Long keel small sailing dinghy?

Discussion in 'Boat Design' started by jacob1234, Jan 24, 2020.

  1. jacob1234
    Joined: Feb 2019
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    Location: wales

    jacob1234 New Member

    Hi guys, I’ve been undergoing a small plywood stitch and glue sailing boat build of a boat I designed myself (my first boat I have built or designed). I’m at the point where I have basically finished the hull except for some reinforcing and coating and am at cross roads. Most builds i have seen fit a dagger board type keel but for many reasons (cockpit space, hull integrity, ease of build, aesthetics) I am leaning toward trying a long keel type design. please could you advise me if this is even feasible in terms of sailing dynamics etc. The lateral area should be enough surely so it won’t matter? The boat is just over 7ft loa made from 8’x4’ sheets of ply. I had planned to go either with a single balanced lug or mini gaff sloop type rig (I like the classic look). Any other thoughts on the overall design are also welcome.

    tdlr: please critique and advise on my design and build so far, any advise welcome
    Thanks!
     

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  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    A long keel on a mini boat will make it less nimble in terms of tacking or gybeing. If the usual center location of the dagger board trunk offends you, then you can offset it to one side without much harm to performance.

    The pictures suggest that you have used a steep rake on the transom. That only tends to shorten the waterline which is an important matter on such a tiny boat. The shorter the waterline the more inclined is the boat to hobby horse. You have used a lot of flare which is fashionable but not particularly advantageous for a boat this small. The righting moments would have been more favorable had you reduced the flare and widened the bottom. Initial stability of your boat will suffer as a result.

    All that said, the boat is a cute little packet and it may give you some pleasing times afloat. Here's hoping that it does.

    The sketches imply that it has sufficient aft rocker. Good for you, that is a desirable feature for displacement boats. I presume that you have done the arithmetic that describes the extent of the rocker which is a function of the anticipated total displacement

    A small skeg at the after end might be worthwhile if you intend to row the boat some of the time. A simple sprit rig such as used on the Optimist pram and many other similar boats is appropriate for your mini boat. Of course you may favor a gaff for appearance sake but that will not be either economical or entirely practical.
     
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  3. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome back Jacob - is this wee boat going to be a tender for the boat you were thinking about here?
    Building cheapest robust safe ocean crossing capable boat - ideas https://www.boatdesign.net/threads/building-cheapest-robust-safe-ocean-crossing-capable-boat-ideas.61754/

    Re your photos above, she looks like a very fine wee boat - you will probably find her rather tippy when you get in, but when she heels a bit her stability should improve a bit (partially due to the flared topsides).
    But note what Messabout says re how increased beam and less flare would have been better for stability.

    Re putting a keel on her - you already have a solution (the boat), and to start thinking about long keels will give you a proverbial solution looking for a problem.
    Keep it as simple as possible, following the same ethos of what you have done so far.

    I would be inclined to just put a leeboard on her - some boats have a board each side, but you could have just one, that can be 'tacked' when you change direction.
    But hang on, you have flared topsides, which means the lee board would not be vertical - that does not really matter.
    Try a single leeboard on both sides - it might work better on the windward side, where the board is more vertical when you are heeled. Be aware though that you don't want her to heel very much.

    Here are some ideas - Types of Daggerboard leeboard centerboard etc https://www.pdracer.com/keel/

    These Sabot dinghies have leeboards - Gresham Marine also make rudders for them.
    Naples Sabot Leeboard - Gresham Marine - Racing Sabot Leeboard https://www.greshammarine.com/shop/blades/naples-sabot-leeboard/

    Lots of info out there - try typing 'sailing dinghy with leeboard' into Google, and see what comes up.

    An edit - ideally it would be nice if you could have some positive floatation in case you get swamped - but the boat is already so small, there won't be much room for you to sail her if you start fitting buoyancy compartments.
    The logical way to start would be one in the bow and one in the stern. Alternatively, you could fit buoyancy bags on the gunwhales (a bit like training wheels on a bike - they would improve stability as well) similar to what Richard Woods did on his 10' Duo design -
    Sailing Catamarans - Duo 10ft Sail/Row Dinghy (nesting option) http://www.sailingcatamarans.com/index.php/designs/1-beach-cats-and-dinghies-/420-duo-10ft-sailrow-dinghy
    Although they would rather get in the way of your leeboard if you just had one which you tack from side to side. But you might not have to - you might find that she will still sail well on both tacks if you leave the leeboard in one place.
     
  4. jacob1234
    Joined: Feb 2019
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    Location: wales

    jacob1234 New Member

    Thanks for the input guys!! I think I got a bit carried away trying to get what I deemed a nice shape in the size constraints I had at the peril of sail handling. Is it still worthwhile putting the effort to finish in your opinion? I dont need her to win any races but would like a functional performance ( I still have two sheets of ply so could restart if needs be but obviously would rather not) The main aim of this boat was to be a little day boat to sail around just off the beach and/or row about.

    You have definitely swayed me toward leeboards I think. Thankyou for the resources bajan. Although the pdr do show stationary bilge keels as an option would this not be sufficient on my boat? (Also yes bajan, this could be a tender to my dream cruising boat but as Im learning I think I need a lot more experience before I even think about starting that project)

    Also messabout I know you suggested sprit rig, but would balanced lug also not be a good idea, from what I can tell it is much simpler than say a gaff or marconi sloop setup in terms of lines and fittings etc. Im not a big fan of the appearance of a sprit rig personally so would prefer a lug if functional
     
  5. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Yes, a small lug will be entirely satisfactory. I do not think it is more simple than the sprit but you can certainly use a lug with good result.........But you are going to need some sort of lee board or dagger board. If you use an Opti type sprit or a lug of similar size, perhaps 36 to 40 square feet, you will need a board of about two square feet of immersed area. I suggest that you give some thought to the offset dagger board trunk as mentioned in the PDR thread. If you put the board case in a fixed place there is no going back. Not a big problem if it is not quite in the right place. You can find a reasonable helm balance simply by moving the mast fore or aft if the need occurs. Do not be bashful about making the rudder bigger than you think that it might need. At this stage of the game make the rudder a generous size and cut it down later if you are compelled to do so.

    Mounting the rudder on that steep transom is slightly problematic. If you let the pivot center of the rudder be parallel with the transom there will be some lift involved when you steer to tack. Never mind all that, finish the boat and go sailing, or rowing, or sculling.........but not swimming.

    You have the boat nearly finished so don't stop now. I think that it will be a little bit tender because of it's relatively narrow bottom. Narrow bottoms can be an advantage in a larger boat. With this boat, I would go for it but be cautious in the first outing or until you become familiar with its behavior.
     
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  6. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Just a thought - have you made any calculations as to where she might float when you are on board?
    A simple way of doing it would be to measure the surface area of the bottom panel, and then multiply this by a vertical draft dimension to give you a volume.
    If you use metres (including for the draft) you will get a cubic metres measurement - multiply this by 1,000 and you will get displacement in kg.
    This will be a rather pessimistic estimate, as it doesn't take into account the significant flare of the hull sides and the transom, or the curved rake of the bottom panel, but it would be a start.
    And 7' long is pretty short really - especially as she has a pointed bow.
    One reason why so many small dinghies have pram bows is to get more buoyancy into them, while trying not to become brick shaped.

    Is 7' long a critical dimension, or can you feasibly lengthen her with your extra plywood?
    If you did lengthen her - say by just under 4', to try and optimise use of the plywood - you would get a lot of extra buoyancy.
    You could simply follow the existing curve re the shape of the bottom panel.
    And she would tend towards a dory with a narrower transom, or even a peapod shape.
    You could cut a section out of the transom, leaving a ring frame - ok, it will be an inclined ring frame, but it will still be effective, and quite funky even..

    OR - you could 'sea trial' try her in the water as she is re her 7' overall length, see how tippy she is, what her buoyancy is like, and how she handles, and if you then want some more length you could build a 4' extension (as per the above thoughts) that simply bolts on to the existing transom - in the same fashion as many two part nesting dinghies.
    And this transom extension could stow inside the main hull when unbolted for transport, if length is an issue for transporting.
     
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