Speed calculation please
 

I found someone with the same boat as mine. I was looking to see what kind of speeds to expect out of the same boat with 575HP . This is what the other guy has


1988 Scarab 28' Excel 4600 lbs dry, Holds 80 gal. of fuel
MerCruiser 454 Mag 365hp
Bravo One outdrive 1:50 gears with 23p Mirage Stainless Prop Max Speed 62 mph @ 4700 RPM


Thanks
Paul

The formula for approximate speed is as follows:

( pitch X r.p.m. ) divided by drive ratio divided by 1056

Try bams site go - fast .com

73 to 75mph in my guestimation

62mph in a 28 ft boat with 365 hp seems a little fast to me. I'd bet that is not GPS or radar.:confused:

Hot Boat
Use Checkmate statement but subtract 12% slippage

speed
 

rpm x pittch
__________ = speed with no slip

gear x 1056


108100
_______ =68.24 - slip ??13% = 59.37 mph

1584

15 hp to go 1 mph????

62 mph would be a 4.4% slip I DON'T THINK SO.

365 was at the crank
if you are 575 at the crank and not the prop it will be apples to apples.

575 - 365 = 210 / 15 = 14 more mph + 59.4 = 73.4

How did I do???????????:D :D :D

Here is a link to our on-line speed calculator.

Speed Calc

Thanks for all the help here. I am going to go with what MIKEHTMSR24 said. It seems well thought out and close to what the base speed of the boat should be. I didn't check out the other sights, Mike seems close enough to me. What I need to find is some honest GPS speeds. Its hard to find someone that will tell the troth on how fast there boat goes unless it's something to brag about. Thanks

Paul

rule of thumb that also works well

Sqrt(new HP)
___________ x old max speed = new speed

Sqrt(old HP)

so:

Sqrt(575)
________ x 62mph =77.8mph

Sqrt(365)

If you start with 59mph which as MIKEHTMSR24 pointed out seems more reasonable then the new speed will be 74mph

And y'all told your 6th grade math teacher when will I ever use square roots in real life :D :D :D

If you use BAM's calculator this hull would fit into Mercury's heavy vee hull/ no steps category. The test data, when adjusted for a believable slip as was done by Mike, give almost the same hull coefficient and slip % as Mercury's heavy vee category. For this kind of hull, in this speed range, the 15 HP per mph is very close.

Now the big question. What prop do I use to get roughly 72-74.......... lets stick with 72mph out of my boat at 5300 rpm. Thats where the motor makes peek power.

In the simulation above I used a HP curve from Dyno 2000, but I made a mistake and didn't build the engine for 600 crank HP @ 5300 RPM. So I added a little more cam and ran another simulation. The best prop was 27" pitch but at only 5100 RPM. If a real 575 SC peaks at 5300 you might expect to use a little less pitch. Or, to turn the problem around, you could raise the drive to reduce drag instead. Then the boat can go faster and the 27 Mirage would run up to 5300. Now you can go 78 mph!

Thanks Tomcat. Those were the speeds that I was figured I would run but I wanted to make sure. Now the big problem is that I am no where near them.

I only got to us the boat for two hours. we first tried a 28 Hydromotive 4 blade . it ran at 4300 RPM and the best we seen was 67 mph. in a slight chop

Then we ran a 26 bravo 1 4 blade ran at 4600 RPM . I never bothered to GPS due to the fact I wasn't at peek RPM.

Then I tried a 24 Bravo 1 , 4 blade and ran at 4800 RPM at about 67 on gps. But That day it was 95 degrees out very humid and water conditions were like glass 3/4 to a full tank of gas. Not a good day for prop testing

after the last run on the 24 bravo 1 I tested the oil and I found water was leaking in the block . The oil was like a milk shake. I didn't see any steam out of the block and the motor ran fine .

could the weather and the water in the oil have a big thing to do with why I am not running the #'s I should see or should I be running 3 blade props instead of 4 blade. Also I have 280 K plains installed on the boat. Could the give me a ton of drag ?????? I would asume that I could be off a 100 HP not running the RPM's up at 5300 and running in those weather conditins . does the seem right.?????

MIlkshake is not a good thing. Where was the oil level on the dipstick? (How much water leaked in?) Don't just look at the dipstick, drain some oil out of the pan too. If the level is up and its all milkshake, you have an engine repair to do. Head gasket at the least, cracked cylinder wall at the worst. Then again, maybe you're just seeing a little condensation on the dipstick. This could be from running the engine cold and never getting the oil temperature up enough to boil off the water. Or it could be a leak inside the exhaust being sucked back into a cylinder at idle, which never shows much water in the oil, but eventually makes the exhaust valves brittle. If the valve head falls off... hole in piston, crack in cylinder, crankcase full of milkshake.

There's lots of reasons why the engine may not be making full power, an overfilled crankcase being just one of them. If you do get the engine back to normal, I would definitely be trying 3 blade props. Check your X dimension. If the propshaft is more than 6-7 inches under the hull, you don't need and can't use a four blade for best top speed. If the boat blows out just a bit in turns or when planing with a load, then the three blades are submerged enough to give you all the blade area you need. Anything more and you are wasting power turning extra blade area. The extra blade does give you better acceleration and lower cruising RPM if that's what you're after.

Using your test data with the 24 four blade (and the full load and smooth water) gives a less efficient hull curve, but you should still be seeing about 71 mph @ 5200 RPM, if the engine is right. The only way I can get the results to fit the curve is if your engine is only delivering about 475 HP to the prop. How sure are you about the power of this engine? Can you provide a few details about it?

The motor was built at typhoon marine. It is a 454 with a 177 blower. cam is a custom ground hydraulic roller 228 intake 238 exhaust @ .050 and a 112 lobe center. Crane roller lifters JE pistons. I don't know the compression. But it was high for your average blower motor. Maybe just under 9:1 if I remember correctly. They are only spinning the blower at 4.5 - 5 psi to keep the temp of the blower down. Heads are lightly ported. Long tube headers. modified 850 Holley I think it is something like a 930 cfm. The dyno sheet said it made 590 HP at 6000 RPM and around 565 at 5300 rpm. if I remember correctly. I don't have the sheet any more.

I talked with typhoon marine today and the leak is from a crack in the lifter valley along the water jacket heading up towards the heads. from what they told me it sounds like just above where the oil runs down the sides of the lifter valley. It is about 2" long.

I ran the boat for no more than 2 hours before seeing that there was a problem. The fluid level in the block was up about 3" on the dip stick. I am using the same oil pan as a HP500. along with a very large oil cooler mounted to the boat and a oil thermostat. That night I drained all the oil, flushed out the cooler and thermostat changed the filter and flushed out the block with about 8 Qts of oil before filling the block again. The filter had no sign of any metal in it and the oil pressure never dropped all day long. It stays right at 52 PSI. The oil temp runs at about 230 degrees and the water temp is at 100 degrees. We want to keep the block cool because of the small blower and keep the oil hot to burn out moisture.

As for the X demission I know it is kind of low like you said. I think I am going to try a three blade prop. but I must get the block fixed first.

The crack is in a non stressful location on the block. I would very much like to braise or silver solder the crack and then cover it in epoxy rather than find another block. We are currently seeing what our option are. money is a big factor at this point. I just want the boat running. Its been a two year project and I want it to come to an end in time for the 1000 Islands poker run. That is also my vacation week we been planning all year. If I need a new block I might as well have the boat winterized and look forward to next year. At this time I am NOT a happy camper :mad:

Sorry for the long post. you guys have been a big help. Thanks

Paul

I have had block problems in the past and believe me, I tried everything to keep my small block going for the rest of the season (a couple of years ago). I was always unsuccessful with trying to band-aid that kind of block failure. My crack was pretty much in the same area and really required pulling the motor, stripping the block down to nothing, and then having some cast-iron welding done. I tried brazing, epoxy, and other combinations of the two (and I know how to do this stuff) and was unsuccessful. You have a piece that is constantly changing with the temperature... without making the cracked area as one by temperature controlled welding, you'll just drive yourself crazy finding water in the oil and wondering what it's doing to your internal parts. My junk 350 was no big deal. I ran it into the ground with the intent of replacing the entire assembly. I don't think you can afford the same luxury with your setup.

Good luck with it...

Sorry to hear about your block problem. I remember you telling me that it was a std. 454 block, but I do not remember if it was a Mark IV or Gen. V. Let me know what type and I will start making some phone calls. I would not even consider repairing the block. (A few years ago I was in the same exact situation that you are.) I trusted a machine shop that assured me a crack could be brazed because it was in a "low stress" area. Well after the motor blew apart all that was salvaged was one cylinder head, an intake manifold, carb/flame arrestor and a distributor. To this day I regret not finding a replacement block. I would hate to see this happen to you.

tom and dave thanks for the advice. I think I found a block I need to make a call shortly to be sure. if not dave please e-mail me your # or call me at 914 906 8091 thanks

Paul

I sent you an e-mail with some numbers. I will call you in the am.