What is the idea prop slip % for V hull at top speed ?
11-17-2012, 06:34 PM
#11
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just for reference...
Here are slip numbers for my 292 with 28" pitch labbed Bravos. The slip may actually be a bit lower than this as I have been told actual pitch on Bravo props are slightly less than advertised. I used 28 in these calculations.
The 292 Fastech is a twin stepped hull with a 24 degree dead rise and 7000 lbs dry.
The 292 Fastech is a twin stepped hull with a 24 degree dead rise and 7000 lbs dry.
11-17-2012, 07:12 PM
#12
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iTrader: (2)
I have lowered the slip on my boat before with 5 blade props while testing, it had a slight gain at cruising, killed the top end, tq rolled like a biotch and sheared off a billet prop shaft. My boat goes the fastest and drives/rides the best with about 16-20% prop slip , if your slip is 12-14% you are already pretty low for a boat like yours. People tend to get hung up on prop slip numbers and start thinking IF they could get slip down the speed would increase but other than very small gains that does not neccesarily hold true. Its more like what is better-if my boat goes 95 mph with a 30 pitch 4 blade at 6000 rpm's and 16.5% prop slip or 95mph with a 28 pitch 5 blade at 6000 rpm's and 10.4% prop slip? Normal thinking would make you think that IF you could get that 30 pitch to slip at 10.4% it would run 101.8 mph instead of 95mph, normally the truth is though unless you were to increase the tq/hp its not going to happen , instead it would just pull the motor out of the power band, tq load the drive more and cause a multi tude of other problems. Now, if your boat is poorly set-up, (too high x, pos prop, major flaw) sure you should work on makingb things effiecient. I tried for years to get my slip numbers lower, all it ever did was make things break and boat to handle worse, I quit worrying about it anymore, Smitty
11-17-2012, 07:47 PM
#13
Registered
35p 5 blade 14-7/8" dia
37p 5 blade 15-1/4" dia
Notice the 35 is smaller dia? Snagged a cable and rolled the edges first time out... Repaired, it still runs within 1mph top speed of the 37 but at higher R's. Both are about 18-20% slip out of the box, no tuning. But here's the kicker, drive centerline is deep, 6" below the haul.
If I raised it 2" with a shorter lower unit, should one expect to see that slip percentage decrease on the same prop? I hear 5 blades perform better closer to the surface than deep.
-K
Charter Member
