Carvel Plank Rowboat Leak Repair

I have an old 12' flat bottom carvel planked rowboat. It has been leaking for years and Im tired of bailing it out every day. It spends winters out of the water and theres 1/4" to 3/8" gaps between the planks. Theres no cotton in the the seams and I have tried polysulfide caulk and Davis slick Seam with little success. The planks are a little soft on the surface but otherwise structurally sound. I dont want to replace the planks because it will ruin the side boards. I would like to fiberglass the bottom but dont want the hassle of gluing hardwood splines between the planks. Instead I would like to fasten a thin plywood to the bottom for reinforcement and glass over it and up the sides. I have no idea on how to attach the plywood or if the planks would still need to glued together. I was told by another source that I could also fill the seams with 4200 and Elastuff over the planks. Im pretty sure that I dont want to CPES the inside so it can breathe. Asthetics are not a factor. I just want a boat that doesnt leak and will last a few more years. Any suggestions on how to go about this would be greatly appreciated.

 

If you can let it dry well you can add glass and epoxy on both sides.
This will make the boat into a sandwich construction, so you can probably remove all frames.

In Norway we use a soft flexible "putty" based on tar and some kind of rubber ( I believe) in the seams on old boats. It is soft enough to let the planks swell. This would be an easier solution, but it will probably still leak a little.

 

A carvel planked boat relies on plank swelling, smashing caulking in the seams, to remain water tight and maintain hull integrity. When a carvel boat goes too long without properly tightened seams of pounded caulk, the framing and planking move a lot. This causes the fasteners holding the planking to the ribs to become loose. The longer this goes on, the harder the repair. The plank edges need to be in firm contact with each other. This makes a solid boat that permits some movement along the seams as the planks take up moisture and dry out. Placing a flexible sealant like 4200 in the seams does not harden the seams, it just keeps out the water temporarily. Because the sealant does not harden the seam lines, the fasteners pull loose from excessive movement. A correctly caulked carvel boat has a solid hull when she has taken up. The planks work as one, spreading loads through out the hull. With 4200 in the wide open seams, a plank can not rely on the one next to it for spreading loads, so the fasteners get pulled out of their holes which probably have limited holding power.

The net result in this repair approach causes the fastener holes to let in moisture, then rot sets in and the fasteners do not do what their designed to do. The planking moves even more, the seams get bigger and bigger and eventually, you need to restore the shape of the badly distorted hull. Without the tight seams and with rotten fastener holes the framing relaxes and the boat opens up like a wicker basket. Usually the hull flattens out athwartship as the frames relax and spread outward and downward. The ends of the boat also begin to droop and these things cause the huge gaps you are seeing.

The seams themselves can become work hardened over time which make the seams appear larger. If the boat has lost it's shape, which is probable if it has not been correctly caulked in a number of years, then the shape will need to be returned to the framing. On a small boat like yours, this is not that tough a job, but takes time, because the hull needs to be gently forced back into shape a little at a time. Some well placed ratcheting straps and blocking may be all that is necessary to push and pull the hull back into shape. This will close up the seams to some degree. Then you can refasten her and pound in some new caulk. It the seams are too badly bashed, then they will need to be repaired too.

Fiberglass will not stay stuck on a carvel hull with loose fasteners and huge seam gaps for long. The plank movement and loose fasteners will cause the fiberglass to delaminate very quickly and the moisture will be trapped between the fiberglass skin and the wood, which will bring on rot. You can skin the outside of the boat with plywood, but the fasteners will have to hold all the carvel planking tightly or the movement will rip these out too. If you use heavy enough plywood with a fiberglass sheath then you'll have enough bulk in the plywood to hold everything together, but the boat will be twice as heavy.

The boat could be used as a mold to build a plywood copy or disassembled to replace the rotted pieces with new. If you are lucky, new fasteners and a proper string looped into those seams will fix her up. If she has gone too long then, hull distortion, fastener hole rot, broken frames, and mashed plank seams will need fixing. I have never seen a "wedge seam" repair on a trailer boat last very long, but I have seen wedge repairs in the seams, that receive new caulking work very well.