Small tinnie with big engine

Discussion in 'Outboards' started by Marco1, Mar 1, 2010.

  1. Marco1
    Joined: Oct 2009
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    Marco1 Senior Member

    Hi there!
    I have an old 3.7m tinnie that we mostly use for rowing. Lately the kids have asked to use the outboard motor more often however the motor I have is a 25 hp Evinrude (1996) long leg and the transom on the tinnie is 400mmm from bottom of the hull to the edge.
    The motor is 570mm from the plate to the inside of the bracket.
    I found an aluminium workshop that can modify the transom and make it higher to fit the long leg motor for little money, but I am not too sure what size to make it.

    The existing transom is aproximately 16 inches. If that is to fit a "15 inch" leg, then to fit a "20 inch" leg I should rise it 5 inches, yet the measurements of my motor are far from 20 inch since it is 22.5 inch from plate to inside of the bracket, and so my new transom should be ... what? 22 inches? 22.5?
  2. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    You will lose some height due to tilt of transom. Chart it out on graph paper then take measurements.
  3. Marco1
    Joined: Oct 2009
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    Marco1 Senior Member

    I understand that I'll lose perhaps 3/4 inch hight because of the tilt, still what hight do I need the transom to be? I thought this woulds be some sort of standard thing. I understadn I can slide the engine up a bit for correction but not much really.
    What is the standard transom hight a long leg boat comes with from factory?
    Is there such thing?
  4. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    Standard transom height on most boats is 20 inch +- 1 inch. But because there are boats with varying transom heights outboards come in 15 inch, 20 inch and 25 inch shafts (see You measure transom height from the transom cutout (where the motor is mounted) down to the lowest point of the transom.

    However I am questioning whether or not this boat can safely take 25 HP. Horsepower is caculated several ways depending on the boat and the steering. see safe horsepower at
  5. Marco1
    Joined: Oct 2009
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    Location: Sydney

    Marco1 Senior Member

    Hi there and thank you for your reply.
    According to the link you posted, my shaft lenght is 22.5 inches or 570mm

    That is what I don't understand, how can it be non standard?

    As for the HP, I have used it on the lower transom, as it is now many times and so has the previous owner. Its is of course all wrong since it sits far too low.
    The question is what size do I make the transom. Most would say 20" or 21", yet that would let the motor hang one inch below the bottom of the hull. If I wanted to lift the motor, there is not much room to go up, so I want to get it right leveled as it hangs on the transom.
  6. tunnels

    tunnels Previous Member

  7. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Hi from Noosa....first thing to do is to hang your motor on the boat and see if its too heavy for it with the crew you usually have . you may give it a run with the motor but I would tuck it under well for a first run or the bow may come up at an alarming angle may find you dont need to modify it at all if it steers ok.(but you will draw more water ) being 25 hp you do not need to use all that power and you could easy limit the throttle movement so the kids could not go too fast for saftey. The 15 20 25 in is based on an old american system which has been slightly adjusted by the jap makers ..yours is quite standard ..just measure from the clamp to the cavitation plate and have him build the back up ( if you need to ) to that distance. or a little less ( you can always pack it up with a strip of wood for final adjustments ).. If the motor is a bit heavy just fit a whale tail for $39 ..let us know how you go
  8. Typhoon
    Joined: Mar 2009
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    Typhoon Senior Member

    For a tinnie, this is sure a lot of hard work! Put the motor on the boat, put a straightedge on the keel, measure the difference, have it added.
    It's only a 25, lift it on there, they aren't that heavy! You could have it done by now.
    And a 25 isn't a big engine for a 3.7m tinnie.

    Regards, Andrew.
  9. Marco1
    Joined: Oct 2009
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    Marco1 Senior Member

    We had this boat with it's motor for one year now. We have used it like that with no mayor issues. It is hard to keep straight and I get a soar arm after 10 minutes from holding it. Also at low tide we hit the sand bank because it hangs so low.

    When I go alone, it gets up to 30 degree before it goes on the plane. It is a pain because if we are at low tide, id digs the engine really down and to lift it up you must go faster, increasing the risk of damage if I hit something.

    I wouldn't like the kids to use it like that. I understand that if I get it to its right height, that is some 5 inches higher, it should improve. I already have that thing you call a whale tale on it.

    All in all I think if I go for 21 inch transom I still am one inch under the hull with the plate, so it is safe it will not ventilate and I can always go up a tad if I wanted to.
    It is strange to read all the different theories as to where to have the plate in relation to the hull. Two inch under, one inch under, leveled and even one inch higher. I suppose there is no fixed rule and it must depend from a lot of other factors, size of the boat, size of the engine, speed, load, prop etc.

    I first considered to do it myself since I have a 250 Trans MIG I use for steel fabrication and blacksmithing, my ex trade now just a hobby, however, by the time I get the right gas and the spool of aluminium, and get a straight weld on alloy I have never welded before, I may as well give it to a professional who uses a TIG (and frowns on the MIG). Since I am at it I'll get him to fit a rail all round it with a few tubes for the fishing rods.
    Next thing is to build a floor between the centre console and a steering thing wouldn't be bad either....:D

    Anyone has a photo or two of a small centre console I can build?
  10. baboonslayer
    Joined: May 2010
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    Location: Colorado

    baboonslayer Junior Member

    Here are some center console photos, these are not my boats but just to give you an idea...

    I am buying an aluminum boat and now that you mention it, I might as well put a few rod holders and a center console on mine too!

    Go to this website, it is very helpful if you want to buy/build a center console.

    Attached Files:

  11. Easy Rider
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Here in Thorne Bay Alaska there are more tinnies than anything else. We run 20hp on 14' and 25hp on 16'. Teach your kids how to load the boat fore and aft. Make them wear PFDs and shut down lanyards. Adjust or modify the transom so the cavitation plate is 1" to 1 1/2" below the keel when the cavitation plate is parallel with the keel. Adjust the steering friction screw so the engine stays set when you let go the tiller at any speed. Then you'll be skiffin in your tinnie just like an Alaskan.

    Easy Rider
  12. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Why modify the transom? At least around here outboard shops are happy to swap a long shaft for a short shaft for a pretty minimal fee. Last time I made a swap it cost me about $25.
  13. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Marco 1 says he's got a transom 16" high doubt a short shaft. If I were Marco1 I'd make a transom board out of 1" plywood that would bolt to the transom ..on the outboard side about 8 - 10" wider than the transom and 2 - 3" below the top of the center of the transom. Overlapping the transom more would be more secure but would weigh more. The new transom should be higher so Marco1 can hang the engine on the transom, measure and cut down until the cav plate is 1 1/2" below the keel or mount the engine and run the boat with the engine tilted a bit high (perhaps 5 degrees above vertical to the keel) Cut the transom down until the engine won't cavatate w the engine 5 degrees up. Or for a very very small penalty in drag cut it about 1/2" lower. could even make better thrust w less turbulence off the keel/bottom. But in a 14' skiff a short shaft would produce a better balanced boat w a lower CG and if the boat wasn't overloaded be a better all around boat. The old OMC engines were easy to change shaft length but I think most (at least) newer engines aren't swappable.

    Easy Rider
  14. Marco1
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    Marco1 Senior Member

    Yes swapping the engine would have been easier, to find an engine to swap may be a problem.
    Well when what you say makes sense, the higher transom makes the boat dryer and also easier to drop from the trailer in the water.
    So I welded the transom leveled with the gunwale, and added a handrail all round. The boat seems bigger and safer for the kids. I made a wooden reinforcment to clamp the motor and covered it with checkered aluminium plate. I made a mistake though. I measured the thickness without cosnidering the checkered plate and I could balrey fit the motor with the clamps fully open. I will have to undo it, plane the wood 1/2 inch down and get the plate back on.
    I also fitted a new propeller, the same size as the original yet the original had 1/2 inch missing all round. so the boat now goes like a rocket
    I get a bit of spray up from the can I describe it...V shaped each side of the leg, some 45 degree back.
    I can see the second plate but not the bottom plate nor the whale tale when underway, so I think the motor is in the right position. I could go deeper once I make the transom thinner, at the moment it sits about 3/4 inch above the edge.
    The boat goes flat, no more bow up in the air and the steering feels much more stable. I have to look for that steering friction screw you mention. Is it that thing at the end of the steering handle? If it is, it does not seem to do much at all.

    I took my grandson to the shops on the weekend and I did 6 km in about 10 minutes and it was not flat chat, only half trottle. Only problem I had was that the engine seems to have a little fault here and there.
    It goes like this:

    well that is how it sounds, like a non regular miss here and there. I thought it could be the engine is sucking air from the fuel line that is old and very stiff. I had the carburator "done" by a champion who has a small test tank in his workshop and assures me that he can only test the engine at low speed or the water flies out of the tank. @#$% &*@.

    I'll try replacing the fuel line and see how she goes.

  15. Easy Rider
    Joined: Oct 2009
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    Easy Rider Senior Member

    Think ignition
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