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Looking at the new Powerboat with the Performance Trials #5.
Some of those fast V's they tested are slipping in the 20-24% range at WOT.
Does this help for speed?
Those X dim. have to be pretty high for that kind of slip, or is that typical of vented bottom boats?
With my conventional V bottom we get 11% at WOT.
I don't think I understand this slippage/X dim. thing as much as i thought.

rv

Lower slip numbers mean the prop is hooked-up with the water better and is not slipping. Therefore, the boat goes forward further per revolution of the prop than one with higher slip numbers. This means the boat is faster with lower slip numbers if it turns the same rpm.

As the X dim goes up you will reach a point were the prop starts to loose its grip on the water and the slip will increase. There is a trade off between speed increase caused by the lower drag of the drive being higher and the speed lose because of the prop slip increasing. Prop selection/labbing with higher X dim can help in this situation. You need to find the sweet spot between drive height and prop design/tweeks.

Good luck,
Rick

Wally

wouldnt running a higher slip number make loose speed.....inturn rev highr rpms.....and inturn make you go up in prop size to compensate? So what i'm saying is...the more prop slip and higher X dim make a boater compensate by running a bigger prop? I know there has to be a happy medium somewhere in the melting pot or we'd all be running swamp boats!

HyperBaja

Isnt that the same as spinning your tires in a car?

Cord

Not spinning your tires. More like having a clutch or trans slip. You are not putting the power the ground.

rv

I agree with HyperBaja, the prop is spinning but is not hooking up with the water. A clutch slipping would mean the prop is not turning as fast as it should.

Cord

I was comparing the spinning tire analogy to having the prop blow out or cavitate. I was hoping the slippage would be compared to a efficency (power) loss, not a rpm loss.

Brad Perry

Come to think of it, I was thinking of prop slip as not really robbing power, but that xx% of water that is being displaced not resulting in forward motion, is still displaced, and it took power to make that happen. I am of the opinion that a low slip number should be pursued. I read an article in PB by Sandler that said you could not have thrust without slip. I don't quite swallow that.

cobra marty

As long as the rpm is raised more than the prop slip, it will go faster, for example:

5000rpm, 24", 1.5, 5%slip = 71.96mph
5700rpm, 24", 1.5, 15%slip = 73.41mph

So increase the rpm by more than the 10% increase slip = faster.
All this is a big it.

WETTE VETTE

But the numbers on the GPS or radar gun are more important than the slip percentage. My fastest prop is a 14.5" x 29 Spinelli that I turn 6200 RPM through 1.5 gears. At 97.8 MPH on the GPS this translates into around 14% slip. Now my 15" x 27 Hydromotive at 6100 RPM and 96.5 MPH translates into about 7% slip. For cruising the Hydromotive wins hands down. I can cruise at 4500 RPM and 72 MPH compared to mid 60's with the Spinelli at the same RPM. As for acceleration the Spinelli wins hands down. It really RPM's quick and as shown earlier produces a little more top end. I like both props and they both have their place on my boat. If I want to cruise I go with the prop that hooks and if I know there will be some racing I go with the Spinelli despite the high slip factor. Slip is a good tool, but don't be surprised if the higher slipping prop is the fastest.