Small Boat Design for Beginners: 2017

No headache here

In the last post I said that I couldn't remember how to develop twisted panels then I remembered an article I wrote for Duckworks, here and here's the quote that put me back on track,

I finally figured out that the reason that the panel development worked on Ode II was that the bottom was flat and that the waterline through the sides was at right angles to the surface of the panel. Problem solved. Go get another sheet of 1/4 ply.

You will also remember that, when developing V bottom panels, I said that the diagonal was at 90 degrees to station 5. Well the same is true of twisted panels but with one important difference there needs to be two lines parallel to each other not just one.

So the design I'm using for this post is 3S. I had designed a boat based upon the Whitehall skiff which I named Sally Blank. White = blanc = blank, hall = room = salle = Sally. 3S is Sally's slimmer sister. Here are the lines.

A slimmer sister indeed!

The panel we're going to develop is the bottom panel which has quite a twist in it. First we'll draw a line from where the chine meets the stem at right angles to station 5.

and then a second line parallel to the first from the transom chine junction. However that won't work as the line exits the panel through the chine and that won't define the length of the panel. Back to the drawing board. The only way to have a parallel line which doesn't exit the panel through the chine is to have that line between the keel and the first line like so.

This line is from the stem knuckle and remains inside the panel. Next we transfer those line to the profile drawing and, as we did previously, draw a baseline with a perpendicular at the middle and transfer the parallel lines to the upright as we did on the previous post. Once that is complete then we can transfer the length of the lines from the profile to the baseline drawing.

One crucial point, the reason the transfer of length works is that we are actually transferring a chord of an arc however if the arc is too tight or the stations are too far apart then the chord does not approximate the length of the arc and is very much shorter.

Here the chord between two stations is very near the length of the arc whereas the chord spanning two stations can be seen to be much shorter. I mention this because one the transferred lines on the profile has quite a tight arc near the bow and so an extra station needs to be put in.

Once the length has been transferred then we can join the station marks and extend them and draw in the ends of the panel.

Now we transfer the station lengths from the section drawing to the station lines as we did previously.

As we did in the last post, draw in the upper curve and take out the big circles and the base line. Extend the station lines downward and draw in the lower curve being careful to get the minute differences as before.