I am trying to improve performance on my boat, and believe a prop upgrade is in order.
Here is current setup:
1988 OMC Cobra, 3.0L
17 foot Renken SS1700, 1988
Running a 13x19 prop now.
WOT rating for the engine is around 4500 per manual.
The boat runs right at 4500 at WOT, but only at about 35-36 MPH, with trim full down. Speed measured via GPS, and my tach is correct, I cross checked it with the diagnostic tach I use for tuneups.
If I trim up a bit I can approach 40MPH, and could possibly get higher, but the RPM gets over the max rating. So, I haven't pushed it as I don't need a blown motor. Not good I don't think.
I also have difficulty getting on plane with 3 or 4 people on board and the top speed also falls quite a bit. If it is just 1 or 2 people this is less of a problem.
The boat has a clean bottom and low hours, it's in perfect mechanical shape.
I am of the opinion that increasing pitch to let's say 21 would improve top end. There would be a corresponding decrease in RPM ( correct?), but I also know that if I trim up to where I get better speed with the current prop, I would have the motor in the correct range. However I am not sure this would help my holeshot/ getting on plane problem. It may be I need a different prop for each operating condition, correct?
My marina does demo props, and they suggested going to a 17 pitch for more speed, but I question this, as it would seem to over-rev the motor, based upon what I have read. Increased diameter might also be a help I suppose.
Any of you that have experience and can comment on this.. please fire away.
Thanks
Craig
Here is current setup:
1988 OMC Cobra, 3.0L
17 foot Renken SS1700, 1988
Running a 13x19 prop now.
WOT rating for the engine is around 4500 per manual.
The boat runs right at 4500 at WOT, but only at about 35-36 MPH, with trim full down. Speed measured via GPS, and my tach is correct, I cross checked it with the diagnostic tach I use for tuneups.
If I trim up a bit I can approach 40MPH, and could possibly get higher, but the RPM gets over the max rating. So, I haven't pushed it as I don't need a blown motor. Not good I don't think.
I also have difficulty getting on plane with 3 or 4 people on board and the top speed also falls quite a bit. If it is just 1 or 2 people this is less of a problem.
The boat has a clean bottom and low hours, it's in perfect mechanical shape.
I am of the opinion that increasing pitch to let's say 21 would improve top end. There would be a corresponding decrease in RPM ( correct?), but I also know that if I trim up to where I get better speed with the current prop, I would have the motor in the correct range. However I am not sure this would help my holeshot/ getting on plane problem. It may be I need a different prop for each operating condition, correct?
My marina does demo props, and they suggested going to a 17 pitch for more speed, but I question this, as it would seem to over-rev the motor, based upon what I have read. Increased diameter might also be a help I suppose.
Any of you that have experience and can comment on this.. please fire away.
Thanks
Craig