Calculating the speed

Of course the acceleration will be better if the hull is lighter. It would be better with a 9" prop too.

I do have a genuine concern with engine RPM. I thought that engine RPM should be governed by selecting a prop that won't allow the engine to over-rev. It doesn't appear that this is going to be possible with the limited selection of props for this engine. I should be able to get 6000 RPM at any weight below 800#.

As far as I know the engine does not have a rev-limiter built in. I had hoped to make the boat teenager and wife proof with prop pitch. Short of adding ballast, what options do I have?

 

Check a Mercury dealer for a REV LIMITER kit to retro fit. Evenrudes have them available. They DO save engines everytime. I know mine does. Then anyone can race around with no worry. Post back if no luck.

 

Cyclops, With virtually NO deadrise, Chine walking is not a problem. I do have to be careful at those speeds though.

Not necessarily overpowering, am only running a small block (302) Ford which falls within the designer,s recommended sizes.

 

But your speeds do not.

 

Appropriate changes have been made to accomodate increased speeds.

 

I had hoped to make the boat teenager and wife proof with prop pitch. Short of adding ballast, what options do I have?

Stick on a prop that is a tiny bit oversized, so even at constant full bore , the kids cant overrev.

 

If you can't find a Rev Limiter, probably have to put on a Larger dia prop or perhaps more blades.
Anything you do to the carb or timing may cause it to heat up.

 

That is exactly what I planned to do.

Mercury offers three props for the motor. 6.5, 9, and 10.5 pitch. I searched the net for hours and found one aftermarket prop with 11" pitch. All the calculations that I've done show that I need a 13" pitch prop.

I'm not going to have a prop custom built for a 15HP Merc.

I built the transom to place the cavitation plate level with the bottom of the Vee then cut it down to drop the propshaft 1/2" below the transom.

The plan was to have a bit of adjustment so I could shim the motor up to get the best performance during sea trials. Given the lack of prop selection for this motor, I'm just going to finish the boat with the engine low and see how easy it is to over-rev the engine. I've never tuned a outboard to a hull before and was looking forward to the challenge.

 

What year 15 hp, or give me the serial # to check it out for you.

 

Hey cyclops, thanks for the offer of help!

Its a 2002 15M #OT517895
Model # 1015201ZD

I've added the electric starter/charging kit and the remote control cable kit to replace the tiller controls. When I search through the Merc parts listing for the motor, I don't see a rev-limiter. There may be one built in to the ignition box, but it's not mentioned in anything I've read. It wouldn't surprise me to find that over-rev protection is built in. Since the motor came with a 9x9 prop, any boat/motor combination under about 800# would allow the motor to over-rev. I can't see Mercury not anticipating this and providing the motor with some protection to avoid warranty claims.

 

RHough,
In case you're not aware, most of todays Props are cupped which acts like an extra Inch of pitch. A 10 1/2 will act like a 11 1/2 pitch and should be calculated as such.

Also with a small lite boat, I would be supprised if your slip would be as high as 9%

You may not have that much of a rev problem.

 

Thanks, The Merc 9x10.5 is not cupped, but my dealer says it can be if I need the extra pitch. He's a crusty old racer and his response to the over-rev question was to use my ears and back off the throttle if it wound up too high ... LOL

From what I've read I thought 9% slip was on the low side? Its my understanding that all props must have a slip angle to work and that 5-7% was about as good as it gets on high performance race boats. Even at 0% slip the engine will be red-lined at 30MPH with a 9x10.5 prop.

It's going to be another month or two before I can get the boat finished and in the water (epoxy takes forever to set this time of year).

I'm keeping the hull reasonably light, but I've stopped trying shave every ounce out of it.

 

RHough,

Prop Slip angle or angle of attack and Prop Slip are not the same thing.

The slip we're talking about here is actually the reciprocal of efficiency or Cal. speed - Actual speed / calculated speed.

It is posible to get much lower than 9%. take a look at:

http://www.boatdesign.net/forums/showthread.php?t=8814

But as you pointed out, you need to get a lot more (Prop?) load.

I believe I would try the boat out with the stock Prop first and then go for the 10 1/2 pitch if necessary. As you indicated the motor may already be limited.

Just a thought - You also could always fabricate a hidden throttle STOP