Prop sice for 29ft scarab with 600hp??
 

Hey Guys.

I am new on OSO. Nice forum by the way.

I have a 29ft Scarab, with a 555cui 600hp as a replacement for my old HP500.
What will the top speed be for a boat with that engine?
I cant seem to clock more than 55mph with a 25in prop
at 5000rpm.
What sice prop do i need??

Regards.
Jens Henrik Vinther.
(Denmark)

Jens,

Specifically what prop are you running? What outdrive and gear ratio? Was your engine dyno'd or are you estimating HP? Something would seem to be off somewhere.

The engine was dynode 597hp at 5200rpm and 654lb-ft trq. I have the dyno sheet, and the engine was build specially for my boat, with a Comp Cam 11-451-8 marine cam and FAST EFI, and fresh water cooling..
The gear ratio I was told is 1,50, and i runs with a 25in mirage plus prop on a Bravo x with imco nosecone.
The engine runs flawless, but no topspeed.

Regards.
Jens henrik

I have a 1994 with 525sc and before the engine upgrade with 525hp i run
67 mph gps with a mirage plus 25p at 5000 rpmand with a bravo 1 26 lab i get 69mph at 5050 rpm all this # with std bravo 1 drive. With 600hp you should get arond 72 mph.
Now the engine is upgrade at arond 650-700 hp and i get 78 mph gps with same 26p lab 4 blade prop at 5500 rpm.

THX for your reply....Keep em coming :)

You have got to figure out why you are only able to turn that prop to 5k rpm first off. With 597 hp you should be able to do better than that. Set-up and rigging issues? Lack of top end cant be a slip issue as you are low on RPM to begin with.

Have you tried dropping to a 23 to see how high you can spin that?

What do you mean when you say set-up and rigging issues??
Sorry if it is a stupid question, but i am pretty new at performance boating...:-). No i haven't tried a 23" cause the 25"
worked well with my hp500.

Can it have anything to do with the imco nosecone i have welded on my lower drive.

That is possible, especially at slower speeds nosecones tend to have negative affects rather than positive. That being said you still have to wonder why your rpms are low for the HP as a nosecone might tend to add slip

At what rpm does the engine make peak torque?

The dynosheet says 594,1 hp at 5200rpm and 653lb-ft trq.
And yes a wonder why it is slower with 600hp than with 500hp...
Trust i am loosing my mind...

Can you post a copy of the dyno sheet? You might also want to try that in the "general q and a section"

Are you making the 653 tq at 5200 rpm as well as peak hp there?