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Old 16 October 2013, 17:56   #11
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Tried this and the results are minimal and with trim set correctly cavitation starts event a 10 mm rise ,you can go from bow to stearin at the standard height with no cavitation turn as hard as you like with no slip of the prop the boat has hit 26 mph it's all good apart from the annoying spray going in the boat and engine bay ,Mercury even know of the problem in sibs that's why they make the deflector
My previous metcury 90 was jacked up to the max But the hull and power trim allowed this but sibs have banana keels that have latent problems and are compromised , but the defects are outweighed by the many great features sibs offer
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Old 18 October 2013, 20:59   #12
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I had a Honwave T40 with a tohatsu 4 stroke and the same problem. 20hp 4 strokes by mariner mercury and tohatsu are all made by tohatsu. The only real difference is the cowling. There is a transom length mismatch with the Honwave because its designed for Honda engines which have a slightly shorter leg than everyone else makes.

I spent ages on this and ended up buying a 10.5mm (if I remember right, look up the transom height in your engine manual vs Honda) deep strip of wood from B&Q and used it as a shim. It effectively made the leg height the same as a Honda engine and nearly completely stopped the splashing and didn't cause any cavitation if trimmed out with the pin in the second position from the transom.
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Old 19 October 2013, 12:58   #13
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Originally Posted by MattD View Post
I had a Honwave T40 with a tohatsu 4 stroke and the same problem. 20hp 4 strokes by mariner mercury and tohatsu are all made by tohatsu. The only real difference is the cowling. There is a transom length mismatch with the Honwave because its designed for Honda engines which have a slightly shorter leg than everyone else makes.

I spent ages on this and ended up buying a 10.5mm (if I remember right, look up the transom height in your engine manual vs Honda) deep strip of wood from B&Q and used it as a shim. It effectively made the leg height the same as a Honda engine and nearly completely stopped the splashing and didn't cause any cavitation if trimmed out with the pin in the second position from the transom.
Hi mattD
The Honda 20 hp 4 stroke measures 431mm (honda handbook)
The Mariner 20 hp 4 stroke measures 415mm (my engine engine lowest setting)
So the Honda is 16mm deeper in the water than the mariner to start with and the Honwave was designed around this engine.
Moving the Mariner up 10 mm Means the mariner prop would be 26mm higher than the design height
The transom is 15 inches ,exactly what mariner recommend ,But the Keel is 25mm lower than the transom and in all instructions the cavitation plate should be level or below "the bottom of the boat" in this case the inflatable keel
technically i have 9mm to play with this would bring the cavitation plate level with the keel line but 24mm above the Honda/Honwave match

This aside my set up is on the third hole up which leaves the cav plate level with the hull level on the plane also 2mph more top speed no low nose drag.

raising the engine 10mm using the third hole gives prop slip galore ,dropping back to the second hole gives cavitation out of the hole and sharp turns and lower top speed and slight low nose also if passengers venture forward on the plane cavitation creeps in

ive come to the conclusion that the standard height gives good overall performance as the boat was designed to do and the splashing is caused because the transom angle leaves a large space between the leg and the transom allowing the poorly designed leg to catch the spray

has anyone with a total Honda setup experienced any problembs
Thanks for posting
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Old 19 October 2013, 17:50   #14
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Hmmm that is odd, I just looked at some notes I made when I was trying to sort it and the Honda engine at the time was 381cm transom height (15"). The biggest problem was that I couldn't raise the engine as much as I calculated because the clamps would have been half way over the top of the indented transom plate but as far up as it would go was sufficient.
It still splashed a little but the splash plate was just above the waterline when on the plane so it was far better and it only cavitated if turning or accelerating to hard. I can't work out what's happening with your setup.
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Old 19 October 2013, 18:08   #15
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Strange. 431 cm = 16.9+"; 415 cm = 16.3"; 15"transom = 381 cm

Sorry you have the problem, but raising the engine would understandably be harmful evidently.
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Old 19 October 2013, 18:35   #16
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thats pretty well where mine was at the uppermost point the clamp plate and on two holes this worked ok but the nose always felt as if it was ploughing a bit which slowed the boat slightly,on the third hole position because of the height of the engine it is picking up the messy aerated water which is causing problems,
The boat handles and goes better than i ever expected it would in standard form trimmed to the third hole out,and the splashing only puts a couple of litres a day in the boat i am not to bothered about that but i don't like the water splashing up the drain holes to the engine bay

on the plane i can always see the splash plate above the water
i am left with two options
to raise the engine and put up with the slightly compromised ride
or leave as it is and put plenty of corrosion guard in the engine bay

i have had boats all my life and i like to get the best out of my rigs and this humble boat is no exception and to be fair is probably the most used boats i have owned and loads of fun
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Old 19 October 2013, 18:36   #17
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Just to clarify it was the splash plate just above the waterline when on the plane NOT the ventilation plate. I expect you get that but wouldn't want you to misunderstand what I have said and wreck your engine.

Loco is your man to speak to but ideally when on the plane the splash plate should be just out of the water and the ventilation plate below the hull. It was fairly hard to achieve with my 2011 tohatsu. I'm not sure but suspect the lack of V shape on the T40 is the root of the problem.
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Old 19 October 2013, 18:44   #18
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[QUOTE=frankc;577036]Strange. 431 cm = 16.9+"; 415 cm = 16.3"; 15"transom = 381 cm


+25mm for the inflatable keel that tappers off before the transom but is the bottom of the boat

Also i quoted the older 20 Honda the latest model is 433 even deeper
This measurement is from the inside of the clamp to the top of the cavitation plate
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Old 19 October 2013, 18:47   #19
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Just to clarify it was the splash plate just above the waterline when on the plane NOT the ventilation plate. I expect you get that but wouldn't want you to misunderstand what I have said and wreck your engine.

Loco is your man to speak to but ideally when on the plane the splash plate should be just out of the water and the ventilation plate below the hull. It was fairly hard to achieve with my 2011 tohatsu. I'm not sure but suspect the lack of V shape on the T40 is the root of the problem.
i call the ventilation plate above the prop the anti cavitation plate
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Old 19 October 2013, 18:54   #20
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Mine used to cavitate like mad on the sea trimmed out 3 notches even before I raised the engine. I never used mine with more than 3 people onboard but it was very load sensitive, I guess cus It's so light. I really don't think the shape at the transom helps it, It's too flat me thinks.

Loved that boat to bits though, so much fun. I'm verging on regretting selling it to buy my current boat.
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