Prop/Pitch Question

Actually, I have a couple questions. Just purchased a 19 1/2′ Starcraft boat with 150 Merc. The prop that came with it is a 24 degree pitch Aluminum prop, and I am having trouble with it cavitating while playing with trim to optimize performance. Once it starts to cavitate, I have to back well off of throttle or quickly tilt down considerably to stop it. RPM’s at full throttle without cavitating are 5100, and speed is not what it should be.

I have two other props for an older Merc 150, both 20 degree props. First, would props for a 1984 and 2000 Merc be interchangeable, and if so would a 20 pitch be better for speed and less cavitation, or worse? How does pitch effect performance? I have one stainless “Hooter” prop and one aluminum prop that, assuming they are interchangeable, could be used. I have never used anything but stainless props much, but I know stainless should give me more top end.

Can anyone shed some light on this dillema for me?

Thanks,

John

Cavitation usually is not related to prop selection as it is motor elevation/mounting. What hole is your motor mounted in on the hull? Raising it may cause cavitaion, lowering it will force the bow down, and let less air into the prop during power up.
Heavy pitched props like a 24 will get you out of the hole in a hurry, and usually provide less top end speed as a 21 or 19. Usually you will see a 24 pitch prop on a heavy boat to get it up and out of the hole. See if your dealer will let you try a 4 blade 21 pitch to provide speed and power. You might like the results. I am not well versed on Starcraft boats, but they should be like most others as far as physics are concerned. You need to balance forward power, and lift, together with the levering effect that causes porpoising and cavitation.
Good luck!
Tuck

A drop to a 21 pitch will increase your RPM’s about 600 and will get you up into the top-end for that motor. A 24 pitch prop is a VERY “big” prop for an aluminum boat with a motor of that size. I’d be willing to bet you’ll be much happier with something in that 21 pitch range. Oh yeah… go stainless!

Thanks Tuck, James, I will be putting on the 21 stainless before my next outing. Unfortunately, that will be a couple weeks due to vacation and high, tree filled water. Would you gentlemen mind turning off the faucets up there?!?
I forgot to mention this outboard has a whale tail also, so hole shots are very fast, but probably losing some speed with it also. The boat is 19.5′, and 8.5′ beam, and was out of the hole and on plane in a couple seconds. I believe I will try the stainless with and without the whale tail and see which I prefer. Hole shots don’t mean a lot to me, would rather have cruising and top end speed. The 4 blade idea is interesting, but I don’t think I’ll have the funds to buy a new prop for a while, so I’ll probably hold off till next year on that. With the current prop, the boat runs consistently at 45-46 mph, which is plenty fast for me, but I will look at mounting and see if I may be able to drop it a notch. I believe that would increase lift out of the hole too, would it not? Thanks again,
John

This is a good thread. Some of you probably have seen my post about my problems with the boat plowing and my rear hanging in the water. Several fellas at coffee time today at the office were thinking that in addition to moving some weight up front, I might want a different prop.

I don’t have a tach yet (plan to get one soon) but I will tell you this, during the water testing (b4 we added the trolling motor and the extra 2 batteries) when I picked up my boat back in March my motor guy took his portable tach and based on the water tests and what the tach was saying, was planning to change my prop to a different pitch. If I remember correctly, we were running too high RPMs for the prop that came with the motor. I beleive it is a 17 pitch, it’s a 3 blade aluminum. Then, after we added the batteries and trolling motor(we did not water test with him at that time ) he decided to leave the existing prop because of the additional weight. I guess he felt it would take care of itself?

I hope I’m not intruding too bad on your thread, John, but it seemed very relevant to my present troubles.

Do you guys think part of my problems might be due to the prop I’m running?

Regards,

Joe

Maybe? Could be? But trying to play around with props without knowing what changes are being made to your max RPM’s is a very tough thing to do. I would suggest buying a cheap tach and hooking it up to aid in diagnosing and fixing your issue. Or you can just do the “swap a prop” routine and chance that you get one that performs the way you want it to.

Whale tails or Dole-fins usually are bandaids for other problems, and can create their own. If you tilt/trim your motor to a point of elevation where your “Tails” leading edge is breaking water, you will force air to your prop, and will cavitate. It happened to me on an old Lowe I had. Raising the motor, and getting rid of the boomerang was the ticket for me anyway.
Tuck

Kornking. A new prop might help get your boat up on plain. But might not help with the problem of water coming in over the transom while slowing down. A shift in weight might still be needed.
Good info on this post. I am still thinking about raising the motor on my boat also. There are no problems with preformance. Its just that in the skinny water I run sometimes a couple extra inches of clearence is very nice.