Tohatsu M70B - how to increase an engine power?

[xandm14]

Hi guys,

I am wondering if is here somebody with the same type of engine. I need to increase the engine power of my boat, as in one month I will be attending the race.
Is it possible to restrict the engine speed and by this increase the overall performance?

I would appreciate any tips and experience:)

Thank you!

[filthy phill]

let me get this right.
you want to make the top speed a little slower, but get more power in the rest of the range ? is that correct.

or are you saying you want more power from the take off all the way to the top but not go any higher rpm than standard engine ?

[xandm14]

The point is that I would need to increase both – the top speed and raise the maximum speed of the engine RPM.

Propeller pitch 17 inches cannot get to the limiter and the ship's speed is 40.3 knots and generally slower start
Propeller pitch 15 inches can get to the limiter and the ship's speed is 39.8 knots and faster start

Ship Weight: Picton Alpha 16 feet with engine 320 kg
Any advice?

[zul8tr]

What are diameters of the 15 and the 17 pitch props noted ?
What rpm was each producing at top speed?

[filthy phill]

Are we talking a uk built boat ?

post a picture of your picton, because picton boats (wales) never made an alpha 16ft speedboat.
you can get the picton Delta, sunsport, picton 140 gts 150 gts 156 gts and the 170 in gts and volante gts. then there is the Royale model in 16 19 and 21 ft.

to be around the 16ft mark and the boat model being an alpha makes it a Riever Alpha not picton. sort of similar to those who dont know,
and a Riever is a lot lighter than a Picton boat.


here is my picton 170 gts Volante, weight of hull only around 400 kilo plus.

[xandm14]

The first propeller is noname stainless steel 11 3/4 "x 17
The second propeller is APOLLO stainless steel 10 1/8 "x 15
Highest speed 40.3 knots at 5700 rpm with a propeller 11 3/4 "x 17
Highest speed 39.8 knots at 6000 rpm with a propeller 10 1/8 "x 15
I need to reach at least 42 knots ... Any advice how to do it?
Can change of the carburetor help? I have the option of carburetor Tohatsu M90A …

[filthy phill]

have you got a picture of the boat ?
if yes show us a picture of the side of the boat and the back of the boat.

where is the cavitaion plate on the motor ? is it level with the hull bottom or above the bottom of the hull ?

are you going to be racing on a river a lake or in the sea ?

need to know exactly what the boat looks like, .

[zul8tr]

The first propeller is noname stainless steel 11 3/4 "x 17
The second propeller is APOLLO stainless steel 10 1/8 "x 15
Highest speed 40.3 knots at 5700 rpm with a propeller 11 3/4 "x 17
Highest speed 39.8 knots at 6000 rpm with a propeller 10 1/8 "x 15
I need to reach at least 42 knots ... Any advice how to do it?
Can change of the carburetor help? I have the option of carburetor Tohatsu M90A …

Are these gps speeds? Do you mean mph instead of knts?
Accuracy of engine rpm?
Are the pitches as stamped on the props? Have they been worked on?
Based on a gear ratio for the 70 hp Toho's of 2.3:1 from engine spec table (is this correct for your engine?) the data supplied appears not compatible with expected speeds.

Ex for 17" pitch at 5700 rpm and 2.3 gear expected best speed is about 40 mph (with no slip and there is always slip so speed would be less). Your 40.3 knts = 46.4 mph is way above that and therefore my questions on base data.

[filthy phill]

totally agree with Zul8tr
you are not going anywhere near as fast as what you think you are. you are only going about 37 to 38mph maximum.

is this definitely the tohatsu M70B 3 cylinder motor ? with 13 spline propeller shaft ?. if yes then its 100% 2.3 to 1 gear ratio.

40mph is possible but NO to 42 knots..

where is a picture of boat and engine ?

[zul8tr]

totally agree with Zul8tr
you are not going anywhere near as fast as what you think you are. you are only going about 37 to 38mph maximum.

is this definitely the tohatsu M70B 3 cylinder motor ? with 13 spline propeller shaft ?. if yes then its 100% 2.3 to 1 gear ratio.

40mph is possible but NO to 42 knots..

where is a picture of boat and engine ?

Pics would help.
11" long rulers that are labeled 12" rulers always measure longer than reality ;-)

[filthy phill]

definitely need a pic of the boat

[xandm14]

I am from Europe. The race is taken place in the river which goes really very slow.

The web page of the race is here: www.racecamp.cz

Pictures of the boat:

61751617526175361754617556175661757

[filthy phill]

That is a Riever Alpha 100% proper vintage late 1970's styling, and weighs a lot more than you think it does. ...
the boat alone weighs around 300 kilo, then plus the motor and tank and battery . (unless the floor and bulkheads have been taken out of the hull.)

to get this boat to 40mph with that motor is pretty good . to go 48mph top end you will suffer big time at pulling away.
these were not race boats, it is more for recreational fun and water ski-ing.
these models were made in the 1970's and 1980's, if the brown is original its around 1980 as that was a very popular colour scheme around this time.
the next model of this boat was slightly more square on the top and windscreen more wrap around , but used the very same well proven hull , they were nice boats.
if I remember correctly these were rated for about 140hp in the day.

seen lots of them with 90hp and v4's on the back, even a straight six 135hp merc.

the race looks like it will be fun.

[xandm14]

Thank you very much for your inputs and overall information. The point is that I ended second last year and this year I want to win in my category:) I really need to increase the speed for 3-5km per hour and this would help. Do you think there is no chance with this kind of boat in these conditions?

Thanks again!

[fs5]

Mate if it was me, I'd try disconnecting the rev limiter and leave the 15p on.
See how many more revs it's got in it. Might pick up a mile or 2

[filthy phill]

can he disconnect the limiter on that motor ? if so how would he go about it ?

I thought its like the omc and cant be touched ?

[fs5]

Dunno mate , pretty easy to disconnect on a 50 .
Be the first thing I'd look at if I was chasing a couple kph

[filthy phill]

it might be easy if you know how.
But how exactly do you remove the limiter on the 50 ? is this just a simple wire or what ?

wish i could get the one off my 56 omc.

[xandm14]

Please can you advise how to disconnect? I am not sure if I can do it as I am not aware how. Thanks!

[fs5]

They just disconnect a wire on the tohatsu 50.
You'd be best off asking a tohatsu dealer about the 70 or try googling it

[Ketzer]

Well, I don't know anything about the Tohatsu, but when I raced Mercs and Konigs, I found that I gained speed if I waxed the deck, vacuumed out all the dirt and gravel, mopped up all the coagulated oil, went on a diet and dropped ten pounds, shaved my arm hairs, and drove with my mouth closed, but then, I've had a few beers, so don't trust me.

[xandm14]

Do you know how they disconnect a wire of tohatsu 50 - any tips? I tried to google it but without success...

[xandm14]

They just disconnect a wire on the tohatsu 50.
You'd be best off asking a tohatsu dealer about the 70 or try googling it

Do you know how they disconnect a wire of tohatsu 50 - any tips? I tried to google it but without success...

[xandm14]

Well, I don't know anything about the Tohatsu, but when I raced Mercs and Konigs, I found that I gained speed if I waxed the deck, vacuumed out all the dirt and gravel, mopped up all the coagulated oil, went on a diet and dropped ten pounds, shaved my arm hairs, and drove with my mouth closed, but then, I've had a few beers, so don't trust me.

I do the same process before each race:D But now I need to do also something more technical to be able to speed up...