Prop sizing

[LuckyPenny]

I know that the subject of what prop for speed, power, etc. But I would like to know the effect on the engine. I have a 100HP Johnson (1979) that runs very good. The prop that came with it is 13.75" X 15" pitch. I don't like running WOT but it will reach 5500-5800 RPM. and I am getting about 28-30 MPH pushing a 21 foot CC. What would the effect of putting a 17 or 19 inch pitch prop have on the engine (strain) and speed (up/down) also fuel useage.

 

[ob]

Re: Prop sizing

I would leave it propped where its at.Overpropping will cause your engine to be lugged which causes it run hotter and accumulate carbon in cylinders.Your rig would also be slower to plane and you would lose allround throttle response.Think of overpropping as climbing a grade in too high a gear.Are you presently using a cupped sst prop or aluminum?

 

[LuckyPenny]

Re: Prop sizing

The prop is cupped, but i do not have a trim/tilt system. It is not SS, alum. Everyone I talk to is telling me to get it resized to a 17" pitch. I am not a hot dog, but a few more MPH would not hurt either. As for getting up on plane, I am thinking of either a whales tail or a set of smart tabs. I was also looking at an after market trim/tilt, or hodro/electric trim tab system, but funding is a problem.<br /><br />

 

[roscoe]

Re: Prop sizing

Sounds like things are fine as they are.<br /><br />

I don't like running WOT

but a few more MPH would not hurt either

Kind of undecided, are we?

 

[ob]

Re: Prop sizing

Lucky,Your engine may be capable of turning a 17" pitch between 5200-5400.If that was so ,the difference in top end speed verses 400-500 rpm is negligible.Not to mention overloading engine.Factory recommended operating range at WOT is where you are at now.The only thing you could do to improve performance is to fine adjust the height that motor is mounted to transom.For optimal wot performance on plane ,you like to mount engine as high as possible without causing prop ventilation when executing sharp turns at around 1/2 throttle on plane.Rule of thumb is to mount where the engines anti-ventilation plate is level with bottom of hull ,as a starting point.

 

[LuckyPenny]

Re: Prop sizing

Actually, I just did that late this afternoon. then engine was down about 2 inches. I jacked it up to the point where it is just at the level of the bottom of the boat. I did notice a bit of difference, but I am going to have to take it out for a real run. I am still considering a whaletail, that may help plane the boat out a bit. Thanks for your help

 

[sharkcat]

Re: Prop sizing

Luckypenny before you get a whale tale /hydrofoil do a search on this site then make a decision.My opinion is that they help on boats that are underpowered & thats about it..I once fitted one to a boat & it produced enough lift to drive the bow down into the waterwe are talking submarines all systems DIVE to put it mildly very frightening .They can also help mask badly set up engines.

 

[trumpetbucks]

Re: Prop sizing

Luckypenny:<br /><br />The prop you have is right in the center of the recommended setups for that motor, so that's why you are presently performing "just about right" - as some of these guys have already noted.<br /><br />I think the difference you are going to notice by moving the motor up is probably about the best that you will do. As some of the guys here have said, if you go one direction you are going to put strain on your motor. You may end up with better hole shot one way and better top end another, but remember the final decision is always a compromise between the two.<br /><br />Good luck!