The Melatelia: light wind dinghy

That gap is horrible for various reasons. If you dont want to cut away the beam where it meets the bouyancy tank surely you can make a notch in the bouyancy tank for the beam to go through. You said you did not want to do that because it would make the bouyancy tank leak, but why should it - epoxy fillet round the joints between the beam and the tank and those joints can be no more likely to leak than any of the other joints in the tank

Alternative could be to leave the end of the bouyancy tank where it was before, in line with the aft end of the foredeck but continue just the side deck under the foredeck so that side deck and foredeck overlap a bit and can be joined together with a wedge shaped block between. I think all we are trying to do here is to make a strong link between the side deck and the foredeck, so that we dont get movement between them. Does Sharpii agree?

 

I went down that design path earlier, and then I was suggested to get rid of bow and stern tanks, and instead, go with side tanks, like now.

It's a never ending cycle. No matter what you choose, someone will tell to do exactly the opposite.

Not that it is a bad idea or anything. But I have to make my mind about eventually.

If I understood sharpii correctly, he said this gap would help, allowing some hull twist without putting strain on side tanks directly from mast reinforcements, so that these tanks wouldn't lose watertighness.

In fact, I don't like that gap too. I'd like to simply extend it up to the curved beam, and then screw+glue them together. I don't think it should fail, but then again, with my lack of experience I could be wrong.

As for the alternative you suggested, do you mean I should extend only the SIDE of side tank, or the SURFACE of it below bow deck?

I'm not sure what would be the point of doing that, if the watertight bulkhead is coincident with aft end of bow deck (curved beam). It would only add complications and weight.

But maybe I misunderstood what you mean.

 

Clarification

The mast partner beam can be attached to the top of the tanks. This is best done with mechanical fasteners and lots of bedding compound. These fasteners can be large diameter screws, or even better, bolts.

There will, of course, have to be a backing timber, inside each tank for the bolts or screws to go into. If using bolts, the nuts can be taped to the bottom of this backing timber.

This is one reason I suggested running the tanks 8-15 cm under the bow deck, so there will be room for this backing timber, as well as for the water-tight bulkhead timbers. The backing timber must already be pre drilled and partner beam dry installed before the tank end bulkheads are installed. The partner beam can then be removed, if need be to facilitate installing the tank end bulkheads.

You are correct. It is difficult to connect the bow deck/mast partner beam to the gunwales. Doing so will not add much strength, so I think that idea could be dismissed.

The idea is to distribute point stresses over as much area as possible, while keeping in mind other practical considerations.

 

Okay, thank you for clearing this up. It's a relief! So, I removed the gap:



And another screenshot with tank top removed to see the timbers inside (they are 2x2cm, let me know if that's not enough):



I'll be generous on screws/bolts and epoxy there.
So I guess you meant that these leaks are a danger only if I had bow bulkhead like before, because basically now the design is as it was then, only without bow bulkhead, with separate watertight bulkhead for each tank instead.

Please let me know if this is what you had in mind.

Also, just a side note - you suggested two non-watertight bulkheads inside side tanks, but I'm thinking that maybe it would be better to have timber cross-beams instead? This would also support deck tops from sailors weight - I'm not sure that 15cm wide 4mm plywood deck will be strong enough on it's own.
A picture of how it would look:



I think they would be easier to install than bulkheads, and they would make installing sides of tanks much easier, acting as a guide on position and angle. I should probably also make some small blocks on the bottom of the boat for this purpose too, like in this picture:



Should I add such blocks?

 

I would make the backing timber deeper (extend down further) and I would make the vertical timber in the tank just as thick as the backing timber. Then I would tie the two together, inside the tank, with a triangular gusset.

I would also install plywood faces on the forward side of the partner beam, which would extend a few cm past the most inboard bolts, on each end. These would be to keep this beam from splitting.

Two bolts on each end would hold the partner beam to the bulkheads.

Better yet, as a further refinement, I might add two triangular gussets, one on each end, on the aft side of the partner beam, then attach them to the tank ends, gluing them there. The arms of these gussets would be equal in length to twice the width of the tank bulkheads, as these bulkheads seem a bit narrow in your drawing. These would do an even better job of distributing the stress.

I would still use the bolts, though, as well as these.

 

To my eyes the gap at the end of the fore deck is not very nice visually and would be better if swept right down the side deck. This puts the panel joint on the king plank, which can have a decorative strip inserted to tidy it up and key it together. It is possible to fill the gap in with a shaped pad, plenty of older designs have done this.

One thing that seems to have been forgotten is being able to move the mast. Both at the heel and at deck level. If this boat balances first time out, you will be very, very lucky. For athwartship support of the deck level mast position, I'd probably put another cross brace to the inwhale/gunwhale. Also make the mast slot a rectangular slot not a circular one to allow chocking and adjustment. When I've used this technique, no fasteners at all have been necessary as long as thought has gone into how the loads are transferred. Primarily, the king plank takes the fore aft - a dovetail into the cross beam is good, and the rear of fore deck brace(s) to the inwhale and gunwhale.

You don't need to be as big as 20 X 20 for the side tank forward supports, 15 X 15 is plenty and they can be triangular for a lot of their length. Easily done with spokeshave or router with 90 Deg V cutter. Normally there is not too much angle to plane to get a good fit for the side tank panel.

 

I hope I understood you correctly. I extended the horizontal timber further down, removed some thickness to compensate for weight. Now it is 3x1.5cm. Vertical timber can only be 1.5x2cm, else it won't fit between the chine and side deck. Also added the gusset (a 1.5x3 piece):



I hope this would be enough. I'm fighting the weight as it is.

I'm not entirely sure if I understood you right, so here's a picture of what I've done:




First image is without the bow deck, obviously, and the second one below that is without curved partner beam (and it's plywood piece). As you can see, I've added plywood pieces to prevent splitting, and the aft one of those pieces includes a gusset, with it's depth for two times the bulkhead width.

Boat is now 0.5kg overweight. So tell me if I got it right, so I'll trim down the weight (reduce boat dimensions a little), and start working on the plans.

A side note - this project will obviously require a lot of carefully beveled timber pieces... It will be a lot of precision work with block plane. I don't know if there is some quick magical way to do that without bandsaw. Please tell me if there is.

I hope this is what you meant.

 

You really came close to what I have in mind, but there is only so much the written word can convey.

So I have drawn a free hand sketch to make my ideas clearer.

(See attachment)

As I hope you can see, a lot of materiel in your drawing can be eliminated. Yeah. My gusset looks uglier than yours, but it is stronger for its size.

You also don't have to clad the entire Partner Beam with plywood, on both sides, to get the job done. Not doing so could pare away a lot of the half kilo extra weight.

So I drew independent Gussets and anti-split patches, and positioned them where I think they are needed. Anything more adds minimal extra strength along with disproportionate weight.

I think most will argue that what I have drawn is too strong, thus too heavy. But if it is, it is only marginally so. I think few will argue its too weak.

There's no bigger heart breaker on the water than a mast step failure, with a free standing mast. Such failures are often very difficult to repair.
With your boat, this is likely to happen at the tank ends. This is why I have been so insistent that this area be re-enforced.

So if there is any place to error in favor weight and strength, this is the place to do it.

The Partner Beam Gusset can be glued to the Tank Bulk head. Or it can be fastened to it with bedding and small mechanical fasteners, such as many boat nails or small screws.

The advantage of this approach is that the tank sides can be installed first, along with their clamp timbers. Then the end bulk heads can be added. Then the Partner Beam Gusset and Partner Beam.

The biggest problem is going to be getting everything to line up just right, where the Tanks and Partner Beam meet. This is one of the reasons I originally suggested putting the Partner Beam on top of the Tanks. The idea looked good on paper, but you pretty much proved it wouldn't work. The tank ends were too narrow, and the beam ends were too thin. So this piecemeal approach is what I think is the best alternative.

If things don't line up perfectly (they probably won't) this method is still likely to work.

 

Thank you for further explanation and sketches, sharpii. You're really putting a lot of effort into a project that isn't even yours. However, your valuable contribution makes you nothing less than a fellow designer - more than a dozen good ideas that are in this boat now came from you. I am most grateful for that. I wish I could invite you to sail together once this is over...

I have modified my design per your recommendations, as much as I could from your sketch, which, by the way, is drawn very clearly.

Using arm length two times of bulkhead width, the gussets came out pretty large, so in the end, it weights pretty much as it did before, but no problem with that - I'll shave off some weight elsewhere.

I haven't drawn bolts in the side tank/partner beam intersection as you did, but rest assured, I have checked that all the needed parts (tank bulkhead, tank timber, gusset, partner beam, anti-split patch) overlap in the right places so that bolts can go right through without being too close to the edges of any of these parts.

The only thing I don't know if can be done is the bolt you drew that connects mast partner to the beam. I don't see how would I install it, considering that the mast partner has a hole for the mast, and a bolt can't be inserted there, else it will interfere with the mast. And I couldn't bury the bolt head in there too, because I won't be able to drill a larger hole from inside of the mast partner to accommodate the bolt head.
Maybe woodscrews + epoxy will be enough to hold both parts together?

Here's an updated picture, with bow deck removed from view:

 

The bolts should go on either side of the mast hole in the partner.

If you drew two lines 90 degrees to the Partner Beam, having each line up with edge of the mast hole at its widest point, you would know where not to put the bolts. The bolts would have to go outside of those two lines. Each bolt would be placed roughly halfway between the line, on its side of the partner timber, and the widest point of the timber.

Being that you decided to use a piece of timber as your mast partner, you can probably get away with using two lag bolts, installed from Partner Beam side.

 

What are these gussets made of? If they're just one layer of 4mm ply like some of the planking, they won't have sufficient stiffness for that job in that location. I'd double them to 8mm, or alternatively give them a solid timber edging to get some reasonable edge stiffness.

Alternatively, use the same amount of 4mm ply and the same amount of solid timber to build a web beam (I beam idea) instead. Split your mast beam in two. Bend the upper section to the deck camber, and run the lower section straight. Use 4mm ply to fill the gap, and to simultaneously give sufficient bonding area to the bulkheads. It'll be as strong as hell and no heavier.

You haven't thought your mast step through either. You have a massive amount of wood sideways and bugger all fore an aft. With an unstayed rig, the forces you have to deal with are much the same in all directions. You're worrying about bolts at the partners but have given no thought to fore and aft forces at the step, which takes loads that are almost as great (since mast bury is such a small proportion of mast length).

Fortunately wood is very strong if used correctly. In engineering terms the loads you have to deal with here are quite low. For example, Douglas Fir has a tensile strength of around 15,000 psi and a specific gravity of around 0.55, so on a tensile strength to weight basis it's almost three times as good as stainless steel.

You could lose the bits of sideways blocking that protrude past the centre block (the one with the hole in it). They're adding nothing useful in terms of strength. You could then bond on some timber with the grain running fore and aft. This can be on top, with a hole in it, or on the sides. Ether would work, and would control any fore and aft splitting forces at the step.

Or, since the sheer strength of even a lightweight wood like western red cedar is up around 1,000 psi, you could realise that a reasonably sized block of that glued straight to the plywood bottom (which has grain in both directions and is your limiting factor anyway for strength) would be perfectly adequate without any additional stuffing around.

Running your keelson underneath the mast step is only raising the mast heel, thereby decreasing bury, thereby increasing forces at step and partners. It'd make much more sense to just notch the keelson in 20mm or so and run the mast heel right to the bottom.

 

They're made of 4mm plywood. Actually I think this whole setup would work even without these gussets, so it's not like they take all the toll. They just help a little.

If I understood it right, I tried modeling it, and it turned out a lot heavier. Remember, my plywood is 700kg/m^3 whereas timber is 450kg/m^3... It also seems more complicated to build, although I'd agree it would be stronger.

Very sound advice. You're right, I haven't thought it through very well. I think I'll re-make the mast step in a cross-shape: a sideway-oriented plank, two shorter longitudinal planks front and aft of the first one, and a long longitudinal plank with a hole for mast on top of that, connecting all three planks.

You see, the problem is that the mast step will have to be beveled, due to incline of bottom hull panel. The more I extend it longitudinally, the more I have to bevel, and the thicker it has to be.

NoEyeDeer, here's a screenshot of re-done mast step, just as I've described before. Everything made with 3cm thick planks. I cannot use any thinner, because of the bevel needed to fit on bottom panel.

Okay, understood. I will add those holes as you suggested.

 

Another quick question, if I may. I'm sourcing materials right now, and found I can buy glued timber boards (with epoxy), which should be very useful when making foils, because I won't be making NACA shapes. I'll just take those wide boards and cut out foils, then round and taper edges, etc.

The problem is that these boards are only sold 18mm thick. So for daggeboard, which is 35x110cm, I'll definitely glue two of these boards together for a total thickness of 36mm, which gives 10% camber. Sounds just about right. I've done my research and found out that similar boats of this size have 1'' boards, which is close to mine.

However, I can't make up my mind about rudder. It is 21x72cm, so, it has chord length almost twice as short as daggerboard. So it would make sense to use 18mm board (NOT glue two together), and have 8.5% camber.

But, thinking of it, rudder often operates at higher angles of attack, so, it makes sense to have larger camber. Again, boats of this size usually have 1'' thick rudders too. In my case, if I were to glue two boards, I come up with same 36mm, which gives 17% camber. That's a lot... So I don't know if I should stick with 18mm or 36mm rudder.

Can someone please give a short comment on that?

 

If you aren't making proper sections, just make the board as thin as possible. This will work better with a rounded nose/tapered tail board. The extra thickness is only useful if you use a wing section so the flow will go around it.

 

You mean, use 18mm even for daggerboard? Isn't that a little too thin in terms of strength? For example, if I'll step on it while recovering from capsize?