Prop Pitch Q: Top Speed for very small runabout (Disney old school)

[top_notch]

I'm looking for more (top end) speed for my little speed boat.


Current (banged up ) prop is a 9.5 x 10 stock aluminum 3 bladed prop.

Motor is a 1974 Evinrude 15hp

Not sure of WOT RPM. (There are no gauges)

Average load: 225# (Me and one kid)

Boat is a 1973 Aqualark, 8 foot in length, weighs about 175# + motor + gas.

So total weight: Boat (175) + me (180) + gas (40) + motor (60) = 455# Total

Is there going to be a big difference in a 9.5 x 11 prop given that the motor is only 15hp? Boat planes decently now and goes about (seat of the pants) 30 mph, but I'd like a little more speed. The boat is only driven at WOT. lol

Will the 11' get me on plane faster or slower? Or will the 11 pitch lug the 15hp too much down low and up top?

For reference...

 

[Robbabob]

Re: Prop Pitch Q: Top Speed for very small runabout (Disney old school)

Slower to plane. Don't know about top speed without knowing current rpm values. There is a likely chance if the 10P is correct for max rpm, then you will lose top speed and lug the engine.

 

[QC]

Re: Prop Pitch Q: Top Speed for very small runabout (Disney old school)

If your current prop is banged up, then I will predict better top speed regardless of the RPM range. You should looking into a TinyTach or similar so that you can get some data.

 

[steelespike]

Re: Prop Pitch Q: Top Speed for very small runabout (Disney old school)

A cite little potato chip.
Boat is probably rated for 2 adults 280 lbs.
Most likely if the motor is healthy it can handle the 11" prop.
And increase speed.Biggest drawback is the small footprint.Will likely be slow to plane.
Passenger and perhaps driver may have to get up and lean forward on the deck to plane
PFD required.Raise the motor on the transom and adjust the trim carefully.
The 15 should be able to run 27-30 in an ideal setup.