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HP Required to Produce mph??

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DLRVelocity1

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Discussion starter · #1 ·
I'm running a 23'6" Regal Velocity, Steven Stepp pad bottom hull. The boat weighs 2850 lbs. dry. It has run a best of 67.8 GPS with the 454/330.

What kind of torque and HP do I need to make with the new motor, to push this boat to 75-80 mph? Any thoughts? Is there a standard mathmatical formula that would give me a reasonably close idea?

Knowing this information will help me make an informed choice. Don't want to under build or over build to the point that this boat can't handle what the motor has to offer... :D
 

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#2 · (Edited)
My 23 Nova did 57 gps with a 7.4 330hp. Boat weighs 4000+,- lbs. With the ZZ502/502, stock Merc exhaust and and a 25 Mirage Plus it would run 68.5+,- gps any day. I expect the ZZ with the Merc exhaust was putting out 460 or so hp??

So, increase of 130 or so hp got me about another 11 to 12 mph. That works out to about 11.3 hp per 1 mph. Your boat is about 1200 lbs lighter. So I would guess with similar hp {460}, you might be in the speed range you're looking for. Also, Revolution Marine exhaust and labbing the prop got me to a top speed of 73 gps. So a decent exhaust and a labbed prop might just get you very close to the 80 mph mark :cool:

ZZ tq is 567 @ 4200 rpm, hp is 502 @ 5200 rpm. No, I'm not saying get a ZZ502/502. I don't want to start a pizzing match about cams and all the rest of the bs. Just giving you the infor so you can prepare.

However.....IMHO a ZZ would be a good choice :D

Oh, what kind of OD do you have?? I don't know if an Alfa would take kindly to 500 hp for too long.
 
#3 ·
Kid is right on. 12 HP per MPH is a good rulke of thumb on single engine boats. I went from a stock 330 MPI to a 502 mag and bumped the can up ever so slightly. The 272 went from 60-62 to 70-72.
 
#4 ·
And before everyone says thats inpossible for a 272 with a 502. It also had headers, labbed prop and a blueprinted hull.

D@MN I miss that boat already,,,,,,,,,,,
 
Discussion starter · #5 ·
Drive..

The drive is a Bravo 1... with 175 original hours on it. Had it apart when I bought the boat a year ago and everything looks new. I'm not a hole-shotter, just ease her up and over. Midrange to wide open is a nice smooth transition too. Work in the aviation industry. Know all about what it takes to break parts..
DLR
 
#7 ·
Go to the prop calculator on the mercruiser site.
Figure the prop and speed for a 454 and then for a 502.
It will hit your mph within 5 mph at the 454 number then when adding the 502 it will figure the mph for you.
12 hp for 1 mph is a good number.
 
#8 ·
I think Kid is right! 500 hp should get you there easily!

I just repowered from a 7.4MPI to an HP-500. The MPI is 310 prop hp and the HP-500 is 470 prop hp.

This 160 extra hp brought my speeds from 63 (best on a perfect day) to a solid 74-75. There is a few more mph in there as well. Maybe 78-80 when everything gets sorted out!

This 12 mph difference is HUGE!!

Take care,
Kent
 
#9 ·
my .02 - 500 hp (at the prop) will get you comfortably OVER 80 mph with the right prop. :cool:
450 propshaft horsepower should be enough to get you just to the edge of 80 on a good day. As Mudball said this will be a huge diffference. You may not stop grinning for days. :D :D
 
#10 ·
DLR, might want to add a decent drive shower. They're relatively inexpensive and good insurance for the $.

Post your results when your done. Not much better in the go fast world than going faster :) .

Like laster says...you'll be grinning for days....good luck.
 
Discussion starter · #11 ·
MPH and Drive Shower

Dual feed drive shower is already in place and does a nice job keeping the op temp. of the drive down. Has anyone got a "Recipe" for a proven motor / (C.I.), cam specs, heads and exhaust, intake, carb, etc. that will produce between 450 and 500 PS horsepower? Wanna build this one myself this winter. Rather than re-invent the wheel, I'm willing to go with a combination that is proven.
DLR
 
#13 ·
Re: Bump

DLRVelocity1 said:
Anxious to start buying parts....Any suggestions??

DLR
Click to expand...
There are a few threads on the site regarding BB's in the 450 to 500 hp range. Good contributers posting such as cfm, cstraub, rouge28, rmbuilder and others. Lots of knowledgeable contributers. I'm sure the guru's will be happy to post suggestions. There's also good info over on OSO's tech site.

Good luck. Sounds like you have something fun to look forward to :cool:
 
#14 ·
I can't answer you really well, but do know what several people on this forum have done.

I bought a used HP-500 engine and freshened the top end of it. This ended up costing more than some other options, I found out after doing it.

Kidnova (and I think others as well now) bought a ZZ502 and is very happy with it, from what I have read. He is running very well, and his installation is impressive.

Others on here have built their own from scratch, or rebuilt for more power.

You might want to search the forum for what Kidnova and others have had to do with the ZZ502. They need some changes for marine use. The price of a ZZ502 is attractive.

If you build (or rebuild) your own, Cstraub, CFM, and FtmBob will have your answers, amoung others!

There are some incredible minds on this forum for marine horsepower!

Please keep us posted as to what you do.
Kent
 
#16 ·
To answer the original question. There is a formula to calculate the amount of HP required to achieve a certain speed. I've used it to project performance increases for customers for many years and it has never been off more than a couple of MPH. It's pretty simple. Take your desired speed and divide it by your current speed. Square that result. Multiply that result by your current prop shaft HP. The answer is the required HP to achieve your desired speed.

Let's take Mudball's boat for instance.
Desired speed is 77.
Current speed is 63.
77/63=1.22
1.22x1.22=1.49
1.49 x310(old hp)=461 PSHP is required to reach 77.
He installed 470 PSHP and ran 74.7 first time out. Pretty close!

Bob
 
#17 ·
Re: Bump

DLRVelocity1 said:
Anxious to start buying parts....Any suggestions??

DLR
Click to expand...
My 502 motors are putting out close to 500 horse. 750 dominator carbs, bumped the cam up a hair ( don't have exact specs but can get them), roller valve train, Inconel valves, and a Junior intake. PM me if you are interested in the specs and I will get them for you.
 
#18 ·
Hey Bob!!

It's great to see you on here at last!!

YOU could help with your input on this forum along with the 'other' one!! This is a fun one!

(Bob, you may shoot me for saying this)
Bob has fun in taking normally considered sled boats and making them extremely fast!!

For instance: I was just outrun by something. Humm, what was that? WHAT, a Crownline?????

You are the king of sleepers!!
 
#20 ·
DLR,
I don't know what your plans are but with the 454 block you have, a stroker kit using that block to make a 496 would get you the power you want and very good reliability. This will be my route in a couple of years.

Chris
 
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