I am in the process of rigging a 33 foot Rib in England for the Round Britain 2008 race. We are looking at the 496 mercruisers however keep reading about the 550 Ilmor engines. would love some comments. The priorty is realiabilty with plenty of HP.

525, done

The Ilmor's are the way to go. They're available up 710hp, the performance, reliability and the sound will just blow you away. I'm Mercruiser through and through, but I gotta say that out of all the powerplants I've worked on, the Ilmor's sound fantastic resonating out of the 6" pipe. A boat with two of them at speed sounds like a WWII fighter plane coming in for the kill. Lamborghini, and Falconer V-12's sound awsomely fantastic as well but may be cost prohibitive for your project.

I asked the question on boat mad in uk 1 guy gave a negative answer possible because they don't run any over here. i think the twin 550 would be enough power as the rib is very light.
abount 1500lbs with out any motors.

Boat Mad? Never heard of it. You do realize your motor(s) will far outweigh (ratio) your rib, is your rib to be a center console similiar to the Prestige 900 or something completely custom with the motor designed into the midship area?

Its a Hunton rib. we have only just put a web page together so only one photo on there www.thebandit1.com
More photos going on this week i hope
Power to weight should be good then.

From all the tests that I have seen, the Merc 525's will burn 10-20 GPH less than the ilmors. If you race endurance, Is this an issue? 4 hour race you will need to carry an additional 100 gallons/700+pounds or need a refill stop.

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That is a very big issue. We were looking at 496 425 hp as top speed isn't what this event is about. It looks as though they may be better as the boat is very light.

If top speed is not a concern then maybe you should consider a Yanmar Twin or single Turbo diesel coupled to a surface drive.

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Not that slow.

And they sound crap.

And no more cheap diesel