outboard conversion

Discussion in 'Boat Design' started by ChrisUK, Nov 29, 2014.

  1. PAR
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    PAR Yacht Designer/Builder

    She's a displacement boat and a 9.9 would be just fine for her, though an idea of her weight would help.

    No need to cut the transom at all, just make a false transom, inboard of the real one and box it in, so it's waterproof. You can leave the top open, so the engine can breath better or cover it, which will handle noise neatly.

    No need for a rudder if the outboard is steered. In fact if the outboard is steered, you'll maneuverability will improve 100%, compared to the straight shaft setup.
     
  2. viking north
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    viking north VINLAND

    These specifically designed family of long shaft outboard sale drive motors are generally lower geared for high torque driving a 4 blade high pitch prop. These class of motors (9.9hp.)easily drive a 26ft. full keel internally ballasted loaded Tanzer (roughly 5000lb.) sailboat to hull speed approx.( 6kts.) Due to the higher torque ratings these family of motors seem to handle headwinds and sea with little problem. They kind of remind me of a modern version of the Old Seagulls. Since your hull looks to be in the displacement/ semi displacement family depending on weight I would think as Whitepointer and PAR suggest a 9.9hp. should easily do the job. However for light weight and reliable long life service I'd look for a two stroke oil injected model. Using one of these long shaft sail drive units, it is quite possible you might not have to cut the transom much and with the lower HP. I suspect other than a 3/4 ply backing plate your GRP transom could handle the loading especially if carried across the entire stern and tied to the side gunnels at the corners with a couple of metal or laminated wood hanging knees. Just curious, what inboard motor is installed presently and why do you feel it needs replacement ?

    A yacht is not defined by the vessel but by the care and love of her owner---
     
    Last edited: Nov 30, 2014
  3. ChrisUK
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    ChrisUK Junior Member

    Thanks for your advice. The weight of the hull is 1300KG and LWL close to 18ft. I think I will go with the smaller engine, cut slightly into the transom then build a box and well inside to cater for the engine when lifted. It sounds like the best option as I don't really want to cut through the bottom of the boat and build in what we here call an inboard outboard system where the engine is mounted inside the boat. But if this sounds wrong, then pleas let me know!
     
  4. fredrosse
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    fredrosse USACE Steam

    "But if this sounds wrong, then pleas let me know!"

    Cutting the transom and mounting the outboard is easier, but building the inboard box for the outboard will keep the good looks of the traditional transom. Cutting away the FRP hull, and glassing in the box is really not such a big piece of work.

    Another option is to make a steel bracket that bolts to the existing transom, so you do not need to reinforce the existing transom, and you do not need to cut it either. The bolted steel outboard mount can be made to pivot, lifting the outboard free of the water without unclamping from the steel mount.
     
  5. ChrisUK
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    ChrisUK Junior Member

    Thank you for your advice Fredrosse. I have looed at these fixed outboard mounts and they do seem like a very practical idea. Do you know if there would be any issue with fitting a steering system to the outboard if it were mounted externally on a frame? Thanks again for your help!
     
  6. PAR
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    PAR Yacht Designer/Builder

    The steering on a bracket mounted outboard is the same as one hanging on a transom. There are generally two types of this portable engine brackets, a light and a heavy duty version. You'll want the heavy duty. It uglies up the back of the boat, but does leave the transom intact. yes, you will need internal reinforcement for an engine on a bracket, but a sizable hunk of 3/4" plywood, glued and fastened to the inside of the hull shell is all it takes.

    Has has been previously mentioned, cutting the boat for an engine well isn't a big deal and is easier than you might think.
     
  7. SukiSolo
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    SukiSolo Senior Member

    Do we assume the steering will be by cable/rod? You will need to allow for the gear change cable/rod and the throttle one too in the runs forward to any control position. Make sure you get a heavy duty steering box for any wheel if you do a lot of manouvreing around lobster pots etc. The lightweight ones are no good for lots of intricate shuffling around, in that they wear too fast, giving a lot of backlash.
     
  8. TANSL
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    TANSL Senior Member

    In my opinion the stern form a harmonious whole with the hull. Do not spoil it placing an outboard motor.
     

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  9. ChrisUK
    Joined: Nov 2014
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    ChrisUK Junior Member

    I really wouldn't like to after hearing members thoughts on the look of the hull. But I am unsure as to the notching out of the engine beds. The beds are probably six inches deep, and I would possibly have to notch out three inches then re-glass over that section.
     
  10. ChrisUK
    Joined: Nov 2014
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    ChrisUK Junior Member

    Gentlemen, I have taken on board all of your comments and advice and it really was top notch! The views on maintaining the looks of this pretty boat have really given me food for thought so I have decided to try again to get the Lister inboard diesel to fit in. if it is truly a no go then I will fit the outboard by cutting out the stern slightly and building in a well to accommodate the engine when its lifted.

    My question and concern is this. The boat has engine beds which are fibre glassed wood, maybe six inches deep. In order to get then engine down to a level to align with the shaft, I would need to notch out the wood maybe three inches, re-glass and then fit the engine to the beds. The beds run from the stern almost right up to the bow. Would notching these out compromise the lateral strength of the hull?

    thank you so much for your advice guys!
     
  11. TANSL
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    TANSL Senior Member

    If you could add an outline of how the engine mount and how you want to leave it, we would advise you better.
    In general, the engine bed has nothing to do with the "lateral strength of the hull", but you need to see how it is connected to the transverse frames for review.
     
  12. PAR
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    PAR Yacht Designer/Builder

    On this hull, I'll bet the engine beds serve two purposes, one to hold the engine, the other to offer longitudinally to the hull shell. I know I use this same trick in many powerboat designs - killing two requirements with the same structural stone.

    With the inboard removed, cutting the engine beds will not be troublesome, though possibly intimating. You can "bridge" the engine beds with more structural elements, both before and after the hole in the hull shell is cut, which will return longitudinal stiffness lost when the beds get cut. Think of this the same as what is done in a house's window. The loads above the window are carried around the opening with a header (bridge), to awaiting longitudinal (studs), which carry the loads down and a sill plate, below the window finishes the opening in the framing structure. The same is done on the boat, around the hole in the hull shell.
     
  13. rasorinc
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    rasorinc Senior Member

    If you have to cut the engine stringers down you can line the insides of both with stainless steel or aluminum (marine grade) plate as far forward or aft as you want and drill and bolt them to the wood stringers and you can tie both together with blocks or channel. Done right you can then mount a 500 hp V-8 AND REALLY ROLL.
     
  14. ChrisUK
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    ChrisUK Junior Member

    Again, thanks very much for your advice guys. I have a marine engineer coming to look at the boat and give me his opinion on the best course from here. I will keep you informed of my progress. Thanks again, this forum is a true asset!
     

  15. viking north
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    viking north VINLAND

    Back on post 17 I queried why you felt you had to replace the present inboard engine, alas the engine is missing.I like the idea of the inboard it matches this classy hull. If necessary to do so, one idea you might use for re enforcing the cut down engine bed /stringers is as I have on my keel/ballast carrying floors using a stainless channel cap over the cut down section. (photos) . I recommend on the forward end of your cut not to end abruptly in a 90% rise from 3 in. back up to the existing 6 in. of vertical thickness but to carry forward from 3 in. to 6 in. at say a 45 to 60 deg rise angle to spread the load change. ( Not applicable in my case as interior framing stringers will be installed in the 90deg cuts) You might even want to follow thru with your stainless cap bent and welded to follow this . I would think 3/16 to 1/8 thickness of stainless channel or even angle would do.
     

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