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Old 23 May 2010, 20:05   #11
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Some props just won't grip the water once they let go, even if the engines are backed off. Nothing less than almost coming to a stop will see them start to grip again. And, you're nearer suffering from ventilation not cavitation. Cavitation is a completely different problem. I don't see a problem with angling the engines except, possibly, a carburation difficulty but if the props are at fault you'll obviously be wasting your time, and I suspect you will be. Sliding your engines further apart will get the props deeper into the water flow if you feel that's the problem.
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Old 09 August 2010, 05:07   #12
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I would suggest to angle the engines.
This is very common in South Africa where most of ribs are twin'ed.

On Tomcat-Stingray cats, it is mandatory - 7 degrees on each engine - otherwise you get cavitation and 1000rpm difference.
Hysucat is probably the same.

You "just" have to remove the 4 lower bolts which fix the engines to the transom, then on each engine the external top bolt, this will allow the engine to rotate slightly.
Then you insert the removed top bolt into the next following hole in coincidence with the transom hole - it should enter without redrilling.
This should give approximately 3degrees of angle - 2 holes give 7.

For testing, the bottom bolts are not mandatory, however I would suggest to install a smaller one, 8mm for instance, for safety if the sea were rough
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Old 09 August 2010, 06:46   #13
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Gona go out on a limb here!
but dose the fact that both engines are on right hand gearbox's not help?
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Old 16 August 2010, 07:03   #14
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Are they parallel? If so, you should anlge them slightly - its because the V hull means the water isn't coming straight off the back, so they should be "toed in" so the skeg etc is running parallel with the water. This isn't a problem with a single, as the engine is sitting on the symmetry centrline.

The front of the engines should be closer together than the rear, I'll leave the threory of the angle to the experts! (I think I read something about the angle calcs on the Boston Whaler chatrooms?)
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Old 16 August 2010, 12:00   #15
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Prop condition?

What condition are your props in? I had a case where the ally prop on my engine over time had developed fine cracks in the blades. Difficult to see without looking really closely. Prop let go really easily. New prop and all sorted.
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Old 16 August 2010, 12:51   #16
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one brand new mercury black max ally, and the other a recon mercury. neither have any paint off them yet so both as good as they can be.

I think i will make do with what i have now and when they need to be replaced, get a bigger diameter yamaha styled blade, with a straight back edge. See if that makes any diff.
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Old 16 August 2010, 13:35   #17
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Black max really not ideal for your set up, Mirage plus or Revolution 4 would probably sort your problems out
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Old 16 August 2010, 14:56   #18
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are they both stainless?

originally i bought 1 to replace one of the damaged ones that came with the boat but it ended up being the same pitch but different diameter, so thought it was better to buy a new one, as i knew exactly the size of it.
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Old 17 August 2010, 07:25   #19
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Quote:
Originally Posted by ed_dash View Post
one brand new mercury black max ally, and the other a recon mercury. neither have any paint off them yet so both as good as they can be.
Woah a minute.....you got two different props? Two of the same would be a good start. Have a trawl thugh the various prop discussions - there's a few posts out there comparing two props of nominally the same spec and the resulting differences in boat handling.

Quote:
Originally Posted by ed_dash View Post
<snip> , get a bigger diameter <snip>
I had much the same problems with a single engine - all cured with a bigger dia prop - Lost a couple of knots off the top speed, but as the enfgine was passing 6.5K & still accelerating with the old prop I could live with that sacrifice! It broke out at the slightest provocation.

Bottom line is unless your boat weighs next to nothing, you need a certain abmount of blade to grip the water.



I'd start by trying to borrow a matched pairt & see how it goes.
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Old 17 August 2010, 07:41   #20
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A new can of worms

Quote:
Originally Posted by 9D280 View Post
Are they parallel? If so, you should anlge them slightly - its because the V hull means the water isn't coming straight off the back, so they should be "toed in" so the skeg etc is running parallel with the water. This isn't a problem with a single, as the engine is sitting on the symmetry centrline.

The front of the engines should be closer together than the rear, I'll leave the threory of the angle to the experts! (I think I read something about the angle calcs on the Boston Whaler chatrooms?)
My single engine is way off the centre line?? Looking around the dry stack, most singles are offset.

I thought that was normal?

Can anyone explain why?

Thanks
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