sealing a buoyancy tank

I've started building my 1st boat (a 12' jon boat), and am planning on having a buoyancy chamber under one of the seats. I'm planning on taping the seams of the hull with 6 ounce cloth, but don't want to fiberglass all around the inside and seat if I don't have to. I'm using Meranti and Titebond 2. Can I get away with careful measurements and thick beads of glue? Is there a caulk I could use? My boat is already getting heavier than I planned, and I'm trying to save as much weight and money as I can, but I don't want to compromise safety.

 

Titebond 2 does not fill gaps.

Did you design it yourself? Have you considered where buoyancy will be located if the boat fills with water?

 

Do some calculations about the amount of positive buoyancy that your under seat tank will provide. One tank may be enough but positioning is more important. Better to have a tank at the bow and at the stern. Better still is to have side tanks that extend the full length of the boat.

Measure the size of your tank in inches......example; 10 inches wide, 13 inches deep, 48 inches wide. That gives us 6240 cubic inches. Now multiply that by the constant 0.0361. That gives us 225 pounds of positive buoyancy. That is probably enough but if you are out of the boat and attempt to enter from either end, the middle tank will do very little to support the ends while you attempt to climb back aboard.

Boats usually capsize by going over sideways. With sufficient side tanks you can easily climb back aboard and there will be little water left in the boat.

Where you put the flotation matters greatly in an unfortunate instance where you might need it.

 

TiteBond 2 (PVA based) is not a suitable Marine adhesive IMHO. There are quite a few decent suitable adhesives depending on the construction technique. Among the favoured ones are epoxies, urea formaldehydes, resourcinols, phenols. The first two would be my personal favoured ones and will yield good results.

My take on the buoyancy tanks is probably better as bow and stern. Side ones are OK but can hold a boat (if at 90 deg) a bit too high in the water to climb into. When righted the water does not slop fore/aft quite as much either with end tanks, in my experience. With respect to Messabout, it is more side deck width sometimes, rather than side buoyancy tanks that prevent water ingress at say 100 deg tipped position. Full inversion is a bit different, and his prognosis is correct.
The boats beam can give you the stability to climb back aboard, so it is important to be able to grasp something inside the boat to help pull yourself back into it. It can be as simple as the thwart or some other rail.

A lot of small boats have used buoyancy tanks built in under the thwarts, across the boat. Usually with one or two sub sections serving as lockers for anchors, safety knife, flares etc etc. This arrangement works OK along with small end tanks. You need to do the calcs for boat, person/people, motor etc to ensure it still floats when swamped.

 

Ix nay to the titebond 11 u glay. Also, don't use titebond 111 either. Too brittle, won't stand shock/movement due to temp/humidity changes and impact, and works poorly on end grain. Tape/epoxy the seams unless you're prepared to use a sealant and screw into frames you'll have to make.
Either that or go with foam flotation and just hold it in with a couple of wood pieces. This is most practical as it doesn't involve a sealed box which might allow the foam to take up moisture over time. Paint the foam and it will look fine.

 

Thank you all for the good advice. DCockey, I took the Puddle Duck design but at 12' long (Puddle Goose) and w/o the rear rocker. Instead of side flotation (as in most), I chose the front and back for buoyancy chambers. At the transom I will have 12" by 12" by 47.5", so 6,840. Up front I have a bow curve, so I'm still working on internal volume calculations. I like the side tanks because it might be easier to right the boat, but it really narrows the inside area.

 

If you use expanded Polystyrene as a form of buoyancy, one trick is to coat it with a Latex adhesive coating to seal it. Believe it or not you can even apply polyester resin with cloth around it without melting the internal expanded polystyrene. If you don't - it melts super fast!.

Strangely enough, I have just split a spar with 'glued' up 'repair' with a Titebond equivalent this afternoon. Wretched stuff, it is a bit like chewing gum in the joints, no strength just elastic. It is an old Sitka Spruce gaff but it will live again when glued up with something more suitable!.

 

As SukiSolo notes, use a "foam", and one that is suitable. We have used foam to create a constant buoyancy chamber on vessels before, works well. Tanks, can be punctured. This is assuming you wish the space (volume) to remain positively buoyant at all times and will not be used for anything else.

 

check the USCG regs- buoyancy tank may not be legal - use foam

 

Tinpin, you have said that you will use the PDR layout except that the rear half of the bottom will be straight. You must be figuring on a planing power boat. The front half of the bottom will have a continuous curve from amidships forward just like the PDR or PDG. I have some reservations about that.

Your better plan would be to take a look at a typical 12 foot Jon-Boat. Their forward bottom curves upward in front but the curvature does not extend all the way back to the mid section of the boat. There are about a zillion of them out there so it wont be hard to find one to scrutinize. The longer straight bottom will work better if you plan to power the boat sufficiently to plane. Never mind that the Jons usually have some flare in the sides. Your plumb sided boat will work just as well.