Fin Toe In

sushipop

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Was it blakestah that attempted that with surf trux?
Ya, I think you’re right. I knew a guy up in San Clemente that was working on something similar as well. I used to see him in the line up dialing his boxes mid session via a little thumb wheel built into the box.
 
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Mr J

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… Another question for your friend is what produces the laminar flow on a foil without overlapping flaps/slats?
...
npsp, I'll have a go at answering my own question. The laminar flow comes from the viscous forces in the moving fluid. The more viscous the fluid the greater the proportion of laminar flow to turbulent flow. The reverse of this ratio is called the "Reynold's number" (ratio of inertial to viscous forces) and there is a simple formula for calculating the Reynold's number of a simple shaped foil - could be wing, fin, hydrofoil. Low Reynolds number means large amount of laminar flow. The greater the kinematic viscosity of the fluid the lower the Reynold's number - the slower the foil/fin is travelling the lower the Reynolds number and the shorter the foil/fin chord length the smaller the Reynold's number.

What is interesting is that the formula uses "kinematic viscosity" not plain (dynamic) viscosity - that is the ratio of dynamic viscosity to density and because water is much heavier than air it works out that air has a higher kinematic viscosity than water. Someone I was following on Instagram was displaying his foil-board foil with a shape modelled on a scaled swallow bird's wing - his reasoning that they are very fast birds, so his foil-board would be fast. So I googled how fast a swallow can fly, enquired about the top speed of the foil board and worked out that to get similar Reynold's numbers from the foil-board as the swallow the foil would need to have a higher aspect ratio than a straight scaled up shape.

edit --> my reasoning for using Reynold's number to determine whether his foil-board was gaining the benefits of a swallow birds wing shape is because Reynold's number can be used to predict stalling and other behaviour. So comparing the two at a similar proportions of laminar to turbulent flow is a more meaningful comparison than just looking at the shape alone.
 
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Senor Sopa

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I like curvier outline with slightly less toe
Made me think of women.

I don't think surfing goes fast enough to get into turbulent flow regime.

J - Interesting to think of the entire flow as a proportion of laminar and turbulent. It's more customary to think of flow as starting as laminar and transitioning to turbulent at some distance based on the velocity.

The water sticks to the surface because, what else is it going to do? Create a vacuum?

When the water releases off the fin, there's still other water getting lugged around by the system, this is one type of drag.

The air/water interface is not well understood by academic types.
We do have to deal with "foamy" water though!
 
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npsp

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npsp, I'll have a go at answering my own question. The laminar flow comes from the viscous forces in the moving fluid. The more viscous the fluid the greater the proportion of laminar flow to turbulent flow. The reverse of this ratio is called the "Reynold's number" (ratio of inertial to viscous forces) and there is a simple formula for calculating the Reynold's number of a simple shaped foil - could be wing, fin, hydrofoil. Low Reynolds number means large amount of laminar flow. The greater the kinematic viscosity of the fluid the lower the Reynold's number - the slower the foil/fin is travelling the lower the Reynolds number and the shorter the foil/fin chord length the smaller the Reynold's number.

What is interesting is that the formula uses "kinematic viscosity" not plain (dynamic) viscosity - that is the ratio of dynamic viscosity to density and because water is much heavier than air it works out that air has a higher kinematic viscosity than water. Someone I was following on Instagram was displaying his foil-board foil with a shape modelled on a scaled swallow bird's wing - his reasoning that they are very fast birds, so his foil-board would be fast. So I googled how fast a swallow can fly, enquired about the top speed of the foil board and worked out that to get similar Reynold's numbers from the foil-board as the swallow the foil would need to have a higher aspect ratio than a straight scaled up shape.

edit --> my reasoning for using Reynold's number to determine whether his foil-board was gaining the benefits of a swallow birds wing shape is because Reynold's number can be used to predict stalling and other behaviour. So comparing the two at a similar proportions of laminar to turbulent flow is a more meaningful comparison than just looking at the shape alone.
Maria got back to me this AM and her first comment was "NPSP, why are you asking me a surfing question?" Her next question was what the temp and salinity of the water that the foil will be moving through as that was needed to determine the Reynold's number to refine the foil's shape for greatest efficiency. She then went on to say that because of the constantly changing AOA of the foil while surfing a wave, that she believes, there will be an almost constant transition between turbulent and laminar flow moving across the foil's surface.

Regarding Sopa's comment, I agree with Mr. J that the entire flow has proportionally more or less laminar vs turbulent flow. When I was in the pipeline industry, we always used the C factor or Reynold's number of the pipe wall to determine efficiency of the entire flow through the pipe barrel. Fluids have turbulent flow next to the pipe wall getting less and less turbulent towards the center of the pipe and eventually (hopefully) becoming laminar flow. In this scenario, laminar and turbulent flow are each a portion of the entire flow. In the case of a pipe, the smoother the pipe wall, the portion of laminar flow will be greater than the portion of turbulent flow moving through a given section of pipe and the reverse for a rough walled pipe.

This is good stuff!!
 

Mr J

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I don't think surfing goes fast enough to get into turbulent flow regime.

J - Interesting to think of the entire flow as a proportion of laminar and turbulent. It's more customary to think of flow as starting as laminar and transitioning to turbulent at some distance based on the velocity.

The water sticks to the surface because, what else is it going to do? Create a vacuum?

When the water releases off the fin, there's still other water getting lugged around by the system, this is one type of drag.

The air/water interface is not well understood by academic types.
We do have to deal with "foamy" water though!
thanks for participating in the discussion :) my crude look at your thought that a surfboard does not go fast enough for turbulent flow:
according to Wikipedia the Reynolds number formula for a foil is Vc/v where V is velocity, c is chord length and little 'v' kinematic viscosity.

Take a surfboard fin chord length at base 0.12m travelling at 10 m/s (36 km/h) kinematic viscosity 0.0000010533 water
Reynolds number = 1,139,276 at the base of the fin.

Cessna light aircraft speed 27.7 m/s (100 km/h) chord about 2.2m / kinematic viscosity of air 0.0000146 = 4,068,493
So such a plane does have 4 times as much turbulence than the surfboard fin base.

However if considering diagonal flow across the hull of a surfboard, we could be looking at about 0.5m of "chord" which would the produce a Reynolds number of 4,746,985 - more turbulent than the Cessna.

Regarding the speeds I chose - I once taped a GPS device to the nose of a CI tufflite New Flyer at Bells beach and got close to 40km/hr - was consistently holding over 30km/hr on the wave so I don't think it was some erroneous velocity spike from hitting a lump of chop or something. 100 km/h for the light aircraft is an approximate take off speed for that aircraft according to the internet.

Water does not always, stick, in extreme circumstances there is a phenomena known as "cavitation" where the low pressure side of a foil will turn the water to steam, however it takes something like an internal combustion engine driving a propeller to do that I am told and definitely no cavitation for our surfboard fins, let alone vacuum. However, what npsp and myself are suggesting is that the flow can stop following the contours of the foil nicely and instead become turbulent - things like vortexes start happening.

However I am intuitively with you on your other comments - surfboards don't travel with fins and hull at constant AoA and hold steady speeds, plus there is all sorts of other stuff happening like foamy water you mentioned. So we cannot simply model our boards on simple formula's such as the Reynolds number and Bernoulli's equation - so shapers will never get replaced by hydrodynamics engineers.
edit--> completely wrong of me to compare flow across a surfboard hull - it has no low pressure surface, its a one sided planning surface.
 
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RTP

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Regarding the speeds I chose - I once taped a GPS device to the nose of a CI tufflite New Flyer at Bells beach and got close to 40km/hr - was consistently holding over 30km/hr on the wave so I don't think it was some erroneous velocity spike from hitting a lump of chop or something. 100 km/h for the light aircraft is an approximate take off speed for that aircraft according to the internet.

...

What period was the swell? GPS would be speed over the ground, so part of that 30 kph is a function of the wave's speed, right? Reckon if you had an olo, it'd be closer to 60 kph.
 
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Senor Sopa

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thanks for participating in the discussion :) my crude look at your thought that a surfboard does not go fast enough for turbulent flow:
according to Wikipedia the Reynolds number formula for a foil is Vc/v where V is velocity, c is chord length and little 'v' kinematic viscosity.

Take a surfboard fin chord length at base 0.12m travelling at 10 m/s (36 km/h) kinematic viscosity 0.0000010533 water
Reynolds number = 1,139,276 at the base of the fin.

Cessna light aircraft speed 27.7 m/s (100 km/h) chord about 2.2m / kinematic viscosity of air 0.0000146 = 4,068,493
So such a plane does have 4 times as much turbulence than the surfboard fin base.

However if considering diagonal flow across the hull of a surfboard, we could be looking at about 0.5m of "chord" which would the produce a Reynolds number of 4,746,985 - more turbulent than the Cessna.

Regarding the speeds I chose - I once taped a GPS device to the nose of a CI tufflite New Flyer at Bells beach and got close to 40km/hr - was consistently holding over 30km/hr on the wave so I don't think it was some erroneous velocity spike from hitting a lump of chop or something. 100 km/h for the light aircraft is an approximate take off speed for that aircraft according to the internet.

Water does not always, stick, in extreme circumstances there is a phenomena known as "cavitation" where the low pressure side of a foil will turn the water to steam, however it takes something like an internal combustion engine driving a propeller to do that I am told and definitely no cavitation for our surfboard fins, let alone vacuum. However, what npsp and myself are suggesting is that the flow can stop following the contours of the foil nicely and instead become turbulent - things like vortexes start happening.

However I am intuitively with you on your other comments - surfboards don't travel with fins and hull at constant AoA and hold steady speeds, plus there is all sorts of other stuff happening like foamy water you mentioned. So we cannot simply model our boards on simple formula's such as the Reynolds number and Bernoulli's equation - so shapers will never get replaced by hydrodynamics engineers.
I may be wrong on the transition to turbulent flow. I'm still pretty sure we can't compare boundary layers between flat surfaces and enclosed pipes though. The flat plate can consider water a certain distance from the body to be at zero velocity. All the fluid in the pipe is moving.

Cavitation, this actually does occur in surfing. We call if "fin hum", and is considered to be a undesirable condition, i.e. you fin suck.

The release of the flow is known as "separation". This is independent of laminar vs turbulent. Golf balls have dimples to induce turbulent flow, which is then better able to follow the contour. The result is less area where the flow release which corresponds to less drag. The theory being more vortex shedding is more water being "dragged along for the ride".
 
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Mr J

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Senor Sopa, you might be right on all those points - I am not an expert so cannot argue with any certainty. I didn't mention the boundary layer - I think I need to do more reading and how it relates to flow separation. However the vortices I mentioned are a type of turbulence - or aren't they?

The Reynolds number equation for a pipe is different from a foil from what I can see - foil being a fin, wing, hydrofoil - I assume that's what you call flat plate? I realise now my mention of surfboard hull is completely wrong for the foil comparison (it has no low pressure surface).

Stephen Hawking said in one of his popular books on physics dumbed down for the masses, that there is no difference between throwing a ball inside the carriage of a moving train or a stationary train - it is still travelling at the same speed relative to the train and all speed is relative to other objects so a flat plate travelling in a stationary body of water would be the same as the body of water moving and the flat plate stationary.

Yes I have had fin hum from badly made home made fins - I didn't know they were turning the water to steam. I have experienced what is thought to be ventilation on a sailboard - when trying to sail too close to the wind and say hitting a lump of chop which would lift the board a bit - air from the surface is sucked down the length of the fin on the low pressure side and hold is lost - the board could go sideways for some considerable distance. Someone I followed on instagram posted quite a convincing picture of his foil board mast ventilating - sucking down air from the water surface - the air is not steam.
 

Mr J

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What period was the swell? GPS would be speed over the ground, so part of that 30 kph is a function of the wave's speed, right? Reckon if you had an olo, it'd be closer to 60 kph.
It would have been long period, but I think you have a point the speed of the flow of water would likely not be the same as ground speed. My vague understanding is that when a wave breaks and turns to white water then water does move towards the beach - however a wave before that point has mainly vertical movement of water so the water flow speed and ground speed wouldn't be too different?

edit --> e.g. when we sit on our surfboards waiting for waves, when we ignore unbroken waves we don't want to catch and let them pass under us we rise up as the swell passes under us, then get lowered down to the rest of the surface level - we don't travel towards the beach. However a wave which is reaching the point of pitching - I don't know about that.
 
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RTP

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It would have been long period, but I think you have a point the speed of the flow of water would likely not be the same as ground speed. My vague understanding is that when a wave breaks and turns to white water then water does move towards the beach - however a wave before that point has mainly vertical movement of water so the water flow speed and ground speed wouldn't be too different?

edit --> e.g. when we sit on our surfboards waiting for waves, when we ignore unbroken waves we don't want to catch and let them pass under us we rise up as the swell passes under us, then get lowered down to the rest of the surface level - we don't travel towards the beach. However a wave which is reaching the point of pitching - I don't know about that.
If you go straight-off-Adolf to go in to the beach, then I can see your water speed and over the ground speed being equal. But I don't think I ever thought I was "going fast" in that case.

 

Mr J

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If you go straight-off-Adolf to go in to the beach, then I can see your water speed and over the ground speed being equal. But I don't think I ever thought I was "going fast" in that case.

Made the wave, well overhead. Bell's breaks like a point break so wasn't straight in towards the beach. If the water is neither moving in towards the beach nor moving sideways , then wouldn't GPS speed be the same as water flow speed when cutting across a wave staying ahead of the shoulder?
 

RTP

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Sorry, didn't mean your wave specifically.

I think this is what is going on, catch the wave at A, kick out at B and the beach is perpendicular to the wave speed vector. To me, I only "feel" the water speed when I'm flying down the line. I remember Roy posting his GPS speeds and them being quite high, but go jump off a boat doing 30-40 kph and tell me if that feels like falling when surfing (I've skipped after hitting the water on a wipeout, but twice max?).

 

Mr J

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... I remember Roy posting his GPS speeds and them being quite high, but go jump off a boat doing 30-40 kph and tell me if that feels like falling when surfing (I've skipped after hitting the water on a wipeout, but twice max?).
Never skipped the surface on a wipeout, although I have failed to penetrate the surface and rolled down the face. Not jumped off a boat, but have water skied twice in my life - once beginner style - two skis - it seems beginner speed is about 30kph. Also mono skied on slalom skis - that would have been faster - took me a couple of attempts to get going because the rope got ripped out of my hands the first time. 30kph seems achievable on a surfboard.

I can't quite picture your diagram, but thinking about what you said about water movement - on a good day no movement at the takeoff zone, but along the length of the wave there is a current in the direction of the deep water channel, so maybe water flow speed on surfboard, not as fast as GPS speed.
 

Mr J

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Yep, that's pretty much my amateur trick. I mark dots 2" either side of the nose, and lay a straightedge from there to the fin dots. Seems to work well for 'Normal' boards - shorties a little looser; guns less so.

I guess if shaping a longer board for smaller waves, you'd have to toe in a little more to retain the liveliness.

What do you pros say? In the ballpark or out to lunch, haha!
Now for a more down to earth post.

Maz you will be pleased to hear that is how shaper designer Dave Verrall of Diverse surfboards Qld does his HPSBs - batten from fins to 2" out from nose. Not well known outside of Qld, but Dave is notable for being an early adopter of CAD and the shaping machine in Aus. Also the first on Swaylocks to propose using volume as a consumer metric when he noted that his team riders seemed to follow a similar body weight to board volume ratio.
 
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griffinsurfboard

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Now for a more down to earth post.

Maz you will be pleased to hear that is how shaper designer Dave Verrall of Diverse surfboards Qld does his HPSBs - batten from fins to 2" out from nose. Not well known outside of Qld, but Dave is notable for being an early adopter of CAD and the shaping machine in Aus. Also the first on Swaylocks to propose using volume as a consumer metric when he noted that his team riders seemed to follow a similar body weight to board volume ratio.
This is so off

You adjust the toe in for what you desire on that board

The distance from tail to these 2" marks at the nose should not determine anything ;-)