Prop advice

[dgehrke]

I am new to boating and really need advice on props. I bought a 87 Sea Ray Seville 17 ft br that has a 2.3L Ford and OMC outdrive 3 weeks ago. My current prop is a composite 4 blade 15 pitch. Top speed is 33 and WOT is 3700 rpms. I think I should be around 4200-4400 rpms. Anyone have suggestions of a prop to buy? Also is there some kind of adapter that allows 15 spline props to work with 13 spline shaft? Thanks for any input.

 

[steelespike]

Re: Prop advice

My info indicates the 2.3 is rated at 5,500 rpm. hp is 128 ratio is 2:00.

I think you have issues other than prop selection. 15 is way down in the lower pitches intended for power/heavy loads.Sizes run up to 24" Motor should easily spin a 15 up to its max rpm.I think the boat would normally run a 17 or 19" prop with speeds close to 40.
If my math is right ? Disregarding slip and drag I think your speed would be about 25 at 3700. So your tach may be reading a little low.verify your tach and check your motor.
Be sure it is in excellent tune,timing set and advancing as it should.Check compression,
Throttle opening all the way.

 

[SparkieBoat]

Re: Prop advice

get a new tach

 

[dgehrke]

Re: Prop advice

Thanks for the help.. could part of it also be the 4 blade prop? I thought I read somewhere that this would decrease rpm's as opposed to a 3 blade.

 

[SparkieBoat]

Re: Prop advice

you almost surely have a bad tach...

 

[bnicov]

Re: Prop advice

Try a 21 pitch prop. That composite prop isn't giving you anywhere near the performance you should be expecting from your boat. You should be able to run around 37-39mph at wot with a light load on board. Have the tach verified with a shop tach to see if that is part of the problem but for sure, you need more pitch on that prop. If you have lots of rock where you normally boat, go with an aluminum prop, if it's a sandy bottom, you can go stainless and pick up a bit more speed.

 

[dgehrke]

Re: Prop advice

Now Im really confused...I thought going from a 15 pitch to a 21 would slow the rpm's and top speed?? I will find out if the tach is bad and probably do a compression check, this is my first boat and its been frustrating! Thanks for the help, I really am clueless about boats.

 

[craze1cars]

Re: Prop advice

Now Im really confused...I thought going from a 15 pitch to a 21 would slow the rpm's and top speed??

You are correct. Bnicov's advice is incorrect, and he/she is apparently thinking backwards in pitch vs. rpm relationship, as he gave similar advice in another thread recently that was opposite of true. A 21 pitch prop would be a terrible choice and your boat would not plane.

If you have a 15 now, you'd need to drop to a 13 or an 11 to get your RPMS up into the 4,000's. If Steelspike is correct and your target is over 5,000 you need a 9 or a 7 pitch. The big problem is that 13's and 11's, 9's, and 7's don't hardly even exist for that outdrive, so I think your only alternative would be to regear the outdrive to handle a bigger prop.

I agree it is very possible your tach is incorrect, and it is also possible you have a sickly motor that is substantially down on horsepower for some reason. It could also be that you have an outdrive that has been replaced, and it has a gear ratio that is designed for a V-6 or V-8 motor....which your 4 cylinder simply can't push. Verify all of those items first, and only then start prop-shopping if you still need to....

I very much believe that your current prop is NOT the item creating your performance problem....something else is wrong.

 

[dragginanchor]

Re: Prop advice

Theres something terribly wrong there. My 17 ft bayliner with the same cobra engine/drive goes 43 MPH GPS @ 5400RPM (trimmed up of course) BTW, reccomened RPM is 5200-5600 RPM. A Sea ray should be faster than a bayliner right ? Just having fun cruisin' lately....