jens henrik

Hey Guys.

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

I have a 29ft Scarab, with a brand new 555cui F.A.S.T fuelinjected 600hp as a replacement for my old HP500.
My engine was made with a Comp cam Extreeme marine cam
XM 284 HR
RPM-range 2200-5800
Duration inlet-284 exhaust-290
The engine produces:
At 4600 RPM - 572,5 hp and 653 LB/FT
At 5200 RPM - 597,1 hp and 600 LB/FT
AT5500 RPM - 581,3 hp and 555 LB/FT

My drive is a Bravo X 1,50 with a Imco nosecone
and a 25" Mirageplus.

What will the top speed be for a boat with that engine?
What sice prop do i need??

Regards.
Jens Henrik Vinther.
(Denmark)

01scarab29

with no slippage 87mph. http://www.mercuryracing.com/propell...calculator.php
check this site , if you know your speed it will give you your slippage % .. then decide if you have the right prop

A.O. Razor

Start by running what you have for a base line and write down the numbers.
You wanna keep it around 5200-5300 rpm. The 25" M+ and a 27"M+ are good places to start. If the 25" is good for 5300+ rpms on average, I'd try a Rev4 in 25" for best overall performance. Do some testing and get back to us. (That is, if you can find a day when it's not friggin' pooring down!!!)

Do you have full hydraulic steering on it btw? You should maybe consider removing the nose cone. At those speeds on a stock length drive, it will most likely slow you down and mess with your steering, especially if you got the stock merc steering. With that power, no matter what, you need full hydraulic steering, just a friendly advice

jens henrik

Problem is that i only goes 55-60 mph at 5200rpm with the mirage+. I have borrowed a 27" tempest from a friend, and as soon as it stops Fuc.... raining, i will try that out.

It is not an option for me to knock of my nosecone, cause it is welded on, and painted, and i think i will crack my drive if i try to knock it off.

Why is it so important with hydraulik steering?

A.O. Razor

To answer your last question. When you install a nose cone, you move the loaded point of the turning pint to the front portion of the drive. Think of it, as trying to push a desk chair with the castors facing forward, they will try to turn around, a bit like when trying to drive a bicycle with the handlebars spun 180 degrees. In other words, it puts a huge load on your steering and if it's not strong enough, things can happen. When putting 500+ hp thru a B1 drive that in itself is quite a bit, there's also a lot of slack in the stock steering. You don't want to swap ends at 75 mph. If the stock steering fails at wot, you're f***k'd. Generally speaking, stock merc steering does not belong on a high performance boat.

Now, 55-60 with that power, well, something's way off. You should be in the mid to high 70's. The interesting part is, you are at peak power now. What kind of power did the boat have before and what speeds did it run? Do you have dyno confirmation on that power? How's the running attitude of the boat? Other things to check for are hook in hull, false rpm reading, false gps reading, engine related issues that can bring down power such as fuel starvation, timing issues and so on. What gear ratio is in your drive? Bottom paint? You can try the Tempest, but it will most likely not work, the geometry of the prop is not what you need on a single 29' i/o boat, especially considering what is going on with the 25" M+ and 55-60 mph. If your drive is a 1.5 ratio drive, your slip is between 33% and 27%. Even with a 1.65 your slip is way too high.

A.O. Razor

Are you suggesting having a prop without slip is a good thing???

jens henrik

With my old HP500 it could reach 68mph. I have the dynosheet on the engine., if you give me your email i will be glad to send you som pictures of the dynosheet so you can see for your self.
Razor do you know the cam that i have in my engine, it should be a mild marine cam made by Comp Cam.( Extreme marine 11-451-8) Whats do you think about that, can it have anything to do with that??
I know that the engine is made with the best parts that money can buy.
Alle the issues that you mention can be a reason for the engine not to perform well, they should eliminated. The bottom paint is new only 1month old, and the boat was also equipped with new Livorsi dials and the engine when it runs WOT it has a AFR on 12,5 and the ignition is set on 36 degree and the engine runs flawless.

A.O. Razor

PM sent.

It does sound like you're down on power. There are better engine guys on the board than me, I'm better at setup and props ect. You can try to contact RM builder (Bob Madara) here on the board, he's one of the best marine cam guys there is. You have added about 100 hp to your motor, so there is a problem for sure. Your 25" should be good for 73-75 @ 5200-5400 rpms, if your gear ratio is a 1.5. What puzzels me is, that you're running 55-60 @ 5200. Has anything on the boat and drive been changed while you built the motor and did the boat run 68 with the 25"?

supermx96

Realy strange, with 600 hp on that boat you should get close to 72 mph

jens henrik

Trust me i am loosing my mind here to. When it ran 68-70mph with my HP500 and now it has a 100hp more , an now i can't seem to get it over 62mph and 5000rpm....