Prop question for mini ski-boat

[Andrew N]

In 2002 I built a 11' mahogany flat bottom ski-type boat called a TNT (the plans are made by Glen L marine). I have done many things to upgrade performance, so i will decribe my current set up:
-25hp 2-stroke merc with boyesen reeds
-lenco trim tabs
-14.5' pitch, stock aluminum prop
With this motor, prop and tabs, I have acheived 37.5 MPH on GPS at 6200 RPM (no passengers). What I'm wondering is, would and increase of pitch to 15.5'' be likely to increase my speed? I don't think raising the motor would be a good bet because the motor already has a strong tendancy to catch air on turns. Hope thats enough info- Andrew

 

[steelespike]

Re: Prop question for mini ski-boat

Not an expert but I'll get you started.Changing to a stainless prop same pitch
should give a little more speed and would probably allow you to raise the motor more.Rpm would probably drop a little but raising the motor might get it back.Have you tried a jack plate makes it easier to fine tune height and gets the motor in cleaner water.

 

[Frank Acampora]

Re: Prop question for mini ski-boat

You really need to go to Hydroracer.com. Those guys really know their stuff.

At 37 MPH and 6200, your engine is turning a high RPM. Even though it would appear that the engine has the poop to turn a 15.5 pitch prop, you may be disappointed with the results. 1 inch pitch even if turned at the exact same RPM is only going to yeild about 2-3 MPH more. Remember, there is only so much you can do with 25 HP and stock parts.

I also suspect that a bit more money invested in a non-stock prop would yeild better results and make you a lot happier than a higher pitched stock prop. Racing props are designed to run on higher mounted engines and utilize the horsepower differently than stock props---remember--ANY stock prop will be a compromise of some sort for general boating while the racing prop will be dedicated to one thing: speed.

 

[Andrew N]

I've had some people recomend a jack plate before and i agree with the theory, but on a boat as small and light as mine, i was a bit concerned about the handeling risks of placing the motor farther back on the boat. Could that alter the thrust angle in a dangerous way? I wonder if adding the weight of a jack plate may also have its consequences? (especially since i've got gas, battery, and electric trim tabs at the back) I would consider looking into a stainless prop if i new a better source for small outboards. I think Merc used to sell some for the 25, but if they do now, they would be just for the 4-stroke 25.

 

[steelespike]

Re: Prop question for mini ski-boat

While its generally good to keep her light in the bow I have to wonder if your
at the extreme.I don't think the guys that run those type boats use tabs.
Do you have trouble with porpoising?If one trim notch seems like not enough and the next seems too much you might try making or buying wedges(go between mount bracket and transom) to help you get half a notch.
If your using tabs to stop porpoising it's probably steeling some speed.I think you need to try everything else weight placement,care full trim.
Another thought, there is a trim and tilt for small motors pricey but would allow you to really fine tune the trim for all conditions.I think it would give you a small set back as well.
As far as the prop you could have a little cup added to your prop.
would help venting might allow a little more height.

 

[Andrew N]

Re: Prop question for mini ski-boat

I don't think the trim tabs, at least in my opinion, are really sacrificing speed. they are intentionally set 1/2'' or so above the boat bottom so when they are retracted, there not actually doing much of anything. The motor is set in the second position, but actually, the boat still porposed when it was at the bottom. I only adjust the tabs down when I want control for turns (for this purpose the tabs are increadible) the boat is equiped with a central fin, which comes out of the water when the boat bounces, leaving me with zero control. The tabs allow me to operate in the second notch and still maintain control (even when tabs are up) Without them, this was an act of suicide. You see, with a 25hp on such a small boat, its power and weight will do more than just cause porposing. Without the hull lengthening of the tabs, the boat was capable of extreame chine walking, nearly to the point of flipping if the motor was trimmed to the second notch. Basically I think the ultimate correction for porposing would be a longer hull (that is if you wanted to run without the tabs. Putting the motor to the second notch has without a doubt, increased speed. This may sound weird, but when the boat is bouncing around the most, it produceces the highest speeds. The way it porposes (with the tabs) isn't much like typical porposing, were the bow plows hard, slowing the boat. When mine bounces, the speed stays constant and there is no big splash. I think the boat is trying to ride on the motor's anti-vent plate or somthing.
The idea of cupping the prop does intrique me however. This can be done to a stock aluminum? Would this possibly reduce some prop slip?

 

[steelespike]

Re: Prop question for mini ski-boat

Is this boat plain flat bottom or is there a non trip chine?Plain flat bottom
can be really tricky especially in a turn.Liable to do anything in the waves.
Have you tried moving a little weight forward even if only mid way might steady the bounce.A little bounce can help speed because it helps cut adheasion,as A boat might run faster on a rippled surface than a glass smooth.
Yes you can cup a stock prop and it will produce more bite.Suggest that they be conservitive I believe you can add more later .May reduce rpm but improve speed.Also may allow more height.The wedges I suggested might allow you 1 1/2 notches instead of 2 I'll grant the tabs aren't causing much drag and may end up being necessary.It is obvious they are acting on the water thus there is some drag.The boat may have a little rocker which they some times use to free the boat.Or perhaps you accidently added some rocker.Is the boat intended for speed and relatively high hp.?

 

[Andrew N]

Re: Prop question for mini ski-boat

My estimate for this boat is that it should probably go no faster than 40mph or so and have no more than 35-40hp (low weight being vital). As for rocker, i don't think there should be much to speak of, at least not according to plans. When building the boat i checked countless times to ensure there was no unwanted additions of rocker or other non specified hull changes. I think its what you would call a plain flat, with no non-trip chines, spray rains, keels, it has a mild vee-entry from the bow, that quickly flatens with nearly no vee at transom. (if your familure with J-crafts, it is very similar to one in design)