trying to get closer to 40 mph

[85fourwinns]

I have a 16ft 85 four winns with a 2.5l. The prop is a 15.75x17 im turning about 4000rpm and doing about 32 any suggestions

 

[Bondo]

Re: trying to get closer to 40 mph

I have a 16ft 85 four winns with a 2.5l. The prop is a 15.75x17 im turning about 4000rpm and doing about 32 any suggestions

Ayuh,.... You've run outa Horsepower,... No way yer gettin' to 40 mph....

Who's motor,..?? What drive,..??

 

[tpenfield]

Re: trying to get closer to 40 mph

That seems like a HUGE prop . . . are you sure you got the measurements right?

Like Bond-o says . . . you are looking for a 25% increase in speed, which will take about a 50-75% increase in horsepower. Prop optimization just is not going to do it. Since the engine is topping out at around 4K RPM, you are already getting about as much HP that the 2.5L can give.

 

[85fourwinns]

Re: trying to get closer to 40 mph

Thats what is stamped on the prop

 

[steelespike]

Re: trying to get closer to 40 mph

These guys are probably absolutely right but maybe we can improve some.
We need the year,gear ratio and hp.Ratio is probably on the outdrive somewhere.
Diameter does seem large.

 

[85fourwinns]

Re: trying to get closer to 40 mph

Its a 2.5l with a alpha 1 outdrive, so if i went to a 14.5x19 it wouldnt help much?

 

[85fourwinns]

Re: trying to get closer to 40 mph

Its a 1985 with a 120hp 2.5l not sure the gear

 

[Maclin]

Re: trying to get closer to 40 mph

Going up in pitch will lower the rpm's and the top speed will be the same or slightly less, and it will take longer to plane out and hit top speed.

If 4,000rpm's is all the engine can achieve right now, and I am thinking the top RPM should be about 4400, then you could go down in pitch some to be able to hit top rpm's for the engine, but your top speed would not change, quicker time to plane though. If 4000rpm is what the engine is rated at then you are at the place where it gets no better.

 

[Maclin]

Re: trying to get closer to 40 mph

As others have mentioned, that is a larger diameter than averag/normal for that engine/drive. Going to a smaller diameter prop may get the rpm's up 200-300 rpm's but that is like 1 or 2 mph.

Right now if we assume a 2.0 drive ratio and that the 32mph is accurate you have .0006% slip which is not "real", not achievable in the real world. Using a more normal 15% slip the top speed calcs to 27 at 4000rpm. My guess is you are not hitting 32 now, or the drive ratio is not for a 2.5l.

Many paths to take and things to try, but all will end up at where you are at now regarding top speed in my opinion.

 

[85fourwinns]

Re: trying to get closer to 40 mph

So if i went down to a 14.5 diam. And up my pitch id stay the same rpms?

 

[steelespike]

Re: trying to get closer to 40 mph

a 14 or 14 1/2 X 17 may help rpm slightly just be sure its listed for your motor/drive.
but::
Before a prop change I would be sure the motor is in tip top tune.
Check the compression. Be sure the timing is set right and advancing as it should.
Be sure the throttle opens all the way.
while you have the plugs out you could turn the motor manually and count he revolutions for the prop to turn once.Probably almost 2 to 1
Again it may be stamped on the out drive.
all these things will probably result in a couple of mph but also may improve performance slightly and get the best mileage.