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Old 16 January 2013, 20:18   #11
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Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
Join Date: Sep 2007
Posts: 1,706
Tohatsu gives worst spalsh back issues compared to other brands engines like Yam, it's a tail design, what stated on owners manual is just a refference from where to start but can vary depending on the boat's hull shape.

The only way to know how high you need to raise the transom is go for a wot spin with well ballanced boat as to have boat near paralell to water level and have a light spotter check how high water flow is passing above small upper plate, that's the distance you need to raise transom for engine to perform as expected.

The tech explanation is very simple, under upper deflector plate there's a triangular area that cuts perfectly water flow, over small plate, where the muffler is, the shape is squared, no water flow is being cut, that's the horrible splash area.

My rib/Tohatsu engine had same problem now that engine has been perfectly height tunned the overall handling, speed and performance is oustanding.

In this example the 30 HP engine was sitting 1" lower, was impossible to plane, excessive water splashed over transom. Wood shimmed the middle cut to have a straight transom and problem was solved forever. This fine tunning height experimentation must be done under trial and error and above all, boater must have a bullet proof patience to do it once correctly and forever..

Happy Boating
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Old 17 January 2013, 10:06   #12
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Some issues :

You are runing a rib, right ? Ribs usually have straight transoms, was that rib bough brand new or second hand ? if second hand probably previous owner must have chopped center transom down to use it with a short shaft engine? Your 60 HP Tohatsu is a long shaft engine.

Can you post your actual transom keel A-B ; C-D distances, check pic.

To solve your splash issues, you have 2 options ;

1-Find or make a well rigid built transom mount to bolt in middle transom so to raise engine. Tohatsu 60 HP weights 108 KG if being 2 stroker, more if being a 4.

1-Close middle cut with wood shim and fiberglass it to have a stright transom, make a wide and long stainles steel inverted U plate so to have inner and outter engine pads, bolt it to transom and go from there, water flow should pass well under upper small deflector plate eleiminating forever water back splalsh.

Happy Boating
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Old 17 January 2013, 15:15   #13
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Country: UK - England
Town: Newcastle, Staffs
Boat name: Just Looking
Make: Ribcraft 5.85
Length: 5m +
Engine: Suzuki 140hp
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Locozodiac

Sorry but my boat is garaged at the moment so can't measure the transom.

The boat was new from Menai Marine and is the 480SR and is supposedly matched to the Tohatsu 70hp.

Thanks for the info it is much appreciated. I get what you say and appreciate yo pointing out where the water should flow.

Nick
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Old 17 January 2013, 15:49   #14
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Nick,

As an example, if you sit same Yam HP at transom, will have no or less water splash than Tohatsu, mostly because both lower tails shapes are completely different. Manufacturers in theory builds transoms to match every engine brand, but in practice on water it's a different story. That's why you need to fine tune engine tail height, preferably on flat no wind waters.

Happy Boating
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Old 17 January 2013, 16:36   #15
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Country: UK - England
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Make: Quicksilver
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[QUOTE=pow2gran;511258]The ventilation plate sits around 20mm below the keel.

It's pretty much at a standard position, so matched as a starting point correctly.


The difference in shaft length from my 30hp from short shaft to long is 148mm, a lot.

My ventilation plate was 62mm above he keel line and it ventilated 5 out of 6 times when attempting to get on plane, with motor trimmed fully down, no joy at all level or trimmed up...
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