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Old 29 August 2013, 22:50   #21
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I'm writting a boating article called Boating ABC Guideline, will have lots of pics, refferences, concepts, myths that will help anyone being newbie or oldie to better their boat/engine overall water performance. Once it's finished will post it. BTW, will take some time.

Meanwhile would like to ask the audience this simple question : What is the small upper plate that every engine has is good for, what does it does, what happens if it's chopped down ?

Happy Boating
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Old 30 August 2013, 04:29   #22
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Meanwhile would like to ask the audience this simple question : What is the small upper plate that every engine has is good for, what does it does, what happens if it's chopped down ?

Happy Boating[/QUOTE]

Well I hope you know because my brain is melting now

My uneducated guess is that it is the lower limit engine height, should not be below bottom of transom??????
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Old 30 August 2013, 04:48   #23
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The plate above the Antiventilation-plate stops splashing up the shaft. Don't chop it down, you (and your engine) WILL get wet.
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Old 30 August 2013, 08:59   #24
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Too low! The cav plate should be level with the water when on the plane and trimmed correctly - ie you should be able to see it skimming the surface.
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Old 30 August 2013, 13:19   #25
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Quote:
Originally Posted by longjohn View Post
Too low! The cav plate should be level with the water when on the plane and trimmed correctly - ie you should be able to see it skimming the surface.
Yep spot on !
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Old 01 September 2013, 16:10   #26
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Correct

I had a humber assuault with a Tohatsu 70 and with the engine sitting on the transom it was just above the hull line.

Had no choice as to lower it would mean chopping the transom, gave great performance but would cavitate on tight turns, so engine had to be trimmed in.

Sold the humber for a bigger boat, have just used it everyday for two weeks and had more time to mess about and watch the engine, i also saw alot of spray coming up the transom unless the engine was trimmed out.

Home now and have measured the height, the ventilation plate is 30mm lower than the hull lines.

If I move one hole which I am tempted to do it will raise the engine 20mm. Should give me better performance and fuel economy.

It will also answer a few questions around prop performance and boat speed.

Moving one hole up will do you no harm, try it and if you don't like it move it back, that is what I am going to do.
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Old 01 September 2013, 16:15   #27
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Correct

I had a humber assault with a Tohatsu 70 and with the engine sitting on the transom it was just above the hull line. Great engine by the way. Was probably only 5-10mm above.

Had no choice as to lower it would mean chopping the transom, gave great performance but would cavitate on tight turns, so engine had to be trimmed in for a strong turn.

Sold the humber and bought an older redbay, have just used it everyday for two weeks on holiday and had more time to mess about and watch the engine, i also saw alot of spray coming up the transom unless the engine was trimmed out.

Home now and have measured the height, the ventilation plate is 30mm lower than the hull lines, so great grip but dragging the leg and plate through the water.

If I move one hole up which I am tempted to do it will raise the engine approx 20mm. Should give me better performance and fuel economy.

It will also answer a few questions around prop performance and boat speed.

Moving one hole up will do you no harm, try it and if you don't like it move it back, that is what I am going to do.
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Old 01 September 2013, 16:15   #28
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Thats my plan. Last run out of the season planned for 20th Sept and am going to try and move it then.
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Old 02 September 2013, 15:08   #29
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Quote:
Originally Posted by Mextli View Post
The plate above the Antiventilation-plate stops splashing up the shaft. Don't chop it down, you (and your engine) WILL get wet.
Not planning to chop down upper plate, was sort of a tech exam. Correct answer, water will travell upwards against rear of leg as seing it from outter transom. Will you experience a outter or over transom water splash and why ?

Happy Boating
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Old 11 September 2013, 11:26   #30
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Just to add to this thread, we have done a boat change but kept our old engine. So far we have had two trips out & the engine is now on its 3rd higher setting than I thought it might need.

The old boat had a deeper V, it handled perfectly with the lower cav plate level with the water at full speed. You could see the trim tab bolt & the water passing by it. The lower cav plate sat dead level with the keel. Top speed was 40mph with the 90hp Merc 2st.

New boat has much less of a V hull. Same length & weight though, but obviously a much different design to the old boat.

1st time out - Loads of splashing engine too low, -10mph ish top speed compared to previous boat

2nd time out - Higher one notch - less splasing but cavitating at speed when trimmed up. Water flow from transom is split & meets the front of the gearbox just at the wrong point & you can see why it cavitates as its not a smooth flow around the gearbox. However we gained 6mph but still 4mph off of previous boat speed. Water flow covered the lower cav plate.

3rd time out - Cav plate now sits slightly above keel line so will test it & see..... & report back our findings.


The next issue is the handling side of things, the boat does'nt like gripping in high speed turns, it tends to hop out of the water which is slighly alarming to say the least. Any suggestions welcome!!
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