I count the attaching of the outwale as a major milestone because it suggests that the hull is shaped and sound. It is also a stressful experience. I have snapped gunwales in the past trying to force them to conform to the curve of the hull. The problem is that the curvature is in two directions at once. The gunwales must go from the widest part of the hull amidships to the narrowest part at the two transoms. At the same time it must follow the very pronounced rocker of the top strake. It is really a strengthening member and calls for every clamp in the workshop – a reminder that one can never have enough clamps. It is not enough that the gunwale be clamped amidships and at the two ends. The curvature of the plywood depends on it being pulled to a constant curve created by the gunwale.
It is also important to do both gunwales during the same session. There can be so much force and spring in the wood that it is easy to distort the shape of the boat if one side is set without the balance of the other side also being applied. I glue up, clamp and then drive 3/4″ inch screws through the hull into the outwales every six inches to really give a firm joint. The S-F instructions suggest that the outwale should be nailed through to the inwale but this then puts nail heads in the rubrail which can take away from the finish of the boat, especially if the wales are varnished. I prefer to leave the outwale unmarred and drive screws through the inwale when it is installed.
My big worry now is that when the temporary bulkhead amidships is removed the sides will spring together although the tension of the gunwales should prevent this. I propose to put a temporary baulk into the bulkhead space as I install the inwale and then the centre thwart which should help keep the shape of the boat. I am probably imagining the worst but I do like to be safe. I am too far along to want to start over.