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Thread: Modify a dory

  1. #1
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    Default Modify a dory

    Seven, eight years ago my son and I built a12 foot Banks Dory from Gardners Dory Book. Not the greatest boat ever but he got to build a boat with little more than a table of offsets and a single page of sketches.

    As Ian warned, its a bear to row and so has not been used as much as it should have.

    Rather than scrap it, we are contemplating whacking the handsome tombstone transom off it and installing a new transom further forward, about halfway to the last set of frames, plumb enough and with a cut out suitable for a small outboard. She will only be used in protected coastal waters for fishing and general mucking about.

    Im not worried about her being stout enough to carry the motor; more concerned about how a long skinny boat will handle if at all. Im a builder, not a designer.....

    Good or bad idea?

  2. #2
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    Default Re: Modify a dory

    maybe a motor well would be easier?

  3. #3
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    Default Re: Modify a dory

    I'm not a designer either. I have cut the ends off of a v-bottom rowboat, yielding a shorter pram. That worked well. In your place I would make a paper model, or at least a drawing to compare to other motor dories. I might also look through traditional plans books, Atkins for instance, to see if there are outboard boats with similar dimensions. You also might consider installing a motor well, or even a small inboard. The longer boat would have more buoyancy and stability. Lastly, you could try to sell the dory and buy an outboard boat. Whatever you decide, please keep us posted.

  4. #4
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    Default Re: Modify a dory

    Quote Originally Posted by donsmarine View Post
    maybe a motor well would be easier?
    ^This.
    You will be able to retain the buoyancy that the designer intended her to have aft. OB Motor boats are fat arsed to carry the weight of the motor and crew fiddling with it right at the back. Sawing off and discarding a chunk of boat will not give you that.
    It really is quite difficult to build an ugly wooden boat.

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  5. #5
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    Default Re: Modify a dory

    motor well for sure...

  6. #6
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    Default Re: Modify a dory

    Thanks gents. I considered the motor well option but kept tripping over how to build the aft end without major modifications to the bottom of the boat and the bottom of the transom to form the tunnel.

    And of course the follow up question is how big an outboard? Not ever gonna be in a hurry, it just has to go faster than we can row. My gut tells me 5 to 7 horse.

  7. #7
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    Default Re: Modify a dory

    No tunnel. The prop and anti-cav plate go below the bottom. A well with a sloping aft side to a rectangular bottom hole will allow the motor to kick up, but it takes a great deal of space. A smaller square hole and well box with all vertical sides, front, and back will require a vertical lift.

    Alternatively, you should be using a low horsepower mill. Like 3HP or less. A simple clamp over the gunnels plank will make a suitable side motor mount. It would probably mount best about 3' forward of the transom. Why work too hard?

  8. #8
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    Default Re: Modify a dory

    I think 5 hp would be more than enough as due to her narrowing bottom aft, trying to push her much faster than rowing (hull speed of only about 5 knots) will make her start to squat terribly.

  9. #9
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    Default Re: Modify a dory

    My assumption was that the aft end of the hole should slope or at least be a tad higher than front, tunnel was the wrong term...
    Sloping the back end to allow the motor to kick up makes a lot sense and since theres not going to be any room behind the well anyway...

    A square box all around with a lift would surely be simpler. I see 1/4" luan mock-ups in our near future....

  10. #10
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    Default Re: Modify a dory

    When you say 12' banks dory I assume it is the one on p 144, with a LOA of 15'6" . What I would do is borrow a 3 hp motor and make a temporary side mount to see how it would work at the after frame. Given the size of the boat, you may be better off putting a vertical transom as you thought, and use an extension to run the engine so you can sit amidships. A simple way that some have done a "well" in a boat this short is put a transom in where you want then, cut out the bottom of the raked transom. rIn this case it would be pretty simple to work with the after sawn frame, cut out the knee and the bottom of the raked transom. Merit of doing this is simplifying the structural considerations. If the boat squats too much with a borrowed motor at the after frame in a test, you will have to do a well between the last and next to last frame. You could use construction details from the boat in Chapter 16.
    Last edited by Ben Fuller; 05-10-2019 at 09:52 AM.
    Ben Fuller
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  11. #11
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    Default Re: Modify a dory

    Pretty much what I had mind initially. No way to sit aft without the bow going skyward with a motor, so had planned an extension to the middle thwart.

    Im gonna mock up the suggestions presented earlier and see which make the most sense structurally and visibly.

  12. #12
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    Default Re: Modify a dory

    As pointed out it would squat too much.

    Also, a long enough well to allow you to tilt would eat up a lot of space, and with a square well you will need to stand up and lift the outboard to go ashore. That can be a real pain when it's even a little bit rough, and you just might find yourself laying in 4 feet of water hugging an outboard.

    I suggest a MinnKota Riptide, that's the salt water model, a bit aft of midships in a rectangular well just wide enough to fit it down with the 2 blades of the prop horizontal. The MinnKota doesn't tilt, it slides up and down. 2 or even 3 deep cycle batteries will give you a nice long ride if you don't go all out, and the ballast down low will be welcomed. It will work much better if you devise some sort of baffle to close up as much of the bottom of the well flush with the bottom when underway.

    A well will be easier to build than the transom modification and it will row better if left as long as it is, and, of course, the trolling motor is almost dead quiet.

    If you do this please do follow up with photos/video and your assessment so that others will get to see how well it works.

    Also, i'm not just theorizing, I have just such a setup. It works a dream. A while ago I crept up to within about 4 feet of a sea lion eating a salmon before he realized I was there.

    One Blue Top Optima AGM battery will get me about 10 miles at 3,5 MPH, but my hull is longer and much more slippery than a Banks Dory.

    https://minnkotamotors.johnsonoutdoo...iptide-transom

    Last edited by Gib Etheridge; 05-10-2019 at 11:02 AM.

  13. #13
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    Default Re: Modify a dory

    Thanks for the input Gib and please understand Im not arguing or disagreeing, just thinking out loud...Im out of my wheelhouse here so all advice is welcome
    There is so little bottom aft the last set of frames, I am wondering how much flotation/lift I will actually be losing by chopping off the old transom and adding a new one. With two people in the boat, the existing transom might sit an inch into the water. Clearly the weight of an engine will pull the stern down all on its own, but with two people now forward; one midship and one forward of midship, how much squatting will there be?

    If the engine goes in a well between the aft set of frames and the next set forward there will be no room behind the well for anything but odd bits of gear. She gets reall skinny real fast back there so Im not too concerned about losing that space.

    I will never pull the engine while standing in the boat. I will be standing in the water next to the boat, or rather the kid will be. Im too old to be hoisting outboards around.

  14. #14
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    Default Re: Modify a dory

    I've had both Willin' and very much prefer the MinnKota. It's quieter and easier all around.

    When you're alone in the dory, and of course that will happen, your weight combined with the weight of the outboard plus the thrust will have that bow so high you won't be able to see where you're going.

    Also, it would be very awkward lifting the outboard from over the side, especially if it's rough and cold and the dory is smashing you or your son in the ribs.

    Back aft is a good place for the lighter of the odd bits of gear, no loss there.

    The well makes a good urinal too, should it come to that. You can even sit on it for a minute or 2 if you have too.

    Wishin' you the best.

  15. #15
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    Default Re: Modify a dory

    I like Gib's idea of the Minnesota electric motor. I had a Banks Dory with a well and 5hp Johnson Motor once. It was more power than the boat needed. But, the main draw back was the exhaust fumes that were inescapable when running down wind. Cough, choke, wheeze! Runny nose and stinging eyes as well!
    Jay

  16. #16
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    Default Re: Modify a dory

    Ive got some ponderin' to do

  17. #17
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    Default Re: Modify a dory

    See if this helps. It accepts a MinnKota or a pedal powered unit, both somewhat modified.

    015.jpg

    029.jpg

    More info later if you want. Gotta go now.

  18. #18
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    Default Re: Modify a dory

    An electric motor would be an interesting choice if you are OK with the range. I'm using a Torquedo ( about double the price of a Minnkota but with an integrated battery to push along my much bigger Romilly (sailboat) and it works just fine. You'll need to size a well based on the size of the motor which is longer than a regular outboard's lower unit, and the prop will likely have a bigger diameter than a gas outboard. If you want to steer with outboard the well will need to be wide enough to have clearance unless you are using the trolling motors which just have a tube.
    Ben Fuller
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  19. #19
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    Default Re: Modify a dory

    Having used an 80lb. thrust Minnkota on a 16 foot dory, I can tell you it is hard to get up to hull speed, maybe 3.5 mph. I would go for an outboard, does not have to be more than 4 or 5 hp. You will appreciate it on a windy day, but i must admit, the quiet of the Minnkota was delightful, just could not outrun the deerflies.

  20. #20
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    Default Re: Modify a dory

    3.5 horse Mercury weighs 41 pounds. Gonna put the boat in the water and sit on the center thwart noting the waterline. Add a 45 pound dumb bell in the proposed location of the well and again at the proposed transom and note the waterline.
    Then get out and see how she sits with just the added 45 lbs. Shift it fire and aft and note the changes.

    As an aside, I watched a Utube of a fairly rotund fellow in a 12 foot aluminum johnboat getting northwards of 15 mph with a 5 horse Mercury outboard. 3.5 horse will be more than enough for our purposes.

  21. #21
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    Default Re: Modify a dory

    Quote Originally Posted by willin woodworks View Post
    Ive got some ponderin' to do
    While you are pondering, consider that the outboard well will reduce your displacement to an amount equal to the size of the hole in the bottom times the depth of waterline. And then consider how much weight you will be adding to the area you just removed displacement from, and that it is near the end of the boat. It is a problem...
    (My own thinking is a 2 hp Honda is more engine than a small dory can use)

  22. #22
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    Default Re: Modify a dory

    In Harry Sucher's Simplified Boat Building / The Flat-Bottom Boat he discusses dories with outboard wells. Apparently they were used in the New England fishery from the late 1920s until the late 1930s, when the use of dories stopped entirely. He shows a scale drawing of a 12' Banks Dory (16' overall) with a well, but no construction details. He shows twin 2"x 2-1/2" skegs ahead of the well which apparently helped with steering. He also says that the boats were found to steer better by leaving the engine straight and using a rudder at the stern.

    He also shows a long-shaft outboard installed through a hole in a dory's transom.

    Incidentally, Joshua Slocum used a dory cut in half with a transom installed at the cut as his dinghy during his around-the-world voyage.
    I will beg you for advice, your reply will be concise, and I will listen very nicely and then go out and do exactly what I want! (Apologies to Lerner and Lowe.)

  23. #23
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    Default Re: Modify a dory

    I favour a well, which in your Dory can be placed dead centre as it has no keel.
    And that 3.5 hp Merc will have plenty of power......it will make the Dory squat at full throttle, but it will be more than efficient than oars.
    I would also recommend a tiller extension so you can use the centre seat when alone. https://www.westmarine.com/buy/ironw...47?recordNum=1

    It is a simple job, follow this example from PBO - https://www.pbo.co.uk/expert-advice/...rn-skiff-60947
    He fitted a 3.5 hp Merc to a 14' Nigel Irens Western Skiff. I think the free downloadable plans (same website) include more details of how to build it in 12mm marine plywood.
    You do not need a tunnel.
    Always remember that when you come into shallow water the skeg and propellor will be the first to hit the bottom, so think ahead.

  24. #24
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    Default Re: Modify a dory

    Why reinvent the wheel. Jeff Spira has already designed a Banks Dory with a well. Buy the plans for that and modify as needed to fit your boat. Will give you some size and location information that is ostensibly more than guess.

    http://spirainternational.com/hp_texa.php

  25. #25
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    Default Re: Modify a dory

    I put a motor well in a 16" Lowell Salisbury Point Skiff and I found it to be more or less a waste of energy and time. I used an 8hp long shaft Evinrude I got from my dad. I build a "fairing" insert to keep the turbulence inside the box. The boat was slow (for the amount of power--very short water line, and the well makes a lot of turbulence, even with a sloped back), it was wet in any kind of chop, and leaned away from the turns (as you'd expect) and needed a fairly long tiller extension to get the driver's weight up amidships to counteract the loss of bouyancy and weight of engine. It was altogether not especially useful nor in any way an improvement on a 12" tin skiff given the narrow beam, the amount of room the engine well took up and the long ends. This was a fairly beamy dory, too. If this one from Gardner is a "real" banks dory, it'll be even narrower on the bottom, tippier, with higher freeboard, and probably even less satisfying.

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