Board suction to water

[davesails7]

It does introduce bubbles so the board has less wetted surface, therefore less drag.

I've never seen any discussion of bubbles reducing resistance at the speeds that a foil board travels at. The only two options I've ever seen discussed are pressurized air being injected under the hull, or reaching a very high speed to force the air in (50+ knots). Even with those methods of forcing air bubbles in, the returns are minimal at best.

[juandesooka]

Endless discussion of this online and on facebook. What is most interesting to me is how many board design experts disagree wholeheartedly with each other on optimal bottom design, concave vs flat vs vee. Being somewhat cynical, what I take from that is no one really knows for sure and, most likely, that the difference is minimal to the point of not being significant in most conditions for most riders.

For concaves, this a very common design on surfboards and SUPS, but as I understand it, the benefits happen once you hit planing speeds. On a foil, planing speed means your board isn't in the water any more. So does it help? Dunno, I'm not about to call Armstrong a kook who doesn't know what he's doing, because of he puts deep concave on all his boards.

I found this interview with Gunnar B quite enlightening:
https://www.listennotes.com/podcasts/wi ... fay3ZUlDA/
Advance to 53:30, where he talks about experimenting with different boards. He decided to try his old kite race board and found he was able to get up in 5kt less wind than any of his sup foil or wing foil boards. Why? Those boards are designed to for maximum acceleration, get planing asap and to disengage the board from the water -- doing that quicker is the difference between winning and losing. Limited power to get quick release: flat tail with mast way back (picture doing a wheelie), vee nose with prominent rocker. A first-hand report from someone who is clearly an expert rider and it thoughtful about design ... holds a lot of weight for me.

Seems to me we are starting see divergence from sup foilboards to wing specific boards. When you think of the means of acceleration and propulsion, they have quite different properties, so that makes sense. The same specialization happened from the early days of SUP foiling on a shorter surf SUP with tracks added, to a SUP foil specific board.

[fluidity]

How could a half-inch of concave pressurize air enough to displace the water under the board?

People have tried to entrap air under planing boats (air entrapment monohulls) with questionable success. However, they're trying to do it at 50 knots which provides quite a bit of air pressure into the tunnels. When you're board is going 6 or 7 knots , there's just no way for the air to be pressurized enough to displace the water under your board.

People have also tried to inject air bubbles under planing hulls (also with questionable results), but it has to be pressurized enough to displace the water.

I don't understand your pressurization comment??

All we're talking here is some pockets of air in chop between the waves that pass under the boardand can't quite make their way out immediately due to some sealing between the sides, rear and high points of chop the board is passing over. Every one is air so it will be up against the board. And realistically, depending on speed this can be anywhere from nothing to "is the board still technically even in the water now??" Imagine that you have 5% under the board is flattened bubbles between choppy wave tips, that's close to 5% less drag so you are already going to go faster. 10% air - now it's starting to make quite a difference! Do you really want to turn down that assist in drag reduction ?

[davesails7]

Imagine that you have 5% under the board is flattened bubbles between choppy wave tips, that's close to 5% less drag so you are already going to go faster. 10% air - now it's starting to make quite a difference! Do you really want to turn down that assist in drag reduction ?

5% air under your board will not reduce drag by 5%. Most drag at slow planing speeds is not from friction. See figure 12.39: https://www.sciencedirect.com/topics/en ... aning-hull

Fanatic seemed to me to have the most pronounced double concave of the boards I've seen recently. However, they note in this video about their 2021 range, that the larger boards in the range (5'4" and up) are flat with almost no concave to be the most efficient planing surface to get up to speed and on foil in lighter winds (starting at 2:35 in the video):
https://youtu.be/_043hr43qHQ?t=157

[Wazza Foil]

I've had a few SUP foil windwing boards.
The first was a heavy crossover 120 l - hard work for me
Second was a light custom 120l concave to shapers design- it just stuck to the water
3rd a production 125l x 31in wide mid weight Windwing board almost flat but bevel sides and long bevel tail - much better.
4th and current board is a 95l Fanatic - flat bottom, no bevels and reasonably light - best so far by a long way

[fluidity]

Imagine that you have 5% under the board is flattened bubbles between choppy wave tips, that's close to 5% less drag so you are already going to go faster. 10% air - now it's starting to make quite a difference! Do you really want to turn down that assist in drag reduction ?

5% air under your board will not reduce drag by 5%. Most drag at slow planing speeds is not from friction. See figure 12.39: https://www.sciencedirect.com/topics/en ... aning-hull
Fanatic seemed to me to have the most pronounced double concave of the boards I've seen recently. However, they note in this video about their 2021 range, that the larger boards in the range (5'4" and up) are flat with almost no concave to be the most efficient planing surface to get up to speed and on foil in lighter winds (starting at 2:35 in the video):
https://youtu.be/_043hr43qHQ?t=157

Some resistance is for sure from actually pushing water out of the way but if you really think that flattened bubbles of air between the board under surface and thw water have no impact on speed then I'm going to already write you off as a lost cause.
Regarding fanatic's video, likely produced by their marketing experts. Just because I have a very annoying habit of usually being right about the claims I do actually make though, I'm going to finish off by mentioning that
1. Air is considered to have about 1/800'th of the drag of water
and
2. Most people familiar with paddling in choppy water have felt the speed increase with light chop.
Meh...

[fluidity]

I've had a few SUP foil windwing boards.
The first was a heavy crossover 120 l - hard work for me
Second was a light custom 120l concave to shapers design- it just stuck to the water
3rd a production 125l x 31in wide mid weight Windwing board almost flat but bevel sides and long bevel tail - much better.
4th and current board is a 95l Fanatic - flat bottom, no bevels and reasonably light - best so far by a long way

This is my current project. It's mostly flat bottomed except for the rear cutaways and the side wedge cutouts intended to reduce water escape from the sides. EPS and it's a pain to work with Every little bump handling and it dents. I think I'm going to have to do an inner glass layer just to properly fair it before I finish clad it.

[downunder]

^
Nice man, I might steal the corners shape if u think on a TT build foil board might make any difference?

Thx in advance...

[davesails7]

Some resistance is for sure from actually pushing water out of the way but if you really think that flattened bubbles of air between the board undr surface and thw water have no impact on speed then I'm going to already write you off as a lost cause.

Not some, most of the drag at slower planing speeds is from the horizontal component of the planing pressure force (induced drag in the reference I cited).

I have no way to prove that concaves don't trap significant air bubbles under your board. I can't prove that air bubbles don't significantly reduce frictional resistance. I can only say resistance of planing is not a new science. This was all figured out >50 years ago and it a well studied topic. When you look at the seminal works in the field (e.g. Savitsky 1964 https://www.westlawn.edu/ReferenceInfo/ ... ls1964.pdf) there's nothing in there about trapping air to reduce resistance. If you have a source that shows air bubbles from the atmosphere can significantly reduce resistance under a planing surface, I'd be interested in reading it. The only discussion I've ever seen of using air bubbles to effectively reduce resistance requires the air bubbles to be injected at pressure, or funneled in by very high speeds (>50 knots).

However, I completely agree that designing a board is just as much about how it feels, not just calculating the most efficient shape. If concaves give a good feel to the board, I say go for it. I just don't see how they can get the board up on foil faster.

[fluidity]

Some resistance is for sure from actually pushing water out of the way but if you really think that flattened bubbles of air between the board undr surface and thw water have no impact on speed then I'm going to already write you off as a lost cause.

Not some, most of the drag at slower planing speeds is from the horizontal component of the planing pressure force (induced drag in the reference I cited).

I have no way to prove that concaves don't trap significant air bubbles under your board. I can't prove that air bubbles don't significantly reduce frictional resistance. I can only say resistance of planing is not a new science. This was all figured out >50 years ago and it a well studied topic. When you look at the seminal works in the field (e.g. Savitsky 1964 https://www.westlawn.edu/ReferenceInfo/ ... ls1964.pdf) there's nothing in there about trapping air to reduce resistance. If you have a source that shows air bubbles from the atmosphere can significantly reduce resistance under a planing surface, I'd be interested in reading it. The only discussion I've ever seen of using air bubbles to effectively reduce resistance requires the air bubbles to be injected at pressure, or funneled in by very high speeds (>50 knots).

However, I completely agree that designing a board is just as much about how it feels, not just calculating the most efficient shape. If concaves give a good feel to the board, I say go for it. I just don't see how they can get the board up on foil faster.

People often have literal blind spot in that which they can't see. To study the action of air trapped underneath you need to either be underneath, have a clear hull or set up under water cameras.
A lot of inferior engine technology has stemmed from human's visual spectrum excluding the infrred wave lengths. Unlike the rainbow for example being recreated from white light with a prism.
Any air trapped below the board will flatten to a large upside down puddle of exceedingly low friction and be very ready to escape at the slightest chance.
Many of the concave hulls are double concaves with a V, the V parts the water to reduce bow wave resistance but likely forms a bubble escape path on each side.
A single concave seals on the sides and potentially traps a bow wave. That may result in stickiness but also slipperyness.