The Variation of Weight with Prop Slippage

I've got a 16' Lund WC short transom, aluminum fishing boat. It's got a Suzuki 15hp 2 stroke, long shaft engine with a 3 gallon gas tank. I'm trying to get the most speed and power out of this set up as I can and need some ideas or guidance.

I figure the boat, engine, gas, and battery weigh right about 500 pounds. The engine has the stock propeller which is a 9.25 x 10.

When I got the boat it seemed like there was a lot of drag on the LS engine and short transom. I had a riser plate built to lift the engine a total of 6 inches. The cavitation plate is now directly inline with the bottom of the boat and it tracks and runs smoother than ever.

With just me in the boat, it will do a maximum of 22.5 mph (GPS) which is very acceptable according to an online calculator.

My question is, when there are 3 people in the boat, it will only do a maximum of 15.2 mph. This seems extremely doggish knowing the potential of the engine. If i went to say a 9 pitch prop, would my top speed be greater than 15.2 with 3 people in the boat or would it just get up on plain faster? With all that weight in the boat, it feels like it never gets on plain with the current 9.25 x 10 prop. Or, would a whale tail cavitation plate be better suited? I'm open to any comments. At this point I'd just like the boat to be consistant whether there be 1 or 3 guys in the boat.

Re: The Variation of Weight with Prop Slippage

You want the same performance from 15 HP with 3 people as you do with 1. Don't you think that counters the laws of physics? Now don't get all huffy. I'm merely trying to point out that you are asking for an impossible solution. What you can accomplish is a "compromise". You can sacrifice a little top end for quicker planing with three aboard. But at the same time you risk over reving the engine at WOT with just you aboard. See how that works? You have 15 HP to pay with and maximum speed will be with the lightest load. As load increases, hole shot becomes worse and top speed comes down as you experienced. By lowering pitch, you essentially shift down as you would with the tranny in your car. As you do that, performance out of the hole is better, but engine speed increases as a result. One would therefore think that if engine speed increases, top speed would also increase. Wrong! Engine speed increases because each revolution of the prop carries you less distance (one inch less in theory - minus any slippage). So the engine simply runs faster at any given speed when compared to the other prop. The danger is over reving as I pointed out. So you can't have it both ways and without a tachometer, you can't be sure you have the correct prop now.

Re: The Variation of Weight with Prop Slippage

You want the same performance from 15 HP with 3 people as you do with 1. Don't you think that counters the laws of physics?

Click to expand...

Ayuh,...

3 people in a 16' rowboat Ain't gonna plane much with only 15hp to work with....

Re: The Variation of Weight with Prop Slippage

As suggested the key to solving your problem is a tach.With your prop size, max rpm and speed a prop selection is possiblle.A tinytach is one solution.Reasonable price,easy to install,will fit any similar motor without regard to stator poles.
If you have no venting problems now you can probably raise your motor higher.Just add 1/4" sticks to the top of the transom.If you can gain an inch you might get 1 or 2 mph and some rpm and perhaps a slight hole shot improvment..Be sure Clamps have a good grip
and it pumps water.Some will tollerate minor venting to get max height.
After your height is set check the Hustler props.They deliver SS performance at AL prices.The few specs I've seen they can improve overall performance.The Hustler may allow even more height.