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boardsurfr said..
Don't overlook that being narrow is just one aspect of the much-liked downwind boards. Many downwind boards also have a very "pointy", wave-piercing nose, and very similar tail. That's a very important part why the downwind boards can pick up speed much more easily than short, fat shapes.
Think of this as pushing the board through the water from the back, with someone standing on it. Basically, you need to push the water around, and the effort required will depend on (a) the projected area, as seen from the front, and (b) the "aerodynamics" of the shape - basically, a friction coefficient. Making the board narrower reduces the projected area proportionally, so you may need about 1/4 to 1/2 less "push" from that alone, depending on which widths you compare. The nose and tail shape reduce the friction coefficient, probably by at least a similar amount. So to get the board to accelerate to, say, 3 or 4 knots, you need only a fraction of the power than a short, fat board requires. Once you get into this speed range, the shape difference comes into place in an additional way, since you are approaching the hull speed (which depends only on board length). A "classical" shape at hull speed has to climb up the bow wave, going into semi-planing and then planing mode. This is literally an uphill battle that requires a lot of energy. The wave-piercing shape of the nose of a downwind board instead pierces the wave, allowing the board to gain more speed much more easily.
One way to feel the difference is by driving on a highway, rolling the window down, and putting your hand out of the window. Have it vertically first (thumb up), then turn it sideways (thumb forward). You'll feel a difference in how much the induced wind pushes on your hand that's similar to the difference between a classical and a downwind foil board shape.
Thanks to everyone for a great discussion. My theory has always been that the width for stability isn't required due to the keel effect of the foil. This is especially true if you ride a shortboard surfing, or you are used to the feel of a prone board. In larger waves a wider board is much slower from rail to rail. I designed a board around the criteria that boardsurfr mentions. After seeing the DW'er's ripping on 7" thick boards I designed my new board to be thicker and narrower. I basically made a micro DW'er. The board is 5'0" 19" wide and 62 liter and 4.9" thick. I am 86 KG and this is my one board quiver. The pros are that it gets my up in the lightest wind like one user mentioned by developing board speed with the wing...not pumping the foil. This is really important for foils that have a higher take off speed like the small HA's that we all love. I have not noticed any negative to the thickness and this is a waverider not a lawn mower.
My takeaway is that too much of most board's design is to assist with balance while on the water and mitigate touchdown friction. Much of those design elements make a board worse for actually riding on foil. My custom board checks all the boxes and provides an amazing foiling ride, but its not for everyone. As users here noted initial climb on balance is poor in rough seas, but I just use the wing as a balance point so its not an issue. The board is a dream in light wind, but is much more of a handful when getting started in rough nautical conditions. In these conditions the narrow element is a bitch on the water, but once you are flying the narrow board is totally superior.
WingOut
> 9 months ago, 11 Nov 2023
