A propellor's pitch determines the theoretical distance the boat would move after the prop turns 1 revolution. Throttleup recommends using 1" less pitch than advertised for BI props. So, for a 30" B1 prop the boat should move 29" per revolution. But, the props are not 100% efficient, so the prop "slips" somewhat.

The formula is:
Slip = (1-actual speed/theoretical speed) x 100%

Theoretical Speed = rpm x pitch/1056/gear ratio

For example:

You are turning 30P BI's at 6000 rpm with 1.5/1 Bravo I outdrives and your GPS/Radar speed is 92 mph:

Theo. Speed = 6000 X 29 /1056/1.5 = 109.9 mph

Slip = (1-92/109.9) X 100% = 16.3%

Theoretical speed is the speed that the boat would go if there were no slip. Comparing actual speed to theoretical speed on a 100% basis yields the % slip.

Once you see how the number work use the go-fast web site calculator. It's much faster.

Slip is usually only applicable at WOT. You need to be honest with yourself and take the rpm and a reliable speed to calculate your slip. People that lie to themselves take away the tools that would enable them to start picking away at a slip/inefficiency issue.

Also, I believe that part of the efficiency process should include an evaluation of the hull straightness. My plan is to have my hull looked at this winter.

Tom

First thing to understand is slip is not bad. Actually you need some or the drive would break all the time. You can run anywhere from 5-25% slip and be setup perfect. As long as the boat handles well, goes fast, and runs economically your good. Don’t get hung up on slip. It's just a gauge.

BTW, No prop is all around perfect. They are all a compromise.

Thanks a lot TOM. Big help there. Only differnece is I am running a alpha 1 with 1.98 gears.. I will try to figure this out tommorow, it is getting late now. To be honest the hull on this boat has lil to no hook in it, surprised me. I cruise the boat at about 34-3800 with a cruise of about 40-45ish., maybe a bit better, but not really concerned on cruisen, but the top end, it cruises good.
All in all I am satisfied with the speed but just would like to have it all fine tuned, to get the most out of it. Would love to have a honest 80 mph 202 stinger with stock power..
John

I've been researching this for a few days and a few good sources have pointed out many alpha's start having issues high 60's and that nosecone is needed. My resdearch is only on smaller single engine boats with alpha's. Hydromotive nosecone's where recommended.

Again, this is what I found (not experienced) and just letting you know. Search here, Hotboat, and SW under 'alpha nose cones.'

So can anybody else verify this, is it true about the alpha, a cone, again this is a small boat with a single.
Thanks John

Also what I came up with on the slip calc, doesn't seem right, can this be right.
GEAR 1.98-1
PITCH 30p but entered 29p
RPM 5300 = a theo speed of 74. Gps is at 74 and some change so it is giving me 0% slip using 29p and 3% using a 30p, can that be right?? Duno is that possible to have 0 slip, I think not.
I will have to go run the boat again and note them to see for sure.

John

Anything lower than around 10% for a V-hull means one of your numbers is off. Test one more time and be very certain of the numbers.

Will do and I am sure 0% isn't freaking possible. So hopefully I will get some good numbers this weekend, hope the the tach is bsin me(gaffrig) and the gps is legit(garmin). I will try on flat water, even though it will run better in a cop, but see what happens.
again thanks
JOHN

You may also want to double check your gear ratio. I know when I bought my boat i was told it had a different ratio than it turned out to be. I was told it had a 1.35 ratio or something like that and that it had been GPS'd between 80-85. This all sounded good being as I was turning it 5400 rpms with a SS 24 pitch 3 blade. However it turned out to be false, it has a 1.50 ratio and only GPS'd 72 with that prop. Also, mine has the old Alpha SS, which is basically an older Alpha with a lower that was made with the elongated pointed case. I actually had a prop guy tell me how big a mistake i made putting a nose cone on that drive. Even after I explained it was original Merc he said I was wrong and that someone had added it. At that point I gave up trying to talk to him.

Are you sure of the 1.98 ratio in the Alpha? That ratio was used with 4 cylinders. Powerboat did a test years back and found the Alpha Gen 1 would experience hydrodynamic blowout around 70mph. I figure there are some factors that could affect the speed but it's around there that they are required to go faster with the Alpha drive. The Bravo has no problem at those speeds as it has a better aspect ratio.

I have Hydromotive nosecones on my Alpha Gen 1's. They require alot of blending, sanding and elbow grease to mount and make look like factory. I have the process down to a science. I'm running a 355ci SBC with around 400hp and am seeing 70-73 gps most of the time.

Give us some specs on your stinger's engine? Seems to be running good.

My research was made because I was tuning an Alpha equipped boat this past Sunday that seems to be experiencing major prop blow out (feels like coupler disconnects) at 66mph (GPS) and just over 5100rpm engine rpm. You can feel it start to do strange things just before 5100rpm. 1.47 ratio '90 older style alpha. We repeated this almost a dozen times in same 2hrs in same water.

Will try other props to see if it helps go past this but will be getting a nose cone anyway.

Ryan00TJ - your name came up in a lot of my searches. Thanks for sharing your info.