The Melatelia: light wind dinghy

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

  1. laukejas
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    laukejas Senior Member

    You're enjoying this, aren't you :D Well, do your worst.

    No problem. Here it is:

    [​IMG]

    Let me know if you need any other views and dimensions.

    Mast heel is complicated? I made it as simple as I could. How could it be simpler?

    If you would suggest how can I make those parts you mentioned simpler, I would gladly modify the design. I have no intention of complicating this project any more than needed.

    As for daggerboard, I can scrap NACA shape easily, and turn it into usual round-and-taper board. But back in the old thread about this project, people heavily suggested that having NACA foil would marginally improve performance, and that I should definitely make it.

    Truth be told, I don't like that NACA shape very much. It is very complicated to build. Especially daggerboard case with all these shaped slots. Very nasty stuff, very hard to fit on Vee bottom, I suspect. Even making these plans was horrible.

    You're sure about this, dumping NACA foil and the complicated housing for it? I'd appreciate if anyone else would also comment on this. Because I suspect that as soon as I trade NACA foil for simple board someone will show up and convince me that I should have kept it :D Happened more than once with this project already.


    3mm sounds reasonable, but 8, 10? A CARPET? In a part that constantly gets wet and gets very little sunshine to properly dry out? Correct me if I'm wrong, but anything that holds moisture in such part spells "rot" to me... Although having sweet sliding would be amazing.


    That's an interesting idea. Do you know any boats that have such arrangement? I could use some pictures.


    Okay, now that is a major re-design. Maybe possible to do, but I don't know. I'll have to go back to DelftShip and try this hull you're describing. I know very little about modeling of such hulls (5 panel, I think?), whereas (I think) I got hang of 4 panel Vee hulls.

    Again, if you could give some pictures of what you have in mind, I could try to estimate if it's within my capabilities in the timeframe I have.

    By the way, do you have a comment about poor forward crew member? Any idea how to give him more space? (I can't move thwart back, it will mess up CLR/COE and LCB/CG balance)


    Other than that, thank you for your criticism. Don't hold back on me. Be relentless. I want to learn.
     

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  2. laukejas
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    laukejas Senior Member

    messabout, please take a look at this. I sketched up this quickly in Delftship. Same LOA (3.3m), same beam (1.3m), same displacement (210kg).

    [​IMG]

    [​IMG]

    Bottom panel width follows 55% of top chine width as you said. I didn't understand what was meant with 22% for lower chine, so I just skipped that and sketched what looked good to my eye.

    Is this what you had in mind? I must admit, I like such hull shapes more than Vee's, but I have very little knowledge on how to design one that performs properly.
     

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  3. John Perry
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    John Perry Senior Member

    A couple of comments:

    I think the two hulls that you have drawn in recent posts will have differing hydrostatic stability in roll. The original single chine one looks to have good initial stability whereas the second one with the flat bottom panel looks more 'tippy', but against that it might well perform better than the first one given a crew that is able to compensate for the tippyness by adjustments to crew position. Presumably the software you are using will output a plot of righting moment vs. heel angle which could be interesting.

    I see that you have made the freeboard at the bow the same, or almost the same as that at the stern. I find that with my own boat, even though the freeboard is higher at the bow than the stern, most of the water that splashes on board comes from forward, so I think there is some sense in having higher freeboard at the bow than the stern.
     
  4. laukejas
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    laukejas Senior Member

    I understand that. The rounder the hull, the less initial stability it has. I don't have a problem with that.

    As for righting moment plotting, I don't really know how to set it up in software. I actually just calculate it with formulas.

    Not a problem with freeboard. I haven't yet worked on that.

    I'm just considering now if it's worth doing this major re-design into 5 panel boat. It would be much easier to build, stronger, maybe even lighter. Would perform better in light winds. Even looks better.

    It's just that it's a lot of work to re-make the model and the plans. I need to be absolutely sure this is worth it.


    P.S. I ran some calculations. It seems that this flat-bottom hull has the same wetted surface as the Vee one at 0° heel, and even slightly larger when heeled. Maybe this is due to the fact that the upper chine is above waterline, so in fact what we have here is 3 panel underwater body?

    P.P.S. It also has somewhat lower stability at maximum heel angle, but not horribly lot (LCB is 26.7cm off the centerline compared to 29.1cm on the original hull).
     
  5. WindRaf
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    WindRaf Senior Member

    Double chine is better;
    double chine is better for two reasons: when the boat is tilted
    the friction surface does not increase as with the single chine, and the shell is more rigid.
    In your design is better enlarge the stern, so it does not dive with the single sailor.
    The problem of static stability and center buoyancy - gravity, of a boat that weighs 40 kg with crew inside of 70-140 kg can not be solved simply with calculation of the centers of the displacement empty boat, or the total displacement like if the crew was exactly sit on the center of buoyancy.
     
  6. laukejas
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    laukejas Senior Member

    Okay, I'll enlarge the stern. Right now, stern is like 80% of maximum beam. What number should I aim for?

    Well, I calculated center of gravity and center of buoyancy with both crew members onboard. If that's not the right way to do it, then how do I do it?
     
  7. WindRaf
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    WindRaf Senior Member

  8. laukejas
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    laukejas Senior Member

    You do seem to love that boat, don't you :D

    Well the problem is if I go with similar hull shape and arrangement, I can't see fitting 2 people in there comfortably with proper trim (no dragging of transom). My boat is 3.3m, after all. That is pretty much as long as I can make without going overweight.

    Let me show my latest attempt. Widened transom, moved rocker 5% forward of midships, moved maximum beam 5% aft of midships (middle of the boat, not the maximum sectional area). Maximum sectional area is 4.5% forward of midships. Not sure if that is the correct way to do it - maybe I should move maximum rocker and maximum beam points forward and aft of maximum sectional area, not midships? Messabout, if you're reading this - I took these numbers from your recommendations a while ago, could you clarify what should I take as a reference, mathematical middle of the boat, or maximum sectional area?

    Attaching linespan and hydrostatics report. WindRaf, I know this isn't exactly like Argie, but I'm trying to get close while making it possible for 2 people to fit. Please tell me if I'm on right track.
     

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  9. tom28571
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    tom28571 Senior Member

    Well, the oysters were great and everyone had their fill. Back to your boat. The construction I favor for small sailboats is like the Spindrift here: http://www.bandbyachtdesigns.com/yachtt3.htm and http://www.bandbyachtdesigns.com/spin.htm It is a design arrived at over a couple years building and using for frostbite racing, rowing and using a small motor. It weighs about 90# built with 6mm occume in S&G. The seat/tanks have the crew low enough to avoid the boom when tacking and keeps your butt out of the water, which invariably gets aboard. Of course they also allow rapid righting and sail-away after a capsize. Make no mistake, small sailboats do capsize.

    It is similar to a combination of your Pic 1 and Pic 6. The numbers of these spindrifts built is well into the hundreds. The basic construction is used in sailboats from 8’ to over 20’ and varies only a little over this range.

    I have also built both a 10’ Spindrift and a 15.5’ Windmill similar to your Pic 5, which I prefer for racing as more comfortable hiking and greater buoyancy for rapid recovery. The Windmill resulted from a redesign I did for the Windmill Class to allow building in wood/epoxy/glass rather than the original open ply on frame method which is difficult to build at the class minimum weight with (or even without) full air tanks now generally required for safety. The resulting boats are easy to build to minimum weight and are stronger and fully competitive with the best fiberglass composite boats. I was able to increase the bottom thickness from 6mm to 9mm which results in a very robust hull, far stiffer than the older one.

    I built an 8’ tender in this design with 4mm occume that weighs 52#. The idea was that I could lift it onto the top of a 24’ pilothouse cruising powerboat. The weight is OK but the bulk is too awkward and it is not used as intended.
    The seat/tanks will give far more hull stiffness than the open and braced construction you show along with other benefits.

    No one seems to be concerned about the effect of adults clomping about on wide 4mm panels on the bottom. Even 6mm has a lot of bending and generally get more support in the form of stringers in similar sizes. The seat/tanks will reduce the bottom panel span and since bending varies exponentially with span, will be much stronger. Your latest design proposal is even more problematic in this regard with a panel span of about 30” of flat 4mm ply exposed on the bottom.

    I agree with others that much of the internal bits of bracing are superfluous, especially on vertical panels where they add weight with little other benefit. Look at the relative weight of backing the seats with foam versus the wood bracing. I use foam glued to stiffen the underside of the 4mm seats in the 8’ dinghy and that works OK.

    I don’t believe I’ve seen whether your plywood is waterproof marine grade or not. If not , the added weight of making it work well in the marine environment may be greater than the added protection needed, not to mention the added weight.

    At 30% greater beam, you boat is subjected to greater torsional stress that Richards dinghy, which is very narrow and needs external buoyancy. Richard, I resist the temptation to call them training wheels and so, will not do that. The catamaran influence, perhaps. Light weight is certainly possible but the attention to structural detail required is inversely proportional to the weight.

    This has perhaps rambled on beyond everyone’s patience and I’ll just add that I do think some time spent in study of materials, structures and stresses would be in order before drawing more shapes that may not be compatible with the needs. Your computational computer skills blow me away.
     
  10. Richard Woods
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    Richard Woods Woods Designs

    Don't worry Tom, I call them "training wheels" myself

    Actually you don't need the inflatable tubes, as this video shows

    https://www.youtube.com/watch?v=IpPCZq5CuWo&list=UUhKTQtbKN5BaXFTg2BjcbqA

    But to me it makes sense to put the buoyancy on the outside of a small boat if possible. And also having all round fendering on a yacht tender is great (after all its one reason for the popularity of inflatable dinghies)

    I have built one of my 8ft Crayfish dinghies in 3mm ply. I think it weighed 30lbs, made from 2 1/2 sheets ply. It worked well, the one in the bottom photo with 3 adults plus luggage is built in 4mm ply and was 6 years old when the photo was taken

    http://sailingcatamarans.com/index.php/designs/1-beach-cats-and-dinghies-/1-crayfish-8

    Too many dinghies are too heavy. The first boats I built were plywood International Moths, not catamarans, so I learnt early on how light a wood boat can be

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  11. tom28571
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    tom28571 Senior Member

    I was just messing with you Richard. Yes I know that we can build some pretty darn light boats and I have worked with 3mm on boats as large as a 20' tri with tortured amas. Built the 40' wing mast of 3mm (15" X 5" teardrop). The 8' dingy I mentioned had a run in with hurricane Irene and got punctured in the bottom. Probably would not have been more than surface damage with 6mm ply. 3 and 4mm ply is not very impact resistant. Large expanses on the hull bottom is my main concern with thin plywood. If I actually use this dinghy much, I would add some reinforcement to the bottom. Its fine as long as care is exercised but stuff happens and dinghys are victim to most of it.

    Edited: Looked at some more of your videos. I always admire your design work. The capsize/recovery looks very good and simple. I'd think the bag buoyancy would need to weigh less than the skipper to allow an inverted boat to be righted. How do you handle that? Vent valve on the submerged bag? Great concept for kids and beginners but, as an old foggy, I can't cotton to it for myself. On the other hand, I probably aught to embrace the bags in my dotage.
     
  12. laukejas
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    laukejas Senior Member

    Well, this looks like a very studry construction, Tom, but if I were to make something like this, my boat would go totally overweight. Spindrift 11, which is closest to my boat in size, weights 38-47kg, and that is using Oakum. The only quality plywood I have access to is 35% heavier. Even if I used 4mm instead of 6mm, there is no way I can make boat with such construction for 2 persons that would weight under 30kg. If you think it's possible, please tell how.

    Well, what would you have me to to solve this problem? As I've said, side tanks weight too much. Again, please keep in mind that I cannot go over 30kg, no matter what. I can change boat length and beam, but it must fit 2 adults comfortably.



    I'm sorry if I appear dismissible of your proposals, Tom, it's just that I have very strict building requirements which I wrote in the first post on this thread, and I can't see how I can meet them by following the path you're suggesting. That may be so because of my inexperience or misunderstanding. Please forgive me if that is the case. I'm very grateful for your help, and I'm trying to be open minded about this, so I'd greatly appreciate if you could tell me exactly how I can make boat rigid enough while staying withing stated requirements.

    For reference, I'll give weight estimation. The hull shape I've attached in my last post (with linespan and hydrostatics file) would weight 15.3kg, including ONLY bottom panel, both chines and transom. Nothing else. That is using 4mm ply (6.5mm for transom). So I've got only about another 15kg for seats, thwart, structural sections, hardware, epoxy, paint, and everything else. Half the weight limit is already spent. I can't see staying within 30kg if I add structure like in Spindrift. Please correct me if I misunderstood you...
     
  13. tom28571
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    tom28571 Senior Member

    laukejas,

    Perhaps our main points at issue is the plywood material you plan to use. Baltic birch is 40% heavier than occoume. That is a weighty penalty to have to accept with your stringent requirements. Several boats built to my Windmill redesign in Finland have used occoume plywood and it seems strange that it is not available to you next door in Lithuania.

    Of course, I think everyone knows when you say oakum that you are referring to occoume but oakum is known to us as an impregnated rope material used in caulking ships. That would certainly make for an interesting boat planking material.

    No worries about having differing opinion from mine as that is sometimes epidemic and not a serious concern. No matter, I would not want a boat with a large unsupported flat panel(s) of 4mm ply on the bottom.

    When most people go through the design spiral, they will need to make some changes to the original goals or requirements in order to make the best boat that they can. If your weight, material, number of ply sheets, and carrying capacity are all set in concrete, the outcome may well produce a boat that is less than your ideal. It may not suit, but that is the way I see it. Richard's demountable side mounted buoyancy bags may be the better alternative but that apparently has a serious expense penalty.
     
  14. Richard Woods
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    Richard Woods Woods Designs

    Finland is of course some way from Lithuania, but I agree it is surprising he cannot get a lightweight softwood ply. They can in Poland and Latvia. Belarus is of course a different story

    One thing laukejas has learnt is that you can go on redrawing and redesigning for ever. The trick is knowing when to stop drawing and start building, which usually means sticking to your original SOR and not getting sidetracked by new ideas

    RW
     

  15. WindRaf
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    WindRaf Senior Member

    Argie has the midship section further back because the two people are sitting next to each other, not one aft and one forward.
    The center of buoyancy is located between them.
    When there is only a sailor sits a few inches forward and the boat is always balanced.
    Your weight problems, space and strength, you can solve them by a boat of 12 feet and 6 mm plywood ... how?,,, Doing the hull a two pieces to be joined with 4 bolts before putting it in water.
     
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