Outboard powered SOT kayak...

Discussion in 'Boat Design' started by retrosub, Sep 13, 2006.

  1. retrosub
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    retrosub Junior Member

    I do a lot of freediving out of an Ocean Kayak Scupper Pro. This is a 15' long 26" wide plastic kayak and is self-bailing with scuppers under the seat. In most respects it's ideal for what I do - stable, large hatches, safe. But for long distances (over 2 miles) it wears me out. After a long day of diving, with fish in the hold and a headwind... it's a tough trip back home.

    I was thinking about building a stitch-and-glue sit-on-top (SOT) kayak with a square stern for a 2hp outboard. I would steer it with foot pedals and use a tiller extension to set the throttle. I would rig a way to raise the engine and kill it simultaneously (when landing on the beach). And of course I'd be able to paddle it in times of need.

    I'm thinking I could make it shorter than my current kayak, because it doesn't really need to track well - I can steer it easily with the motor. And I can go a little wider for more stability. So probably 12' long and 36" wide or so.

    What thickness plywood would I need for the hull? I will fiberglass the outside completely. Do I need any rocker in the hull or is flatter better? What about the sides, should they be flared like a dory, or staight up and down vertical? Any other comments, suggestions?
  2. Poida
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    Poida Senior Member

    I used a kayak too for a while but as you say the paddling especially coming in against the wind was a bit of a bummer.

    What I bought myself was a second hand wind surfer and I made a transom on the back for an outboard. Of course I didn't need the sail.

    What I also wanted to get away from was putting my gear on and off in the water, and the wind surfer would have been big enough to get on and off fully rigged.

    A wind surfer also comes apart, so you can take the hulls down first then bolt on the frame and then finally the motor.

    However I built the transom, then I bought a boat so I never used it.

    Other than that it was a good idea.
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    FAST FRED Senior Member

    Depending on the range you need an electric trolling motor is lighter and quieter than a gas outboard.

    Some of the new AGM batterys can be inverted with no hassles , cant say the same for gasolene.

  4. Crag Cay
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    Crag Cay Senior Member

    I agree - I think this is one application where electric power is not only a viable option, but probably represents the best solution.
  5. retrosub
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    retrosub Junior Member

    I was researching battery power and the smallest marine battery I found was 35 pounds, and that doesn't include the motor. A small outboard is 28 pounds, that's why I was initially thinking gas. But I'm OK with electric or gas, that's not my primary concern right now. I just want power, not paddle power.

    I'll have to post some pics of the model I made last night so you can see where I'm going with this.

    What thickness plywood do you reckon I can use if the boat is fiberglassed on all the seams inside and out, and then is completely fiberglassed on the outside as well?

    The construction and design are similar to this boat, but with a full top:
  6. skyl4rk
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    skyl4rk Junior Member

    1/4" will work.
  7. retrosub
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    retrosub Junior Member

    Can I go thinner than 1/4"? What about 4mm for the sides and top, 1/4" for the bottom?
  8. messabout
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    messabout Senior Member

    You will have to get near enough to the outboard to start it. Your back end of the boat will need sufficient bouyancy to float you and the outboard. That implies that it will be fairly wide at the transom end. A possibility is to find a garage sale canoe, cut the sides down , chop off the back end, and add a full deck. The deck can be 4mm ply if suitably supported. Save a lot of messing with gooey fiberglass this way.

    The electric is a better idea than gas because you do not have to get close to the motor to make it go. In the electric case you can make the boat a little more kayak like. For simplicity you can build a flat bottomed hull pointed in the front and almost pointed in the back. You will give it a bit of rocker on both ends. The sides need not be more than 8 inches high amidships. at low height you can crawl in and out of the boat easily. Give it some sheer by raising the bow and stern 4 or 5 inches or so.Not too much because windage will be a problem if you over do it. Let the sides be plumb. 30 to 32 inches of beam will be plenty stable if you use the plumb side option. You can still paddle the boat when the battery runs down. It will be a piece of cake to build because of the vertical sides.
  9. skyl4rk
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    skyl4rk Junior Member

    1/4" is almost too light to put your weight on, it needs a lot of framing underneath. However if you build a lapstrake boat, or a boat with a lot of curvature to the plywood, you might get away with 4mm marine ply.

    When you bend plywood panels and then glass/epoxy them together, they get stiff. But the load is carried in the chines so make sure you fillet and tape it good.

    I built a lapstrake Wee Lassie canoe with panels about 3" in width, it was very stiff and could hold my weight. But only becuase there were a lot of epoxy joints and bend to the ply.
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    Many canoes and kayaks are built from 4mm ply. 6mm sounds heavy. Fiberglass will add a few pounds. Do you have to carry it far? It is surprising what a fully enclosed hull like a SOT will take; I built a 7ft Ama using 1/8 ply for sailing my canoe, it did not work well so I put bricks under each end and two of us (415 lb) jumped up and down on the middle trying break it; didn't even creak. Finally I had to use a saw and axe to make it fit in a garbage can. I have built a couple of lightweight canoes using really cheap 1/8 (3mm) ply and a 3/16 ply floor to step/sit on; they're stiff enough even without decks but are meant only for rivers and small lakes. Stiffness is greatly increased by decks. Strength comes from the joints; I dislike using epoxy and glass so I use use bilge blocks and construction adhesive. After many tests I concluded the optimum thickness for these is 4 to 6 x ply thickness, the higher number for wider planks. These hulls are not expected to last long, a few seasons at best; when I am happy with the design I will rebuild them using marine ply. A small boat doesn't have to weigh a lot. A motor and battery could mean an inverted boat may be hard to right. The motor shaft can be passed through a well so it did not have to be right aft. I don't have any experience with SOTs, personally I would go for a regular sit-in with a nice wide transom for a motor and a ladder, and lots of room for diving gear. Hope these thoughts are useful.
  11. JEM
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    JEM Senior Member

    Lot depends on the shape. 1/4" could be overkill or not enough. For a wide spanned flat bottom, it'll oil-can quite a bit with light fiberglass coverage. You could add some internal stiffeners to address that.
  12. alexlebrit
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    alexlebrit Senior Member

    You might have seen this:

    But just in case you haven't here's a bunch of people mounting electric trolling motors on their kayaks.


    Another option might be to stick with your existing, and side mount a small outboard with counter weighting on the other side, à la pakyak motormount.


    It also has the advantage that you're not miles from the motor to start it.

    And HERE'S a man doing something odd to his kayak with a motor (again side mounted and outriggers too.

    And finally for now, why bother with ply? You say you want to build a sit on top, so why not build it like the surfboarders do with a foam core and a glassed outer skin? That way you can carve it to match your bottom, cut holes in for hatches/barrles/storage wells to suit all your gear. While it's nothing at all to do with boats, you can find info about GRP over urethane foam HERE
  13. mikebair
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    mikebair New Member

    outboard powered kayaks

    Sure alot of people have done it and many want to but.....

    I have actually fabricated a mount that (only fits Heritage kayak models) fits an electric motor and a gas motor at the same time. On electric only lakes I use two Minn Kota 30#T electric motors.

    The gas motor is 12# and the electric is heavier. The optima blue top battery is 80# but the boat still floats nicely. The gas motor helps me go long distances up the river without draining my battery so I can troll and navigate quietly with electric once I get there.

    For $1000 total I got the neatest, most effective little bass boat I could easily store while significantly increasing my chances of getting to the monster fish hang out areas with ease.

    My catching verses fishing ratio has gotten much better since the added dimension of this powered craft set up.

    Attached Files:

  14. retrosub
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    retrosub Junior Member

    I've changed my design and now I'm making an outboard powered proa. The motor will be mounted on the outrigger spar, which will be easy to reach. I found an Evinrude 4 hp light twin that had only been used twice for $175. The engine is very light, 35 lbs! It's about twice as much power as I need, I doubt I'll ever need full throttle.

    I used the Cheap Canoe plans from bateau.com and stretched the hull a foot to 15'. The beam at the chine is 22" and it flares to maybe 30" on top. An interesting modification I made, was to use a 7' section in the middle and butt blocks at either end, so there's a 4' panel, then 7', then 4', instead of two panels joined by a central butt block. There are bulkheads fore and aft that cover the butt blocks, so you don't see them. The inside room is 6'6", so I can lie down inside.

    I've cut the panels, stitched them up around a midform, and glassed the chines with biax, and will glass the inside and bottom with light fiberglass. I just need to make the decks in front and back and the main hull is done. The sealed decks/bulkheads will serve as flotation chambers and have room for storage. I'm still debating what kind of storage hatches to use: buy or make?

    After that, I'll start on the ama, which should be very easy.

    I've been fooling around with a sailing option as well, probably a lateen rig with the mast step aft of the bulkhead.

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    Mike: from the picture that looks a tiny kayak for all that hardware, I wonder how much freeboard you have. I worry about the stability although have to admit my own 9.5 ft kayak handles a sail quite well although there has been odd moment of terror.

    Retro: Sounds great; I also have put the butt joints near the stems instead of the middle and it worked as I figured there would be less stress there and I got more boat with less ply waste.

    What did you plan to use for the outrigger spar? I experimented with an ama for a sailing canoe but have not found anything good for the spars so I appreciate any tips. I suggest you leave enough space between the hulls to paddle home if the motor gets moody. The ama creates drag, my canoe paddles dead straight with a single-blade paddle on the ama side, a double paddle may not be the best option here.

    I notice Mike has the motor mount behind the seat; I assume that helps steering under power.

    I have read that the cheap canoe is a bit tippy, but an ama with the weight of the motor on the spar should fix that and it's easy to reach. An excellent solution to several problems. Don't forget the photos!
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