Twin Outboard Setup Question

[ed_dash]

Hi

As my profile says i have BWM deepsea 18 with twin 60hp 2 stroke Mariners on.

It tends to cavitate quite easily and i was wandering whether anyone has tried tilting the engines slightly so that the shafts are angled out away from the hull. Im only talking a few degrees nothing major, just to see if the inch or so extra separation would help the cavitation.

It was very noticable when it was choppy, say 1m sea that when i landed i would cavitate a lot unless i backed right off the throttles losing all speed. It could have just been the angle i was landing. Was fine when wave hopping down the waves but up them the stern seemed to dig in a disrupt the water flow over the props. I was trimmed all the way in. Maybe going faster is a good idea next time, see if the bow comes down more.

Any ideas about the cavitation or engine angling?

Thanks

[chewy]

The props will tend to cavitate if you keep WOT whilst air born. Try easing back then giving it some throttle just as you land?

[ed_dash]

Originally Posted by chewy

The props will tend to cavitate if you keep WOT whilst air born. Try easing back then giving it some throttle just as you land?

I was trying to do that as best as possible, i wasnt WOT, maybe that was the issue trying not to land too heavily may have made me over compensate and drop the stern in causing the cavitation. Its a bit diff to the SR4 i used to fly in.

What about the angled engines? it ever been tried before?

[chewy]

Them hysucat things have the engines angled out like your saying but I'm not sure the reason behind it?

[Bigmuz7]

Those shafts look quite short, I know its twins so thats ok, but where is the cav plate in relation to the hull, with the legs trimmed down, any photos of this ?

[willk]

What happens in flat water when you go WOT at standstill, any cavitation?

[ed_dash]

wilk, on a flat day when i start from a stand still they dont cavitate but i get the sucking / slurping noises that ive always associated with the start of cavitation. Make sense or just make me sound like a weirdo?

Sorry no pics of the legs down, but from memory the caviation plate is clear of the hull just to one side and not below the bottom as you would for a single set up.

[willk]

Originally Posted by ed_dash

i get the sucking / slurping noises that ive always associated with the start of cavitation. Make sense or just make me sound like a weirdo?

If it does, you're in good company here.

I'm gonna go out on a limb here (imagine ) and speculate that your props might have too short a pitch. If you're not completely sure about the setup, I'd suggest giving someone like Steel Developments a call. They'll need the boat details, weight and your current prop size.

[ed_dash]

Im only getting 5500 revs at WOT with the engines trimmed out, which is the max RPM but i was light on load and doing 38kts. Im currently running standard mercury 10 1/8" 15"p ali props, not sure if i go any higher in pitch whether i would lose out on both hole shot and top end. Also at 38kts and trimmed out she was starting to Chine walk a little.

Id be up for trying diff props but with twins i dont really want to buy 2 new props just to try and i dont think many people have 2 mercury / mariner 16". I would like to be proved wrong though!!

[willk]

It won't cost much to check with Steel Dev. Even if they do recommend a change, they sell recon allys for c.£40. Handier than wigging your motors off...

I'd be interested in hearing the outcome.

[jwalker]

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.

[yorfuoj]

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

[Andy B]

Gona go out on a limb here!
but dose the fact that both engines are on right hand gearbox's not help?

[9D280]

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?)

[Searider]

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.

[ed_dash]

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.

[martini]

Black max really not ideal for your set up, Mirage plus or Revolution 4 would probably sort your problems out

[ed_dash]

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.

[9D280]

Originally Posted by ed_dash

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.

Originally Posted by ed_dash

<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.

[JABS]

Originally Posted by 9D280

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