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How much prop slippage while slalom??

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Orlando76 View Drop Down
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    Posted: August-21-2016 at 12:00pm
We got into this discussion yesterday on how much prop slippage is there, if any, while pulling a skier in the course? I don't suppose there's a way to measure but my guess there's almost zero slippage once on plane. I ski with a very accomplished skier who is kicking the idea around about buying a new boat but another boat we ski behind, '16 SN 200 with 6.0, is whipping his a$$, having to go from C3 to B1.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 63 Skier Quote  Post ReplyReply Direct Link To This Post Posted: August-21-2016 at 1:46pm
There has to be slippage, a propeller in water is far from a perfect screw and while it isn't cavitating, it certainly is slipping to some extent. I'm sure it has to do with prop rpm and blade area as much as anything, a reduction drive with a larger prop would slip less. At least that's the way I visualize it working.

I'm wondering if your friend is experiencing motor capability more than prop slippage. In other words, a strong slalom skier can slow down a lower power motor even if the speed control is compensating. Is it just that the 6.0 is more rock solid with power output?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8122pbrainard Quote  Post ReplyReply Direct Link To This Post Posted: August-21-2016 at 8:26pm
Todd,
There's always slippage. The amount varies depending on the load on the hull. Yes, pulling a skier will increase the slip.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote 75 Tique Quote  Post ReplyReply Direct Link To This Post Posted: August-21-2016 at 11:35pm
I built a quick little spreadsheet years ago that figures out prop slip. You could use it to compare normal cruising slip to slalom pull slip is you want me to send it to you.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote 63 Skier Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2016 at 2:48pm
Larry, if I follow this correctly ......

pitch in inches/12/5280 x tranny input ratio / tranny output ratio x engine rpm x 60 = theoretical speed.

For my boat with 1442 (13.25" x 15") and 45.4 mph at 5,180 rpm:

15"/12/5,280 x 1 / 1.23 x 5,180 rpm x 60 = 59.8 theoretical mph

59.8 - 45.4 = 14.4        14.4/59.8 = 24% prop slippage

Did I miss something? It doesn't seem like that amount of slippage would be correct. Probably a math error somewhere ........ sorry, the geek in me likes this stuff!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TRBenj Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2016 at 3:33pm
That's correct. Put your Gt40 in a 16', 1600lb boat and it'd run into the low 50's (call it 52). Half as much slip- you're back down around Larry's 12%.

The bigger, heavier, more wetted surface of the hull = more prop slip.

Don't confuse prop slip with cavitation... It's better thought of as inefficiency (i.e., rotation doesn't translate to as much forward speed).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GottaSki Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2016 at 3:47pm
At speed, ever notice the propwash exits the back of the boat a bit faster than the water around it? That's where the energy is going.

Just as Tim sez, The less efficient the hull, or more loading, the greater that delta-speed.



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blamey Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2016 at 6:04pm
Isn't this what Zero Off does, it works to counter the additional slippage by adding RPM when a skier initiates a turn?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 63 Skier Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2016 at 7:35pm
Originally posted by TRBenj TRBenj wrote:

That's correct. Put your Gt40 in a 16', 1600lb boat and it'd run into the low 50's (call it 52). Half as much slip- you're back down around Larry's 12%.

The bigger, heavier, more wetted surface of the hull = more prop slip.

Don't confuse prop slip with cavitation... It's better thought of as inefficiency (i.e., rotation doesn't translate to as much forward speed).

Got it, I understand its not cavitation.

Using my '63 by comparison, I did get gps numbers a decade or so ago. I believe it was 40.6 mph, so let's say 41 (60 year old boats should get the benefit of the doubt!). It spins a 12 x 12.

12"/12/5,280 x 1 / 1 x 4,200 rpm x 60 = 47.7 theoretical mph

47.7 - 41 = 6.7        6.7/47.7 = 14% prop slippage

Interesting stuff. Curious how a G21 or G23 numbers would compare.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AAM196 Quote  Post ReplyReply Direct Link To This Post Posted: August-24-2016 at 6:48pm
Originally posted by Orlando76 Orlando76 wrote:

, '16 SN 200 with 6.0, is whipping his a$$, having to go from C3 to B1.


I think your friend is noticing the difference between RPM and GPS based speed controls. I too like PP.
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