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 Junior 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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    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 Junior 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
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    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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    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.
     
  7. jacob1234
    Joined: Feb 2019
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    Location: wales

    jacob1234 Junior Member

    Sorry to keep you guys out of the loop for so long! Been a very hectic year so far.... out of all this chaos with coronavirus I am trapped at home like many of us so have finally got time to work on the boat. I decided with all the extra space in my garage after my flatmates left for home to extend the boat after all! So far I think it looks much better for it. I have also started work on the framing and some oars aswell. And with the new found space in the boat I have decided to bite the bullet and give a centreboard a go as it does seem like the best option really. Please have a look at the new progress pictures and a sketch I made of what im envisioning the rest of the layout will be. (Ignore the fact theres no floor on the new aft section, Im going to flip over first to install.

    The added extension brings the LOA to just shy of 10' . Any more and the stern got too skinny I thought. Its already a little 'fish headed' shaped as it is, which from my understanding isnt ideal but its not extreme so hopefully will be alright. I think with the better run out due to extension the transom wont dredge as it would have done before!

    Things Im still a bit unsure about
    - extension join. the old transom is going to be cut away and form part of rear seat and the join glass taped inside and out. will this be sufficent?
    - will the seats be okay where they are in the sketch? I want compartments for buoyancy fore and aft but dont want to ruin my sailing balance by not having seats far enough each way?
    - theres quite a lot of rocker at the front now realtive to the length, i guess if i just load the boat enough itll have the pro of increasing stability and submerging flat to avoid pounding?

    As always, any help is much appreciated! Hope youre all doing well
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    Last edited: Mar 29, 2020
  8. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Jacob, you are overbuilding that cute little dinghy. All those ribs are unnecessary in a boat of this kind. It appears that you have also been very generous with the epoxy joints. For sure it is going to be a substantial but somewhat overweight boat. I presume that you will be adding some cross members on the bottom. A boat of that size typically has one main frame somewhere near the middle and that is sufficient. Stiffening the sheer line can be done easily with a modest stringer on the inside and another on the outside. Too late to not turn back now so on with the project.

    It appears that you will be ready to splash the boat soon. Cheers..................
     
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  9. jacob1234
    Joined: Feb 2019
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    Location: wales

    jacob1234 Junior Member

    Ah, I think I was worried about whether it would be strong enough as I intend to beach launch it potentially in surf and maybe overcomensated with all the framing haha. I also hoped that any extra weight would add stability possibly to stand against sail without having to use body weight so much? I am just securing the daggerboard case to the hull now and hope to have the hull essentially finished in the next couple of days. I will post pictures in case you're interested in how it turns out! thanks for your help so far.

    Some more questions
    - how big of a skeg would you recommend on the underside? I was thinking something about 2'' deep most of the way along, extending to flat bottom line at the aft rocker? And also does it make much difference having two offset skegs vs one central. Two central ones would mean the boat can rest on its skegs not hull when i beach it
     
  10. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Extra weight of all those frames is not going to do anything positive for your stability - in fact they are raising the centre of gravity of the boat a bit, rather than lowering it. They will only increase the weight.
    Are you going to add gunwhale strips as well along the sheerline?
    Re beaching, I think that having a centre longitudinal skeg will be much more beneficial re strength. Make sure it is long enough that it is the part of the boat that first comes into contact with the beach.
    Yes please re more photos showing your progress!
     
  11. jacob1234
    Joined: Feb 2019
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    Location: wales

    jacob1234 Junior Member

    It appears the frames were a bad idea indeed then haha. I suppose too late now, ill see how she sails and make adjustments somehow afterwards.
    Re gunwhale strips yes! i want to add scuppered inwhale and standard outwhale. However i am having trouble bending any of the strip i have around. (18mm or 12mm) I do not have access to anything to make a steam box due to lockdown so what are my options? I was thinking just make it up of tiny sections glued and shaped as if it were a single smooth bent piece but you sacrifice on looks under clear varnish (and the actual strength of a single bent gunwhale??)

    Here are the pictures from down tools yesterday. (Id already put more frames in before I saw bajan and messabout's replies... forgive me)
    [​IMG]
    [​IMG]
     
  12. messabout
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    messabout Senior Member

    Jacob it is also common practice to laminate strips in order to negotiate a severe curve. Three 6mm strips will make up the 18 mm width. Four 4mm strips will be even more flexible. Rip the strips, attach the first one to the curve. Let the adhesive cure. Now you can add the next two strips all in one go. Use plenty of clamps and clean off the squeezed out glue before it hardens. If you are using epoxy, a rag wet with lacquer thinner will clean the squeeze out nicely. A rag wet with water will clean white glue and I expect that you know all this cleanup routine. When cured the strips can be sanded to produce a smooth and attractive appearance. If you want to gild the lilly a little bit, use a different color wood for the middle ply or as you see fit.

    The relative stiffness of a piece of wood, such as the stringers, can be approximated by calculating the cube of the thickness. If the strip is 4mm thick, then the cube is 4 x 4 x 4 =64 If the thickness is 6mm then the cube is 6 x6x 6 =216. Thus the 6mm is more than three times stiffer than the 4mm. The 18 mm single strip would be beastly stiffer by that calculation.

    If you want to cover the edge of the plywood, rabbet the first strip so as to cover the edge of the ply. Or you could raise the laminated strips a few mm above the edge of the ply and then install a narrow strip to cover the edge. Judging by the pictures it is clear that you have the ability to do a first class job of this trim work. Keep up the good work.
     
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  13. jacob1234
    Joined: Feb 2019
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    Location: wales

    jacob1234 Junior Member

    Thats a good idea about the covering the end of the ply! Ripping and laminating strips sounds very doable, however I am stuck at how to rip them? most videos I have seen show people using table saws/circular saws or bandsaws. The only tools I have at my disposal are a handheld jigsaw (that needs fighting to cut in a straight line) and basic hand saws. Im worried i'll waste my limited supply of strips trying to get any near straight enough to laminate nicely. I suppose Ill have to figure out a way to use the jigsaw effectively.
    Thanks for the encouragement as well messabout!
     
  14. jacob1234
    Joined: Feb 2019
    Posts: 12
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    Location: wales

    jacob1234 Junior Member

    Hey guys, just a little update for you - managed to bend round solid pieces for the rail with a lot of patience! (although the joint between the two did make a bit of a kink but its not terrible so have accepted it) Hopefully only a couple more bits of gluing/coating and the inside rails then itll be ready for paint! I have bought white hempel multicoat for the hull inside and out and hempel vrnish for all trim and seats etc. I got very confused over paint tyres re application over epoxy but 3/5 stories I read said alkyl enamel paint such as multicoat should be okay over epoxy as long as scrubbed and keyed first?
     

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  15. jacob1234
    Joined: Feb 2019
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    Location: wales

    jacob1234 Junior Member

    Hey guys!
    Tested it on the water today! Very pleased ith the performance and think it was definately the right idea to lengthen the hull. Only seen it row so far but will raise the sails next time I am down.
    1045f6e1-581e-4165-a3f6-0d88035db625.jpg IMG_0622.JPG IMG_0614 (1).JPG
     

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