Tohatsu 18hp test run - bit slow.

[Bote]

I have Tohatsu 2stroke 18hp on small 11ft aluminium Stacer
dinghy V type boat (max rated to 20hp in Australia).
I installed outboard as per Tohatsu manual with cavitation plate
about 1 1/2 inch below hull botom.
Tested yesterday. Boat pickup and going into plane was very quick and responsive but I expected this with this size outboard motor.
Top speed was rather low at maximum of 36km/h or 22.3 miles/h.
I was with my 9 year old son while 3 friends on 13ft aluminium boat
with 20hp Honda 4 stroke were keeping up.
I expected to be bit faster in straight line. Not sure if problem could
be related to outboard height. Should I lift outboard so cavitation
plate is level with botom of the hull as it is recommended on US Tohatsu/Nissan website ?

Any tips ?
Andrew

 

[JB]

Re: Tohatsu 18hp test run - bit slow.

Yes, Bote. Raise her at least that much.

A perfectly positioned anti ventilation plate (often called cavitation plate) has it exactly at water level when on plane.. .not below water, not out of the water.

That means it is never positioned below the bottom of the transom and sometimes 1/2" to 1" higher.

Good luck, mate.

 

[TOHATSU GURU]

Re: Tohatsu 18hp test run - bit slow.

What JB said....Tohatsu is more concerned about people mounting them too high and exceeding the RPMs or losing water for the cooling system. Hence the cautionary advice from them about mounting them so low.

 

[Sea Rider]

Re: Tohatsu 18hp test run - bit slow.

Bote: I have a Tohatsu 18 HP 2 strokes mounted on a 380 meter sib, when on plane on calm waters with 2 mates onboard, water flow passes 1 mm under cav plate, performs perfect, triim angle is set in middle third hole and weight is correctly balanced towars sib middle to bow. On flat waters, no wind, my max speed is 25 miles/hour, about 40 km/hour with factory delivered prop. Your achieved speed is not that bad using a heavier boat than mine. If in persuit of top speed, make a wot test and change propeller accordingly to match 5800 max rpm at wot

Happy Boating

 

[Bote]

Re: Tohatsu 18hp test run - bit slow.

Thanks for the tips.
I had a look today and outboard will need to go up
about 1 1/2 inch - it makes sense.
I also need to trim it a bit better. It was on second inside hole
but it will need to go to third hole at least.
Explains poor stability and heavy steering at speed.

On positive side old tohatsu performed very well. Started first pull.
Ran without any problems at slow speed or WOT for long periods of time.

Andrew

 

[Sea Rider]

Re: Tohatsu 18hp test run - bit slow.

Thanks for the tips.
I had a look today and outboard will need to go up
about 1 1/2 inch - it makes sense.
I also need to trim it a bit better. It was on second inside hole
but it will need to go to third hole at least.
Explains poor stability and heavy steering at speed.

On positive side old tohatsu performed very well. Started first pull.
Ran without any problems at slow speed or WOT for long periods of time.

Andrew

Are you experimenting water splash issues inside boat, besides heavy steering ? is your transom at leat 40 cm height ? Your engine must be perpendicular to boat and cav plate must be parallell to water flow once on plane, try adjusting to third hole and move extra mate towards bow to compensate weight distribution.

Happy Boating