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Old 21 May 2010, 06:55   #1
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Twin Outboard Setup Question

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
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Old 21 May 2010, 07:01   #2
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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?
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Old 21 May 2010, 07:10   #3
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Quote:
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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?
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Old 21 May 2010, 07:54   #4
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Them hysucat things have the engines angled out like your saying but I'm not sure the reason behind it?
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Old 21 May 2010, 14:38   #5
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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 ?
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Old 21 May 2010, 15:06   #6
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What happens in flat water when you go WOT at standstill, any cavitation?
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Old 21 May 2010, 16:46   #7
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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.
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Old 22 May 2010, 04:40   #8
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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.
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Old 22 May 2010, 04:55   #9
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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!!
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Old 22 May 2010, 05:55   #10
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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.
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