Diagonal slices in AKU

When I worked at the boat builder I did what was called “lofting”- taking the designer’s “lines” drawings and “offsets” (measurements), and redrawing them full scale on sheets of plywood placed on the floor. It was a back and forth kind of operation- you clean up the curves of the water lines and then see what effect they have on the curves of the sections. Actually you have 4 sets of lines to balance out- sections, waterlines, buttocks (would be like rocker) and diagonals. From the full scale drawings we’d make a mylar pattern of the sections and then construct the mould.

As I shape my new bonzer I’m trying to fair everything out, get rid of bumps and I’m always thinking of the flow of water.

A long while ago Craftee did some testing with regard to water flow- http://www2.swaylocks.com/node/1010045 -

When a board is turning the water does not flow parallel to the stinger. It’s going at angles. This made me think of the diagonals on the yacht “lines” drawings.

In this example they are the dashed lines. Looking at the boat head on they are diagonals. But they become long flowing lines when you look at the plan (the boat from above). It makes sense to examine these lines since boats almost always sail heeled over.

So I’m thinking it would be very informative to see slices of boards according to the direction of water flow. Like, if on AKU you could draw a line from the center out towards the rail at 10-15 degrees (according to Craftee’s findings). Is there any way to do that? It would be interesting to see what concaves do to the foil and curves of a “diagonal” section.

The water flows diagonal even when you are in trim. IMHO the only time it is (close) to parallel to the stringer is when you are dropping in.


While his test is interesting and useful, I don’t think that the Craftee test is indicitive of what is happening on a surfboard riding waves. On waves the water moves UP, toward the hull, and the hull is moving along the (curved) surface of the wave.


As far as, adding the flow vector lines to aku to predict how hull contours manipulate the flow, I think it would be difficult (but not impossible) because you also have to account for the speed of the flow of the water, and the speed of the board (most often at different angles, whose vectors have to be summed), and then also the angle of the board in three dimensions (pitch, roll, and yaw) relative to the flow.


I think understanding this is essential for surfboard design. I also think that the vast majority of surfers, and even surfboard designers get this wrong because they use the boat analogy. While the boat analogy is useful, it doesn’t provide the entire picture of what is happening under a surfboard because of the way the water moves on the wave.


i remember in an old thread where someone (Josh Dowling, ie speedneedle maybe???) said that on a single concave bottom, they put the straight edge diagonally across the bottom of the board, the way that the water would flow, and the straight edge sat flat on the board.

The straightedge diagonal on the concave is an old trick used by good shapers to illustrate the concave. Josh knows this because he's been around some good shapers. And he is a good shaper himself.

The diagonals are and have been considered by good designers. I've had several conversations with other shapers about it. Good on you guys for getting onto it.

hey guys im struggling to get my head round this diagonal flat concave thing, anyone able to either photo or draw an illustration??


The concave is rail to rail. There is also the rocker which is convex from nose to tail. There is an angle at which (depending on the concavity from rail to rail, and the convexity (word?) from nose to tail) a straight edge will lay flat against the bottom of the board. It is the intersection where the two curved planes create a straight line.


If you have boards with concave, go put a straight edge on the bottom and find the line/angle where it lies flat. It will vary from board to board depending on the concave and rocker.


It is hard to draw in 2D, since it deals with curved planes in 3D.

I can't do a photo or illustration, but it's simple enough:

On a contemporary shortboard with a reasonably deep concave, take an 18''-24'' straightedge and put one end by one of the side fins and the other end forward, diagonal toward the center. At some angle it will sat flat across the combo of rail-to-rail and rocker curve.

This is somewhat seperate from the diagonals illibel referenced, however, but related.