14-15 ft sailing dinghy project

Discussion in 'Boat Design' started by kerosene, May 29, 2009.

  1. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    1st - this is not a "real" project. I have a plywood dinghy project on the way and maybe one day I will make a strip planked boat along the lines presented here.

    Small boat for pick nick sailing in closed calm lake waters. Big enough for 2 adults and 2 8 year old kids or so. Mast needs to come off (bridges) easily. Easy to sail no need to be too thrilling. Light - easy to pull out of water or carry - doesn't have to be easy to carry but reasonable to manage by 2 adults. No car topping dreams.

    What I am thinking is strip planked pine (scandinavian pine - that's where the boat would be built) with proper glassing in and out - ie pine cored composite.
    For rig I am thinking a catboat style setup. Just a decent sized main. Daggerboard. Thats about it.

    I think the 14 footer might be a little small and a foot more might make it more versatile and fun.

    I am hoping to make a scale model of the boat but would rather work on something that isn't totally wrong. So feel free to critique.

    the boat:

    Volume properties:
    Displaced volume : 0.251 [m3]
    Displacement : 0.257 [tonnes]
    Total length of submerged body : 3.666 [m]
    Total beam of submerged body : 1.258 [m]
    Block coefficient : 0.2718
    Prismatic coefficient : 0.5051
    Vert. prismatic coefficient : 0.4313
    Wetted surface area : 3.106 [m2]
    Longitudinal center of buoyancy : 1.794 [m]
    Longitudinal center of buoyancy : -5.624 [%]
    Vertical center of buoyancy : 0.140 [m]
    Midship properties:
    Midship section area : 0.135 [m2]
    Midship coefficient : 0.5381
    Waterplane properties:
    Length on waterline : 3.666 [m]
    Beam on waterline : 1.258 [m]
    Waterplane area : 2.907 [m2]
    Waterplane coefficient : 0.6302
    Waterplane center of floatation : 1.664 [m]
    Entrance angle : 90.000 [degr.]
    Transverse moment of inertia : 0.253 [m4]
    Longitudinal moment of inertia : 1.859 [m4]
    Initial stability:
    Transverse metacentric height : 1.150 [m]
    Longitudinal metacentric height : 7.554 [m]
    Lateral plane:
    Lateral area : 0.463 [m2]
    Longitudinal center of effort : 1.958 [m]
    Vertical center of effort : 0.123 [m]

    Attached Files:

  2. Guest625101138

    Guest625101138 Previous Member

    Looks beamy for the length but the WL beam to length is reasonable.

    Would be interesting to see a heeled waterline at say 15 degrees.

    I have little idea of strip planking so cannot comment on ease of building and that is something that needs considering.

    If you make a scale model then make a good size - maybe 1m+. This give a lot of freedom on choice of materials rather than skimping or building over weight. Also gets closer to real life.

    Now need to detail the structure to handle the loads.

    Rick W
  3. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    The design appears to be too deeply veed to have much initial stability, though she will stiffen up once heeled. In a small boat like this, a flatter bottom works better especially where several people are boarding/aboard.
    changing the midsection profile to flatten the bottom will reduce rocker as well, no doubt increasing the CP.
    The extreme would look something like an O'day Daysailer design, though you might prefer a more "wholesome" displacement hull somewhere in between.
    I would probably take about half of the midship deadrise out.
  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Strip planked boats are labor intensive. You will use up part of the life of your rip saw when making strips for a boat of this size. Consider glued lap strake for strength, light weight, and reasonable labor content, and distinctly salty appearance. Strippers are, or at least can be, very light, clean inside, and beautiful. But they are a lot of work in building.

    I'm with Alan. Way too much deadrise. The aims that you have suggested can be largely fulfilled by a boat with near flat bottom section. Flatter bottoms will improve initial stability while making the WL beam larger. Flatter bottoms let the boat take the ground easier and are also easier to get on and off a trailer. Simpler floor boards too. What you have drawn is reminiscent of older Moth class boats of the Cates design. They were very swift but needed athletic skippers. A family boat should not require athleticism.
  5. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    And i'm with the others on strip-planking. At the outset, it has appeal because it is a simple process. Just begin stripping and continue til done and sand/glass inside and out.
    The truth is, a lot of user-friendly construction techniques are out there, most using plywood, which is also very light and strong.
    It's not necessary to glass more than the chine and keel seams so the use of epoxy with all of its issues of cost and health concerns is minimized. Less ripping of wood (very hard on the nerves if not the saw) is required. Weight of ply construction can be competitive with strip construction. Internal frames are straight, so while there are sometimes more internal parts, they are simple ones.
    A single chine and some shallow deadrise is perhaps the most bang for the buck. More sophisticated than a simple flat-bottomed skiff, a shallow vee has less wetted surface area and, of all of the simple constructions, most efficiently approaches a chine-less (developed) hull when it comes to sailing dinghies.
    Yet, the shallow vee is actually stronger than the flat-bottom due to the panel spans being halved. No wonder so many novice builder designs use it.
    Many well respected designs have been built this way. Some, in my opinion, are better looking than the round-bottomed equivilant.
  6. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    Thanks for all the suggestions

    The scale model I will be building would be more for joy of building than realistic case study. Ie RC sailboat or bookshelf ornament.

    I hear you on the deadrise - i will adjust.

    Messabout, Alan
    Strip planking. yes I know that the amount of strips gets really high quickly and covering area with thin strips means more waiting for adhesives to set than actual working. I have made on 4ft scale boat hull and I know the tedious factor - at the same time I love the look of it and the round bigger boat lines you can get. Also if/when I actually get to build this (or other boat) it will be in Finland where I will have access to 150 acres of forest - family farm - and a bandsaw for turning trees to lumber. What I will lose in epoxy cost I will gain in cheap wood.

    Also I started a plywood boat last summer (will continue on this August on my holiday). This is a design I drew sort of following the general dimensions of a uffa fox's round chined dinghy. My project got a little complications as the lumberyard was out of 1/4 inch plywood and I had to use thinner (4mm). It will probably not be the greatest boat ever built but the fact that I got it started and get to continue this summer means that one mental health function is already fulfilled.

    I will post lines for that later today - Though I had to change the deadrise in the transom a little - plywood didn't want to bend the way computer claimed it would - not big change in the form but the panels don't fit perfectly as is.

    Attached Files:

  7. alan white
    Joined: Mar 2007
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    alan white Senior Member

    Pardon those of us who are used to rank novices with ideas of their own despite almost no knowledge of boats in general. I see you are somewhat experienced.
    I've seen a lot of ameteurs build strippers without too much complaint. I think you know the downside (time and tedium and epoxy exposure). It wouldn't have been honest to have avoided making sure you knew what you were up against. If you love the look, that makes it all worthwhile. Not everyone is moved by the same ideals.
    Regarding waiting for adhesive to set, you don't really have to wait if you use a stapler and various other fastenings like brads or small nails to hold strips in place as you go. My suggestion is to use something like Titebond III to glue the strips rather than epoxy. Being water-cleanup, it's less of a mess compared with epoxy or polyurethane glue. You can swipe inside and out as you go with a rag kept in a bucket of water. That makes sanding less torturous.
    A common mistake in sanding the hull is the use of too high a grit number. Without the stiffening of the inner glass, the outer surface is easily pressed inward a bit while sanding with a longboard that's too stiff or a grit that's too high. The result is what is almost invariably seen on typical strippers; unfair surfaces especially on flatter areas. Start with a good amount of extra "meat" on your strips for sanding. Then use a very coarse grit like 36 grit floor sanding paper.
    I'm not a big lover of cedar strip grain, and would be happy if the 36 grit finish were the sum total of the sanding done. I would be painting, which is far less work to begin with and to keep up, and ensures that the epoxy is totally UV protected forever. Once glassed you can't tell the difference by feel. The idea is to have a bottom as smooth as a well-built fiberglass hull, though some very minor "telegraphing" of strip seams might occur over time, something also seen on fiberglass-only hulls.

  8. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member


    I agree with the concensus above that your boat has more deadrise than is the norm. But it is not a bad boat, just a tender one. In exchange for some initial stability you get a hull that is more responsive to the crew's position and one that will handle seas and chop with more aplomb. Attached are some photos of the first boat I ever built as a teenager. It is 15'9" overall, and is chined, But the underwater profile is nearly identical to the boat you pictured. She is a very capable sea boat in skilled hands, and has carried sail rigs up to a Hobie 16 main with H18 jib with twin trapezes and outboards to 25 hp. She is nearly 30 years old and has had the versatility to serve my needs from small creeks to to weeks of coastal camp cruising. If you want to get a feel for a tender boat, find an old Mac Dinghy or Windmill. Mine is based on a Mac 14 (only 5 built) enlarged to 16'. As mentioned by others, your prismatic coeficient is very low. if you plan on even the smallest of kickers, or even rowing, a higher Pc of around .57 would give you more speed potencial, and would also likely improve the performance when heeled.

    Attached Files:

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