Low-speed sailboat hull

Maybe you have gone too far down the design spiral now. Since light weight was the main concern, how about eliminating all the buoyancy tanks etc, and replacing with floatation bags? You can buy ones for optimist dinghies, for example. Or make some ripstop nylon bags for around styrofoam blocks. Like NoEyeDeer says, a floor grating can be removeable. Put all the extra available weight into length. Why? This will make it easier to load onto the roof rack.

For some reason I was reminded of Dave Carnell's $200 sailboat, and came across this interesting blog:
http://ginzorama.blogspot.co.nz

 

I'm wondering why strict 30 kg weight limit, If you set empty hull weight limit to 40 kg you could make it maybe 4 m long and have larger more comfortable boat that potentially could take 3 people for sail. Loading 40 kg shouldn't be that much harder than 30 kg boat. I have managed to put 7 m long 60 kg canoe hull on a roof rack alone without too much trouble and I am not super strong person.

 

He mentioned it back here:

 

That's strange, I thought as long as your roof cargo aren't massively oversized (like putting 10 meter mast on a roof of a compact car) cops won't bother with it. I have friends who often travel through Lithaunia with lots of stuff on the roof and I have never heard anyone complaining about cops aking to remove their stuff and put in on a scale.

 

Maybe he's worried that the boat will look like it's more than 35 kg, so figures it had better weigh less, just in case.

 

I don't know. So it's either side decks, which allow something to sit on when hiking in higher winds, but without stern and bow tanks (so I need some different mast step setup), or bow and stern tanks with double outer gunwale like Sharpii2 suggested.

I'm really not sure which one is better.

I looked up that boat. It surprised me that it has no strong structure to support such length. Even gunwales are just simple timber pieces. Yet this boat weights a lot. I guess Dave used very thick plywood.

Well, I'd rather have built-in flotation. Somehow it makes more sense, and it adds structural integrity if made right. Anyway, tough styrofoam is not available in our country. I looked for it intensively several years back, and the hardest stuff that I could find was easily broken with bare hands.

I'm pretty sure cops won't actually weight the boat to check if it's under the limit, but I'd rather not risk having to argue with them, trying to prove that it is. I know a guy who got severe fines for overloading his car top in terms of weight, not dimensions.
My car manual clearly specifies that 35kg is the limit for roof rack. So, 30kg for the boat, and several more kg for spars, oars, etc. And maybe a kg or two for reserve.

Anyway, 30kg seems already heavy enough to be difficult to put on a car single handed. I work out at the gym currently, and I know how much a dumbell of 30kg weights. Not very easy to lift, and raise above head. It would be even more difficult if such weight was spread over 3 meters, I believe. Or to carry from car to the lake for several hundred meters.



So, after trying so many design options, I'm at loss again. Should I go with bow and stern tanks and outer double gunwales, or get rid of them and install side decks, running most of the length of the boat? I had hoped to combine these two, but it seems it's too much for my weight limit. First option seems more attractive, but I'm not sure.

 

Third alternative: bow and stern tanks, with side decks that don't have tanks under them but can have oars stowed under them.

Seriously, try a few mock ups. See how the spaces work with what you want to carry in the boat. If you want a pair of oars and two adults, see how they all fit. Figure out what you are happy with, since you will always have several options to chose from.

 

Oars under decks? But decks are curved, as you can see from screenshots. How could oars fit?

 

Depends on length of oars, amount of curvature, width of decks, etc, etc.

Anyway, main point still stands: mock some stuff up and see what you think would best suit you.

 

Easier said than done. I'm deep into studies right now, I have absolutely no place to do this mock up, no garden or workshop or anything. Nor enough time, for that matter.

Maybe after New Year. Hopefully. It gets very cold here.

By the way: if you suggest having decks without side panels (which turn them into tanks), then how should crew sit in light winds on the bottom of the boat? The decks will carve into our backs/necks. If not them, then the structure to support decks (frames).

 

I was just suggesting another option, which has been used on some boats. My preference would be side tanks, with decks carried to the ends, and lose the bow and stern tanks. You'd still have a bit of foredeck to hold the mast partners, of course.

Oars have been stowed along booms, but it's not something I'm all that keen on. You could just have a couple of paddles instead, which are shorter.

 

I tried designing it. Is this what you had in mind? Weights 22.5kg. Not much of a gain... Looks kind of weird. Previous design (with bow, stern and side deck tanks looked more boaty).

http://static.dyp.im/XJtexSXkOf/f997b772ac7287dec4f769a135b97e41.JPG

 

A problem I see with such long, narrow side air-boxes is that if any water got in them, through a leak in the chine joint, perhaps, it may never be discovered. Inspection plates are the answer to this, but you may need two to three on each air-box.

For me, adequate flotation is the order of the day. This can be done even with foam that you can destroy with your bare hands.

In the attachment below is a section drawing of one of my scow designs.

As you can see, it is exposed, but placed under the narrow side decks. It is adequately secured to the sides of the boat, so it can't float away.

Except for were it touches the sides of the boat, it will never trap water, and is easy to inspect and remove, if necessary.

The long narrow air-boxes you have drawn will add considerable rigidity to your hull. They probably will need two or three internal bulkheads, per air-box, to prevent twisting.

 

No, with the same foredeck you already had. This makes sense for structural purposes and ease of building, and will look better.

Basically, carry the side tanks forward to where the mast is. At the mast, have a partial bulkhead that caps the ends of the tanks and provide a bit of support for the foredeck. Deck it in forward of the mast. Or you could have a full bulkhead at the mast if you want to, turning the area under the foredeck into a bow tank, but I don't think it's necessary. None of this should cost you any weight over what you have just drawn.

By losing the stern tank you get more room in the boat. This will give more room for two adults to arrange themselves while rowing and will make fitting the rudder, or clearing weed off the rudder blade, easier. The side tanks will provide plenty of buoyancy, so the stern tank isn't required.

The other thing is that if you end up pushing the thing offwind in a breeze (and nobody can resist that sometimes ) not having the stern tank will allow the crew to sit out while getting their weight aft.

 

You could get away with one, and if the boat is built properly it shouldn't leak anyway.

Web frames, not full bulkheads. With limber holes too, just in case.