I forgot about this thread and about the need to consult naval architects, and about how adding extensions will actually be a negative buoyancy and sink in the stern, as well as somehow also pushing the bows underwater because of too much buoyancy, and how I should be buying
a new boat instead.
So I foolishly went and started extending our sterns without all the above advice. Just zipped up the first one yesterday. Needs the seams ground and faired yet and then needs gelcoating. Non-skid also.
Extended the stern 27" and changed the entire staircase - fewer, but longer steps with a different step height than before. Added internal dorade structures for the blower vents so that there are no longer any clamshell vents intruding into the sterns like before.
It was a bit more complicated because of the 3D shape and complex curves of the step rails and hulls in the original. Not to mention that the sterns are mirror images
of each other, so one hull can't be used for the other.
The new extension is made in polyester, nidacore (steps and hull sides - hull bottom is solid) and 1808 biax.
The main connection structure is a nidacore box beam that connects to the old steps and catilevers onto the new hull. The new steps are taped to it. The new extension butts onto the old hull and is taped inside and out.
The new extension was built similar to how Brad described wanting to do his - a mold
was splashed off the old hull and rails, a wedge was cut out of this mold
to fit the necessary dimensions for the extension and rejoined. The steps were made with a plywood plug
and the rails and hull molds joined to it. Glass was added as needed to these pieces to form a complete stern mold, then that mold was cut horizontally so that I ended up with two molds - one containing the steps, rails and most of the hull sides, and the other containing the hull bottom and partial remaining hull sides.
The new stern was laid up inside these two molds.
Worked great! Once everything is faired, I will splash a hull and staircase mold off of it, splash rails off the other transom like before, connect those rails to the hull/step mold and make the other transom. This should be much quicker because there will be no making wood staircase plugs, fussing with getting the fitting of the wedge cutout and rails correct, etc. And much less post-mold fitting and fairing to exact shape.
The new build came in at 135lbs (that's 61kg for you cutting-edge measurement people). The added volume gives 650lbs (295kg) of buoyancy, with 500lbs (227kg) of that within 10" of the waterline. I could have saved weight by cutting off the covered part of the old transom, but I did not.
The first pic is dry-fitting the two molded parts
of the extension - you can see the box beam cantilever already glassed to the old hull. The new hull and staircase will later be glassed to it also. The second picture is everything all glassed in. You can see the complicated step rail shape, although the compound curve in it and the hull side is not very evident in the pic. The holes are just access holes for glassing and will be glassed over.
Without this complex and mirror image shape on our boat, the stern extension done similar to how Brad proposed would be very easy and work well. In my opinion only, of course.
Please don't tell me how much I screwed up doing this! I read back through this thread and already know how much peril I am in…