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MPH slow - what should I be doing?

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Old 12-27-2010, 07:12 AM
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Default MPH slow - what should I be doing?

Hi guys,

I recently bought a 34' Scarab - Aussie version made under licence by Fastlane. Same hull as Wellcrafts, although I suspect it's heavier.

It's powered by twin Mercruiser Magnum 350's - later vorect headed models. I've just replaced the fuel system with Mallory pumps, fitted air gap Performer RPM style vortec intake manifolds, and had the carbs (holley 4150's 650cfm) rebuilt. The boat uses Alpha drives.

WOT I'm doing 40 mph. Engines are revving at about 3400 rpms. I've checked I'm getting full throttle.

My next step is to read some numbers off the Props. They're Mercury Mirage Plus, but I don't know the specs yet.

The hull is painted with anti-foul as the boat lives in salt water. The drives and props are clean.

What MPH should I be doing with these power plants in this boat? I assume I should be aiming for around 5000 rpm at WOT / Top speed via a prop change ?

Thanks for any advice or suggestions!

Craig
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1990 Scarab 34
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Old 12-27-2010, 07:38 AM
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Start playing around with props it sounds like. You may loose a couple MPH with the paint but when you leave it in the water you need it. Find out what pitch you are running but I would guess you want to be in the 21 range.
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Old 12-27-2010, 03:44 PM
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Originally Posted by earlysport View Post
Hi guys,

I recently bought a 34' Scarab - Aussie version made under licence by Fastlane. Same hull as Wellcrafts, although I suspect it's heavier.

It's powered by twin Mercruiser Magnum 350's - later vorect headed models. I've just replaced the fuel system with Mallory pumps, fitted air gap Performer RPM style vortec intake manifolds, and had the carbs (holley 4150's 650cfm) rebuilt. The boat uses Alpha drives.

WOT I'm doing 40 mph. Engines are revving at about 3400 rpms. I've checked I'm getting full throttle.

My next step is to read some numbers off the Props. They're Mercury Mirage Plus, but I don't know the specs yet.

The hull is painted with anti-foul as the boat lives in salt water. The drives and props are clean.

What MPH should I be doing with these power plants in this boat? I assume I should be aiming for around 5000 rpm at WOT / Top speed via a prop change ?

Thanks for any advice or suggestions!

Craig
------------------------
1990 Scarab 34
Hi Craig,
Are you sure that you have the timing set correctly on the engines? I would put a light on them and run the rpm up in neutral and check your total advance, should be 32 to 36 degrees, if the timing is retarded it won't rev.

Also check prop pitch as discussed. I am quite sure that your engines are rated at 4800-5200rpm per Mercruiser specs.
Good luck on the project.
Dave
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Old 12-27-2010, 04:06 PM
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Salt water with outdrives? Are they gunked up with growth inside?
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Old 12-27-2010, 04:30 PM
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Originally Posted by Audiofn View Post
Start playing around with props it sounds like. You may loose a couple MPH with the paint but when you leave it in the water you need it. Find out what pitch you are running but I would guess you want to be in the 21 range.
x2 - try starting at 14 3/4 x 21 prop
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Old 12-27-2010, 04:34 PM
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Thanks for the feedback so far guys.

I have checked the timing at idle and set both engines to 8 BTDC. I will check at higher revs - I fitted timing tape to the balancers to make this straightforward. The engines run the factory mercruiser ignition module.

The drives are regularly cleaned to remove marine growth. I learned this the hard way as 3 months in the water saw a bunch of barnacles on there, and a 10+ MPH drop in speed. the subsequent cleaning saw the MPH come straight to where it is now. Not sure about the insides of the drives though! They were rebuilt less than 2 years ago however.

I'll post the prop numbers when I get them - maybe today.

I was hoping this engine / boat combo might be good for 55 or 60 MPH, but maybe I'm being ambitious.. (with it currently doing 40)

Cheers,

Craig
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Old 12-27-2010, 07:19 PM
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If you are saying you have 300HP small blocks in you 34' Scarab and doing 40mph. Then I say you are doing the best that you can expect, you need more torque. I saw a 30' Scarab with small blocks and it didn't do more than 49mph. Sorry
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Old 12-27-2010, 07:38 PM
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Quote:
Originally Posted by Sonic30ss View Post
x2 - try starting at 14 3/4 x 21 prop
I'd listen to Joe... He's a race winning, proven good guy!!!

http://www.offshoreonly.com/forums/g...yesterday.html


Also for some help on numbers I have used Marc's BAM pages for a long time to play around with numbers...

http://www.go-fast.com/boat_speed_predictions.htm

And this one...

http://www.go-fast.com/Prop_Slip_Calculator.htm


DO YOURSELF A FAVOR HOWEVER READ ALL THE TEXT AND UNDERSTAND IT OR ELSE IT WILL SEEM LIKE WITCHCRAFT...

But it works and helps a lot!

Hope this helps.

GL
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Old 12-28-2010, 12:26 AM
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Default My prop numbers

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Originally Posted by Sonic30ss View Post
x2 - try starting at 14 3/4 x 21 prop
Ok, I have the numbers off the props.

They are Mercruiser Mirage Plus 48 13700 19P, which I decode to be a 15 1/4" 19 pitch 3 blade.

Given I'm maxing at 3200-3400 rpm at WOT, what props do you recommend?
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Old 12-28-2010, 12:47 AM
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Originally Posted by earlysport View Post
Thanks for the feedback so far guys.

I have checked the timing at idle and set both engines to 8 BTDC. I will check at higher revs - I fitted timing tape to the balancers to make this straightforward. The engines run the factory mercruiser ignition module.

The drives are regularly cleaned to remove marine growth. I learned this the hard way as 3 months in the water saw a bunch of barnacles on there, and a 10+ MPH drop in speed. the subsequent cleaning saw the MPH come straight to where it is now. Not sure about the insides of the drives though! They were rebuilt less than 2 years ago however.

I'll post the prop numbers when I get them - maybe today.

I was hoping this engine / boat combo might be good for 55 or 60 MPH, but maybe I'm being ambitious.. (with it currently doing 40)

Cheers,

Craig
I have the same boat with 454CI 365 Mags and it sees 58-60. Sorry to say but I agree with Rookie with that set up you may be at your limit, might get 45 playing with props but without big block torque you will never see 55-60 range. Its an eight thousand pound boat dry from Wellcraft. Do you have any previous owner info on the boat? If it has ever done better?
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Last edited by glassdave; 12-28-2010 at 12:59 AM.
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