I need a faster prop

[smsscuba]

I have a 1998 Johnson three cylinder 35hp outboard with a four blade "whale tail" prop
(10-1/2 x 12).I can only get 26 knots on my 12ft speedboat (WOT at 5000rpm). Is there another prop type I can use to achieve any more speed from the engine?

 

[QC]

Re: I need a faster prop

I understand that motor has a 2.15 gear ratio. If that's true something is wrong with the numbers. I'm getting -13% slip which means at 26 knots you are going faster than the propeller can take you. My guess is that the tach, the speed, or the pitch is wrong and maybe more than one of those. How are you getting your speed number? GPS? Define "whale tail" prop. Is that maybe "elephant ear"?

 

[smsscuba]

Re: I need a faster prop

I used a Garmin GPS to measure the true speed. To be honest, there is no tach installed, but the outboard is certainly producing high revs.I have attached a pic on the original post for you to see the (whale tale?)

 

[steelespike]

Re: I need a faster prop

Sure looks like your motor is 2 to 3 inches too low.And you are not only dragging too much motor through the water but the whale tail as well.The antivent plate where tail is attached should be even with or about an inch above the bottom.Raising the motor alone could add a couple of mph.
A tinytach will give you your rpm can't make a correct selection without.They have a web site.
With the motor raised and the correct rpm and speed we can help select a prop.

 

[QC]

Re: I need a faster prop

If the speed and pitch is right, I think you're spinning well over the WOT range. 10% slip puts you at 6300 . . . so the tinytach thing is important if you are serious about getting this right. If you raise that motor, and get rid of the whale tail, then it will be even higher RPM. "Whale Tail prop" is what had me confused . . . Got it now. MIGA (make it go away) at the very least for testing.

 

[Maxz695]

Re: I need a faster prop

Might also be why theres no paint left on that prop must be hitting ground constantly. Raise that engine and retest it

 

[smsscuba]

Re: I need a faster prop

guys, I don`t understand.The cavitation plate is 100% on the same level as the bottom of the hull.It does not show so well in the pic.Where am I going wrong????????

The other question is, what type of prop would suit me better, could someone give me info please.

 

[smsscuba]

Re: I need a faster prop

would a 10 1/2 x 14 three blade prop be any good?

 

[steelespike]

Re: I need a faster prop

If the motor is over reving yes a 14 would likely help. Keep in mind if the motor overevs it will eventually self distruct if it lugs constantly you will have premature repair problems beside having poor performance. A 4 blade is a poor choice unless there is a venting(cavitation) issue.
You should raise the motor to a point just before venting becomes a problem and you want the foil clear of the water at speed.Usually the antivent (cavitation)plate ends up about an inch above the bottom. Boat/motor design will determine the final demension.

 

[Silverbullet555]

Re: I need a faster prop

would a 10 1/2 x 14 three blade prop be any good?

There is no way to know until you know the RPM the motor is turning at. You have to know that number so you can determine if you are in the right area or not. Just by sound isn't going to cut it.