Fin Toe In

Mr J

Gerry Lopez status
Aug 18, 2003
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SdSurfrat, that is not me, he introduced himself as a Mr Knight (I don't know him). I am Mr Jones.

Regarding the vid, I think he is misusing "coanda effect" for the reason why the water follows the low pressure curved surface - I was once told on swaylocks that the coanda effect is for flow of a fluid on a surface that is not submerged in the fluid.

My non expert understanding is that it does follow the surface though - laminar flow - at least up until up to the point of stalling.

AoA is also generally referring to the line from leading edge to trailing edge and its angle with directional movement - so there is only one AoA not one for the bottom and one for the top - he seems to be illustrating a tangent to the top and bottom surfaces - but maybe I was misunderstanding his intent.

Nevertheless, his explanation of the pull in towards the rail seems right.

My thought/guess on a reason we don't toe the fins out instead of in is the direction of thrust (we can't call it lift because lift is upwards and this "thrust" is horizontal - so by toeing in we point some of that thrust in a forward direction. He could well be right that the "flow separation" aka stalling is a reason or even the main reason and also his explanation about the outer fin releasing more easily in a turn makes complete sense to me - hadn't thought of that one. I think he knows more than me.

PS been flat out surfing and working heaps, so not much time/energy left to post here, but thought I better clear up your question on identity - then ended up writing a bit.
 

waxfoot

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Apr 21, 2018
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Surf simply's audience is mostly Silicon valley VALS, but I do like their scientific approach to coaching. This latest video series aint too bad, though I can't comment on this specific video ... it's all a mystery to me. Reckon he's on the right track @griffinsurfboard ?
 
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npsp

Michael Peterson status
Dec 30, 2003
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SdSurfrat, that is not me, he introduced himself as a Mr Knight (I don't know him). I am Mr Jones.

Regarding the vid, I think he is misusing "coanda effect" for the reason why the water follows the low pressure curved surface - I was once told on swaylocks that the coanda effect is for flow of a fluid on a surface that is not submerged in the fluid.

My non expert understanding is that it does follow the surface though - laminar flow - at least up until up to the point of stalling.

AoA is also generally referring to the line from leading edge to trailing edge and its angle with directional movement - so there is only one AoA not one for the bottom and one for the top - he seems to be illustrating a tangent to the top and bottom surfaces - but maybe I was misunderstanding his intent.

Nevertheless, his explanation of the pull in towards the rail seems right.

My thought/guess on a reason we don't toe the fins out instead of in is the direction of thrust (we can't call it lift because lift is upwards and this "thrust" is horizontal - so by toeing in we point some of that thrust in a forward direction. He could well be right that the "flow separation" aka stalling is a reason or even the main reason and also his explanation about the outer fin releasing more easily in a turn makes complete sense to me - hadn't thought of that one. I think he knows more than me.

PS been flat out surfing and working heaps, so not much time/energy left to post here, but thought I better clear up your question on identity - then ended up writing a bit.
More on the "Coanda Effect" from my friend Maria's Fluid Dynamics blog, Theory Bites: (3) : ** Coanda Effect **

Coanda effect is the phenomena in which a jet flow attaches itself to a nearby surface and remains attached even when the surface curves away from the initial jet direction. The phenomena derives its name from a Romanian born aeronautical engineer Henri Coanda.

In free surroundings, a jet of fluid entrains and mixes with its surroundings as it flows away from a nozzle.
When a surface is brought close to the jet, this restricts the entrainment in that region. As flow accelerates to try balance the momentum transfer, a pressure difference across the jet results and the jet is deflected closer to the surface - eventually attaching to it.
Even if the surface is curved away from the initial direction, the jet tends to remain attached. This effect can be used to change the jet direction. In doing so, the rate at which the jet mixes is often significantly increased compared with that of an equivalent free jet.

Many devices use Coanda effect. One notable application of the Coanda effect is the NOTAR™ (No TAil Rotor) helicopter and the Dyson Airwrap.

#knowledgesharing #knowledgethatmatters #fluidmechanics #BasicLearning #fluiddynamics #aerodynamics #turbomachinery

Source: S & C Thermofluids Ltd

Coanda Effect.jpg
 
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Mr J

Gerry Lopez status
Aug 18, 2003
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More on the "Coanda Effect" from my friend Maria's Fluid Dynamics blog, Theory Bites: (3) : ** Coanda Effect **
...
The blog excerpt describes the coanda effect resulting from a stream of fluid being projected from a *nozzle* over the foil - I have read this elsewhere too - the stream can be sheet shaped, but thin. So my question for your friend Maria is that a plain foil - such as the side fin of a thruster doesn't really have a nozzle streaming water over it does it, so the laminar flow does not seem to me that it comes from a nozzle effect?

If I have understood right, then my interpretation of the picture posted in the blog excerpt is that it has a "front slat" overlapping the front upper surface near leading edge and overlapping "extended flaps" on the trailing edge - these would produce a nozzle effect from the overlaps. The word "additional" is used which I take as meaning augmenting the usual laminar flow from a plain foil. So coanda does come into that picture, but not a regular thruster fin? Another question for your friend is what produces the laminar flow on a foil without overlapping flaps/slats?

NeedForSpeed twinzers have some overlap on the front fins, don't they? So looks like a twinzer would produce a nozzle effect as described by the diagram.
 
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griffinsurfboard

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Oct 31, 2004
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More on the "Coanda Effect" from my friend Maria's Fluid Dynamics blog, Theory Bites: (3) : ** Coanda Effect **

Coanda effect is the phenomena in which a jet flow attaches itself to a nearby surface and remains attached even when the surface curves away from the initial jet direction. The phenomena derives its name from a Romanian born aeronautical engineer Henri Coanda.

In free surroundings, a jet of fluid entrains and mixes with its surroundings as it flows away from a nozzle.
When a surface is brought close to the jet, this restricts the entrainment in that region. As flow accelerates to try balance the momentum transfer, a pressure difference across the jet results and the jet is deflected closer to the surface - eventually attaching to it.
Even if the surface is curved away from the initial direction, the jet tends to remain attached. This effect can be used to change the jet direction. In doing so, the rate at which the jet mixes is often significantly increased compared with that of an equivalent free jet.

Many devices use Coanda effect. One notable application of the Coanda effect is the NOTAR™ (No TAil Rotor) helicopter and the Dyson Airwrap.

#knowledgesharing #knowledgethatmatters #fluidmechanics #BasicLearning #fluiddynamics #aerodynamics #turbomachinery

Source: S & C Thermofluids Ltd

View attachment 93602
Looks like we should all be towing our fins outward by 1/2" in 4 1/2 length !!
 

GromsDad

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Jan 21, 2014
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Looks like we should all be towing our fins outward by 1/2" in 4 1/2 length !!
For me I have noticed through the years and countless boards that the boards that perform the best have a tow in that intersects on an imaginary line about 10 inches in front of the nose of the board. Going back to the 80s glass on days the first thing I've always done when checking out boards is to flip it over and sight down from the nose on the center line and see where the toe in was and if both fins were set strait.
 
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npsp

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Dec 30, 2003
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The blog excerpt describes the coanda effect resulting from a stream of fluid being projected from a *nozzle* over the foil - I have read this elsewhere too - the stream can be sheet shaped, but thin. So my question for your friend Maria is that a plain foil - such as the side fin of a thruster doesn't really have a nozzle streaming water over it does it, so the laminar flow does not seem to me that it comes from a nozzle effect?

If I have understood right, then my interpretation of the picture posted in the blog excerpt is that it has a "front slat" overlapping the front upper surface near leading edge and overlapping "extended flaps" on the trailing edge - these would produce a nozzle effect from the overlaps. The word "additional" is used which I take as meaning augmenting the usual laminar flow from a plain foil. So coanda does come into that picture, but not a regular thruster fin? Another question for your friend is what produces the laminar flow on a foil without overlapping flaps/slats?

NeedForSpeed twinzers have some overlap on the front fins, don't they? So looks like a twinzer would produce a nozzle effect as described by the diagram.
Good questions Mr. J. If I can get in touch with Maria I'll ask her. She lives in Spain and is an engineering director for a large pump and motor company so it may take a while to hear back. That said, your comment regarding the lack of a nozzle to direct flow over the foil surface may not be correct as the bottom of the board forward of the fin could be considered a nozzle which directs flow over the foil. Emphasis on "may not be" because I'm not sure....

Regarding your second question, while I'm not a fluid dynamics expert, at some point without the aide over overlapping flaps to maintain/induce laminar flow I think there will always be a point of separation where laminar flow turns into turbulent flow as the fluid moves over the foil towards the trailing edge. Your Twinzer example is a good one as the canard fin acts as the flap (creating a nozzle) allowing increased laminar flow over the foil before separation and conversion to turbulent flow.

Again, I'm not an expert and could be way off base but that's my $0.02.
 

Maz

Billy Hamilton status
May 18, 2004
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For me I have noticed through the years and countless boards that the boards that perform the best have a tow in that intersects on an imaginary line about 10 inches in front of the nose of the board. Going back to the 80s glass on days the first thing I've always done when checking out boards is to flip it over and sight down from the nose on the center line and see where the toe in was and if both fins were set strait.
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!
 
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