[makowish]

I have a 24 Mako. Approx. 2500 lbs with a Johnson 235 outboard on it. I'm running close to 6000 rpm WOT! Top speed on the GPS is only about 40-44mph and she seems to really spin hard on takeoff with out going anywhere. I think my pitch or my prop is totally wrong. Anyone know what I should get or what I need. Maybe someone could come out and see what they think for themselves? I'm in Harrison Twp. Call me up or post back. 586-292-1770

 

[wildbill]

Do you have a stainless steel prop or an aluminum prop?Aluminum will flex and tend to run your motor at higher RPM's, while stainless will not give and will give you better performance. Pitch also has alot to do with your RPM's. I believe the higher pitch props will give you more top end performance but also could raise your RPM's, and the lower pitch a quicker lift off on to plane. Check your engine for the RPM rating at WOT. I'm not sure about out board motors my I/O is between 4200 and 4600 at WOT.Some marine dealers will let you experiment with different pitches until you find the correct one. Just a little food for thought. Check out Mikes Marine on jefferson.
Good Luck!

 

[damn yankee]

When you run WOT @ 6000rpm, what kind of weight--including fuel load is in the boat? Boat manufacturers will and do "prop" boats so that when loaded up with people, gear, and fuel so that it will run close to the recommended rpm at WOT. So, you might have the right prop for what the expected use of the boat was built for. But if you run with no fishing gear, an empty live well and 1/4 tank of fuel, and it's just you on board, I could understand that you're running too high of rpm. Try a different prop. You'll get a better hole shot, and you'll run rpm in the right range.

 

[Guest]

We definitely need more info. Like your O/B's recommended WOT range for rpm and your current prop pitch. Assuming the Johnson is like a Merc, they list their WOT range for a 225 as 5000-5600 or 5800 rpm. You may be overreving a bit at 6000. If you know your gear ratio, you can feed your numbers into a prop slip calculator to see what the theoretical ideal speed would be for your setup.

http://www.go-fast.com/boat_speed_predictions.htm
http://www.go-fast.com/Prop_Slip_Calculator.htm

If you're over you max WOT range, you should go up in pitch and the common theory is that for every 1" you go up, you will lose 200 rpm. So if you're spinning a 15" at 6000, a 17" should max out at 5600 or so. That would probably be better for your motor, but may not give you any increase in speed because you'd be spinning a bigger prop at a slower speed.

Is your prop in good condition? Nicks and bent blades will have a negative effect on performance, too.

 

[makowish]

 

[makowish]

I have a 15X17 prop. It is aluminum and we ran this at 6000 with 4 people and a full tank and 1 - 1/2 tank. Recommended range is 5-6 and the dealer said I should run no more than 5400-5600 safely.
Someone said try a 4 blade and go up one higher pitch? an 18 maybe?

 

[damn yankee]

Like Bill said, go up in pitch to a 15X19. You're not always going to run with 4 peeps. Also, in the fall and spring when there's cool, dry dense air your motor will make more HP and thus turn more rpm than in the middle of summer.

You might be able to borrow one from the dealer and try it out before you settle on the one that you want.

 

[damn yankee]

I just had another thought. Has that prop ever been in for repair/refurbishment? If so, it's common to fix small dings by grinding down the prop. This means the overall diameter will decrease, which will allow the motor turn higher RPM. It's an old trick that racers use to get more RPM. Measure it and see.

 

[makowish]

I called michigan wheel and I'm going to give one of their vortex 4 blades a try. 14.25x18 I think it was.

 

[damn yankee]

Let us know how you make out with the new prop!

 

[makowish]

should be in today! I'm giving her a rip tonight or tomorrow. I'll let ya know how she goes