I’m restoring an old royalex Penobscot 16 that was stored upside down on the ground in the Pacific NorthWet. All wood is completely rotten and being replaced, but the new parts I have need to be cut to fit and I want to do it right.
The seats were in decent enough shape that I can get a measurement, but the thwart and yoke were rotten back to the screw holes. I think I can salvage a measurement from where the screw holes were, but have no idea how long the original wood pieces were. I figure knowing the screw to screw distance is actually the most important part- or does the thwart and yoke touch the sides of the hull?
My plan was to cut the new ones just a bit bigger so the new drill hole is not too close to the edge of the wood and the piece fits without touching the sides. Sound ok?
I am assuming your boat has synthetic material for gunnels since you did not mention anything about replacing them.
I would suggest cutting the thwart and yoke slightly longer than the measured distance from gunnel to gunnel, keeping in mind that the finished ends of those pieces will probably be at an angle other than 90 degrees. Take a bevel square and determine the angle of the boat relative to wood ends at the fasten-in points for the wood. Then trim the wood at that angle, nibbling off a bit at a time until you’re happy with the fit. The proper fit should have the ends of the wood just kissing the hull. Then mark your bolt holes and drill the wood accordingly. If there is an appreciable amount of hull flare you may have to drill the holes at a slight angle to make sure you can clear the hull to get the nuts on the bolts.
The yoke is designed to be a specific length and that will determine the overall width of the canoe. I have a Royalex Pen 16 at home. I can get you the measurements for the yoke and thwart after work if that helps.
Once you are a confident in the length of your yoke, you’re best bet is to use a clamp to hold the yoke in place (if the pressure of spreading the hull is not enough) and drill down and slightly towards the center of the canoe, using the existing holes in your gunwales as the guide. This ensure two things; 1) you have enough space for the lock nut and washer to not interfere with the hull and 2) you have enough wood on the outside of your holes so that your yoke will not break or pull free during use.
You want the yoke to sit flush against the hull. Any wiggle space can result in excess stress applied to the hardware, and that is more likely to cause a failure on a portage trail.
Once the yoke is placed, your width is set, and you can install the thwart. Since you have holes in the gunwales from the original, you can simply lay the uncut thwart over top of the gunwales and mark your end cuts and angles by comparing to the hull below. Place the thwart and use a clamp to hold if necessary. Now you can drill the thwart holes using the gunwale holes as your guide.