The Melatelia: light wind dinghy

Thanks, Suki, I'll keep that in mind when choosing adhesives. It's not easy to find anything marine grade here, so maybe it's safest to stick with epoxy.

Please take a look at my current arrangement. 4mm floor is supported by 3.4x1.5cm keelson and two 3x1cm bilge runners (blue).



The largest unsupported span of bottom is 15cm.

Is that enough, or do I still need to add something to strengthen the floor?

 

Re. the rudder on my proposed rowing boat you may be right. I am hoping that it will be big enough for straight line rowing on the sea but I guess that I might need to make an alternative larger rudder for rivers. This is a pure rowing boat, a sailing boat would definitely need a much larger rudder.

A couple of years ago I had the chance to try out a sliding rigger rowing boat and even though I only rowed it for a couple of hundred yards I liked it. However, for me and Josephine I would like to be able to carry an optional passenger and also some lightweight camping equipment, hence the design shown in my earlier post which will also provide a choice of sliding rigger or sliding seat rowing modes.

The picture below shows me trying out the sliding rigger boat - it was built as a prototype by a boat builder who was thinking of making the design a production boat but I dont think that worked out and a friend of mine acquired his prototype. The 'rudder' blade on that boat is just a small aluminium plate, I think it was about the size of the rudder blade shown in my drawing. It did not steer the boat, it just dropped down in a slot at the stern and was quite effective in helping to row in a straight line. If you needed to make a sharp turn it was best to pull a string to raise it first.

In 2012 Josephine and myself rowed our heavy sailing dinghy the length of the river Thames in the UK. We were in company with some other row boats and canoes and the advantage of having a second person to steer was very obvious - the row boats with single occupants were frequently hitting the bank or getting stuck in trees after failing to get round the tight bends in the narrow upper reaches of the river. For that kind of trip I think it helps a lot to have a second person to steer, and also so that you can change rowers every half hour or so. Hence I have provided a rudder control knob to steer with, this can also be used to lock the rudder for sea rowing.

Re your latest proposals for stiffening the floor of your boat I think it is looking a lot better now, but I would not like to promise you that it will be OK, I suspect it may still be a bit marginal. I am trying to remember how the floor of a Mirror dinghy is stiffened - I think that is also 4mm ply and I think it has two internal stiffeners plus external strakes and it is a Vee bottom which provides stiffness at the centreline. Presumably the bottom of a Mirror dinghy is strong enough for normal use but I did hear of a heavy person stepping right through the bottom of a Mirror dinghy! Maybe the floor was rotten, I dunno.

 

Cool boat. Looks kind of futuristic. It reminds me of those long and slim sculling racing canoes. But I'm not sure why you chose such method of rowing. If I were to make such a hull, I'd choose usual face-front canoe seating and a single double-ended oar. Unless you're racing, of course

Well, what else can I add to make it even stiffer? Cross-members like in your boat, or some more longitudinal ones?

I should get some cheap 4mm ply and try to put my foot through it. I'm trying to search for some boatbuilding (not canoe-building) videos with 4mm to get an idea of how flexible and strong this material is, but I can't find anything.

 

The early Mirrors are actually about 5.5mm floor and had a single floor batten. the later ones use two full length floor battens. I repaired one which had holed on a trailer because of this lack of second batten, so after doing the floor panel, I put an additional (matching original style) batten each side.
A bit easier than removing the inner transom mahogany and replacing that....

I think your floor with external batten will be just about stiff enough. Make sure your 4mm ply is tight maybe a 4 ply? The Mirror ply early type is 4 ply and good quality gaboon. Watch the edge of the deck to hull joint - I'd glue a very small triangular fillet piece along the inside of the hull to help with glue width and stopping the deck ply cracking. Only needs to be where you will sit on the sides. Too hard to get in there and fillet later plus lighter!.

 

That's a relief to hear. I hope others will agree with your opinion that floor is stiff enough with current arrangement.

As for those triangular pieces, I think I have it now, if I understood you correctly:



A simple timber strips, 1x1 on tank side and 1x1.5 on sheer line (a bit thicker because of high bevel angle, to give more meat for screws to dig in).

Is this what you had in mind?

 

The pic above shows the concept just fine. You only need this where the weight is. I have found this to prevent early longditudinal cracking of the deck ply. You will probably need a couple of braces to help vertical support too. If you were to combine a ply (lightened with holes or scalloped) vertical at the point where the daggerboard is you will significantly increase the torsional rigidity. One reason the inwhale helps is that it allows you to shape the gunwhale (outside face) to a comfortable angle to help you sit out without losing strength. I've seen pro built boats without this (inwhale), crack in 3 months from new, but where I have used this technique (with 5mm deck ply) it has stood up for years even with guys 110Kg+.

I'd probably put one there and another further aft where you will sit out when one up. Also a few small cross tie supports. It is important to work out the gunwhale layup and capping to help keep a light strong structure. As you are finding, especially with small boats, every little detail counts......

Your illustration should show the foredeck ply overlapping the gunwhale, I think? You won't need the inwhale under it unless you plan to sit on it! If you have the deck ply extend over it you can plane it back and then you have a choice of how to do the edge capping and create a nice flattish angled down gunwhale. This allows better comfort for the back of your thighs, a simple round or hard edge is pretty painful even for a few minutes....

 

You sure about those vertical braces? They will be hard to make (you know, beveling the ends of them to fit angles of bottom and deck panels. Nobody mentioned that up to now, I thought ply would be strong enough as it is now...

The inwale terminates at bow bulkhead, so it doesn't go underneath the bow deck. The bow deck joins with side panel without any timber. I think I should be able to access bow compartment enough through the bow bulkhead hole to give enough epoxy filet on the inside.



There is a practical building issue that I'd like to ask advice for. As you can see from my drawings, the side decks are curved shapes that will be glued and screwed together from pre-cut plywood panels. I'll make sure that the ends of these panels coincident at transom, but the question is, will they coincident at bow bulkhead? If the difference is only a few millimeters, then no problem, the small gap between the edge of shorter panel and bow bulkhead can be filled with thickened epoxy. But these ends misalign by a lot, I'll have to trim longer panel, and then I'll have to move bow bulkhead too much aft - and it will be too small there to fit between hull panels.

I'm not sure if my explanation is clear, so here's a drawing of what I'm asking.



I don't know how should I proceed with this. Maybe I can cut those side panels a little longer, and then trim those ends with block plane little by little until they align and the bow bulkhead is aft enough to fit. But this is extra work, I'm not sure if it's necessary.

Could someone share some experience on the this? If not for the bow bulkhead, then I wouldn't worry about it. But it has to be at it's exact location, or else it won't fit. So I must be sure that those deck panels will coincident without a gap that can't be filled with epoxy.


P.S. With all the warnings about how hull should be reinforced if I am to use 4mm ply, I'm wondering why no one is expressing any concerns about transom. It is also made from same 4mm ply, and has another 4mm ply shape sandwitched together to provide more strength and backing for rudder hinges.



I actually was talked into using 4mm ply for transom. But is this really enough? Just wondering.

 

End capping the side tank.

If it were my boat, I'd make end bulkheads for each tank and then place them back at least 15 cm from their front ends.

I would not attach the end of the side tank to the bow bulkhead and expect a water tight seal

The reason for this is because the mast partner is attached to the bow bulkhead, and the bow deck is separate from the tank tops. This in effect creates three separate (two tanks plus the bow) box beams that are joined at the very point of maximum torsion stress. On top of being expected to stay intact, this joint is also inspected to stay water tight.

I would then do away with the cambered bow deck in favor of a flat one, which could follow the sweep of the sheer. This way the side tank tops can be joined directly to the bow deck. I would even consider adding re-enforcement patch, which would extend at least 15 cm past this joint in each direction.

(See attachment below)

There seems to be little justification for making the bow deck cambered or pitched. Doing so simply adds more complexity to the building process, and maybe even more weight. If you insist on keeping it, I suggest you eliminate the bow bulkhead and replace it with with a timber beam, which will sit on top of the side tank ends.

If you decide to do it that way, I suggest you move the water tight tank end bulkheads an additional 15 cm back.

As your boat is presently drawn, you are going to need at least two additional bulkheads inside each tank in order to hold their sides in the correct position. These inner bulkheads need not extend to the bottom of the boat, nor do they have to be water tight. They have three jobs:

1.) maintain the angles between the tank sides and top,
2.) define the curves of of the tank sides, and
3.) provide some support for the tank tops (side decks).

Here's a partial bulkhead I drew for a pdracer design of mine.

(see attachment)

As you can see, I avoid side air tanks like the plague.

I prefer foam or bow and stern compartments.

The foam is guaranteed float, no matter what. and end compartments do not have to be absolutely water tight, because they are much easier to bail out. They also double as much handier storage compartments.

 

Forgive me, I do not understand the reasoning behind this. It seems logical to me that if parts extend to meet at common joint, the joint is stronger. If these side tanks are terminated before they reach bow bulkhead, the torsional forces are no longer transferred directly, but through hull panels and gunwales. Around.
It almost seems to me like you're suggesting me to prepare the boat for some disaster in which the parts around the mast would suffer such a catastrophic failure that would violate the integrity of side tanks and sink the boat. If so, what are the odds of that happening, anyway? I can't see a reason for the mast reinforcements to fail. In any kind of over-the-top wind the boat would capsize long before something would break.
My reasoning for this common joint is that the strength added by it would help preventing breakage in the first place.

Please tell me if I'm wrong, I'm just trying to understand the reasoning behind your suggestions.

I was told that cambered surfaces are much stronger than flat ones, that's why I chose cambered. I don't think it would be hard to make this. I'll install bow bulkhead, add some timber strips, and then simply bend, glue and screw the bow deck piece. Then add some epoxy fillets from inside.

Another reason why I had to make cambered deck is because I need mast partner considerably higher than sheer line. The larger the distance between step and partner, the lesser leverage mast forces produce.

As for joining side tanks with bow deck, I am right now attempting to curve the side tanks up a bit so they can be made flush with cambered deck. It isn't working out well, though (too heavy, and parts don't nest on plywood sheets).

You're probably right, I should add this. It would also simplify installing side tanks onto the hull - using those bulkheads, I could align and glue both side tank pieces before I mount and glue them to the hull.

I'll try to make some bulkheads with the same 4mm plywood. Hopefully I can do it within weight limit (I have started to shrink boat dimensions to stay under it, and it isn't a good thing).


Thanks for your detailed suggestions. Please forgive my suspicious questioning, but as someone said, I can't build boat by consensus. I have to choose which advice to take, so I'm trying to understand the reasoning behind every piece of advice.

 

Sharpii2, please take a look. Designed some bulkheads from that 4mm ply, with holes in center to reduce weight.

Even with them, these ******** add 0.4kg to the total boat weight. I'll have to reduce beam or something to compensate.

Is this what you had in mind?

 

Something along those lines, referring to the small frames. However you need to make sure that the bottom is not a complete stop - water must drain to the stern/drain or you will get rot in the tank. So you need to create a drain path with limber holes or a suitable passageway. For a drain 'bung' just use a small doubler of ply (seal with epoxy) and put a Ø22mm or so hole in stopper with a wine bottle cork - one of teh lightest and cheapest effective solutions.
You don't need to touch the middle plank with the support, but do need more depth directly under the deck. I think your positioning in line with the daggerboard case (bow side) is good and will help transverse stiffness a lot.

Regarding the bow decking, the ply can extend over the inner part of the (outer) gunwhale to increase glue area. The transom will need light outer framing to be able to take the loads ie rudder etc properly.

 

If you take a good look at my last screenshot, you'll see that those side tank bulkheads (frames) do not extend down to the hull. So the water will be able to flow nicely along the entire length of those tanks.

I'll add those drain holes as you suggested. I thought having those big hatches is enough to ventilate the moisture, but if for any reason a larger amount of water accumulates there, it makes sense to have a drain.

I'm reading this for the 10th time now, and do not understand which part of the boat are you talking about. What middle plank, which support? Could you please clarify?

Okay, I'll think of something there.

P.S. I'm not sure what kind of transom framing on the outside would be best. What shape am I looking for? Right now, I made a quick preliminary sketch. Those bluish points is where rudder hinges will be attached. Is this what you had in mind?




P.S. Another idea. As I've extended bow deck over gunwales as you suggested, I thought - maybe I should extend side decks too? Then I wouldn't need inner gunwales (inwales), because side decks can have decent gluing and screwing surface on gunwales.

I haven't done it yet, but here's a picture to better visualize what I mean. The side decks would overlap gunwales just like bow deck now does. Should I do this and get rid of inwales?

 

Your boat is starting to look quite 'normal' now. Indeed, it is becoming so normal that you could probably find any number of similar boats on the second hand market for next to no money, but I can understand you wanting to make your own.

Re the stiffeners on the transom I would have thought that it mainly to top edge of the transom that might need stiffening, so I am not sure that there is much advantage in having stiffeners elsewhere.

Re the support of the daggerboard case, below is a possible arrangement which might be a bit lighter and less of an obstruction for your feet, just a suggestion.

I think you do need some timber to reinforce the joint between the sides of the daggerboard case and the hull bottom. The inside bottom corner of the daggerboard case will be subject to wear so you may want extra fibreglass tape here, but first cut away some of the wood around the corner since if you allow the fibreglass to build up on the corner I think it tends to stop small stones falling out, then the board jams (I have seen this problem).

If you stop the keelson at the end of the daggerboard case without tying it in to timber along the sides of the daggerboard case I think you will likely get discontinuities in the curve of the plywood around the ends of the daggerboard case.

ps - I forgot to show it in my sketch, but you might like to include 'limber holes' in the cross members fore and aft of the daggerboard case

Just thoughts

 

2 sailor boat under 30kg? Haven't seen much of those But anyway, what you say might be true in US, or some other large country, while in Lithuania, boat market is pretty much empty with as much as 1-2 boats showing up each year...

Okay, I'll re-do it. The problem is the sculling notch, but I'll think of something.

Thank you for this very detailed model you made, John. It must have taken some time. As for the keelson, I'll update my design as you suggested, that makes sense.

But as for the knees, I'm not sure. When I made those knees before, people here said that I should extend them up to the top of the daggerboard case to maximize the lever arm. That makes a lot of sense. If I were to make this as you drew, then the top of the daggerboard case isn't supported. Could you please comment on that?

As for using plywood for those knees, well, I'm not sure either, because since I use 4mm plywood, it might be not strong enough to survive an unfortunate kick during some involuntary gybe or capsize. That's why I wanted to make those knees really big and strong (better to hurt feet than break the boat).

I'll include limber holes too

 

Here's another approach to reinforcing a thin ply (3mm in this case) transom...