Prop advice

[SHickey]

Hello,

I have a '87 Blue fin with a 2.3 Cobra OD. Currently I am running a 13.25 X17 aluminum prop ( mfg recommended) but I could not get any top end out if the set up. To make a long story longer, I discovered the LU was changed out and did not have the recommended gearing 2:1 and has gearing most likely for a 5.0 more like 1.59:1.

I am not in the position to change out the LU with the proper gearing and was hoping someone could recommend a different prop size so that I can run at higher RPM with out laboring the motor.

I am thinking of going to a lower pitch like 15 but I was also wondering if I should reduce the diam slightly if I am going to buy a new prop anyway.

Current top speed is 31 MPH at 4500 trimmed out and no speed increase from 4500-5000 except for a lot of motor laboring.

Any help would be most appreciated.

SH

 

[steelespike]

Re: Prop advice

Unless the Blue Fin is an amazing light fast boat I find it unlikely you are making 4500 with a 1.59 gear ratio it indicates 31% slip very high normal slip is about 10% to 14%.Is the speed by gps? really the only accurate method.
To figure it another way at 31 mph and 14% slip the rpm would be about 3500.A more likely scenario.
You should probably verify your tach. Perhaps borrow a shop tach.

 

[SHickey]

Re: Prop advice

Thank you Steelespike for your response. My tach or more likely my boat speedometer is probably off when I did my last test.

Tonight I was messing with my trim settings and had the GPS on board. You were on the money because the best top speed was at @ 3500-3700 RPM trimmed out. When I increased throttle from there I got no more speed and alot of laboring.

A friend of mine is letting me borrow his prop for some testing. He has a 12.5 X 15p which is a smaller diam and less pitch that the prop I have. My hope is that it will work better for my motor and LU miss match. After testing that size for a few rides then maybe I can figure out the best size and pitch for my set up.

Any guesses on how the smaller prop will perform? I will be careful not to over rev my motor which is one of my fears.

SH

 

[steelespike]

Re: Prop advice

Do some test runs with the 13 1/2 X 17 It should plane. You can tell better what you need with numbers from the 17 lightly loaded.
A 12 .5 X 15 seems like an odd size. usually as pitch goes down diameter goes up.Might be from an outboard.

 

[SHickey]

Re: Prop advice

Thanks for the response.

I would be a little afraid to use a 13.5 diam because the out drive prop tip clearance would be @ 1/4 inch. The prop on now is a 13.25 X 17 and I would think a larger diam would make the problem worse. But I am no expert when it comes to matching good size and pitch with a small motor and an outdrive gearing for a motor twice the size as I have.

I hope that the smaller, lower pitch would get me in the ballpark.

Getting to plane is not a problem with the current prop, getting the motor to max out to higher end RPM's is annoying to me.
SH