Prop Slip as an indication of power...

[longshanks]

Can anybody shed some light on whether or not an underpowered boat would typically show higher prop slip #'s? Or is there no correlation between prop efficiency and power?

I am showing 12% slip with my present setup, but my holeshot is very slow, and my top end is around 25% less than the published top end test speeds for this type of motor and hull combination. While it may seem that I am underpowered, hull slip is minimal/average, in my estimation.

thoughts?

 

[QC]

Re: Prop Slip as an indication of power...

Can anybody shed some light on whether or not an underpowered boat would typically show higher prop slip #'s?

A very heavy work boat would have higher slip than a very light cat hull . . . Power to weight may be a good indicator, but it is not cut and dried for sure.

Or is there no correlation between prop efficiency and power?

It's more about hull efficiency. Least wetted surface for the given weight should have the lowest slip. The prop's design definitely matters though too. A lot of science, but a decent chunk of Voodoo . . .

I am showing 12% slip with my present setup, but my holeshot is very slow, and my top end is around 25% less than the published top end test speeds for this type of motor and hull combination. While it may seem that I am underpowered, hull slip is minimal/average, in my estimation.

thoughts?

WOT throttle RPM vs. recommended range? Could be a crap combo, but 12% is pretty decent. My boat goes 62 on paper, same engine, same drive they pull 2 inches more pitch. Best I've done is 57. If I threw everything out, everything, only 10 gallons of fuel and me, maybe, but not real world for sure. I don't recall anybody getting the same speed as test data.

 

[longshanks]

Re: Prop Slip as an indication of power...

Thanks for the reply QC. In the example concerning your boat, you are around 8% less than the test boat's speed, with a 2" difference in pitch.

In comparing my boat to the test data, I am pulling 1" less pitch than they are, and getting around 25% less speed than they can. This seems significant to me. And My WoT only reaches 5200, while it should be reaching 5500-6000 (ideally, 6000).

It seems to me that there is a problem with the re-built powerhead, but at this time, the dealer is saying I am underpowered...

More information is here: http://forums.iboats.com/showthread.php?t=219498

 

[Texasmark]

Re: Prop Slip as an indication of power...

Some data from my boat (all boats won't react accordingly):

Just planing out, mine is 18 mph and 40% slip; course this is where the load is the highest. If my engine was so small that I could just run at "getting on plane" speed my slip would be very high. Once on plane it drops really fast.

WOT on this boat, using it's 90 hp is 50 mph and slip is less than 10%.

So slip is a function of load on the prop caused by boat drag among other things, and the faster you go, usually the less boat is in the water, the boat drag is minimized, the prop doesn't have to push as hard because the load is lighter so the inclination to slip is reduced.

Mark

 

[QC]

Re: Prop Slip as an indication of power...

^^^^^^ Exactamente, puuuurrrrrrfect 'splainin there ^^^^^^^

 

[Texasmark]

Re: Prop Slip as an indication of power...

Why thanks QC. Appreciate the support. Grin

Mark