The Melatelia: light wind dinghy

Discussion in 'Boat Design' started by laukejas, Mar 20, 2015.

  1. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    Hello everybody once again,

    With a certain amount of pride, I can finally say that I have finished the theoretical stage of designing a 2 person dinghy which I'm going to build in the beginning of June. It is probably unusual to produce plans before building the prototype, but I believe it was worth making myself a clear step-by-step manual to aid in complicated building process to avoid at least some part of the problems (some can't be helped anyway on the first serious project), since as some of you know, my experience in boatbuilding is really quite shy. Also, making this building manual helped me notice and correct errors in design before I started building. Also, it will be easier for you folks to stop me before I do some horrible nonsense which is presented in these plans as a valid design idea :D

    I've been working on this design for nearly a year, and I must acknowledge that I had a lot of help from a lot of people, including many good people on this forum. I thank you for your patience, understanding and time you spent guiding me through my own ignorance. During this time, I red countless books and articles on the subject, consulted with dozens of other sailors, engineers, sailmakers. I even had the honor to have my sail reviewed by the famous Todd Bradshaw.

    For those who are unfamiliar with this project, you may wish to see how it all started. If I were you, I wouldn't want to surf through 20+ pages of never-ending discussions, so I'll just summarize what is this all about.

    The requirements for this boat are as follows:
    2 person sailing-rowing day-sail dinghy (no racing);
    Designed to sail in the lightest of winds, yet manageable in gusts;
    Car-toppable (parts that go on the roof cannot exceed 35kg legal limit);
    Comfortable enough for whole day trips;
    Easy enough for a novice to build;
    Build-able with 500$ budget (excluding tools);
    No expensive tools needed (such as table saw);
    Life-span of at least 10 years;
    Low maintenance costs.

    The solution I arrived at (with a lot of help from some good people) is what you can see in plans I'm attaching to this post.

    If someone could take a look through and comment, it would give me a last-minute check before I start buying materials and build (and possibly mess-up horribly). I re-checked technical solutions and building procedures dozens of times, but there is only as much a poorly experienced guy like me can anticipate. I know it is a lot of pages to go through. I am asking a lot.

    I'll list some of the things I'm not yet sure about this project so you can know what to look for when viewing the plans.

    1. As it happens, marine plywood is not available in my country. Exterior grade ply's are laminates, and only fir/spruce, which, as I researched, have serious problems with getting wet. Also, laminates make it difficult to work with. However, there is Baltic Birch plywood available, which, although not graded as marine, is described as boil-proof, and available in excellent quality (clean, inspected, knot-free). Baltic Birch, as I've researched, is very strong and sturdy (compared to oakum). However, it is also very heavy (700kg/m^3), and as I've mentioned, weight is a concern in my boat. This is why I decided to use 4mm plywood for bottom and side panels, while 6.5mm is used for things like transoms, bow cross-section, and so on. Generally, it is not recommended to use 4mm plywood on anything but smallest boats, but I'm hoping that the strength qualities of Baltic Birch plywood will compensate for lack of thickness, keeping in mind that I don't plan hitting on rocks (there are practically none where I'll sail). I plan to coat the exterior of the hull with single layer of epoxy, but no fiberglass cloth, because it's not available in m country, and would make my boat overweight anyway. So, with this in mind, I'm not entirely sure about structural integrity of my boat. I discussed this project with many of you for a while now, and none expressed major concerns about this, but I must ask again, just to be sure - should I be worried?

    2. I'll use NACA 0008 daggerboard, which will fit in a daggerboard case which has a slot with 1mm offset. I'm not sure if that offset is enough. Daggerboard blade will get 3 coats of paint, while the daggerboard case slot end-grain of plywood will get substantial amount of epoxy to prevent rot, as well as the same 3 coats of paint. I'm not sure how much thickness does paint and epoxy add. Is making slot 1mm larger than the daggerboard enough? Snug fit would be ideal, but I don't want it to be too loose or not fit in at all.

    3. There isn't a lot of space for bow crew member, as you can see from the plans. I made what I could (widen forward part of the boat, move thwart aft somewhat, raise boom), but still, it might not be very comfortable. Is there anything I can do about it? (Note: boom downhaul will be only rigged in downwind sailing, and strong winds, so most of the time, it won't get in the way)

    4. As of now, I haven't found solution for what shall sailors sit on. They sit on floor, obviously, because gunwales are not comfortable for that, but there is no grating (because it weights too much, and creates a lot of problems). It would be ideal to make some kind of removable padding there so that sailors wouldn't have their bums wet all the time. Cheapest and lightest solution would be inflatable pillows. The problem is that they are often rectangular, and the space in the boat is more like trapezoidal. Either they won't fit, or they will move all over the place. I could buy some hard styrofoam, cut to shape and sew PVC fabric cover on it so that styrofoam doesn't get damaged, but the water and all kind of nasty stuff will probably get inside through seams and stay there. Any better ideas? Just please nothing heavy or complicated to make, I've got enough work already!

    5. I want to build this boat in 1 month, including everything that is in the plans. I'll work for about 12 hours every day, since it'll be holiday time for me. That's at least 300 hours in total (maybe I'll have bad days), and that is still a pessimistic figure (I can work more than 12 per day). Is that enough to build this boat (including the sail)?

    6. I don't plan to keep this boat in the water all the time. Maybe leave overnight sometimes during sailing season, but most of the times, it will stay on shore. During non-sailing seasons, I'll keep it in the garage. Keeping in mind that hull will get 1 coat of epoxy and 3 coats of paint (so, no fiberglass), is it realistic to expect 10 years lifetime?

    7. Note that in bill of materials, I haven't decided on amount of primer and paint. Can someone help me estimate? I'll coat everything (interior and exterior) with 1 coat of primer and 3 coats of paint, including rudder, daggerboard, insides of bow and stern compartments, etc. Tiller and spars will get varnished. How much primer and paint do I need?

    All right, so that's about it. Proud and loud, I present my boat plans:

    The Melatelia

    Booklet - presentation of the boat, specifications, some information on building.
    Building manual - step by step guide for myself what to do, how, when and in what order;
    Templates - a set of patters for some of more complicated parts that would be time-consuming to plot by dimensions. I made templates for A2 sheet and 914mm roll, but haven't decided yet if it's worth spending money and printing 4 meters of that roll so that I don't have to plot hull panels on plywood by hand.

    Expect some left-over mistakes (including spelling). I checked this more than a dozen times now, but nothing is ever error-free. I'm not printing this yet, no doubt I'll make tenths of corrections later on.

    Okay, that's about it... Awaiting your judgement :rolleyes:

    Attached Files:

  2. tom28571
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    tom28571 Senior Member

    Nice exercise. I doubt that the min weight goal will be met even with the very thin scantlings listed. With these scantlings and the construction I see, I also doubt the boat will be stiff enough to perform satisfactorily. There does not appear to be adequate torsional stiffness for one thing. Other issues are bound to appear but this is enough for now.
  3. laukejas
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    laukejas Senior Member

    Thank you for taking time to look through my work.

    Why do you doubt about the weight? I calculated all weights exactly with software, taking 700kg/m^3 for plywood and 340kg/m^3 for timber. Everything is included, every piece of hardware. I left 4kg for epoxy and paint.

    What do you mean by "perform satisfactorily"? If it twists a little, how will that affect performance? As I've said, I won't race it. I just need it to stay strong enough so it won't break in the conditions it can sail before capsizing.

    Torsional stiffness... I discussed that with several people on this forum before. They said that spaced gunwales, along with two compartments and thwart should be enough. If you don't believe so, what would you propose? What can I add to make it stiffer? Only that I'm already at the weight limit.

    P.S. Re-uploading plans with some minor fixes in the first post.
  4. Sailplan
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    Sailplan Junior Member

    That's an incredible amount of work and detail. I'll dig deeper into the structural question as I work through your documents but I first wanted to commend you unreservedly on the effort.
  5. tom28571
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    tom28571 Senior Member

    I distrust the weight data because it is considerably lighter than similar small boats in 4mm built or seen by me. I think all preliminary software estimates of weight are low because of unknown data inputs. My thoughts are based on both experience and calculations of similar boats.

    Twisting is nothing to overlook in a boat and your structure appears to have minimal torsional resistance. If built as shown, I think you will be disappointed, especially with two aboard as shown and in gusts. Torsion will be highest when sailing with two aboard in gusts.

    I specifically looked for some spacing or widening of the gunwales and can see none in the PDF drawings. As to what I might suggest, I think some side seat/air tanks on each side using the material in the aft and forward tanks and the thwart will give a box structure on each side that offers a maximum of stiffness at minimum weight. To stiffen the seat tops, which will not be very stiff at 4mm, I would glue some 1" or 2" thick styrofoam underneath, A couple of intermediate bulkheads would also be good.

    These recommendations are base partly on an earlier design which attempted to go open similar to yours for minimum weight and simplicity. After a redesign to incorporate the side seat/tanks as I recommend to you, it turned out both simpler, stiffer and lighter.

    The message is that a box structure will always be stronger than a non gridded but reinforced open structure. The side tanks can be made like a triangular box which has the highest strength to weight ratio available.

    You spent much effort and time on turning out an impressive amount of detail in your plans. As I said, its impressive but I have serious reservations that it will do what you want.
  6. Richard Woods
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    Richard Woods Woods Designs

    Agreed, you should be very proud. A lot of work there, nicely done

    However, (sorry!) I think you have far too many small bits of wood to cut to length, so try to simplify the framing

    It is very similar to my Duo design, as you know.

    I have had no problems with strength/torsion etc using 4mm okoume plywood. I have sailed mine now for nearly a year with no problems at all, so it is possible to have a lightweight (under 30kgs) hull that is strong enough. I can carry mine under one arm, as you have seen in my first video (at the end)

    But there is no way your crew can sit where you show.

    And I don't think you have enough built in buoyancy, but you will of course check that in a practical capsize recovery test before sailing it for the first time. Both of you need to get back on board remember if it is to be a 2 person sailing boat

    You can easily build the boat in a month, after all two of us built a Duo in 35 hours (2 days), so I could build 4 Duos in a month

    Richard Woods of Woods Designs
  7. laukejas
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    laukejas Senior Member

    Thank you for your compliments. It was indeed a lot of very hard work. A lot of thinking. For the time I spent designing this boat, I could have made at least 2 such boats. Anyway, it's winter, so there's not much that I can do outside. Sure, these plans look cool. But it will all be for naught if the boat won't do what it's supposed to do.

    Well, I've inputted every bit of data I could think of. As I've said, even screws and epoxy is counted in. But maybe you're right, I can't be sure. It will be clear only when boat is finished. But what else can I do?

    Maybe my terminology is wrong. What I meant by spaced gunwales is that they will be laminated with 3 layers: inner gunwale (a 3x1cm strip of timber), spacer blocks (1x3x12cm), and outer gunwale (same 3x1cm). This is shown on page 16 in Building Manual.
    If I misunderstood you, please clarify!

    At a very early stage of design, I have attempted to make something like what you're describing. I had to scrap that idea because it provided no head clearance from boom (crew sitting too high), too much weight, too little space in cockpit, and way more plywood than can be fitted in 3 oversize sheets.

    Here are some screenshots from that stage:








    Please tell me if any of these designs look better to you. As I've said, I had to scrap them due to reasons stated before, and also because they were criticized heavily by community here. After the Pic6, I arrived at current design, which received least criticism :D So I stayed at it.

    Well, I did that to compensate for lack of thickness in my plywood. To add more structural integrity. Maybe even help twist somewhat. tom28571 says that my boat has too little structural integrity, while you're suggesting to simplify framing. Or do you mean different things?

    Yeah, I won't even try to hide it: I took a lot of ideas from your wonderful boat. Basically I just tried to upscale it somewhat so that it can be sailed by two, while staying under weight limit.

    I see. But do you think my boat won't have problems with torsion too? tom28571 says it will twist. Could you please comment on that? It seems you two are not in consensus, and I find it difficult to decide whom should I believe :)

    These words strike fear into my heart, and I'm being serious. Why do you say so? Do you mean bow crew member, or both? I'll try to make some mock-up in my home with furniture to see how much space there actually would be in the boat, but from my plans, it seems that crew should fit. What am I estimating wrong?

    Well, compared to your Duo, I don't think there are a lot of boats that could compete in terms of buoyancy :D

    Anyways, I'm not sure what can be done about it, except going back to side deck design.

    P.S. I just calculated that if I used oakum plywood (I guess it's density is 450kg/m^3, correct me if I'm wrong), with 6.5mm and 9mm sheets instead of 4mm and 6.5mm respectively, I would arrive at the very same weight. But, as I've said, oakum is not available here. I hope that Birch plywood is that much stronger than Oakum as it is heavier. If so, then I should expect normal boat rigidity, as 6.5mm oakum ply is what most people use on boats of this size. If not, well, I'm screwed. Anybody got any comment on comparison of these two plywoods?
  8. tdem
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    tdem Senior Member

    You can't design a boat by consensus!
  9. WindRaf
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    WindRaf Senior Member

    I'm wondering if you know what is the center of buoyancy and the center of gravity with the crew on board
  10. laukejas
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    laukejas Senior Member

    Yeah that's true, but it ain't easy to go forward with building when there are some experienced people telling me that my idea is doomed.

    I forgot to include that in plans, but it is accounted for. At full load (210kg displacement) center of buoyancy and center of gravity coincident vertically, with some freedom in crew positions. Same in solo sailing. When rowing with 2 crew members, transom is slightly submerged, but only a little.
  11. WindRaf
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    WindRaf Senior Member

    Looking at your drawings I doubt that the centers coincide.
    Another thing I wonder is: what is the point draw a hull exasperated for close-hauled and then put on a sail not suited to windward?
  12. laukejas
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    laukejas Senior Member

    I re-checked just now. Centers do coincident with only 3cm difference. That's close enough, I think...

    Why is standing lug not suited to windward? Sure, it isn't as effective as triangular sails, but if made well, it will work reasonably. I chose standing lug because it allows to spread a very large sail area (requirement for low winds) with short mast. Mast cannot exceed the length of my car, and that is 4 meters. Also, standing lug is not self-vanging, so it can spill winds in gusts, which adds a lot to safety.

    And it is quite high-peaked. Aspect ratio is reasonable. I don't see why it wouldn't perform adequately to windward.
  13. tom28571
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    tom28571 Senior Member

    laukejas ,

    First, I apologize for not having studied all of your drawings completely so I missed the inclusion of gunwale widening spacer blocks that you intended bu t do not appear on the first drawing.. The time element. There are similarities in some of your study constructions to boats I have designed or built. I missed any reference that you are limited to a particular number of sheet of plywood. That is a practice that, in my opinion, belongs only to academic exercises and not to actually building a boat.

    I will reply later after looking further into your drawings but need to go and steam a couple of bushels of oysters for a crowd of people who will expect them to be ready for a yard party.

    I do applaud you work and do not intend to be only negative. It seems that you understand that which makes the time spent worthwhile.
  14. laukejas
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    laukejas Senior Member

    That's okay :) I hope that these gunwales will suffice to give adequate twist resistance. If you could look at them in more detail when you have time, I'd like to hear your opinion. Also, as I've said, I'm using thinner plywood than usual in boats of such size, but again, this plywood is heavier, and, by some reports, stronger than oakum. I hope that would compensate for lack of thickness, but I can't find enough data on these types of plywood to make a proper comparison.

    As for number of plywood sheets - well, they cost a fortune. I plan to use only the highest grade, top quality plywood (so I can be sure it has best strength-per-weight ratio), and it is very, very expensive (more than 100$ per sheet). That is why I'd really like to keep number of those sheets to a minimum. Sadly I'm a poor student and have to make do with what I have.

    Have a nice party! I'm not in hurry here, take your time. There are still 2 months left until I have to start building (and about a month until I should start buying materials), so there is some time to make some re-designs. I just hope I won't have to make major changes, like turning hull shape into pram, for example, or completely changing major parts. That stuff takes time.

    Hope to hear from you again! And thank you for the time you spend to help me. I really value it.

  15. messabout
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    messabout Senior Member

    Laukejas is a glutton for punishment so I will pick on his work a little bit.

    First let me say that you have done a lot of good work for this project. Kudos.

    I can not find a section view or a body plan in the PDF files. The pictures look good but they give no specific information. Give me some section views with dimensions,

    You have drawn some pretty parts that are unnecessarily complex. Parts such as the mast partner, the mast heel, boom yoke, board case, and the board itself. The NASA section will be pretty but tedious to build, and not measurably better than a far more simple board for a 3 knot boat. There is no compelling need for fancy sections, including the rudder. The inside of the case can be a simple box. No need for making it fit the NASA profile. Even if you feel that you must use the NASA board.

    Incidentally, you asked in another thread, whether one mm would be enough clearance between board and case. No! Three mm is more like it. Maybe 8 or 10 mm and lined inside the case with carpet. The board slides sweetly and stays in place when done that way.

    A suggestion for the board.....At the top, cut a triangular piece out of the leading edge part that will be enclosed by the case when the board is full down. That would allow the board to be positioned straight down or raked aft according to the need of the moment. If you don't get too fancy with the board, it can be reversed in the case so that it can have forward rake if you need it.

    The stiffness of the ply is far more influenced by thickness than it is on material density or strength. For a SWAG estimate of relative stiffness, use the cube of the thickness. 4mm cubed = 64, 6mm cubed = 216 so the extra thickness increases stiffness by 3 times and more.

    OK you have to go with the ply that you can get. Sometimes it is useful to make the bottom have more than the flat two surfaces that the vee has. Breaking the panels into narrower sections will improve the rigidity if some of the panels are angled upward from the bottom.

    Think of a bottom with a flat center plank that is about 55% as wide as the upper chine. Now add lower chines that are about 22% of the total upper chine width. Angle those parts upward to somewhere slightly above the water line where they meet the sides. You have now effectively stiffened the bottom of the boat, you have gotten the upper chine out of the water, you have probably made the boat a little more stable, It now has a partial flat bottom that lets the boat rest on saw horses on the ground, or on the car top without tipping. You have also given your feet a flat place to be when you are in the boat. The boat will draw less water, and the wetted surface is not more than the vee bottom would involve. Because the boat will draw less water you might be able to lower the sides a little bit which would save material and therefore a wee bit of weight.

    The boat will still have an entrance angle that is as narrow as the vee bottom would allow. By fiddling with the forward end rise of the beveled chine, you can adjust entry angle and also area distribution which will show up on your area curve. It is also possible that the boat will turn more happily with that kind of bottom configuration.

    Sorry about that Laukejas. I do not suggest that you change the whole design of your boat. I am just sayin' that there are other ways to do it that have advantages. It might be fun to play with that alternative during some of those cold winter nights.
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