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Old 10 May 2013, 08:13   #1
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Engine height

I guess from the height of the cavitation plate bottom of transom, my engine is a couple of inches too low? Image imported upside down by the looks of it.

Looking at engine mount, I've probably got enough adjustment to bring it within an inch, but do I put anything under the engine mount, like a strip of wood to take the weight of the engine off of the bolts and onto the top of the transom? Image seems to have rotated 90 degrees on import.

Not sure why the images are rotated, but you get the picture.
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Old 10 May 2013, 09:25   #2
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Which engine brand is it, is it a S or L shaft leg, with ? HP mounted in which type of boat, sib, rib ? Post a complete pic as first one, but showing the prop too. Correct inversion as not to turn my monitor up side down...

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Old 10 May 2013, 10:41   #3
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Zodiac 3.6M Fast Roller with Mariner 9.9HP (4 stroke) short shaft (approx 36KG)

Images annotated and corrected hopefully

According to engine manual, I need to get cavitation plate within 1" (25mm) of bottom of transom. So probably need to raise engine 1 to 1-1/2" above height of transom, which is probably within the level of adjustment allowed by the mounting plate on the transom?

If I had the choice, do I edge nearer cavitation plate to 1/2" below bottom of transom or stick to 1".

But this will leave engine aloft of the transom, so I guess it is wise to put something beneath it and top of transom to support it? Any suggestions? Strip of 35mm wood is a bit heath robinson. I nice block of soft acrylic plastic would probably do the trick nicely - of course, someone may sell these types of things.
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Old 10 May 2013, 18:13   #4
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I think you're ok there-the tubes are lower than the bottom of the transom.
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Old 11 May 2013, 04:49   #5
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Thanks. That makes sense.

I'll check the offset with a straight edge below the tubes.
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Old 11 May 2013, 10:28   #6
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I'd try to run it higher. May take experimentation, but some boats run the plate up to an inch above the keel level. Higher is generally better up to the point where you get ventilation. As you get close to ideal, it's a balance between drag and splashing on the low end, and blowing out in turns at the higher end.

I don't think the tubes really come into the equation. Has to do with how the motor sits on the water as it passes off the bottom of the hull.

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Old 11 May 2013, 16:31   #7
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I measured level against bottom of tubes. Cav plate was higher than bottom of tubes. So cav plate is below transom but above bottom of tubes. Not sure where waterline is.

I've got quite a bit of running in where I can examine levels in water. For time being, I will assume that Zodiac made transom correct height for their short shaft engine requirements.
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Old 11 May 2013, 16:44   #8
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Ian - stick a bit of 2"x1" in there and try it out. Works for me.
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Old 12 May 2013, 14:14   #9
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Anticav plate/boat's bottom height parameters are just a refference from where to start testing under trial and error as correct engine/transom height match will depend entirely on boats bottom shape, tube diameter, correct pressure on air deck & tubes, brand engine used.

Try this simple guide :

-Inflate air deck and tubes to factory working parameteres, usually 3.0 PSI minimum and evenly for tubes, 12 PSI for air deck.

-Sit engine trimed so to have a perpendicular engine and anticav plate paralell to water level once sib is floating. Test on flat calm no wind water condition preferably.

-To ballance well sib, sit a mate at bow, driver must seat on air deck next to transom. Use your kill cord and hold tiller firmly to go on a straight water course when at wot.

-Check water flow passing at speed through lower leg. Ideal water flow height to have excellent hole shot, superb tight turns and good top speed is achieved when flow passes slightly under small upper deflector plate.

By visually checking this flow issue, will know for sure if in need to raise the engine and by how much if needed. Test with different wooden shims thickness untill you achieve the described sweet engine/transom height match position.

Check this video for clear understanding :



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Old 13 May 2013, 12:14   #10
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Permanent Modified Engine Heights Examples :

Most sib/rib boaters assumes that all boats are factory deliverd with correct transom heights that will perfectly match any brand engine. But this sometimes isn't so, if you have the time and patience a wot spin under trial and error to achieve best engine/transom height match is mandatory.

Problem : Excessive water splash over transom at speed.
Boat : Sea Rider 380 sib/Tohatsu 18 S.
Tech issues : Transom being 38 cm high when needed to be 40 cm.
Correction : Raised transom to 40 cm to find sweet engine /transom height match.
Performance : Engine has gained extra 250 RPM , has perfect hole shot, excellent close turns at any speed, better fuel economy needing to throttle much less to maintain plane.

Happy Boating
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