The Melatelia: light wind dinghy

I'm not sure I understand, but I modified my model according to what you wrote in regard to side decks. I also copied John's suggested model with those gussets modified to reach halfway to the top of the daggerboard case. I also added timber and gussets to the tank sides.



The inside view:



I'm not sure whenever inwale is required, since tank tops extend over gunwales anyway.

Is this what you had in mind?

As for the flatness of side tank tops, I don't think that's possible. The sheer line has sweep, and side tank tops must follow that sweep so that they can rest on gunwales. If I were to make them flat, they'd have to terminate below sheer line. That would create a ridge which makes no sense and will hurt butts of sailors when hiking.

Anyway, the sweep is really minimal, I don't think there would be problems bending timber strips and deck tops.

I'm not sure what to do. I really want the ends of side tanks terminating at bow bulkhead. I tried terminating them before they reach it, and it looks horrible, creates a gap, and adds weight (because side tanks now need additional caps to make them watertight.

I would prefer to go with second option, making side and bow decks flush. But I'm not entirely sure what you meant there. Do you know some photos or of some boat that has the arrangement you propose? I could try something, but right now, I'm not sure what to do exactly.

It sounds like it would be curved inwards, then. When I try to picture it, it doesn't look like a boat...

Please take a look at my previous screenshot (side view). To have upper mast support this high, I'd have to make this raised mast partner really high. It would need some gussets or something. It would look really horrible.
I know aesthetics isn't number 1 in case like this, but still... Not to mention this arrangement is 1kg heavier compared to curved tanks.

Isn't it just possible to strengthen the side tank-bow bulkhead joint to make sure it doesn't lose watertightness from mast forces? I don't know, more epoxy, or some timber ant joints?

Why have it on the inside when it can do two jobs (structural and abrasion prevention) on the outside?

I'm not sure what are the arguments for having those runners on the inside. After all, I'll have to car-top this boat. If I have bilge runners, they will take the beating while I raise the boat on roof rack (I will transport it upside, not upside down). It would protect the plywood. And even though I won't drag the boat over rocks, things happen, don't they?


I don't know, the more I look, the uglier this boat looks now. It looked much, much better with curved tanks. Sharpii2, are you sure there is no simple way to install curved tanks? I thought of marking a line on bottom panel where tank side must lie, then bend that tank side, stick in place with heavy duty duck tape from outside, angle using those bulkheads and by sticking with tape or some temporary wood blocks at each end (at transom and bow bulkhead). Then add epoxy fillet on the inside, wait to cure. Then add timber strip on the inside top edge, bevel it like you suggested earlier, and at last, put tank top on it.
Wouldn't that work?

P.S. Damn, I forgot to add holes in daggerboard structural timbers for draining. Don't tell me, I know! Will add them now, just too lazy to update screenshots.

 

While waiting for reply, I've also updated transom. Removed double-ply and added external framing instead. These two frames will be cut from 1.5cm thick board:



Added two sculling notches, because one looked very bad (I can't put in on the centerline because the upper rudder hinge must then be lower, which reduces rudder force lever arm, which is bad, I guess).

If I use Birch, transom will be 4mm, if Okoume - 6.5mm. In either case, is such framing sufficient? Maybe 1.5cm is over-doing it, maybe 1cm would be enough?



P.S. by the way, I calculated that changing daggerboard box supports from the large gussets to the current arrangement didn't save any weight - in fact, it is slightly heavier.

 

I prefer the curved sides, no more difficult to build if built in and supports the first chine. The transom framing does not need the full vertical pillar if you go over the top and round the edges ie inside the tank(s). It IS strong enough without the vertical or even a knee, well at least 37Kn on the sea says my own craft is strong enough.... Mind you I think I'm 35mm thick on these rails but a lot more powerful boat and the rudder fittings (fixed blade) are screwed not bolted.....

If you transport right way up, is it on a trolley? Better upside down for aero on the car. I just don't get slowing a boat down under water I suppose. A bit of glass on the edge of the chines is probably enough reinforcement for most reasonable usage. Older designs used a brass or aluminium reinforcement band screwed in along the chine and along the keel for protection. In fact the latter is still worthwhile and very small penalty for some types of even racing craft.

Again personal, I preferred your earlier daggerboard case supports, but you can put lightening holes in them.

 

What do you mean by "built in"?

I am sorry, I totally didn't understand what you meant by "go over the top and round the edges inside thanks". Could you please re-phrase what you meant about transom reinforcement?

No, no trolley. The problem is that the boat is a bit wider than roof racks. If I transport it upside down, roof racks happen to partially reach top of side decks. I'm not sure it's a good idea of having the whole boat weight concentrate on a very small surface area of these thin side decks.

If I transport it right way up, then the roof racks are wide enough to accommodate the whole bottom panel, or bilge runners, if present. Weight pressure spreads across larger area. I believe that is a good thing. It also leaves ends of roof racks free so I can lash ropes to them.

As for aero, I can't think of a reason why boat orientation (right way up or upside down) should influence air drag. There is a considerable distance between car's roof and roof racks (10cm or so), so the air will flow through the gap between roof and boat anyway. So as long as the bow points the right way, what's the difference which side is up?

Oh, man You're not making this easy for me. I'll think about it. Anyway, it's not the timber that makes up the lot of weight, it's the plywood (because it's denser).

 

You put the side tank sides in frames (and curved) before you build the hull round them, this gives perfect internal joint. Note as previous post you need to get the boat off the frames so will need to sacrifice temporary frames or design collapsable frames - except transom and bow tank.

[/QUOTE]
I am sorry, I totally didn't understand what you meant by "go over the top and round the edges inside thanks". Could you please re-phrase what you meant about transom reinforcement?[/QUOTE]

Put a rail across the top of the transom as per your sketch but inside face. However also continue the thickness around the inside of the transom down all the chine faces and across the floor. Then remove the vertical support in your sketch, the cross rail which ideally is one piece across the floor is strong enough. Currently your design would most likely part at the transom to floor joint if you step in the back of the boat. Certainly would after a short time.


[/QUOTE]
No, no trolley. The problem is that the boat is a bit wider than roof racks. If I transport it upside down, roof racks happen to partially reach top of side decks. I'm not sure it's a good idea of having the whole boat weight concentrate on a very small surface area of these thin side decks.

If I transport it right way up, then the roof racks are wide enough to accommodate the whole bottom panel, or bilge runners, if present. Weight pressure spreads across larger area. I believe that is a good thing. It also leaves ends of roof racks free so I can lash ropes to them.[/QUOTE]

You'll need some sort of custom cradle then, so as not to stress it and hold securely. Not forgetting if you load the bilge runners by lashing down, it may distort the hull, there is no water to support the ply.

[/QUOTE]
As for aero, I can't think of a reason why boat orientation (right way up or upside down) should influence air drag. There is a considerable distance between car's roof and roof racks (10cm or so), so the air will flow through the gap between roof and boat anyway. So as long as the bow points the right way, what's the difference which side is up?[/QUOTE]

Won't matter if a few Km but 1000+ will affect car handling in strong cross winds and fuel economy. Boats on trailers behind cars are already in 'dirty' air of car and actually behave very well right side up.


[/QUOTE]
Oh, man You're not making this easy for me. I'll think about it. Anyway, it's not the timber that makes up the lot of weight, it's the plywood (because it's denser).[/QUOTE]

Yup, the ply weight is actually more in the glue as well as species. I'd prefer the solid with good grain orientation because it has less end grain to seal and you know exactly which direction it is stong in.

 

Well, I'd like to make this boat by traditional stitch&glue method, without any jigs or frames. Stitch and glue the hull, and then add side decks. First the side, with lots of epoxy fillets on the inside, then add timber strips to the top edge, and finally glue and screw deck surface on top.
Is it possible to do it this way?

So the timber pieces would go around the perimeter of transom, right? I can do that, sure. But there will be epoxy fillets between transom and hull panels, as well as floor. Why would this area fail? A lot of boats don't have any kind of timber between floor and transom, and they seem to work fine.

I hope I can find a way to do this without any cradle or transporting beams. Just put the boat on racks in some position, and lash it down.
Do you have any ideas how can I do that?

Well, if the difference will be felt only after thousands of kilometers, I don't have a problem with it

 

You will be using the transom as a frame? Bow tank? If not it will be pretty hard to get the hull fair. I have found that if you build the side tanks in whilst upside down and add the side internal tank/deck supports the hull will stay very stiff when you turn it over. Do you intend a cradle to support the shape when turned over? You'll need something I'd guess, I've had enough problems with stuff moving unless held in some sort of jig/cradle etc which is partly why I prefer to put the side tanks (not deck) on whilst upside down.

The Arabs sew their Dhows together then add the frames, but generally building in the West is the reverse. Maybe personal but I prefer the control and accuracy of frames, even stiff cardboard is enough for some parts. Try working out how to hold the correct Datum horizontal for your build ie vertical especially as no floor or ground is truly flat. The transom and bow tank give you the X and Y dimensions but how are you going to hold them correctly in the Z? and parallel too......

Epoxy fillets are heavier than solid timber so add more weight if you want. A combination of reasonable size solid and small fillets is more effective. You need area for the glue joint at the transom especially as that joint is not in shear. Also a reason why the cohesive strength of epoxy is a good idea there.
Microballoon (maybe mix with a little microfibre) the internal fillets ie in the tank to save weight. Use saw dust (gaboon coloured) for the ones that show to get a neat effect.

 

Well, I plan to do this the "traditional" stitch&glue way, like in this excellent video I found some time ago:

https://www.youtube.com/watch?v=Wuoy1dGOxFI

No jigs, right way up building on table or two sawhorses, using spreaders to hold hull panels in correct shape until gunwales and bulkheads are installed.

So, I plan to do it in this order:

1. Stitch hull panels and transom together, using spreaders and bow bulkhead to hold it in correct shape;
2. Use epoxy to glue hull panels, and transom together;
3. Glue bow bulkhead in place;
4. Glue and screw keelson and gunwales;
5. Install daggerboard box with all reinforcements;
6. Remove spreaders.

By this time the hull should be plenty rigid to serve as a mold to install side tanks. It would look like this:



7. Using duct tape, bend and fix the tank sides in place. The daggerboard case support beams would serve as a perfect tool to ensure correct angle;
8. Epoxy fillet side tanks from inside;
9. After it cures, remove duck tape, add timber strips to the top of the inner tank sides;
10. Glue and screw tank tops on top.

Well, that's about how I plan to do this. Seems logical, I can't see how this could go wrong, but if you think this is a bad idea, please tell why.

 

Some clarifications

Your most recent top view drawing shows pretty much what I mean with the tank sides. The only difference is I wouldn't have the tank top seam, if any, in the same spot as the tank side angle joint.

As for the bow deck, I meant that, by just following the the sweep of the sheer, it would have some fore and aft curve. It would serve all its usual functions, other than shedding water over the sides of the boat.

designed as I suggested, it would shed water into the cockpit.

To mitigate this problem, I would put a timber piece across the aft edge of it, on top. This piece could be made sturdy enough to hold the mast partner as well.

This flat deck would stiffen the hull more than a pitched or cambered one, and would vastly simplify construction.

Yes, you can make your tank/bow joints strong enough to be water tight, but it would take more massive timber clamps, on the top, sides, and the bottom of this tank end to do so.

I would not trust fillet/tape joints to do the job, as this joint is bound to flex under press of sail. These timber/plywood joints would be best held together with mechanical fastenings and some kind of flexible filler, such as roofing cement, to keep the water out. This is all doable and not overly difficult, but it does add weight.

By going with the flat deck, with a timber beam across the top of its aft edge, I believe you can eliminate the bow bulkhead completely, and just have a cap on the end of each tank.

Previously, I suggested putting the tank ends bulkheads 30 cm aft the tank ends. Now that I think about it, that's not such a good idea. It would be very difficult to reach them, while they are being installed.

Now I suggest installing them only 8-15 cm back, or even at the ends, providing these bulkheads fit inside the tank ends rather than over them. Although this may seem a subtle difference, structurally, it is definitely not.

I would cap the bow deck/ tank top joints with external patches, which the timber beam, mentioned above, would notch over.

Although this idea may not seem very boat like, it should be quite strong and economical in weight and material usage. It would also be much easier to build.

 

I'm really sure how exactly these flat two-part side decks would be stronger. Easier to install - agreed. But I was taught that any curved surface is stronger than a flat one, because flat one has no pre-defined bend side which it "chooses" when a pressure is applied to it, so it may wobble like a flapping sail. Curved surface, on the contrary, cannot be forced to bend to opposite side, and is hence more stable.

And as I've mentioned, flat two-part side tanks are much heavier than curved ones...

I'm really not sure about this, sharpii2. Your advice on bow bulkhead makes a lot of sense, but as for side decks, I don't understand your reasoning.

An example I know well:

Since I'm a guitarist, I've seen a lot of classical guitars. As you may or may not know, guitar body is subjected to a constant and significant forces from string tension, as much as 60kg. Designed to withstand this tension for decades, and to achieve this with minimum amount of structure, the top and back decks of guitar are slightly curved. The amount of curvature varies per instrument but in general, it is proven to work.

But I've seen guitars which had these decks completely flat. It only takes a couple of years for these decks to deform. Sometimes they curve outside, sometimes inside (this is what I meant by no pre-defined bend side), and sometimes they just wrinkle. It always leads to instrument becoming unusable, and sometimes even up to the point of breaking.

I believe same principles should apply to any construction. This is why I'm trying to have as much curved surfaces as possible, as long as it doesn't aggravate building process too much.

While I want to have a curved bow deck, I could eliminate bow bulkhead like you suggested, replacing it with timber beam which runs from gunwale to gunwale, and cap the ends of side tanks with plywood patches.

I don't know. I'll try to stay with bow bulkhead for now, and add these big timber clamps on the bulkhead-side tank joint. I'll see if I can come up within weight limit.


Bad news - the Okoume guys refused to sell me any of their marine plywood. When they heard I only need a few sheets, they said they don't deal with small fry as myself. They sell 50 sheets at a time as a minimum. And they ensured me there are no re-sellers.
I also found a small Lithuanian sailor boatbuilding community, asking if they know any sources of Okoume. They confirmed there are none.

So I'm back to Birch ply.

 

Update

I tried to solve problems you sharpii2 and SukiSolo mentioned without making major changes to the design.

1. Added 2x2cm timber clamps to the side tank - bow bulkhead joint to make it stronger and hopefully less leaky (side tank top removed for better look inside):



2. Added inside framing (2x2cm) for transom-bottom-side tank joint to save on epoxy, permit screwing between those parts and make it easier to install side tanks:



3. Reduced external transom framing to 1cm thickness, but left it basically the same. My reasoning for top edge frame is to strengthen it for sculling, and vertical pillar is meant to give some backing for rudder fittings to screw to, as well as transferring forces to top edge and keelson:



4. Moved side tank bulkheads so that they are roughly between transom-daggerboard case and daggerboard case-bow bulkhead, since those are least supported areas:



5. Added backing for mast partner in form of bow bulkhead top edge reinforcement (mighty 3x5cm plank), as well as king plank (1x5cm). Hopefully this will reduce the load on the bow bulkhead, allowing side tanks to stay watertight:




The total "dry" weight (no epoxy, no paint, no hardware) reads 25.7kg, which is a little too much, but hopefully I'll be able to save on epoxy, since there are now far less fillets to make.

EDIT: maybe I could make some lightening holes in parts like mast backing plank, king plank, etc ?

Please understand that I cannot please everybody, and much of the advice given here, no matter how sound and reasonable, often contradicts itself between different opinions. In the end I have to choose one thing or another, never both.

I don't mean to sound like an ungrateful ******* who comes here, asks for advice, takes everyone's time, then says "you're all wrong" and does it his way. I really appreciate all the help you give me. It is a lot, and I'll probably be never able to repay it. If it weren't for you all, I'd still be stuck with a project that is doomed to fail before it even reaches water.
It's just that it ain't easy, being the one to decide and ultimately, taking responsibility for these decisions.

I know there is always a better way to do the same thing, and people always disagree on it. But it can lead to a never-ending pursuit of design perfection. And the time is running out. Please, just tell me if there are some serious problems with my current design. I want to finish it at last. As they say in my country, "Better a bird in your hand than a moose in the forest".

 

Classical guitar tension (6 string standard scale) is normally closer to 40Kg. Steel string on 645m scale length closer to 80Kg (mediums) and 70Kg (lights). If the top starts to deform whether domed or flat, it is more a function of bad bracing. I've seen a few too! I can assure you that my first prototype carbon backed acoustic (spruce top) steel string is still fine after over 20 years. The later ones are true professional instruments and have not displayed any structural faults.

I've reset necks and changed bridges too on a number of domed top guitars so they DO move over time, not just flat tops. The worst fault is dishing down above the soundhole - then the guitar is doomed.

 

you might ask the supplier who their regular customers are and see if they will piggy back your order on top of theirs. If they for example order 100 sheets, and you need six, they place the order for 106 sheets, and you pay the big wig for the six sheets. you might even offer to drive the delivery truck if it does not take any special commercial license in your country.

Not sure it is worth the trouble, the birch ply is perfectly good too, but it might be away to get a few sheets of what you want. You would also get a better price since the usually the larger order gets a lower per sheet price.

if you are concerned about weight you might just eliminate all of the boxing of the side tanks, and just depend on the fore deck and under floor for floatation. It means you would have to sit on a gunwale if you hiked out, it is not so bad if you are not up there for very long, for pleasure sailing you usually site down on the floor most of the time anyway.

 

Well said Petros. I've doubled up with builders before to get 'free' delivery of adhesives and stuff before now. Seems a bit odd if the ply manufacturers are local that there are no local stockists, somebody is using it, maybe a boatbuilding company you can buy it off.

Here's a link to another stitch and glue multi purpose dinghy. Note the floor battens and the under side deck solid cross supports. As this is relatively close in concept to your craft it may be useful. For accuracy of hull shape I prefer to use frames, but then I'm after +/- 1.0mm on shape at this sort of size. I don't think you can control it tightly enough with stitching to get this. Also ply panels tend to bend a little differently, even from the same batch.

www.youtube.com/watch?v=bEe8n4ML-P0

 

More clarification

I've gone ahead and sketched a top view of my proposed fore deck ideas.

(see first attachment)

I've also sketched a section view showing how I think the dagger board case should be supported. I don't think you need the support running up the side of the tank, like you have drawn. I'd end this timber just short of the tank sides, rounding off or even tapering the ends, just after the bottom stiffeners.

It has occurred to me that you may have too much buoyancy in your tanks. The risk here is that, once you right the boat after a capsize, you may not be able to get back on board. This is because the boat will weigh significantly less than you. Trying to get back on board over the sides may simply capsize the boat over you. Trying to get in over the transom may flip the boat over on end.

So, in my sketch, I show the water tight tank bulkheads far inboard from the tank ends, making the enclosed length about 1.25 to 1.5 m long. This would enclose approximately 43 L in each tank, which, combined, is more than double what the boat is supposed to weigh.

The re-boarding procedure would be to right the boat and then pull the windward transom corner down, then pull yourself in.

This should work, because a significant amount of water would still be in the boat, once it's righted. This water will slosh to the stern as you try to pull yourself in. The bow will tilt up high, but should get nowhere near vertical. kind of like a motorcycle doing a "wheely". Once back on board, you can move toward the middle of the boat and bail the water out. Keep in mind that, while there is a lot of water aboard, this water itself will support a lot of your body weight.

As drawn, my sketch may require three bulkheads per tank. Two water tight ones, and one non-water tight one at each tank end.

The portions of the tanks which do not enclose air can have permanent openings cut in their sides, which could reduce weight some, but also help them serve as handy cave lockers to stow gear in.

If you want to keep the the cambered bow deck, just run the open tank ends past the end beam or bulkhead. Attached are drawings of a stitch and glue scow on which I wanted a pitched deck. Hopefully this will give you some idea of what I mean by "running the open tank ends past the end beam or bulkhead". As you can see, I was dealing with a very similar situation to the one you are.I ran the side decks through the end bulkheads, tapering them to a point well past these bulkheads.