Outboard shaft length question

Discussion in 'Outboards' started by hallywood, Feb 23, 2010.

  1. hallywood
    Joined: Feb 2010
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    Location: Up North

    hallywood New Member

    I "acquired" an old 40hp Mercury, measuring from the anti-ventilation plate to the inside of the mounting arm is 17.5", to the center of the prop is 25"

    Measuring from the top / bottom of transom = 22"

    Will this work or should I be looking to sell this motor before the open water season arrives here. If this will not work as is, are there any options I have to make this work?

    The motor would be sitting on a 1977 14' Starcraft.

    Thank you in advance.
  2. tunnels

    tunnels Previous Member

    Basic method for inital set you of any outboard of any size

    Hook the outboard over the transom and use a couple of Gee clamps to hold the motor against the transom , Get a straight edge and hold it along the bottom of the keel and adjust the height of the motor till the straight edge is just touching the underside of the cavitation plate .Check the motor for centre and level and straight, tighten the clamps and drill your holes , This is your starting point !! but could be the magic place when you take it out and test drive :p
  3. hallywood
    Joined: Feb 2010
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    hallywood New Member

    First off thank you for the reply.

    My concern in the motor shaft is not long enough since the anti ventilation plate would be sitting 4.5" above the bottom of the boat when I hook it over the transom.
  4. tunnels

    tunnels Previous Member

    you need to put it back in a museum , It must be a old 16 inch short shaft , what you need is a 20 inch shaft !! after that is a 25 extra long ! :p
  5. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    Mercury still make 15" engines?
  6. tunnels

    tunnels Previous Member

    I have been making boat in this part of the world since 1972 and have never made any boat ever to take a 16 inch , I can only remember seeing one way back in the early 1960s :confused:
    Sure they even cut 20inch legs to 12 inchs as well for racing motors ! But for every day use 20 or better still 25 to get the motor up and away from the sea water and spray etc . i think there is even a 30 inch on some brands but have only seen referance in books not in the flesh so to speak . :p
    I have a old 1975 14ft 6 in boat and it was origanally set for a 16 and had been packed to take a 20 but whoever had done the job had jacked it 1.5 inchs to high so the motor used to cavitate si some conditions .
    I chopped the whole transon off and completely changed everything !. Now it has a 25 inch yamaha and a 450mm stepped back transom .Made a huge differance to the preformance and lots of room for fishing !!:D
  7. Hunter25
    Joined: Mar 2006
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    Location: Orlando

    Hunter25 Senior Member

    Hallywood you have a long shaft transom and a short shaft outboard. You can cut the transom down, to accommodate the short shaft which probably involves making or modifying the splash well or using a long shaft engine, which you don't have to cut the transom for. The ventilation plate usually is about 1" below the bottom of the boat to even with the bottom of the boat. If it is above the bottom of the boat you will have problems.
  8. tunnels

    tunnels Previous Member

    One inch down is to much!!!! 1/4" or even less is ok .
    If you get going fast the spray from the front of the leg goes every where and can even find its way into the splashwell so make sure the slashwell drains are always clear !!:p
  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The cheap solution would be to bolt a motor bracket and the engine to it

  10. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    They made 15" outboards for small boats for decades w no splash well. Older boats (light and narrow) up to at least 18' were fine for short shaft. The transom is stronger and stiffer or lighter (or both) and less torque is applied to the transom and mounting clamp. The lower center of gravity is always a plus. Thats why they make short shaft outboards .. to fit the boat. I've run many short OBs on skiffs 14 to 18' and never had a problem w splash over the stern. Running a small boat w a short OB is smart.
    Your'e correct. One to 1 1/2'" below the cavitation plate.
    Cut the transom down and go enjoy your boat.

    Easy Rider
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