Mr Havasu Cat prop slip calcs

[MrHavasuCat]

Mrpumps,
I am running 9 pounds of boost at 5800 rpm. Blosdale says the motor is good to about 15 pounds so I might bump it up just a little.
SVO540,
I know what you are talking about with the "overdrive" feeling. When I ran the bigger numbers with the 1.50's and the 32 I was very light on fuel (towards the end of the day)and you can definitely feel the difference. I had a mach 26 last year with outboards and it had a much different feeling. You could air the boat out a lot more and get a very "light" feeling out of the boat. This is similar to how my current boat feels when it is light. I am sure this is the same feeling you are describing. I always end up putting to much fuel in the boat because I am running av gas and don't fill up at the marina, therefor I don't want to get stuck on the water with not enough gas. I had to be towed once with my last boat when I ran out of gas and it kind of sticks in my head. As far as rpm goes, I am running a Crane hydraulic roller cam with Jesel valve train. Jeff Blosdale says the motor can spin up to 6200 rpm, but he would like to see it kept under 5800.
I am going to use the smaller prop in the summer, and I will also try it in the cooler weather with less trim and see how it runs. I think the excessive positive trim I am running might be a reason for some of my slip numbers. After having an outboard boat it is hard not to want to trim the shit out of the motors. The outboards loved a lot of trim. Thanks for the reply.
[This message has been edited by MrHavasuCat (edited December 05, 2001).]

[SVO 540]

Mr. Havasu Cat,
My guess is that you will end up with the same prop selection as me. That is, a non-lab 28 for lake cruising and a lab 30 for the top speed runs.
Here is an interesting stat from my mach. At 3000rpm using a non-lab 28, my speed is around 55mph and my boost is -1. At 3000rpm using my lab 30, my speed is around 50mph and boost is +1. The non-lab 28 is much better in this cruising speed range. It is 5mph faster and makes 2psi less boost. Less boost because there is less load on the motor with a smaller prop and more mph because the non-lab has less slip than a lab in the mid range. I have run 112mph with my non-lab 28 (had to lift because of rpm limitations) and 132mph with my lab 30 and I can honestly say that I like the non-lab 28 better, except for getting the big mph number. By the way, with my non-lab 28 I can over rev the motor at WOT which is my concern for you with a 28.
Mr. Pumps, Three questions:
1. Do you have any interesting slip data from your HTM you can share?
2. What props do you use and for what purpose?
3. What is your trim setting at cruising, drag racing and top speed runs?

[gimprvr]

SVO 540 : A 1.5 ratio is correct. I am also running a Max Machine Worx Lower unit with nose cone. As afar as the MAX RPM, make a recomendation. I am anly running 4-lbs of boost.
Duck, I was running it all the way down because with the drive trimmed up I would run out of RPM quicker.

[SVO 540]

Gimprvr, The answer is .... 32pitch lab.
Ok, so how did I get to that prop. I assumed the following: You are looking for top speed only, this prop could be sloppy out of the hole. The desired engine rpm is 5000.
I don't know what kind of boat you have, but I know that a 25 Daytona with a HP500 (470 prop shaft HP), 1.50 gear and 28 pitch prop will run low 80's at around 5000 rpm. Using this as a benchmark for your boat gives you 100 more HP with the same 1.50 gear.
The next statement is somewhat of a guess but I think every 1 inch increase in prop pitch will decrease wide open throttle (WOT) rpm by 200. Also, a lab finished prop will turn 150 more rpm at WOT than a non-lab prop. Although this will be different depending on gear ratio, 1.50, 1.36, 1.25 etc.
Going from your 28 to a 32 will decrease WOT rpm by 800. I think this will be too much of a reduction. This is why I suggested the lab finished 32, because you will pick up about 150 rpm, also it is faster. Therefore going from a non-lab 28 to a lab 32, I would guess that you will decrease your engine rpm by 650.
I am not suggesting decreasing your engine rpm from 5300, but decreasing it from what it would have turned had it not had the rev limiter and putting it in the 5000rpm range.
What if I am correct? Here are your stats. 5000rpm, 1.50gear, 32pitch prop, 10% slip factor gives you a top speed of 91 mph.
I hope you like it. If any other readers have comments or disagree please post. Keep in mind, this is only a best guess estimate.

[gimprvr]

Thanks SVO, you seem to have a thorough wealth of knowledge and I really appreciate your willingnes to share. I had planned on trying a 32. It just seems so big for this application. Do you see any way to hit the 95mph or even the elusive 100! I have a brand new imco 1 inch shortened gearcase as a back up. what do you think this will provide?

[SVO 540]

Gimprvr, Before I do any more speculation, tell me what kind of boat you have and if it is open or closed bow. I also need more info on your motor...as much detail here as possible. More info on drive, standoff box, yes or no, etc. Have you tried the 1 inch shorter IMCO lower? The more detail you can give me the better I can guess at it. With this info, we can work backwards from 100mph to see what it will take to get you there.

[kab]

I like that SVO 540 work backwards from 100. I have a DCB Mach 26 open bow 502/ whipple charger not broke in it yet,but when it is I'll be postin some info on it to see what I need to reach 90.

[riverliver]

SVO540
would it not be better to run a 1.35 and try to get to the magic 28 prop then a 32 with a 1.50, I think the chance of grenadeing that 1.50 with a 32 are very high.
I think we need to change your name to mister SLIP. LOL
nice talking with you on the phone today, tell Bug and the little one Hi.

[mrpumps HTM]

Originally posted by SVO 540:
Gimprvr, Before I do any more speculation, tell me what kind of boat you have and if it is open or closed bow. I also need more info on your motor...as much detail here as possible. More info on drive, standoff box, yes or no, etc. Have you tried the 1 inch shorter IMCO lower? The more detail you can give me the better I can guess at it. With this info, we can work backwards from 100mph to see what it will take to get you there.
SVO540... could you calculate my slip on this iam curious about it 1.5 gear 32 prop/7000 rpm

[FastCats]

Hi SVO,
I would like to commend you on your observations and calculations on most of the boards boats and set ups. Very informative and quite accurate. Do you ever take into consideration, water conditions, water temp, current, wind, etc. I know I do.
I have currently been testing with a 1:30 gear, 36 labs, at 6000rpm, average 9% slip, and am finding that the conditions mentioned above do come into play along with the obvious.
Again, good work with the "blue oval", I find that part interesting.
Scott
[This message has been edited by FastCats (edited December 05, 2001).]