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light wind surfboard vs foil board

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Matteo V
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Re: light wind surfboard vs foil board

Postby Matteo V » Sun Feb 18, 2018 3:35 pm

matth wrote:
Sun Feb 18, 2018 5:37 am
Larger volume will plane sooner without question because it has more float , but low volume is more manageable once planning.
Again, this is the misconception that almost all windsurfers carry over to kiteboarding. But even in windsurfing, volume over anything that floats with just a bit of freeboard is luggage at sub planing speeds - even on a shortboard. This excess volume IS helpful when displacement windsurfing as it adds stability when a rail is pressured at these non-planing displacement speeds. But once the force of the water pushing up on the (sub)planing surface, even at sub planing speeds, becomes the dominant force holding the board up - volume is no longer a part of the equation.

The best way to conceptualize this is if you have a completely cut away deck down to the bottom layer of the board (no volume) and you start moving. Once you are moving, the water on top will fall off the back of the board and only planing forces will be holding the board up. Obviously this will not work in windsurfing, but it does work in kitesurfing. And it happens all of the time with TT's. Look down at you TT next time you are slowing down to a crawl. There is no volume there, so only the planing forces are working to keep the board up.

Volume is displacement and for displacement to have an effect, the volume must be submerged.

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Re: light wind surfboard vs foil board

Postby Bille » Sun Feb 18, 2018 3:40 pm

*
Last edited by Bille on Mon Feb 19, 2018 2:26 pm, edited 1 time in total.

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Re: light wind surfboard vs foil board

Postby Peter_Frank » Sun Feb 18, 2018 7:12 pm

The reason why volume plays a huge role, at least on a hydrofoil in marginal winds, is the fact that we DONT move forward to start with, we stand on the board and go nowhere at first, board is sinking - and even if we moved ever so slightly, it would not be planing on the surface as this requires a lot more power than present.

The thing is, to transition from this submerged position, to foiling, we need to have the absolute maximum available power from the kite, "fed" into the foil wing, also timed exactly at the very split second it is present.

Thus, if you have a board without volume, EVEN if big area, you will have to use your small amount of available peak power, to drag it up to the surface, as your FULL bodyweight has to be lifted.
In fact, even when on the surface, a really big area might have more drag than a smaller one (with more volume) in terms of letting go from the water surface and maybe also chop.
It could and would most likely be more difficult to pump the wing if a huge area sucking to the surface, instead of a big area more volume.

Having volume, and also some area indeed - it is possible to make this transition very fast, thus not requiring extra kitepower, so instead of losing your small amount of powerspike available to "get you up" and "get you free" - it can be directed into the wing so you can foil in the most marginal winds possible.

In these winds say sub 6 knots, even our special light kites can not really hang in the air nomore - not possible to park, so you will have a lot of time and many times, where the kite gives you absolute zero power, when trying to get up foiling.
Then even the biggest area non volume board will sink, and you are back to start, not a chance getting up and foiling :cry:

Another advantage is, that in lulls you can stand centered on your higher volume board, not moving but not sinking (fast at least), so you get another chance to try to go in the next slight puff or chopwave - instead of wasting the puff to do a waterstart (which is WASTE of energi only)

Once up and blasting along foiling, kite can be parked and good power now :naughty:

There is a very good reason why marginal wind foilboards have a lot of volume, and still need to be light :D

For other boards, say windsurf boards, I think some of the same parameters is present, thus here more area but definitely also high volume superlight boards can be pumped onto planing the easiest.

It is all about the transition into foiling or planing, to have an easy board to handle musclewise into the flying state, and not wasting energy :thumb:

8) Peter

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Re: light wind surfboard vs foil board

Postby Slappysan » Sun Feb 18, 2018 9:02 pm

Matteo V wrote:
Sat Feb 17, 2018 7:13 pm
If you weight 75kg, you would need a board that displaces 75 liters of water to "float" your weight (mass above the water, plus a little extra for the weight of the board). Even a 50l surfboard can't "float" you if you come to a full stop. A 35l board functioning in full displacement mode WILL SINK, not float you - nor does it provide any usable flotation to your body unless it is completely submerged and you submerge yourself to start displacing some water too.

Here is a challenge for you, try to ride your kitesurfboard completely submerged. One thing you will find with a rockered board, is that it tends to come up out of the water pretty much immediately when you start moving. That is not float doing that, but rather the rocker and surface area of the bottom of the board as the movement of the board through the water come into play. No movement - the board stays sunk - and pretty deep too, likely up to your armpits or at what ever point which equilibrium is reached when you add 35l to your body.
Here's what you are failing to realise Matteo, you don't need to support the full 75 kg.

Ever notice how once you get in to chest deep water you can't walk upwind anymore? That's because you don't have enough relative weight anymore to counteract the pull of the kite, it just lifts you up off the bottom. This is because the bottom half of your body is essentially weightless now.

The same is true on a surfboard, even when stationary, the kite will hold up 30%-50% of your weight. That leaves only 37kg of weight the board needs to float.

Now factor on top of that, once you start to sink your weight rapidly goes down. I can stay standing on my board with my knees touching the water during lulls, then just pop back up once the wind freshens, never needing to overcome the massive drag of an actual water start with my whole body in the water.

This works exactly the same for low volume SUP boards:
sinkerSUP1.jpg
sinkerSUP2.jpg
Like I said, this volume is just for keeping you out of the water during lulls, but it makes a big difference between swimming and slogging when it's light.

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Re: light wind surfboard vs foil board

Postby evan » Sun Feb 18, 2018 10:23 pm

Peter, with my 2cm thin 150*50cm board I can fully submerge it on its side getting maximum grip and line tension when waterstarting. This means less Downwind drift and more power out of the kite thus easier waterstarts compared to boards with so much volume that you can't submerge and grip the water.

I find that grip on the water and planing area is more important than volume when foiling in marginal winds.

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Re: light wind surfboard vs foil board

Postby dylan* » Sun Feb 18, 2018 10:30 pm

Slappysan wrote:
Sun Feb 18, 2018 9:02 pm
Matteo V wrote:
Sat Feb 17, 2018 7:13 pm
If you weight 75kg, you would need a board that displaces 75 liters of water to "float" your weight (mass above the water, plus a little extra for the weight of the board). Even a 50l surfboard can't "float" you if you come to a full stop. A 35l board functioning in full displacement mode WILL SINK, not float you - nor does it provide any usable flotation to your body unless it is completely submerged and you submerge yourself to start displacing some water too.

Here is a challenge for you, try to ride your kitesurfboard completely submerged. One thing you will find with a rockered board, is that it tends to come up out of the water pretty much immediately when you start moving. That is not float doing that, but rather the rocker and surface area of the bottom of the board as the movement of the board through the water come into play. No movement - the board stays sunk - and pretty deep too, likely up to your armpits or at what ever point which equilibrium is reached when you add 35l to your body.
Here's what you are failing to realise Matteo, you don't need to support the full 75 kg.

Ever notice how once you get in to chest deep water you can't walk upwind anymore? That's because you don't have enough relative weight anymore to counteract the pull of the kite, it just lifts you up off the bottom. This is because the bottom half of your body is essentially weightless now.

The same is true on a surfboard, even when stationary, the kite will hold up 30%-50% of your weight. That leaves only 37kg of weight the board needs to float.

Now factor on top of that, once you start to sink your weight rapidly goes down. I can stay standing on my board with my knees touching the water during lulls, then just pop back up once the wind freshens, never needing to overcome the massive drag of an actual water start with my whole body in the water.

This works exactly the same for low volume SUP boards:
sinkerSUP1.jpg
sinkerSUP2.jpg

Like I said, this volume is just for keeping you out of the water during lulls, but it makes a big difference between swimming and slogging when it's light.
great comparison

man i really want one of those short sups, my 7'10x32 is feeling huge these days

Matteo V
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Re: light wind surfboard vs foil board

Postby Matteo V » Sun Feb 18, 2018 11:08 pm

Slappysan wrote:
Sun Feb 18, 2018 9:02 pm

Here's what you are failing to realise Matteo, you don't need to support the full 75 kg.

This works exactly the same for low volume SUP boards:
sinkerSUP1.jpg
sinkerSUP2.jpg

Like I said, this volume is just for keeping you out of the water during lulls, but it makes a big difference between swimming and slogging when it's light.
Excellent pics! They illustrate the principle of buoyancy perfectly. They fall flat on their face if you put a kite or even just a wave in the picture. In both of those pics, the rider would need to paddle hard enough to get a bare minimum of their submerged body out of the water to catch a wave. The reader here must realize that you can paddle as pictured, but you can never catch a wave without having planning forces take over completely in the system. That means you need the board at the surface to ride a wave. Speed with your body submerged is a huge hurtle to get over also.

With a kite, it would be an almost insurmountable task to go from hip deep to the surface with the board while managing the kite pull (light winds). Think of having inflated balloons for booties. The buoyant force at your feet is very unstable. Even submarines have weight in the bottom for stability, and flotation up high. The pics above are reversing that system making it extremely unstable. Related to this, but harder to go into right now, is that having lower volume on your feet in those pics would actually make the system more stable. That is the main case for low volume kitesurfboards in all wind speeds.
KiteboardVolume1.jpg
KiteboardVolume2.jpg
KiteboardVolume3.jpg
KiteboardVolume4.jpg
The above diagrams illustrate how things work in kiting - the only wind sport where a surface vessel reaches sub planing speeds nearly instantly on a single sine or loop of the kite. Hydrofoil or not, volume or not, if one stroke of the kite does not move you fast enough to shed water off the back of a zero volume board, you will be body dragging the whole time. More to come - I am hoping to have an illustration of how planing forces take over at sub planing speeds. An animation would be better, but for now, I will have to stick with just sketches.
Last edited by Matteo V on Sun Feb 18, 2018 11:42 pm, edited 1 time in total.

Matteo V
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Re: light wind surfboard vs foil board

Postby Matteo V » Sun Feb 18, 2018 11:17 pm

evan wrote:
Sun Feb 18, 2018 10:23 pm
Peter, with my 2cm thin 150*50cm board I can fully submerge it on its side getting maximum grip and line tension when waterstarting. This means less Downwind drift and more power out of the kite thus easier waterstarts compared to boards with so much volume that you can't submerge and grip the water.

I find that grip on the water and planing area is more important than volume when foiling in marginal winds.
And this is another case for lower volume = more performance. To explain this simply:
What is so hard about using a SUP with a kite??? - Too much volume to sink the board to grab an edge/rail.
Why do we not use old windsurf boards (even short boards and sinkers) to kite with??? Because even the sinkers have too much volume to sink/control the rail at no speed or starting speeds from above 0knots to 2 knots.
Why were old kite raceboards (finned, not hydrofoil) limited to around 55l when old windsurf lightwind raceboards were never smaller than 95l???Excess volume is a liability, not an asset in kiteboarding, but it is necessary in windsurfing, though performance was maximized by having the minimum volume needed to occasionally displacement windsurf.

And most of all - WHY IN WINDSURFING WAS IT ALWAYS THE GOAL TO GET TO THE SMALLEST BOARD POSSIBLE??? -WAAAYYYY BETTER PERFORMANCE/SPEED/CONTROL/JUMPS!!

More volume means you can't be as aggressive in light winds with the kite power stroke or a loop because the power makes you slide out downwind if the volume holds the board up on the surface while you are laying in the water next to it on a water start(not necessarily on a hydrofoil with a foil section for a mast as that can be your edge instead of the rail of the board). If you are on a board submerged, you have the equivalent of balloons attached to your feet. That is as unstable as you can get!

The whole point to kitesurfing/kiteboarding is that you can achieve sub-plaining speeds where planing forces overtake buoyancy as the dominant force keeping you out of the water. And this is achieved extremely quickly, with one stroke or loop of the kite even in light winds. If kiteboarding took place at speeds of less than 2 knots of water speed, we would all be back on longboard windsurfers. If you can develop more than 2 knots of waterspeed, planing forces take over and there is absolutely no need for buoyancy - save the nostalgia for windsurfing.

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Re: light wind surfboard vs foil board

Postby Matteo V » Mon Feb 19, 2018 5:33 am

Peter_Frank wrote:
Sun Feb 18, 2018 7:12 pm
The reason why volume plays a huge role, at least on a hydrofoil in marginal winds, is the fact that we DONT move forward to start with, we stand on the board and go nowhere at first, board is sinkingthe next slight puff or chopwave - instead of wasting the puff to do a waterstart (which is WASTE of energi only)
You should move forward (somewhat perpendicular to the wind direction) on the first dive or loop of the kite as described below when on a hydrofoil to make instant apparent wind.

I must admit, there is a possibility of my foiling experience being less than yours (strapped backrolls is where I am at, with terrible [like 40% success rate on] transitions). That said, I have to go for the hat trick here.

The mast, being a foil section, is the tool you use to use the dive of the kite to make pretty much instant apparent wind. If you use the float of the board, you have a huge problem in light winds, which is the same with a board with too much volume without a hydrofoil. That problem is that the board will slide off in the direction of the of the kite pull - always very much downwind in light wind situations. If you travel in the direction of the wind (downwind) on a waterstart, you loose any chance of creating apparent wind. Actually, you will almost completely loose wind if you travel downwind (with the board) on the dive or loop of the kite.

Funny thing is that if you have 2 round poles or tubes as your mast, the above does not work and you wind up sliding downwind since there is no lift (sideways push) generated from those 2 poles standing in for a foil section mast

What you want to do with a hydrofoil, regardless of how light the wind is, is dive the kite, and immediately point around 45° down wind of a beam reach. As you instantly change from buoyant to sub-planing on the board (board on top of the water), you want to curve from 45° downwind of a beam reach to less than 5° downwind of a beam reach. This is accomplished by using the foil section of the mast of the hydrofoil as the "rail" of the board. Given that this rail is a foil section, it develops lift (sideways resistance stronger than a rail) in the upwind direction so long as you maintain it's angle of attack to the water flow consistent with or near it's maximum AOA to produce lift in the desired direction. On the next sine or loop of the kite, you want to take the angle of travel up a bit higher to point higher than a beam reach.

If you do the above, you are upwind on 2 strokes of the kite in light wind or any winds.

If you try to use the float of a board, and not waterflow/vertical foil section, and you are losing ground and kite power instantly since you will begin to travel directly i the direction of the wind.

My theory on why you may be doing what you have described in your first statement, is that you are using too high of a volume a hydrofoil board AND trying to "edge" it. Don't edge a hydrofoil board deck, use the mast as your edge. (Once you get really good, use the wing itself to edge once you can water start on an angle so the wing provides some of the "upwind" push/direction of travel to load up the kite)

High volume hydrofoil boards exist because extremely talented racers can make use of the added volume to stay higher on a "touch down" - typically on a badly executed tack. Regular riders like you and me....well we would have to work much more than we do to make use of that technique to make up for a badly executed tack (please note that this is more of a racing thing, not necessarily a freeride thing). Most high volume production boards out there are high volume because there is no real detraction from performance at the freeride level, and it is eaier/smoother to have some rocker on a flat deck if you put a bit volume into the board and shape off of that volume (instead of rockering a flat deck). But you are killing your hydrofoil "waterstart to upwind" if you stand on any volume of board without moving perpendicular to the wind to some degree in the first stroke of the kite.

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Re: light wind surfboard vs foil board

Postby joriws » Mon Feb 19, 2018 10:27 am

Matteo V wrote:
Sun Feb 18, 2018 3:35 pm
Volume is displacement and for displacement to have an effect, the volume must be submerged.
Exactly. Windsurfers need volume board to allow rider to stand up and haul sail up while standing on board. It also needs to support weight of the rider, the sail, trapeze etc.

You can imagine throwing flat rock on water surface which skims. Density of rock is higher than water so it will submerge immediately, let's call it anti-volume board. But the speed of rock (aka planing hull only) keeps it on top of surface skimming. Immediately if it catches water (rock volume is "applied") it will stop & submerge. There is the physics as simplified as possible.

Image

On kite-hydrofoil (high) volume of board is not required, on water start you are sitting on water. Only enough volume of board is needed that you can press deck with heels to have foil tilted sideways, heavier foil more buoyancy you need. So anyhow you need the pull from the kite to get on top of board but at the same time have immediate speed that board is lift off from water to foil.

If it is low wind and you have small kite rigged and you try to use foilboard in displacement mode, it is the same story as with all boards - you'll sink because you don't have power to accelerate foil to lift off speed and drag of the submerged board is too much for your kite to overcome. Remember - riders mass&density - volume of board&density = how much water needs to be displaced. 35l board and 100kg rider = if you are sitting on top of your board you'll be on buoyancy point where your body displaces roughly 65kg of water. Waist down? Simple Archimedes-stuff.

Sure surf foils are a bit different story as with surf/sup you need displacement & hull speed (=hull length) high enough that wave or paddle can accelerate over the foil lift off speed. If hull speed is less than foil lift off speed you cannot paddle yourself up to foil. (Again overly simplified as hull speed can be broken if you have force to do it)..


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