In Part 1 I wrote about using ash that I milled to strengthen Curlew’s deck beam. In this post I detail another complex project using more of the ash to strength and maintain the centreboard case. This work was undertaken over the last two winters, and culminated in replacing the case batons which was one of my most challenging amateur boatbuilding tasks to date!
Wooden Centreboard
Looking up the centreboard case from the bottom where the ply was starting to delaminate (the boat is lying on its side in this photo).
Curlew has a wooden centreboard pivoting on an 3/4" bronze bolt. The centreboard doesn’t look original although the case is. After our first season with Curlew I wanted to check on the condition of the centreboard as it is one of those hidden parts of a woden boat that are hard to inspect. The case has a lid with eight wood screws and is home to a cleat and line that holds the centreboard in position whilst underway.
On inspection I found that the centreboard was in good condition although still damp even after a fortnight in the boat shed (taking the lid of the case and removing the centreboard to dry out and have a touch-up has now become a routine task over the winter layup).
I noticed that the plywood at the bottom of the centreboard case adjoining the keel was starting to delaminate (see picture), this was a “hot spot” for rot if the ever was one. I also noted that the glue between the centreboard case knee on the thwart had failed and that the batons atop the case were cracking, and the glue holding them to the case was beginning to fail. These discoveries prompted my work to maintain and strengthen the case fixings.
As an aside I note that Naval Architect Francois Vivier has designed a clever system where a pivoting centreboard can easily be removed like a daggerboard. Something to add to my wishlist for my perfect boat.
Maintenance
Testing the fit of the centreboard prior to repainting
Last year with the boat upturned for other maintenance I addressed the issue of the deteriorating ply. The centreboard case is integral to the structure of the boat as the forward end is beautifully curved and jointed into the mast step - replacing the whole thing would be a major project and keep the boat out of the water for a season.
Further poking and prodding revealed that the wood was still in good condition and I did not detect any rot. Therefore I decided to maintain the existing structure. To do this I sanded the inside of the centreboard case where it meets the keel as best as possible and re-sealed and faired the joint between the two using epoxy (after letting it thoroughly dry out over the winter).
I then re-varnished the interior of the entire case using a “behind the radiator” brush. A little sanding of the inside of the case and the centreboard (coinciding with its repaint) and the centreboard was refitted. So far the interior of the case has held up well and the case hasn’t sprung a leak.
Strengthening
Glueing and filleting the centreboard knee to the thwart
“That is a rather important bit to wobble”
…said Matthew (fellow DCA member and boat builder) when I described the failed glue joint between the aft end of the centreboard case and the thwart.
This was readily fixed by cleaning out the old glue and prizing the knee from the thwart long enough to inject an epoxy mixture, and then re-clamp and fillet the joint. Luckily it is under the thwart so not visible to the casual observer. Once complete the knee was secure and the centreboard wobble reduced.
The top of the case has two batons running fore and aft to strengthen the case and to house the screws affixing the centreboard case lid. The batons were coming away from the case sides and were splitting where I had missed previous pilot holes when re-screwing the lid - a curse of my own making by removing the centreboard annually - I wonder whether anyone else does this every season?
Dry fitting replacement batons along the centreboard case
I had puzzled on the solution for these batons for some months - should I replace or restore? I had previously filled all the ’extra’ holes and re-varnished. However, I noticed that there were also affixed to the case with horizontal screws, and these were causing the plywood of the case sides to spit as well. This along with failure of the glue holding the batons to the case promoted me to lean toward replacement.
Timely as ever the March/April 2024 issue of Watercraft magazine (#164) dropped onto the doormat. Julian Wolfram’s article on dinghy construction describes methods for transferring load from the centreboard case into the hull to mitigate force of impact and reduce the chance of splitting one’s centreboard case (sounds bad), and helped me design my new batons.
The re-glued knee had improved the situation but the case still lacked rigidity and strength that the boat deserved if was to be used for cruising in unknown waters. Thus a plan was formed, I would cut fresh batons from my stock of ash and fix into the thwart using a dado joint. In theory this would help transfer any force along the centreboard case into the thwart and thence to the hull.
Glueing replacement batons along the case after the captive nuts had been fitted
After sketching a couple of diagrams as a plan I cut 3/4" batons to fit the centreboard case and with additional length to accommodate the joint into the thwart. I wanted the lid fastenings to be durable to help with annual maintenance, so I went with a captive nut approach using epoxy to secure the nut in the holes in the batons. My thinking here was that if I had to remove the lid “at sea” I’d have fewer pieces to lose. Interestingly, I learnt that vaseline works as an effective epoxy inhibitor to keep the inner thread of the nut clean while the glue sets.
I ripped the lengths from my milled ash using a table saw and then cut the dado joint notches in the thwart and dry-fitted the batons. A sharp chisel was the essential ingredient and allowed me to peel away the hardwood of the thwart a layer at a time.
I opted to glue the batons to the centreboard case to avoid the problems of screws causing the ply to split (having also filled and faired the old holes and cracks along the way). I knocked each baton into its notch and then glued and clamped them into place alongside the centreboard case.
The new batons have greatly improved the rigidity of the centreboard case and the additional joint into the thwart has added significant strength to the structure. The lid is now affixed with M4 A4 stainless bolts into the captive nuts and sealed with butyl tape.
The refurbished case should provide good service for some years and the added strength will hopefully prevent this becoming a weak spot on the boat. Indeed we ran aground during a recent trip and the centreboard and the centreboard case held fast, the rudder however was a different matter…but that is a story for another day.