HP question

[johndmay]

My new 502 showed 550 HP at 5300 rpm. Does anyone have an approximate idea of what the horsepower is at the prop?

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John D. May
Manotick, Ont.
www.jackmay.com

[GeneD]

You know...that is a good question.
Forrest should have an idea.
That is a real good question.
Who has the formula?

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GeneD
007
Melbourne, Florida

[RickR,GroveCity]

johndmay
I am not an engineer but;
I figure the loss caused by the geartrain would be almost the same(@ the same RPM,same drive)
454 Mag Bravo(5000RPM redline) is 385@crankshaft,350@prop

502 Bravo Merc(MCM 465)(5200RPM redline)
500@crank,465@prop

So 550-35=515


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RickR mailto:riggerb@aol.comriggerb@aol.com</A>

[This message has been edited by RickR,GroveCity (edited 05-28-2000).]

[GEOO]

John What boat and drive are you running?? GEOO

[johndmay]

GEOO, it's a 1988 Z-25 with a Bravo 1. It's running about 65 mph (on the GPS) at 4800 RPM. The prop is a 26P Bravo 4 blade. I'm pleased with the new motor (although I was hoping for low 70's) but was just curious what the true HP at the prop was, after the dyno showed 550.


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John D. May
Manotick, Ont.
www.jackmay.com

[GEOO]

John, Sounds like you have a nice strong motor. What rpm's did it make 550hp?
You are running at 17.5% slip. That's a little high, you may be able to gain a couple mph by making the boat more efficient.
GEOO

[johndmay]

GEOO, it was peaking at 5300 rpm when the 550 hp. was indicated. My rigger says that either blueprinting the last 8 feet of the hull, or installing a shorter shaft will help. Both are pretty expensive options though. Is there any other way to lessen slip? I'm happy with the way the boat runs, and I'm sure I'll get better speeds on the St. Lawrence River, which is a LOT choppier than the Rideau River, where I live.

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John D. May
Manotick, Ont.
www.jackmay.com

[Frank Civitano]

John
What prop are you spinning? My Dyno was at 540hp and put out 510hp. My rpms are at 4800 with a 30pitch. My top is 77mph with a load. 22ZX with 4000lbs. 1/4tank no load Im over 80. Your hp and your pitch prop are not equilizing? Sounds more like a 502/415hp then a 550hp. You still should have 500hp at the prop. Your size boat should put you in the lower 70s? I ran a stock 24pitch and I was high 60s after the Supercharger I went to a 30pitch. Your prop should be a minuimum of 28pitch for 500hp. What gears are you running in your drive?

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Frank Civitano

[GEOO]

Frank, I think the Z-25 is harder to push then the new ZX's with stepped bottoms.

John,
How do you get your top speed out of the boat. Tabs up, drive up? Does the bow come up nice so you can air out the boat? Check and make sure the tabs lift completely out of the water. You can check this by putting a straight edge on the bottom of the hull and you should have at least 1/2" clearance to the tabs.
Is there any weight in the bow which you can move to the stern?
What engine, prop, rpm, speed did you have prior to the new power plant? Did you gain 8-10 mph per 100 extra hp?
Is the engine getting enough air? What aircleaner did the engine have at the dyno? Does the engine room have enough fresh air?
If you can safely, run the boat with the engine hatch open and also with the aircleaner removed; does your WOT speed change? You may be able to gain a few mph. I think you are close to the correct speed for your boat & engine combo. I've run in an older Superboat 24 foot with a 502/550 hp it ran 68-70 mph. GEOO

[johndmay]

Originally posted by GEOO:
Frank, I think the Z-25 is harder to push then the new ZX's with stepped bottoms.

John,
How do you get your top speed out of the boat. Tabs up, drive up? Does the bow come up nice so you can air out the boat? Check and make sure the tabs lift completely out of the water. You can check this by putting a straight edge on the bottom of the hull and you should have at least 1/2" clearance to the tabs.

Top speed comes with the lower unit and tabs all up. Then I have to apply tabs just enough to calm down the porpoising.

Is there any weight in the bow which you can move to the stern?

Nope, other than to drain the porta potti.

What engine, prop, rpm, speed did you have prior to the new power plant? Did you gain 8-10 mph per 100 extra hp?

It had a tired Magnum 454 with a 21p three blade Mirage. It turned 5200 and the old VDO speedo showed 62 mph (which may have been 58 mph realistically). It sure doesn't look like I gained 8 mph per 100 hp.

Is the engine getting enough air? What aircleaner did the engine have at the dyno? Does the engine room have enough fresh air?

It's getting lots of air (I think). An 850 cfm Holley with a massive flame arrestor.

If you can safely, run the boat with the engine hatch open and also with the aircleaner removed; does your WOT speed change? You may be able to gain a few mph. I think you are close to the correct speed for your boat & engine combo. I've run in an older Superboat 24 foot with a 502/550 hp it ran 68-70 mph. GEOO

I'll try that, with the hatch slightly open. But what do you think of the riggers suggestions? Can curing a hook really add 5 plus mph and eliminate porpoising? When we put the straight edge on the hull, it showed a gap of *maybe* 1/8 to 1/4 of an inch in the 6 feet ahead of the transom.

I also have to consider new motor tightness, and lack of chop where I boat. I know this new motor is stout, but I have to admit I'm just a little disappointed in the speed.


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John D. May
Manotick, Ont.
www.jackmay.com

[Forrest]

Where on the hull was that 1/8" to 1/4" hook? Between the two inside lifting strakes? If it is, that will kill your top speed. If the hook is between the chine and the outside lifting strakes, that's not as bad. Six feet forward of the transom on an 18' Donzi and that area of the hull would be out of the water at full speed, but on a 25' hull, six feet forward the transom is definitely in the water at full speed.

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Forrest

[This message has been edited by Forrest (edited 05-30-2000).]

[johndmay]

Originally posted by Forrest:
Where on the hull was that 1/8" to 1/4" hook? Between the two inside lifting strakes? If it is, that will kill your top speed. If the hook is between the chine and the outside lifting strakes, that's not as bad.


It's pretty much the width of the bottom. Not real severe, but visible with the straight edge on.

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John D. May
Manotick, Ont.
www.jackmay.com