2014-02-14

Post VI - Lengthening the Design

Here is where I cross into completely foreign territory for a short while. For some unknown reason originating in my reptilian brain, I thought it would be a good idea to have a longer boat. Plans are for a 16' boat, but since I intend to use it with two rowers I thought the extra length would be an asset in efficiency. I also read (can't recall the reference) that 16' is maximum for 1 rower in a whitehall and bare minimum for 2 rowers. Maybe it will be imperceptible overall, but fuck it, who cares. I want a longer boat. From what I read, it's not generally an issue as long as you don't increase more than 10% or so. Usually you only want to change length - not beam.

This is pretty easy to do in cedar strip. Simply set the forms on the strongback with some extra space between each and voila! You have a longer boat. This presented a small problem at the bow when I realized that a stem faired for the original dimensions might not be quite fair if it was simply translated farther from the first station mold. The stem shape itself would have to be re-drawn and stretched slightly for the new hull dimensions. 

This presented an opportunity to practice with battens. I've done a lot of reading about lofting and how to draw fair curves based on a limited set of reference points using battens. To generate the reference points I took offset measurements from the paper plans and multiplied each offset by the before/after length ratio. I re-plotted them on my MDF board (preserving the vertical dimension), punched nails in as guides and sprang a batten around them. Figures 10-12 show the process.

Figure 10: Constraining the batten using nails. Real boatbuilders have weighted hooks to do this--but I'm not a real boatbuilder.

Figure 11: Pretty damn fair if you ask me.

Figure 12: Here I constrained the batten much farther than the stem itself using offsets pulled from the bow molds. This ensures a fair blending of the curve from stem to keel. 


I can totally see now why battens are so heavily favoured for drawing curves in boatbuilding. Once you have it coarsely constrained to the reference points, you can see plain as day where unfairness lies, and adjust your constraints slightly to relieve areas where the batten doesn't curve right. It's an interesting process too, because adjustment of constraints doesn't always have the desired effect. You might adjust the constraint nearest to an unfair section and realize that it just creates a new unfairness farther along. Then you try adjusting in a different spot farther along the batten and the whole thing just magically springs into a perfect curve. Neato!

Important caveat: don't overconstrain. I was having limited success until I called my buddy (who is a former boatbuilder) and he told me to pull a few of the nails. It should take the desired shape using only 2-3 nails to hold the curve.

EDIT: I realize that the same proportional stretching should be done on the transom, but by by calculations it amounted to adjusting the rake of the transom by a mere 1 degree. Given the extra work that would be involved in adjusting the angle and re-drawing the whole transom mold, I decided it would be easiest to just translate the transom and leave its shape/angle the same. I can always make adjustments to the molds if the hull gets wonky in the aft end (though I doubt it will).

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