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Thread: Tohatsu M70B - how to increase an engine power?

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    Default Tohatsu M70B - how to increase an engine power?

    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!

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    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 ?

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    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?

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    Team Member zul8tr's Avatar
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    What are diameters of the 15 and the 17 pitch props noted ?
    What rpm was each producing at top speed?
    " Three may keep a secret if two of them are dead" Ben Franklin
    " ------- well Doctor what have we got a Republic or Monarchy? A Republic he replies if you can keep it"
    Benjamin Franklin, 1787 Constitutional Convention, as recorded by signer James McHenry's in his diary at the Library of Congress

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    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.
    Attached Images Attached Images  

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    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 …

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    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, .

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    Team Member zul8tr's Avatar
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    Quote Originally Posted by xandm14 View Post
    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.
    " Three may keep a secret if two of them are dead" Ben Franklin
    " ------- well Doctor what have we got a Republic or Monarchy? A Republic he replies if you can keep it"
    Benjamin Franklin, 1787 Constitutional Convention, as recorded by signer James McHenry's in his diary at the Library of Congress

    Location: SW Orlando, Fl

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    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 ?

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    Team Member zul8tr's Avatar
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    Quote Originally Posted by filthy phill View Post
    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 ;-)
    " Three may keep a secret if two of them are dead" Ben Franklin
    " ------- well Doctor what have we got a Republic or Monarchy? A Republic he replies if you can keep it"
    Benjamin Franklin, 1787 Constitutional Convention, as recorded by signer James McHenry's in his diary at the Library of Congress

    Location: SW Orlando, Fl

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