I'd like to try a 4 blade prop..

[Kwdog75]

I have a 1975 Starcraft Cheiftain with a 1977 Evinrude 115hp, it has a 13.25x17 prop on it now but i would like to try a 4 blade. It currently goes 32mph at 5300 rpms. Any suggestions are apprecited.

 

[TunaFish389]

Why a four blade? I find them usefully for keeping bow rise down, up on plan a little sooner. Other then that fours have more slip (it uses more of the energy to lift stern), less speed. I like a four for pulling sports, it keeps my bow down so I can see and I can plan at lower speed. Other then that I run a three blade.

If you have a good running engine, proper tach numbers and GPS speed. I would try to hit 5750 rpm's at wot. A three blade 13.5x15p should do the trick.

 

[Kwdog75]

My 3 blade has seen better days lol it has nicks and pieces missing out of it.

I,d like to try something new so i figured why not try a 4 blade.so it can be a little more responsive turning.

 

[TunaFish389]

Ok then the 13x15p four blade will get you close.

 

[Kwdog75]

Would it be a wise idea to get a 3 blade 13.25x17 and a 13x15 4 blade?

I'm thinking that way i will be able to switch back and forth depending on what i'm doing?

How much top speed will i loose with the 4 blade?

 

[TunaFish389]

I went from a 19p three blade to 17p four blade same diameter, same brand and model and lost 4mph on top and gained 300 rpms. This was with a 3.0 I/O though but stayed within my max rpm range. The four blade is at the top of rpm range and three near the bottom.

 

[jimmbo]

You are pretty close to the top of the Engines WOT RPM range. which is 4500 - 5500. An extra blade will drop your RPM and your top speed. Since I avoid 4 & 5 blade props like a Plague, I would check to see if your old/current prop has a cupped blade. If it does, I would just put a new 17 on it. If the blade is uncupped, a new 17 with cup would drop the rpms. Changing to a 15 with cup should place your rpms very close to 5500