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Cuda
02-24-2002, 04:29 PM
I finally got a chance to take the boat out today and get a speed reading on it. Boy, am I dissapointed! I only got 47.2 mph (gps) turning a 24" prop at 4500 rpm. I ran the numbers through the prop slip calculator, and came up with 47% slip! That sound excessive! What is the average prop slip?

marcdups
02-24-2002, 05:07 PM
Cuda approx 8 to 10 %, what engine and what gear ratio, maybe completly incorrect prop for your application.

?????????? :confused:

Cuda
02-24-2002, 05:55 PM
The engine is a 330 hp 5.7 Vortec. The outdrive is a 1.5:1 Apha with a Bob's nose cone.

Shanghied Again
02-24-2002, 06:06 PM
Cuda, What boat are you running

Formula Jr
02-24-2002, 06:24 PM
((4500/1.5) x 24) / 1056 = 68.18 You're about 10 MPH under what is expected.

however

Plug in a 1.98 drive ratio and you get 51.65 which is within the usual slip factor.

I'm assuming your F 20's hull is clean, fare and you are using a 14.5 dia SS prop?

You will have to rotate the engine with the plugs out and actually check to see if your engine spins about two revolutions to one revolution of the prop (1.98), or if you engine just turns 1 and 1/2 revolutions to each prop rotation (1.5) You may have a tag that says its a 1.5 drive, but it could have been altered to a 1.98. There are many more of the high ratio drives out there cause that is what the smaller four cylinder engines used.

If you are absolutely certain its a 1.5 to 1 drive, Check to see if you have the old or new hub design. If its the older press fit rubber hub, you might be slipping on the hub. Also how did you determine your RPMs. Regular boat tachs are not very reliable, I use a digital handheld attached too the battery, coil and number one plug lead.

Bryan Tuvell 33ZX
02-24-2002, 06:45 PM
FYI, the lower half actually decides gear ratio on Alpha's, gen II's. My last brand new alpha on the old boat came 2.00 label on top (merc label, not 1.98) anyway, long story short, the bottom half sets the ratio.
Bryan

GEOO
02-24-2002, 07:44 PM
Cuda,
Sometings funny their, miss marked prop or gear case. I have found normal slip to be 12-16%. GEOO

HP 600SC
02-24-2002, 08:57 PM
Cuda, not possible, something is wrong in the math
or mismarked part's

Cuda
02-24-2002, 09:30 PM
I am pretty sure it is a 1.5 ratio. The guy I bought it from, bought it with big block engines, but he installed a Bravo. If it wasn't a 1.5, then I should be turning more r's shouldn't I? The pitch of the prop is stamped in it.
I only made one run down the lake, before I threw a belt. One guy asked me if I trimmed it out. Oops! No I didn't. I was busy watching guages and forgot. I know on my outboard open fish, I gain 500 r's trimming out. I can feel the boat get "light" when it hits the sweet spot. I also think that my bottom may be a little rough. I used a pressure washer with a sand attachment to knock off the bottom paint some idiot put on it. :mad: , but I have read where some people claim that they are faster with a little rough bottom.

Formula Jr
02-25-2002, 04:02 AM
Cuda, when faced with such a big difference of what is predicted and what is, you have to go back to basics. Your first basic is "knowing" your gear ratio. A F 20 with 330 Hp should run in the high 50's, so someth-in aint right. The fastest way to find the easy answer is to start with the drive's gear ratio. It is the least invasive of the alternatives. It may be a total waste of time, and your drive maybe 1.5 but you gotta start somewhere. Once this is "known good" we can look at other things. The SS prop in the pics looks to be in the area of a 21 to 23 pitch prop so that is still over the parameters that would put you at 47 mph with that rpm reading. Aside from all this, yes, proper trim can make a 10 mph difference. So get back when ya have run it through.

By running a rough bottom, which is a proven fact, not a belief, we are talking about 150 grit sanding in the area of the wetted surface at WOT. Some people just do the leading edge of the wetted surface. Its a way to introduce micro bubbles and break the friction of water conforming to a perfectly smooth and flat surface. This is why steps were introduced to later performace boats.

Chris can tell you the difference better than I, about hydro-scopic and hydro-static hull surfaces.

Would it be insane to think that at WOT, there was enough vacuum to scavange the exhaust stroke on well placed exhaust vents and that also created the bubble slide afforded by steps?

Like put the whole exhaust system under the the boat , on the face of the steps.

Woodsy
02-25-2002, 06:13 AM
Cuda,

Forrmula Jr. Has it it right. Manually spin over the motor and figure out what ratio drive you really have. I am with you, its probably a 1.5, but check it anyway. Then rerun the test. This time trimming the boat out. At WOT, trim it out till just before it starts to porpoise. You should be seeing about 4800 RPM on the tach. I am sure you will be picking up anywhere from 5-7 mph over your current results. Just my .02 worth.

Woodsy :D :D