The Melatelia: light wind dinghy

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

  1. messabout
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    messabout Senior Member

    Eliminating the transom tank or seat makes good sense. Many a hot rod boat has no rear seat and have an open space between the side tanks. Phil Bolger was most philosophic and slightly ascetic when he complained that people want the boat to look like what they think it should look like, design considerations be damned.

    I recall that my Windmill had an open space back there. I had installed a good sized transom flap made of Lucite. I could see what was going on at the transom, and that sometimes caused me to make slight adjustments to boat trim. Not only that but when the flap was allowed to open it could bail the boat if the boat was in motion or on the trailer.

    While I am on the Windmill recollection, class rules allowed the board to be tapered one and a half inches at the leading and trailing edges. Otherwise the board was a slab of three quarter inch ply or an equivalent thickness of glued plank construction. The Windmill never ever had to apologize for its windward ability when sailed well. That slab board apparently did the job.

    In a fit of ambition and enthusiasm, I built a snazzy NASA section board to use in the many non Windmill class, all class bashes, that we had at the time. That board was probably a better one but the boat performance did not change much, if any. At best it might have partially compensated for missing one windward leg shift. My M20 scow had big ole flat slab, 5/16, aluminum plate boards. They were tapered a little at the leading and trailing edge, The Thistle, if I remember correctly, did not have a sophisticated board. My A- cat did have some pretty elegant ones. The Snipe and many other good little boats get by with elementary boards and rudders. ........There I have attempted to make the case for practicality? Please do excuse the well intentioned rant. With all that I confess that the board on my flat bottomed skiff has a nicely configured board and rudder. To what avail, I do not know.

    Carpet in the case has not been a problem. My pet little skiff is nine years old, has an okumee case, and shows no sign of deterioration. OK so the interior of the case was carefully epoxy saturated and it is always dry sailed.. That probably matters.
     
  2. laukejas
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    laukejas Senior Member

    Interesting story! You surely sailed quite a lot of boats. I envy you! I hope my experience is yet to come.

    I've eliminated aft seat. Visually, boat is missing something back there. Looked better with a seat. But at least I got weight under limit. It is now 24.7kg. A screenie:

    [​IMG]

    Now I'm wondering if this structure needs any more reinforcement, or is it finally okay. There is nothing beneath those plywood panels, except for a single 1x15cm plank behind bow cross-section to reinforce against mast forces:

    [​IMG]

    I think transom needs a little timber piece for backing of rudder hinges. Aside from that, do I need to add anything? 0.3kg to spare, technically speaking. Of course, I can still shed some weight. Lower the sheer line, for example (it is now 27cm above waterline, I can lower to 25), or make beam narrower a bit. But lines are starting to love lovely, least for me, so hopefully I won't have to do that.

    EDIT: Damn, I miss that aft seat...

    P.S. Messabout, if you want to pick on me about the choice of sail, now's the time. Just please remember that I must as large sail as possible with mast no longer than 4m. I'd really like not to complicate this any more than necessary.
     

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  3. Richard Woods
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    Richard Woods Woods Designs

    well surely you have room to get an aft seat plank in your car? Why 6mm box sides, 4mm is plenty, all the load comes top and bottom. I'd also have a removable central thwart to sit on when rowing and to stiffen the case when righting after a capsize.

    Agree re foil shaping, look at the Mirror dinghy and what they do.

    RW
     
  4. sharpii2
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    sharpii2 Senior Member

    I think you have a little too much timber in the stern seat. You can probably get away with just two uprights and maybe three seat top stiffeners. Just make sure they attach to the paneling on their edges and not their flats.
     
  5. tom28571
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    tom28571 Senior Member

    OK, I know it is not done to please me but the boat is as I always thought it should be unless you insist on the flat bottom panel which I think is a mistake. Nothing wrong with a V bottom. I also don't understand the attachment to a rear seat thwart since you can't sit in the middle when sailing anyway. I like the open transom for some of the reasons messabout gave. Even when I have a rear thwart, it is open to the transom underneath to allow drainage trough the transom. The idea to have removable thwarts and a transom passenger seat for rowing is good.

    You don't need the vertical piece behind the mast collar and step. You do need a glue strip across the top of all enclosed areas to have something to glue the top panel to. The bow top panel should extend over the mast collar. Ideally it should also have a central king plank but you can't stand the weight. Stress from the mast is borne by the bulkhead, top panel and hull bottom and that vertical piece does almost nothing. You do need a top piece at the transom to resist bending moment from the rudder.

    Richard is right about needing only thin box sides. Even my Windmills only use 4mm on tank sides on far larger panels than these. None of you hull panels need interior stiffening unless they are subject to loading other than in the plane of the panel.

    Speaking of Windmills, the class now allows 3" shaping of fore and aft edges of the DB as well as shortening the bottom of the DB (by 9" I think). Rudder is allowed to have a full foil shape and can slide in a vertical case. Things change but whether the new foils are significantly superior to the old may be argued. The shorter DB is clearly easier to handle and has not been seen to have any negative effects.
     
  6. Skyak
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    Skyak Senior Member

    A fine bit of work on very well considered requirements. Whatever the input here this looks like a fine first design.

    About the current state and issues

    -I favor the three panel bottoms from the previous page for the increased stiffness mentioned and the heeled displacement shape. It is a bit more work but you might find it fits the sheets better. There is one other way to make a nice hull known as tortured ply that makes a stiff rounded bottom but it's a bit of an art.

    -about flotation -the primary function of flotation is that when you roll the boat back over after a capsize it floats high enough that you can climb back in and bail it out. With that in mind, the place to put flotation is high on the sides -flotation elsewhere does not matter. To the absolute minimum, you only need flotation on one side but that may require turning the capsized boat to the right direction (more chance to get cold and drown). With this in mind there is another measure of a sailboat -the recovery angle, how far you can heel without taking on water. If you had decks on the side of your boat and sat on those you could have a recovery angle greater than 90. Good crew can scramble over to the daggerboard and avoid capsize altogether.

    -about weight - my minimized weight boat would have
    -shorter topsides
    -decks down the sides to sit on
    -floatation bags under the side decks
    -the mast would be directly forward of the main (only) bulkhead and the daggerboard would be directly aft of it.
    -the 'deck' over the bow could be fabric

    You are likely concerned that the CE and CLR are unbalanced, particularly for a lug -well this is where I recomend a sprit rig with a jib. We have been through this before I know. To be honest I think balance is overrated in such a small boat. As long as you have weather helm a big rudder fixes all.

    Well, once again, you have done some fine work here and I will be disappointed if you don't think it is good enough to build.
     
  7. laukejas
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    laukejas Senior Member

    That's true, such seats could fit. I also thought of using a simple plank for this job. Only that I don't know how I could make those seats fixable in their places without violating airtightness of side tanks. I'll give it a though.

    So, if I use 4mm for box sides too, that leaves only transom and bow bulkhead with 6.5mm. It doesn't make sense to buy a whole plywood sheet just for these two items (and those guys don't sell scraps of sheets, I checked). Can I go with 4mm for transom and bow bulkhead too? I could reinforce them with some timber, or I can double-layer them. What do you think?

    You mean the triangle cut-out at the top so that daggerboard can be tilted?


    Geez, I was right. I said that once I'll set for flat bottom, you guys are gonna try to convince me to go back to V... But why? Why is flat bottom a mistake? I thought having more smaller panels is stronger, this is why I switched to 5 panel (one bottom piece, 2 lower chines, 2 upper chines) hull. Messabout recommended it. WindRaf also recommended it when he showed me Argie. What's wrong with such a hull?


    Okay, so I got rid of rear seat and tank. No big loss.

    I thought that this piece would give more "meat" for mast partner to screw to. It would also support some of the compression from halyard. But if you're sure it's not needed, I can get rid of it. But what if I used 4mm ply for this bulkhead instead of 6.5mm (as I've said before in this post, there are only few 6.5mm pieces needed, not enough to justify expenses of buying whole 6.5mm sheet)? Should I leave this vertical piece then?

    All right, that's great to know! One less thing to worry about.

    Three panel bottom? I'm sorry, I'm not sure I follow what you meant by that. My hull is now five panel in total.

    [​IMG]

    Do you mean I should break down the bottom panel into three? Seems like a lot of work, is that really needed? Messabout said that having bottom panel 55% of beam width would work. You disagree?

    So you're saying that I should raise side seats to sheer line? Well, I can see virtues of such design, but that adds a lot of weight. I'd probably need another plywood sheet to fit parts that big, which adds to expenses. Also, I'd need to raise boom even higher to give proper clearance, which lowers sail area. I'm not entirely sure all of that is worth it... But I'll consider it nonetheless.

    By topsides you mean sheer line? I can lower it (it is now 27cm at lowest point), but I've been advised against having it lower than 25cm earlier. How low would you go?

    Mast in front of bulkhead - that's interesting. But I guess it violates airtightness of bow compartment, unless I make some kind of "tunnel" for mast to fit into. Which sounds like a lot of work and weight. Or would you forgo the bow compartment as buoyancy tank altogether?

    What advantages would this have over lug? I can consider it, but lug seemed like an ideal rig for my requirements already (large sail area, no need for long mast, easy to make, easy to rig, easy to take down). Could you please try to convince me otherwise? :D
     

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  8. laukejas
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    laukejas Senior Member

    I attempted to make side decks coincident with waterline, like some of you suggested. Here's how it looks:

    [​IMG]

    I don't know, but it looks ugly to my eye. Weights 27.2kg, way over limit. Is this what you had in mind? I can't convince myself this is a good idea.
     

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  9. tom28571
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    tom28571 Senior Member

    Quote:
    Originally Posted by tom28571 View Post
    OK, I know it is not done to please me but the boat is as I always thought it should be unless you insist on the flat bottom panel which I think is a mistake. Nothing wrong with a V bottom.

    Geez, I was right. I said that once I'll set for flat bottom, you guys are gonna try to convince me to go back to V... But why? Why is flat bottom a mistake? I thought having more smaller panels is stronger, this is why I switched to 5 panel (one bottom piece, 2 lower chines, 2 upper chines) hull. Messabout recommended it. WindRaf also recommended it when he showed me Argie. What's wrong with such a hull?

    My objection to the flat bottom panel is that a flat panel is the weakest kind of skin structure. The span of unsupported flat panel actually increased when you went to the multi-chine sections. Going back to the V bottom cuts the unsupported span of this panel in half, well, almost in half. Bottom panels on either side of the V support each other because of the angle at the joint.. Arguments about hydrodynamics of different hull sections have almost no importance in a boat like yours.
    ----------------------------------------------------------------------


    Quote:
    Originally Posted by tom28571 View Post
    You don't need the vertical piece behind the mast collar and step. You do need a glue strip across the top of all enclosed areas to have something to glue the top panel to. The bow top panel should extend over the mast collar. Ideally it should also have a central king plank but you can't stand the weight. Stress from the mast is borne by the bulkhead, top panel and hull bottom and that vertical piece does almost nothing. You do need a top piece at the transom to resist bending moment from the rudder.

    I thought that this piece would give more "meat" for mast partner to screw to. It would also support some of the compression from halyard. But if you're sure it's not needed, I can get rid of it. But what if I used 4mm ply for this bulkhead instead of 6.5mm (as I've said before in this post, there are only few 6.5mm pieces needed, not enough to justify expenses of buying whole 6.5mm sheet)? Should I leave this vertical piece then?

    As I said, 4mm is fine for vertical panels and vertical piece on the forward side is not needed. Without a jib, compression loading of the mast is minimal so horizontal stress from the mast is easily handled by the bulkhead and bow deck panel. The step loading is also horizontal and supported by gluing to the bottom and bulkhead. The mast collar does need to be strong and I would add ply piece (a partial king plank) over the collar and forward several inches over the bow deck panel in addition to extending the deck over the collar. The mast collar and step are the highest loaded parts on the boat. This construction will be as light as possible and will handle the load.
     
  10. laukejas
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    laukejas Senior Member

    I see. That makes sense. Nonetheless, it would be great if I could stay with single bottom panel, because it makes it easier to put daggerboard box on, easier to place boat on sawhorses when building, flatter surface to walk or rest feet on when sailing... Maybe there is some other way I can support that bottom panel without splitting it in two?

    Okay, I see. Just to check - does this apply to transom too? Because I've never heard of 4mm ply transom. People usually go with 9mm stuff.

    As for extending bow decks onto the mast collar, I'm not sure how can it be done, since my deck is V shaped, just like in Richard Woods' Duo dinghy. To extend it, I'd have to make bow deck flat. Could you please explain how you intend this to be done? I still can't piece it in my head.
     
  11. tom28571
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    tom28571 Senior Member

    If you have a problem installing the DB on a V bottom, maybe boatbuilding is not for you.

    "Flatter surface too walk on or rest your feet on when sailing." I'm confused. Have you done much sailing? I have never considered sawhorse limitations as a determinant of the design of a boat. Wedges work.

    Transoms are often over built. I use 6mm on sailboats up to and beyond 20' when weight is an issue. Reinforce where it is needed not all over the place. What is needed is support for the rudder gudgeons and that is usually calls for a vertical reinforcing piece. Some support across the top of transom is also needed to stabilize it. Transom bottom is firmly braced by bottom, sides and tank walls. 4mm is fine for your transom panel.

    As for extending the foredeck over the mast collar, just do it. 4mm is flimsy stuff and will conform just fine. If it seems awkward to you, just curve the top of the collar. You are making some very simple jobs complicated. Build the boat. Nothing like some practical experience.

    Many boats have actually been built out of paper but none have ever been built on paper. Yes, I am messing with you a bit.
     
  12. WindRaf
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    WindRaf Senior Member

    to stiffen the bottom plate just raise slightly the knee on the stern and then force the panel during assembly
     
  13. laukejas
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    laukejas Senior Member

    Not my idea. I just happen to agree. I'll just quote:



    I can't take the risks here, sorry. I can't just "go and build" something I don't understand. The requirements and limitations (including financial) for this boat doesn't allow me the freedom of try-and-see approach. If I mess up, I won't have enough materials, time or free weight left to do it another way.
    I'm sorry, I don't want to appear rude. But I have to plan this very carefully if this project is to succeed.

    Do you mean I should bend the bottom plate at stern into V shape?
     
  14. WindRaf
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    WindRaf Senior Member


  15. laukejas
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    laukejas Senior Member

    I see. Well, that complicates things. You sure this is absolutely necessary?
     
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