7-9 knots depending on your weight and foil size

Jethrow said..
My GPS tracks show board-speed jumping from ~5 knots to 9 knots about where I first take off, so I'm saying my takeoff speed is ~5 knots.

I'm 100kg's and using a PNG1150 in flat-ish saltwater, advanced beginner (I foil easily but can't gybe).

This take off speed hasn't changed from using my 6'10" wide board compared to my 7'6' skinny DW'er board, the DW'er just gets there easier.

Edit: Looking forward to using my new (to me) 1300PNG and see what that does to takeoff speed.

Hi, thanks for your contribution, although I believe your figures are incorrect.Do you think you can pump x2 or x3 walking speed ? 5-9 knt as pre-foiling speed is a too high and GPS is not accurate in short distances. Many people tend to walk at about 3 Knt (1.42 metres per second 5.1 km/h; 3.2 mph; 4.7 ft/s from wikipedia) .... I guess with 4 Knt a regular foils starts to fly.
Of course "take off speed hasn't changed from using my 6'10" wide board compared to my 7'6' skinny DW'er board" simply with the DW board you need less eford (less wind, less wing size, less pumping technique...)

JonahL said..
7-9 knots depending on your weight and foil size

let's assume weight, board, foil size, etc.. are balanced.the question is still valid.I don't think we can pump a board to 7-9 knt.... 7Knt is 13 Km/h .... I don't think we can achieve this speed by just pumping, before releasing the board to foil.

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 .

boardsurfr said..
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 .

Thanks boardsurfr !

...I've just learned from Wikipedia about bow waves. en.wikipedia.org/wiki/Bow_wave.

The negative effect of the bow wave is it just a board nose issue ? This does not seem to be a major drag factor, compared to the board bottom as you pointed.

Will it work better a kayak racing nose style (no rocker and ultra sharp nose) ?

Rounded bottoms like the KT downwind board seem to be the answer, although they are hard to balance.

What about tails ? How to make them shorter and with enough volume ?

FranP said..
The negative effect of the bow wave is it just a board nose issue ? This does not seem to be a major drag factor, compared to the board bottom as you pointed.

The problem with the bow wave is that a normal boat/board shape will climb up the wave. The faster you go, the bigger the bow wave, and the steeper the uphill angle gets. The creates a limit how fast a traditional boat shape can go in displacement mode, the "hull speed". To go faster than the hull speed requires a lot more power, or transition to planing/semi-planing, or a wave-piercing nose. For a given volume and weight, the bow wave gets smaller as the board/boat gets longer, causing the calculated hull speed to increase with the square of the length, IIRC.

Hull speed is a simplistic concept; multi-hull race boats and planing boats and windsurf boards can go a lot faster than theoretical hull speed. But for the board you want to design, it's highly relevant. For example, lower nose volume and rounded or V-type shapes can reduce how much the board rides up the bow wave, and more length reduces it (think of 14-17 ft long race SUPs!). I've seen videos of guys being very happy with 8-ft downwind boards.

FranP said..
What about tails ? How to make them shorter and with enough volume ?

This kind of thinking is what led to the traditional short and fat board shapes. If you look at current DW boards, they tend to have a long tail, typically a pintail. Basically, the outside rails are pulled back together for a smooth water flow in displacement mode. That minimizes turbulences and the resulting drag. A benefit it that the tail volume is reduced, allowing wingers to sink the tail easier when pumping the board to increase the angle of attack on the foil. That generates more lift to get the board out of the water.

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.