Looked down yesterday to realize that my work table is currently the Noah’s ark of measuring devices. What I’m doing here is trying to figure out how high I can raise the sheer at the ends of the boat. From personal experience, I know that this Shearwater will take waves over the bow in only moderate chop and waves over the stern when trying to land with even a small sea running. This is unbelievably frustrating when you are trying to pull the boat up on a shoreline and trying to get camping gear out while the boat is simultaneously filling with water.
I don’t want to mess with the design too much, but an extra inch of freeboard and an extra inch of sheer on top of that shouldn’t mess up the lines and should make the boat more useful. However, I need to make sure that raising that sheer won’t require me to flex the gunnel upward, which would put hogging force into the boat.
You can check this by measuring the angle of the sheer strake at each of the frames and then averaging them and then multiply the tangent of this angle by 1/2 of the width of the boat. The number you end up with is the depth of sheer curve that a straight sheer plank would create when wrapped around that boat width at that gunwale angle. If this number is less than the sheer you are trying to achieve it will introduce hogging tension into the boat, in which case you will have to take countermeasures by either sawing the sheer curve or laminating it. Not such a big deal in this boat because of the massive keel assembly, but this is absolutely critical to understand when building up off of a plank keel and is the reason that so many skin boats end up hogged. Often times I will even introduce extra sheer to compensate for the natural flattening effect of the skin tension.
In this case, the natural sheer matched perfectly to what I want to do with the boat so everything is good to go, and I can draw my new sheer line onto the stems and keep building.
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