speed boat prop

[boostedmax]

i have a 16' speed boat with an 1983 115hp evinrude and i need some opinions on a prop

im looking mainly for speed

current prop that came with the engine is an aluminum 12 3/4 x 21 and the rpms got around 4300-4400 today no speed reading yet forgot the gps

what prop should be put on and what is the WOT for my motor?

thanks for the help.

 

[Faztbullet]

Re: speed boat prop

What type of boat?? Sounds like your set up or tach is off as you should be able to turn that 21 at least 5000rpm or better. The WOT rating for that engine is 4500-5500rpm. A 16ft light hull with a 115hp should be able to pull a 22 raker at about 5200rpm.

 

[boostedmax]

Re: speed boat prop

Its a light weight fiberglass boat that looks like a Glastonbury

I'm looking to try to get as much speed as I can would I be better off going with a different prop?

If so what pitch?

 

[Bob_VT]

Re: speed boat prop

Before you go jumping props you need to make sure your tach is correct and your motor is set up properly. At the current rpms you posted the motor is lugging.

How old is the boat? What is in it? Is there any hidden extra weight such as waterlogged foam? Are you running T&T?

If you can borrow a 17" pitch I would try that and check the rpms. That motor should turn 5500 to be at it's optimum.

Way too many variables to look at before you look for speed.

 

[boostedmax]

Re: speed boat prop

Its an older fiberglass boat no foam it was removed by the previous owner and I put a new floor in it

Basically it is just the motor in the rear with one battery

2 fold down seats in the front with a 11.5 gallon tank

It has a cmc plate on it and currently the motor is mounted on the top holes

I had a 13x19 prop on it the other day and the top speed was the same as the 12 3/4x21 prop (50 mph) on gps

 

[Faztbullet]

Re: speed boat prop

According to prop calculator motor rpm is 5850 RPM with the 21. Where is the cavitation plate in relation to the bottom of boat and does boat have a pad or "vee" bottom?

 

[boostedmax]

Re: speed boat prop

It is a v bottom boat and I had the plate mounted in line with the lowest part of the boat but the motor
Looked way to close to the water so I raised it back up

 

[boostedmax]

Re: speed boat prop

Took the boat out today and got 51.5 mph on gps and running 5340rpm trimmed out

 

[boostedmax]

Re: speed boat prop

do u think i can gain some mph but going with a different prop or do i need to some more hp to get it faster?