load limit...

Just go for a Yamaha 13 pitch aluminum prop to get you closer to the 6000 desired revolutions per minute. You may be lugging a little with your current 15 pitch prop.

If you look at the better performing pontoon boats, you'll find 3 blade propellers. 4 blades work best on bass boats that have ridiculous horsepower/weight ratios.

Remember that pontoon boats are all compromise. They're asked to be fast cruisers, tow tubers, handle ridiculously heavy passenger loads and ride good. They just don't do everything good.