[foil geeks] Minimum speed to lift a board.

7-9 knots depending on your weight and foil size

A few years ago we were playing behind a boat w a older Naish 2000 foil, which had a huge high lift foil section. We brought the boat up to speed slowly and at intervals and everyone was logging w various watches plus the boats navigation system. The board was the old Naish multi-purpose foil board w the pretty Tequila Sunrise Red to Yellow fade. You could feel the first lift round 4.5 to 5.0 knots. Lighter riders around 65 kg's came up around the 4.5, us 80 kg's around 5. We didn't do any pumping.
That said, I was trying to flat water start a few days ago and my watch said I peaked at 5.6 mph, but I didn't come up. Haven't got my cardio up to sustain that yet.
If I had the time and resources I would love to conduct some more scientific experiments, I would think these bigger companies would be pulling boards in some tanks for data.

Always better to look at actual data. Here's a graph showing sorted speeds for a foil session on a 1500 cm2 foil (Axis HPS 1050), 90 kg sailor:


The jump between 4 and 8-10 knots shows the region where the foil first starts to contribute lift, and later contributes enough lift for board and sailor. The board in the case is 228 cm long, 85 cm wide, 140 l. Next, a sorted speed graph for a lighter winger (60 kg) on a 1250 cm2 foil:



Here, the curve is shifted downward a bit, and the "rapid acceleration" part starts at 3 knots. The board is shorter (150 cm) so it has a lower hull speed. It requires active pumping to fly (unless it's crazy windy).

For the answer to your question, you'll need to look at the parts above ~ 8 knots where the curve flattens out again. For a 90 kg sailor on a 1300 cm2 foil, you'll need about 8-10 knots. But keep in mind that the foil push is not an "all or nothing" thing. Even below 8 knots, the foil will provide lift that causes the board to rise partly out of the water and accelerate.

The data above are from using a very accurate GPS recording at 5 Hz. Data from watches recording at the usual 1 Hz will be a lot less accurate, and not have enough data points for a clear picture to emerge.

Foil lift is proportional to the square of speed. So if a foil lifts an 80 kg sailor at 8 knots, then it will provide 20 kg of lift at 4 knots. With an efficient enough board, that will reduce drag enough to allow acceleration. On a short and wide board, you may run into hull speed issues, which need to be overcome with pumping.

For a "self starting" shape that gets going without much pumping, one issue is how to get the volume. You want excess volume, otherwise you get a lot of drag from water coming over the top of the board. To keep the length near 6 feet, you would need to make the board quite thick, which has it's own drawbacks. A longer board will also have a smaller bow wave, and resulting drag. For foil boards, the bow wave may not be quite as terrible as for other water crafts, since riding up the bow wave increases the angle of attack of the foil. That said, we probably only want an increase of a few degrees. I have not run any numbers on this, but my gut feeling is that this points toward quite long boards (> 7ft), with a bow shape that does not ride up much (low volume, wave piercing, and so on).

All else being equal, board width should have a (roughly) linear effect, so going from 20 to 22 inches should increase drag by about 10 percent. More interesting is the design of the underwater structure near the bow. Just about anything will work, but what works best - flat like most traditional boards, rounded like a D2 windsurfing board, sharp Vs like catamarans, or even wave-piercing shapes with a downward slope at the top (like on the Slingshot Flyer), or a mix of these? This would be a great thesis project in fluid dynamics, with a theoretical and practical component. Maybe we should get Fangman and Flex2 into winging to see what they come up with .

Not sure there is such a thing as an optimum board shape; every decision has advantages and draw backs. I'd play around with a board design program to get an idea what's possible. Personally, I'd consider going to 22 inches wide, maybe even a bit more, and at least a bit beyond 7 ft, with the foil close to the center. But I'd also aim for at least 110 l to have +20 liters. Underwater shape is something that would deserve some experimentation, but definitely round or V in front, and some kind of pin tail in the back. A lot of that seems to be the current standard, except that I'd be willing to go longer and wider than many boards out there. I don't think I'll get a chance to try enough shapes to base a design on that.