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join wood top to steel hull

Hi, a year ago we bought a 72ft wood top narrowboat and after struggling with the continuous leaks decided to recover the top with marine ply. The wood joints worked out fine with fibreglass after a few failed attempts but with the expantion etc. the join between the top and the gunwale does not want to stop cracking! Appart from replacing it in steel £££ does anyone out there know of a solution?
Any feedback would be helpfull.

Asked by: Daniel Greenway  | 11.54am, Friday 12 August

WW says:

Since wood and steel expand and contract at different rates, I am afraid it is almost inevitable that a rigid joint between the two will fail and leak. You need to devise a way of allowing the movement to occur but keep the water out.
The only way I can think of doing this is to have an upstand on the inner edge of the gunwale and allow the cabin side to overlap this like the tiles on a roof.
The bottom of the cabin side would need to finish slightly above the gunwale to prevent water landing on it being drawn up into the gap by capillary action. The gap could be filled with a flexible sealant strip or a rubber gasket positioned slightly higher than the bottom of the cabin side so that water drips off.
The ideal way to form the upstand is to weld a two or three inch wide strip of steel to the back edge of the gunwale but this is probably only practical if you are adding a new superstructure - not trying to amend an existing one. It may be possible to fix a steel angle bedded on mastic to achieve the same effect but, without seeing the boat, it is difficult to be more specific

Graham Booth  | 10.41PM, Friday 12 August

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