The Melatelia: light wind dinghy

Interesting, thank you.


A breakthrough. I've managed to find the sole supplier of Marine Plywood in my country. They have a monopoly, and they make only the highest grade. And they have Okoume.

I have contacted them, and they said they make custom size sheets. Any size.

So I've quickly re-made my model, using 6.5mm ply for bottom panel, transom, bow deck, side deck sections and side deck bulkheads. Total weight came up at 23kg due to lighter nature of Okoume.

I've sent them dimensions of my nested plywood sheets ( 6.5x3000x1500, 4x3200x1300 and 4x2300x1300), asking for prices.

I'm waiting now for them to calculate how much this order would cost, but my preliminary calculations show that it might cost twice as much as if I were to use 3 sheets of 4mm birch plywood, as I planned to do up to now.

I should also include transportation costs - these marine plywood guys are based 100km (62 miles) away, and transporting plywood on car top is very fuel-consuming trip.

Could someone please give a figure of how much marine plywood costs in US? For example, 4mm Okoume. How much $ per square meter? I estimated that it's about 11$/m^2 of 4mm ply. Is that right? And these guys in my country charge around 17$.

All in all, I'm not sure if it's worth it. What do you guys think? Should I pay twice for lighter Okoume and have some parts on my boat thicker, or should I stay with heavier birch, pay less and strengthen thinner plywood with timber and stuff, like in current model?

 

depends on how long you want the boat will exist

 

Well, 10 years would be a good start. I'm pretty sure that birch plywood is also very high quality stuff in terms of longevity.

P.S. I estimated that the ply would cost me around 245$ if I choose Okoume, and around 130$ if I choose Birch. I'll have to wait until Okoume guys write me back to know for sure.

 

here there is a big big price difference between the full sheet and cut pieces, I fear that it will be so to you

 

What do you mean? Full sheets cost more per square feet, or vice versa?

The Birch guys sell normal and oversized sheets at the same price, only full sheets, no pieces.

The Okoume guys sell only custom size sheets.

 

i mean that the cost, in mq, is more in cut pieces

 

Okay, I see. Thanks for information.

Does anybody else have any comment in this Birch VS Okoume choice?

 

Well, maybe five minutes or so. I have been doodling with computers for decades, not for work but as a nerdy sort of hobby, so I have probably gotten quite quick at it.

Well, you should be able to check the strength of the daggerboard case. For highest stress while sailing, the righting moment of the boat with the crew as far off centre as they are likely to be, together with the estimated height of the center of effort of the sail will give you the side load on the daggerboard. For a safe calculation you might assume no side load on the rudder. The estimated depth of the centre of pressure on the daggerboard together with the case height then gives the side load on the top of the case. I assume that you have a vertical piece of wood in each end of the case, you might assume, again for a very safe calculation, that these pieces take all the bending load due to the side load at the top, this load acting on the cantilevered length from the top of the case down to the top of the plywood that links it to the cross member. The safe bending load for various species of timber is available on the internet and in some text books. Alternatively, since you said you are using Solidworks, perhaps you could use the built in FEA?

My guess is that the arrangement shown in my previous sketch would be strong enough for normal sailing loads, but if it is not you could add a few more bits of unidirectional wood as sketch below. I was assuming that the plywood linking the daggerboard case to the cross members would be more than 4mm thick. If you used two thicknesses of 4mm glued together would that be enough not to break if it gets kicked? But if you prefer your original design with knees carved from lumps of timber that's fine, it does look more traditional.

Loads higher than normal sailing loads could occur in a mishap, but if you are trying to save weight you may not want to design for such a situation. I once watched a dinghy sail into a quayside that was invisible being submerged at high tide. The course of the dinghy was perhaps twenty degrees from the edge of the quayside, so the daggerboard was forced sideways as it slid along the edge of the quayside. The daggerboard snapped off and floated behind the boat, which is probably better than having it smash up the boat. To design for that situation just base the side load on the bending strength of the daggerboard.

 

Thank you for those suggestions. Right now, weight is right on the target (25kg with Birch 4mm plywood, 23.5kg with Okoume 4mm and 6.5mm), so I don't need to lighten the boat unless I make some major changes. Right now, it looks like this:



If I'll go over the weight for some reason, I'll reduce the height of knees like you suggested.

Added some more support to the transom:



So, even if I use Birch 4mm ply, the top edge of transom is 12mm thick. I guess that's enough.


John Perry, what's your take on the plywood choice? Should I spend twice as much for Okoume marine plywood so that I can have thicker bottom, transom and decks, or do I stick with Birch highest-grade plywood, pay less and have thinner yet somewhat stronger parts?

 

Personally I would be much happier with 6mm Gaboon (Okume) floor WITH internal batten, and forget the external bilge keel. I Know this would last 20+ years without excessive weight. If you get really crafty you can laminate a light timber with a thin durable capping to save weight. John's suggestion about the board case is totally valid, and with a modified added gusset in the side tank would significantly increase stiffness across the craft.

Transom only needs enough to brace the rudder loads to the hull without twisting as well as grounding etc Again I'd probably use a 15 - 18mm (light) solid part frame to achieve this. Make sure the bottom pintle area has reasonable support, this is the one that takes most load.

I'm with Sharpii vis a vis capping all end grain ply where possible. This is not only good practice, it removes a lot of open end grain that water might (read will) get into eventually. It makes for a much nicer looking boat (and longer lasting), integrated. I think my last one only had four bits of open grain (solid timber) in total, the internal ends of the box and the transom ends of the gunwhale laminations.

 

I'm sorry, what do you mean by internal batten and external keel? In my current design, I have keelson (internal) and two bilge runners. What were you refering to?

I can add that gusset, if I understand what you mean right.

Okay, I can attempt to make it as you suggested. A vertical solid piece on transom, and another piece along the upper edge. All on the inside.

 

Very much so. I see they don't extend all the way down to the chine. This is exactly what I had in mind.

I'd make them a little more sturdy because they have to serve as molds, as the inner tank sides are installed.

I still don't like the tanks ending at the same bulkhead that the mast partner is on.

I can easily imagine the water tight joint there failing, as the boat twists under full press of sail.

The reason I suggested putting the water tight tank bulkhead further aft is so the non-water-tight tank ends could have some give to them, allowing some hull twist, probably just centimeters, without the water tight bulkhead joints failing.

I suggested having a flat bow deck joined directly to the tank tops, as another way of moving the high loads away from the structural joints. The bow deck would not really be flat. It would follow the sweep of the sheer, so would be curved fore and aft.

The mast partner itself could be made of timber and installed on top of the bow deck, kind of like a fat bow sprit, facing aft, with a hole in it for the mast. This would help make up for the height lost by having a flat deck. The timber would have to be about 1.9 cm thick. I would then put a layer of plywood on top of that to keep it from ever splitting.

I think the massive timber dagger board case gusset is a good place for weight cutting. A plywood gusset on either end, extending to about half the case height, and that for out on each side, attached to a timber bottom frame, each, should be quite sufficient. It would look somewhat like what was drawn in an earlier post, except the gussets would be somewhat larger. These gussets could be two layers of plywood, if your worried about durability.

As drawn, I think this boat is going to be a bit of a pain to assemble. A major reason for this, IMHO, is the curved tank sides (you can forget about placing an inspection plate on them too). It looks to me like this boat is now going to need a timber building jig, which the transom, the side tank bulkheads, the bow bulkhead, and maybe even a temporary stem piece, are going to have to attach to. This is so both the hull sides and the tank sides will be forced into the correct curves.

This is not normally a big deal, as boats have been built on these for centuries. But this structure, as temporary as it will be, will cost some money and time to build

I think a useful simplification would be to have the tank sides be two pieces each, joined at an angle, where the maximum curve is. Then they will both be straight and flat. And more importantly their clamp timbers on top will be straight also. The tank sides now being flat will need no force to hold them in place, and will now be able to accept inspection plates. The non-watertight bulkheads could then be as light as drawn, because they willno longer be forcing any bends in the tank sides. Built this way, you may be able to get away with straight stitch and glue construction, with a minimal cradle type temporary jig.

This cradle jig will have two upright pieces, one for the forward half of the boat, and one for the aft half. they will be cut to the shape of the boat section that will sit on them. Their purpose is to keep the hull from twisting as it is assembled.

 

From the structural point of view the most important property for the plywood is probably the panel stiffness, although it is not the only thing to consider. The panel stiffness is proportional to the modulus, which represents the basic elastic stiffness of the material in tension and compression but it is also proportional to the cube of the panel thickness. So, for a given weight of panel, a less dense timber allows a greater thickness and that gives a big increase in stiffness. More dense timbers do tend to have higher modulus but that is unlikely to compensate for the greater thickness possible if lighter timber is used. I think that is the main reason why gaboon plywood is the most popular kind of plywood for lightweight boat construction.

The really light timbers, e.g. balsa wood, are not very suitable because their surface would be too easily dented but balsa wood with the grain normal to the skin surface is certainly used as the middle layer of a sandwich with grp or plywood skins. I don't know about paulonia, I have heard that is used on the other side of the globe but is not readily available in the UK.

So, gaboon is probably better than birch, but either could make a usable boat, only you can decide how much you want to spend.

 

I'm with Sharpii on a framework to build it on. This can be made from scrap, except for the parts that remain such as the transom, and bow bulkhead. The angled tank sides will be a pain to accurately shape if put in afterwards. Best solution is to build them in and use them as part of the structure. However becauase they are angled, it means the internal frames will be more complex because you can't just lift the hull off - I'm assuming upside down build. You could either saw through the temporary frames to release the hull or make the frames with screwed on parts that can be released and become reuseable.

Yes, I meant get rid of the bilge runners - replaced for strength by internal floor batten. Nice smooth outer skin. I'm guessing you don't want to drag it over rocks....

You have the two gussets (per side internal tank supports) as shown, but you have not yet allowed water to flow freely to drain so they are potentially rot traps. Apologies, I'd do a drawing but the machine with that software has just lost it's hard drive!

Sharpii has also outlined the mast support area as potentially problematic, which I tend to agree with. It either needs a small king post just forward of the mast or a secondary cross brace to gunwhale behind it, which you do not really have space for. I'd recommend a very small kingplank too, higher vertically than wide to tie in the structure. Without shrouds there will be a reasonable load in the mast support area especially the top of it.