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PeterP said..
I think it might have been discussed elsewhere as well - but essentially the reason why a shorter mast is easier to launch is because of the foils surface interaction, more than the amount of mast in the water. The reason for this is that the foil displaces and pushes water up towards the surface when moving through the water. The closer the foil is to the surface the less water the foil has to displace (air is lighter than water) and the less draggy it will feel. You can see it on some of the pump videos in glassy conditions, the water displaces up and creates a wake behind the mast.

this is not the case. A foil wing becomes less efficient as it gets close to the surface and creates more drag. In this case it is not profile drag or induced drag, but wavemaking drag, as the foil wing will be displacing water upward when it is close to the surface.

if this seems counterintuitive, keep in mind that a submarine can go far faster underwater than it can on the surface, and that most of the drag of a ship at anything but low speeds is wavemaking resistance. One of the reasons that hydrofoils are so efficient is that they generally have very low wavemaking resistance and they leave virtually no wake on the surface

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Pacey said..

PeterP said..
I think it might have been discussed elsewhere as well - but essentially the reason why a shorter mast is easier to launch is because of the foils surface interaction, more than the amount of mast in the water. The reason for this is that the foil displaces and pushes water up towards the surface when moving through the water. The closer the foil is to the surface the less water the foil has to displace (air is lighter than water) and the less draggy it will feel. You can see it on some of the pump videos in glassy conditions, the water displaces up and creates a wake behind the mast.

this is not the case. A foil wing becomes less efficient as it gets close to the surface and creates more drag. In this case it is not profile drag or induced drag, but wavemaking drag, as the foil wing will be displacing water upward when it is close to the surface.

if this seems counterintuitive, keep in mind that a submarine can go far faster underwater than it can on the surface, and that most of the drag of a ship at anything but low speeds is wavemaking resistance. One of the reasons that hydrofoils are so efficient is that they generally have very low wavemaking resistance and they leave virtually no wake on the surface

@Pacey that goes against all the advice I have seen for pumping. Everyone else says keep the foil high to maintain speed and efficient pumping.

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hilly said..

Pacey said..

PeterP said..
I think it might have been discussed elsewhere as well - but essentially the reason why a shorter mast is easier to launch is because of the foils surface interaction, more than the amount of mast in the water. The reason for this is that the foil displaces and pushes water up towards the surface when moving through the water. The closer the foil is to the surface the less water the foil has to displace (air is lighter than water) and the less draggy it will feel. You can see it on some of the pump videos in glassy conditions, the water displaces up and creates a wake behind the mast.

this is not the case. A foil wing becomes less efficient as it gets close to the surface and creates more drag. In this case it is not profile drag or induced drag, but wavemaking drag, as the foil wing will be displacing water upward when it is close to the surface.

if this seems counterintuitive, keep in mind that a submarine can go far faster underwater than it can on the surface, and that most of the drag of a ship at anything but low speeds is wavemaking resistance. One of the reasons that hydrofoils are so efficient is that they generally have very low wavemaking resistance and they leave virtually no wake on the surface

@Pacey that goes against all the advice I have seen for pumping. Everyone else says keep the foil high to maintain speed and efficient pumping.

as far as the actual data goes, it's well established that for foils operating near the water surface, efficiency goes down and angle of attack has to go up to provide the same amount of lift, and more energy gets dissipated in the wave system. Both of these result in higher drag.

a good recent reference is here: www.taoxing.net/wp-content/uploads/2021/04/jhd_2020.pdf, but the key sentence from that paper is "It is found that nearly the same maximum lift coefficient can be produced by the shallow foil in the modeled condition as by the deep foil, but much higher attack angle is required near the free surface, which also results in larger drag"

note that when pumping the foil is following a sinusoidal path, with its highest angle of attack occurring when it is deepest and its lowest angle of attack occurring when it is nearest the surface. This means that it is generating its lift when it is at a depth where it is most efficient and is generating little lift and drag when it is least efficient. And of course Kane is correct about the length of mast in the water adding drag. So yes you want to be fairly high on the foil when pumping as you are not actually asking the foil to produce much lift (and therefore induced and wave making drag) when it is at its highest

should also mention that the loss of efficiency of a foil as it nears the surface starts to kick in about 1.5 x chord, once you are below that the losses are negligible

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Pacey said..as far as the actual data goes, it's well established that for foils operating near the water surface, efficiency goes down and angle of attack has to go up to provide the same amount of lift, and more energy gets dissipated in the wave system. Both of these result in higher drag.a good recent reference is here: www.taoxing.net/wp-content/uploads/2021/04/jhd_2020.pdf, but the key sentence from that paper is "It is found that nearly the same maximum lift coefficient can be produced by the shallow foil in the modeled condition as by the deep foil, but much higher attack angle is required near the free surface, which also results in larger drag"note that when pumping the foil is following a sinusoidal path, with its highest angle of attack occurring when it is deepest and its lowest angle of attack occurring when it is nearest the surface. This means that it is generating its lift when it is at a depth where it is most efficient and is generating little lift and drag when it is least efficient. And of course Kane is correct about the length of mast in the water adding drag. So yes you want to be fairly high on the foil when pumping as you are not actually asking the foil to produce much lift (and therefore induced and wave making drag) when it is at its highestshould also mention that the loss of efficiency of a foil as it nears the surface starts to kick in about 1.5 x chord, once you are below that the losses are negligible

Super-interesting, cheers Andrew.

Epic to have someone able to join the dots with hydrodynamic science

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mcrt said..

frenchfoiler said..
Cedrus with art999 :

www.instagram.com/p/CcLdht_IsCr/

Check the water flow behind the mast, i don't know what to think about this ? good, bad ? Any idea, theorie ?

You mean the water climbing up the mast and trailing behind?.
All masts do this to some degree, maybe with the Cedrus it is more noticeable.
Reason could be the thicker section,or the joints between the leadin&trailing edge and the carbon central part,or both.

IMHO it is drag ,lifting water takes energy,my guess is it is the result of a more turbulent vs linear flow over the mast.

Turbulent is draggier but has more energy,it might even be desirable to prevent flow separation and ventilation.But i really have not read much at all about surface piercing foils.

It is not the seams of the non-structural leading/trailing edges. Check out the website, ask any owner, you can't even feel the transition from rubber to carbon with your eyes closed.

The point at which a foil (mast) pierces the water contributes a significant amount to the total drag in the system. Yes lifting water takes energy. As you mention, all masts have spray off the trailing edge but you may notice it more with Project Cedrus due to the thicker profile. The math again supports ~5% drag increase of the system for a 19mm thick mast vs. 16. Plenty of clients don't notice a difference, but then there are some who claim they notice it. Could be confirmation bias, could be extreme sensitivity, and could also be their setup. Please note that heavier riders have much more lift-induced drag (at the wing) and will be far less sensitive to mast parasitic drag. Heavy riders also benefit most from a stiffer mast. This all being said, there are quite a few very happy lightweight female riders who appreciate a light/stiff mast and again don't notice or mind a slight impact to speed.

Turbulent flow does create more drag along the surface of which it is attached to. But a turbulent boundary layer is thicker, and will stay attached to the surface longer, reducing pressure drag and the likelihood of ventilation or stalling. It's why airplanes have vortex generators on the leading edge. It's why golfballs have dimples. But the top 3' of the surface of the ocean is about the most difficult set of boundary conditions for fluid flow analysis or CFD, and by difficult I mean impossible. I worked with some of the best America's Cup foil designers on Project Cedrus and even they can't predict the characteristics of the boundary layer between water and the foil. It's a combination of laminar flow transitioning to turbulent at all times, and also depends on temperature, salinity, wind/wave conditions, speed, currents, and more.

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BayAreaKite said.. I worked with some of the best America's Cup foil designers on Project Cedrus and even they can't predict the characteristics of the boundary layer between water and the foil. It's a combination of laminar flow transitioning to turbulent at all times, and also depends on temperature, salinity, wind/wave conditions, speed, currents, and more.

As Heisenberg said:
"When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first"

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BayAreaKite said..

mcrt said..

frenchfoiler said..
Cedrus with art999 :

www.instagram.com/p/CcLdht_IsCr/

Check the water flow behind the mast, i don't know what to think about this ? good, bad ? Any idea, theorie ?

You mean the water climbing up the mast and trailing behind?.
All masts do this to some degree, maybe with the Cedrus it is more noticeable.
Reason could be the thicker section,or the joints between the leadin&trailing edge and the carbon central part,or both.

IMHO it is drag ,lifting water takes energy,my guess is it is the result of a more turbulent vs linear flow over the mast.

Turbulent is draggier but has more energy,it might even be desirable to prevent flow separation and ventilation.But i really have not read much at all about surface piercing foils.

It is not the seams of the non-structural leading/trailing edges. Check out the website, ask any owner, you can't even feel the transition from rubber to carbon with your eyes closed.

The point at which a foil (mast) pierces the water contributes a significant amount to the total drag in the system. Yes lifting water takes energy. As you mention, all masts have spray off the trailing edge but you may notice it more with Project Cedrus due to the thicker profile. The math again supports ~5% drag increase of the system for a 19mm thick mast vs. 16. Plenty of clients don't notice a difference, but then there are some who claim they notice it. Could be confirmation bias, could be extreme sensitivity, and could also be their setup. Please note that heavier riders have much more lift-induced drag (at the wing) and will be far less sensitive to mast parasitic drag. Heavy riders also benefit most from a stiffer mast. This all being said, there are quite a few very happy lightweight female riders who appreciate a light/stiff mast and again don't notice or mind a slight impact to speed.

Turbulent flow does create more drag along the surface of which it is attached to. But a turbulent boundary layer is thicker, and will stay attached to the surface longer, reducing pressure drag and the likelihood of ventilation or stalling. It's why airplanes have vortex generators on the leading edge. It's why golfballs have dimples. But the top 3' of the surface of the ocean is about the most difficult set of boundary conditions for fluid flow analysis or CFD, and by difficult I mean impossible. I worked with some of the best America's Cup foil designers on Project Cedrus and even they can't predict the characteristics of the boundary layer between water and the foil. It's a combination of laminar flow transitioning to turbulent at all times, and also depends on temperature, salinity, wind/wave conditions, speed, currents, and more.

Great to have feedback of this level, thx for taking the time Kyle.

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Pacey said..

hilly said..

Pacey said..

PeterP said..
I think it might have been discussed elsewhere as well - but essentially the reason why a shorter mast is easier to launch is because of the foils surface interaction, more than the amount of mast in the water. The reason for this is that the foil displaces and pushes water up towards the surface when moving through the water. The closer the foil is to the surface the less water the foil has to displace (air is lighter than water) and the less draggy it will feel. You can see it on some of the pump videos in glassy conditions, the water displaces up and creates a wake behind the mast.

this is not the case. A foil wing becomes less efficient as it gets close to the surface and creates more drag. In this case it is not profile drag or induced drag, but wavemaking drag, as the foil wing will be displacing water upward when it is close to the surface.

if this seems counterintuitive, keep in mind that a submarine can go far faster underwater than it can on the surface, and that most of the drag of a ship at anything but low speeds is wavemaking resistance. One of the reasons that hydrofoils are so efficient is that they generally have very low wavemaking resistance and they leave virtually no wake on the surface

@Pacey that goes against all the advice I have seen for pumping. Everyone else says keep the foil high to maintain speed and efficient pumping.

as far as the actual data goes, it's well established that for foils operating near the water surface, efficiency goes down and angle of attack has to go up to provide the same amount of lift, and more energy gets dissipated in the wave system. Both of these result in higher drag.

a good recent reference is here: www.taoxing.net/wp-content/uploads/2021/04/jhd_2020.pdf, but the key sentence from that paper is "It is found that nearly the same maximum lift coefficient can be produced by the shallow foil in the modeled condition as by the deep foil, but much higher attack angle is required near the free surface, which also results in larger drag"

note that when pumping the foil is following a sinusoidal path, with its highest angle of attack occurring when it is deepest and its lowest angle of attack occurring when it is nearest the surface. This means that it is generating its lift when it is at a depth where it is most efficient and is generating little lift and drag when it is least efficient. And of course Kane is correct about the length of mast in the water adding drag. So yes you want to be fairly high on the foil when pumping as you are not actually asking the foil to produce much lift (and therefore induced and wave making drag) when it is at its highest

should also mention that the loss of efficiency of a foil as it nears the surface starts to kick in about 1.5 x chord, once you are below that the losses are negligible

Very interesting, most of the PDF was way over my head but the conclusions are clear.

How it applies to us in a practical way is another matter.
If wave creating drag kicks in at depths of 1.5 x chord then i would say it fits with what we all know by experience..."high" on the mast is glidier.

1.5 x chord in my Kujira 1210 is 24cm deep, barely a handspan.And that is the max chord.With the average chord we would be at a depth of 19cm.
That is pretty high on the mast for me.

I cannot ride that high in anything but very smooth water and any roll would breach the tips.

So our Goldilocks depth is pretty shallow i would say.

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jondrums said..
I side by side tested Cedrus and NoLimitz (own both) with Axis 999 and 1099. Both surf foiling and wing foiling. I couldn't tell any difference in drag - and that's with a super low drag foil setup so it should be obvious if it was. The stiffness difference is immediately noticeable. I'm 100% back on Cedrus now.The Inde Foil study shows a 2-5% difference in total drag when going from 12mm to 19mm mast thickness. We are talking about 16mm vs. 19mm so much less difference. I think the extra drag on the cedrus could possibly just be in folks heads.

Formal tests aredefinitely the way to go, it is hard to trust biased opinions because they are alwaysbiased in some way or another.

WakeThief tests results sort of match what Inde foil mentions about 5% increase at 0 degrees. You can see the glide is exactly the same between 19 and 16.



Indefoilalso mentions something about the drag substantiallyincreases when the mast starts generating lift (whensetatan angle). In that sense its profile and chord would affect it as muchin the sense that a thinner mast with abig chord will probably have the same effect as thicker and narrower chord. When people are carving in and out in waves the mast definitely goes in all weird angles so maybe they could be triggering some issues therefor sure.

I have a Cedrus mast and find it feels differently depending on the foil i use with it. I use it mostly with SAB gear and interestingly when I ride it with the thin w1110 the mast is almost inexistent, however when I ride it with the w940 i can feel some weirdness depending on the wind/swell conditions. Is this really drag produced by the mast or is the mast sort of resonating and giving me a false impression of drag?

I am yet to rid it with an f-one 980 and sab w740 and see what happens. But at this moment i find this thick vs thin business is not black and white.

Interesting stuff , Appears mast stiffness is more important overall, Different question.. Do you think lighter riders have an advantage overall as they can get the required lift from smaller foils therefore reducing the size of their rigs and the drag ? Or is it relative.

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kobo said..
Interesting stuff , Appears mast stiffness is more important overall, Different question.. Do you think lighter riders have an advantage overall as they can get the required lift from smaller foils therefore reducing the size of their rigs and the drag ? Or is it relative.

For pumping there is no doubt stiffer is way better. It's once you start going faster the thicker mast becomes more draggy. Lighter rider always wins - not relative.