Installing new 454 on new (to me) Donzi 24' - best prop for top speed?

[TorchedGT]

I got the boat with a dead engine, and I was laughing a bit at the fact that it came with a Viper 3 blade 21p on it - the guy said it ran in the 70s? The engine was a Gen VI roller cam w/ holley 800 and stainless marine headers. I just have doubts that a 21 could get that boat up to the 70s.

A friend recently got a 25' Nordic (Rage I think - mid-cabin) and with a stock 496HO he was hitting mid-70s at 4700rpm (i think that's what it was) - with a 26p 4 blade? Most of that doesn't add up to me! I thought those props were for hole-shot and not top end? Also seems like a lot of boat for a 425hp engine to hit that speed stock. I know they've got good hulls though.

The 454 going into my boat put out just under 500hp and 620lb-ft torque on dyno, so it's a significant increase over stock (not that I even know what prop that would have been) but for a Donzi-type offshore hull, OMC King Cobra (cone clutch, less drag as the main gears are up top) and a focus on top-speed, could I get away with a 26p 3-blade, or would I have planing issues? The boat does have trim tabs. I'm still miffed at my friend's Nordic performance with that 4 blade though.

Most of what I do will be cruising, short top-speed runs, and (least likely) some water sports and hole-shot stuff, coming out of frequent no-wakes, etc.

The drive is also reverse rotation, so it'll be a pain to find anything useful - as most of the reverse OMC props were for the smaller Cobra drives and max out at 23p.

 

[tpenfield]

Re: Installing new 454 on new (to me) Donzi 24' - best prop for top speed?

Maybe float that question over on the OSO site (offshoreonly.com) as there will be lots of folks with high performance boats.

I would think that you could be running at least a 25P, but maybe 26P.

You have an OMC King Cobra outdrive ??? Interesting. . .

 

[TorchedGT]

Re: Installing new 454 on new (to me) Donzi 24' - best prop for top speed?

Maybe float that question over on the OSO site (offshoreonly.com) as there will be lots of folks with high performance boats.

I would think that you could be running at least a 25P, but maybe 26P.

You have an OMC King Cobra outdrive ??? Interesting. . .

Thanks, I just registered - I'll post in a bit.

Yeah the boat is a '90 (mostly restored) but the drive remains. Apparently this last generation of the King Cobra is incredibly strong; closer to a Volvo SX.

 

[bnicov]

Re: Installing new 454 on new (to me) Donzi 24' - best prop for top speed?

Parts and expertise on those drives are getting harder and harder to come by. Make sure you maintain it and don't abuse it.

 

[TorchedGT]

Re: Installing new 454 on new (to me) Donzi 24' - best prop for top speed?

Parts and expertise on those drives are getting harder and harder to come by. Make sure you maintain it and don't abuse it.

Yup, I went through the research phase before I got the boat - in the end, since it was the cone-clutch late model I was okay with it. From what I understand, the knowledgeable Volvo Penta techs can work on this model with no problem.

Hopefully abuse doesn't count as a new 500hp motor.. :redface: (literature stated they're rated at 600+)

 

[Philster]

Re: Installing new 454 on new (to me) Donzi 24' - best prop for top speed?

Planing issues? Okay, stop that.

With 500 HP, you need a 4-blade like you need higher gas prices.

Get at least a 26p 3blade prop. Will Bravo props fit? If so, that would be a good 4-blader if you insist, but you might be in the 27p blade zone if you have just 24' of boat with near 500HP!

The numbers for the Nordic make sense. That's not a heavy boat with a very deep V is it?

Okay, general guidelines for single-engine big block are that a 25P will be good for 67-70, so a 26p is good for mid 70's for sure. A 4-blade prop can beat a 3 blader on the top end in an outboard application where the outboard is on a jackplate and the 4 blader can be raised to run at the surface. In the applications you discuss, the rule of thumb is a 3-blade, cupped, maximum bow-lift prop. Mirage Plus comes to mind.

(27P props are generally for 75-80 in your type of applications, but gearing and other hull designs introduce variables that must be water tested. Because speed returns at the higher MPH ranges get smaller in increments, but props jump up in the same steps, you often find that you need a prop customized. If prop A is good for 70 and prop B is good for 80, but your engine RPM and boat combine for 75 and you'd like more RPM, you might need a prop shop involved. Playin' with the big boys gets expensive, and this stuff becomes the hobby)

All fun.

(4-blade hole shot versus top end: A 4-blader might knock a 76 MPH boat down to a 74 MPH boat. It's not as if it destroys the top end. Just trims it a little).