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Old 29 October 2012, 21:24   #1
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Optimum mounting height for outoard engine on RIB?

This is related to my thread "Positioning of a helm station on a RIB" but different enough that I thought I should start a separate thread.

AB 9.5AL RIB, Yamaha 15MSH 2 stroke engine. What's the optimum mounting height for the engine?

AB doesn't specify. Yamaha says in general, the anti cavitation plate should be between 0 to 25mm below the bottom of the hull of the boat.

Has anyone done any experimenting with this to see what works best in your situation?
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Old 29 October 2012, 23:54   #2
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I suspect you're going to have to find out for yourself by shimming the top of the transom with bits of wood 'til you find the optimum height for your particular boat.
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Old 29 October 2012, 23:59   #3
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As Nos said, it's pretty much trial and error. Starting height is generally considered anti-ventilation plate even with the hull, but usually best all-around performance comes with it up to an inch or two above the hull line. Depends on a lot of factors: normal sea state, load, weight distribution, and about a thousand other things.

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Old 30 October 2012, 08:49   #4
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Start by ballancing rib properly, trim engine to be perpendiculat to rib, full throttle, at plane on flat calm waters, ideal would be to have water flow slightly passing under small upper deflector plate, that's the sweet spot. Engine will deliver best rpm at wot with minimum tail drag and rib will turn fantastic at tight turns in all water conditions. Can play with different wooden shims thickness untill the sweet spot is found. Once there could screw that shim on top transom.

To have an overall idea :

Height stated on owners manual is only a refference from where to start, but if wanting boating perfection a trial and error water test must be done.

Happy Boating
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Old 30 October 2012, 10:55   #5
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Thanks for the replies gentlemen.

The boat is being set up primarily as a two person runabout with a minimum of fuel and gear aboard. The secondary purpose would be tender duties, ferrying people and freight to and from shore. Performance really is not an issue for the secondary duty so I'm assuming that all set up experimenting should be done with two people aboard.

As I understand it, if the engine is too high it will result in cavitation. If it is too low, it creates drag and is inefficient. I thought that I'd start with shims in place so that the anti cavitation plate on the engine would be in line with the bottom of the hull assuming that this would be the highest point I could go. If I have no cavitation, it's done. If I do, then I'd probably move it down in about 6 mm increments.

Sound reasonable?
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Old 30 October 2012, 11:11   #6
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Quote:
Originally Posted by Hocus-Pocus View Post
. I thought that I'd start with shims in place so that the anti cavitation plate on the engine would be in line with the bottom of the hull assuming that this would be the highest point I could go.

Sound reasonable?
A good start, but try going up from this point in increments - until it starts to caviate on tight turns. Thats too high - so drop it back down by an increment - I think this will give you best all round .
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Old 30 October 2012, 20:52   #7
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The learning process is at work.

The Tohatsu video was interesting and seemed to show the anti cavitation plate just below the surface.

Another video suggestion that came up was "Yamaha 90 optimum height? This showed a test where the plate was completely clear of the water. I wonder if he tried a hard turn at this height?

So now I'm thinking that the limit of height will probably be the point at which it cavitates on a hard turn at WOT. Expanding on that, perhaps the tests should be done with a light boat, that is just operator and fuel because that is a situation that can come up, albeit probably not often. The downside might be that with a normal load the height might not be optimum. If the height were optimised for normal load than it would likely cavitate with a light load, which would take some of the fun out of the whole experience.

Decisons, decisons, decisions.
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Old 30 October 2012, 22:26   #8
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measure anti vent plate to the bottom of the outboard clamp.
measure transom top to bottom of your stern.
both heights should be identical. if they are place the trim holes on setting 3 and be done with it.
you should see a wake tail like this at plane.
BTW if youre near van with your sib let me know and i can show you a 15HP 3.2m SIB at plane.
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Old 31 October 2012, 19:23   #9
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Originally Posted by Hocus-Pocus View Post
The Tohatsu video was interesting and seemed to show the anti cavitation plate just below the surface.
The upper small plate is a water deflector, the AC plate is doing it's work some centimeters down. The height test should be performed at plane, at non planing speed it's a different story.. But once sweet spot is reached, rib will perform better at idle speeds.

Engine tails have a cutting edge right under small upper plate, water flow must be cut there, if flow is passing and hitting the non cutting edge (over upper small plate) will have water splashes and tail drag.

Happy Boating
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Old 07 November 2012, 03:43   #10
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According to my Merc manual, the Anticavitation plate should be set 1/2-1" BELOW the centre (lowest point) of the hull. In other words the AC plate should always be slightly below the water surface. In my case, the engine water intake sits flush on the bottom of the AC plate so it must be submerged at all times.
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