Need ideas for a new prop for my 74 Starcraft

[eckhoffb]

I have a 74 Starcraft 21 aluminum boat. Motor is a 74 Johnson 115. Prop i believe is original prop. It is a 15" pitch. At WOT I'm getting 34 mph @ 4700RPM.
I been looking at maybe getting a four blade prop, something to help get on plane faster and hold it there at lower speed. Any help is really appreciated.

 

[Chris1956]

In theory, a 13" pitch prop would give you 5100 RPM. An 11" pitch prop would give you 5400 RPM, plus some extra, for getting more into the motor's powerband. I am not familiar with your motor, but 11" pitch is pretty low by today's standards. Is the motor running properly? Is your tach correct?

 

[eckhoffb]

I believe the motor to be running well. I have spark to all 4 cylinders. Its been recently rebuilt. I also believe the tach is working properly too. Its brand new and show idle at about 1000 rpm. Any idea what a 4 blade with 13 pitch would do? I think if the boat could get further out of the water it would move easier and bring rpm up.

 

[buckinrut25]

First thing to do is get your current pitch confirmed. Most boats run an average of 8-15% slip and your is not showing any. Running the numbers accounting for slip your current prop would be a 17. I would confirm the pitch and get a GPS speed to know your working with good numbers. In general a 4 blade requires a pitch drop to keep Rpm about the same and will help with hole shot and lower speed cruising.

 

[eckhoffb]

Finally found out what this prop is. It's a 13 3/8 x 17. So I am thinking of ordering a 13 four blade prop. Any objections to that?

 

[eckhoffb]

And it is stainless

 

[buckinrut25]

That move will get you to the top end of your rpm range. It should improve the areas your wanting but will cost you a little speed. You will prob find yourself around 5500 and 30mph.

 

[steelespike]

A 17" prop is pretty typical. And stainless should work well.
Don't assume all is well because of the rebuild.A 74 I wouldn't be surprised that timing advance and carb sync is all mechanical under the hood.
Throttle may not open all the way.
Your rpm range is 4500 to 5500 So at least your within range. I think a 13 is over kill a 15 is more reasonable being a 4 blade itself should
improve hole shot, reducing pitch could add about 300 rpm putting you at about 5000.
All that being said changing from ss to al and 3 to 4 bld. can make the results more unpredictable.

 

[eckhoffb]

Thanks for the help. I ordered a 15 inch four blade. Figured I should try that before jumping to a 13 inch.

 

[MikDee]

I agree with steelespike!