How much prop slippage while slalom??
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URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=39732
Printed Date: April-29-2024 at 7:43am
Topic: How much prop slippage while slalom??
Posted By: Orlando76
Subject: How much prop slippage while slalom??
Date Posted: August-21-2016 at 12:00pm
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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. ------------- Please support The Cystic Fibrosis Foundation 1976 Ski Nautique 351 Escort 1993 Ski Nautique purple and black 351 HO PCM |
Replies:
Posted By: 63 Skier
Date Posted: August-21-2016 at 1:46pm
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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? ------------- '63 American Skier - '98 Sport Nautique |
Posted By: 8122pbrainard
Date Posted: August-21-2016 at 8:26pm
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Todd, There's always slippage. The amount varies depending on the load on the hull. Yes, pulling a skier will increase the slip. ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: 75 Tique
Date 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. ------------- _____________ “So, how was your weekend?” “Well, let me see…sun burn, stiff neck, screwed up back, assorted aches and pains….yup, my weekend was great, thanks for asking.” |
Posted By: 63 Skier
Date Posted: August-22-2016 at 2:48pm
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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! ------------- '63 American Skier - '98 Sport Nautique |
Posted By: TRBenj
Date Posted: August-22-2016 at 3:33pm
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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). |
Posted By: GottaSki
Date Posted: August-22-2016 at 3:47pm
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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. ------------- "There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing." River Rat to Mole |
Posted By: Blamey
Date Posted: August-22-2016 at 6:04pm
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Isn't this what Zero Off does, it works to counter the additional slippage by adding RPM when a skier initiates a turn? ------------- 96 Super Sport Previously: 95 Sport Nautique, 1980 Ski Supreme |
Posted By: 63 Skier
Date Posted: August-22-2016 at 7:35pm
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. ------------- '63 American Skier - '98 Sport Nautique |
TRBenj wrote:Posted By: AAM196
Date Posted: August-24-2016 at 6:48pm
I think your friend is noticing the difference between RPM and GPS based speed controls. I too like PP. |