| View previous topic :: View next topic |
| Author |
Message |
boardsurfr
Joined: 23 Aug 2001
Posts: 1266
|
 Posted: Fri Dec 12, 2014 10:53 am Post subject: |
 |
|
| rigitrite wrote: | | I think that viscous forces are far greater than any lift created |
The usual definition of "viscous force" is that this is the force "in a direction so as to oppose the flow of the fluid past the object". For a windsurfing fin, this force would be directed to slow you down, not to prevent side slip.
I'll assume that when you say "viscous forces", you mean the resistance that the fin offers against being pushed sideways that is due to the viscosity of the water. I agree with you that this is one of the forces that makes a fin work; I said so in my initial post. When you pump a fin at low speed to get planing, you are using this force. You could say that it is the dominant fin force at this point in time.
The relative contribution of the two forces will depend you your speed. Forces related to viscosity increase in a linear fashion with speed; lift from foil effects will increase with the square of the speed. So the relevant importance will increase at higher speeds. Maybe you need to sail faster  .
There are quite a few empirical observations that indicate that lift is the dominant force at planing speeds. isobar's example that spinouts at deep downwind angles are much easier it one of them. I tried to give an explanation that relies of lift and reduced angles of attack, and I think it's reasonable. A "viscous force" explanation would not work, however. Speed is higher on downwind angles, so "viscous forces" would be higher, and spinouts should he harder, not easier.
Another one is the fin profile (or "foil shape"). For just providing resistance to sideway drift, the profile would be irrelevant (or even slightly detrimental). A board without any profile would do. Indeed, some of the old Starboard beginner boards had boards without any profile that you can just attack to the side. These did work well enough as a dagger board alternative - at non-planing speeds. The faster you want to go, the more the foil-shape of the fin becomes important. The better it is, the smaller a fin you can use for a given set of conditions (assuming flat water).
Fin sizes used in speed surfing are another example. For 30 knot speeds, a 35 cm fin would be typical; for 50 knots, the fin would be 20 cm or smaller. Looking at fin areas for Black Project speed fins, the 20 cm fin has about 50% of the area of the 35 cm fin. Any "viscous forces" that increase linearly with speed would be smaller for the the 20 cm fin at 50 knots than for the 35 cm fin at 30 knots. But lift increases with the square of speed, and would be about 50% higher for the 20 cm fin at 50 knots, despite the smaller area. Without any doubt, the forces at play to reach 50 knots are higher than the forces needed to reach 30 knots.
| rigitrite wrote: | | I pretty much only ride wave or free-style fins |
Typical board speeds in waves and on freestyle gear are below 20 mph. Freeriders can easily reach 30 mph on halfway flat water. The lift produced at the higher speed would be more than twice as much for the same fin. But free ride fins are typically much larger, so the actual lift the fin produces may well be 4 times as high when freeriding. Typical freestyle fins are so under-dimensioned that they do not provide enough lift or "viscous forces" to let you sail upwind; even a small upwind angle requires digging the rails in. Sailing this way, the "viscous forces" are indeed what keeps the board on track (albeit mostly viscous forces from the board, not the fin). But that's the exception, not the rule. Interestingly, the trend among high-level freestylers is towards slightly large fins, compared to just a few years ago. They need some lift from the fin to reach the higher speeds for the new power moves (like Air Kabikuchis).
So rigitrite does not believe that fins provide lift. Fine. The companies that design and make fins believe that fins work as hydrofoils, and produce lift. These companies are often founded by passionate expert windsurfers with a background in aerodynamics or fluid dynamics - for example Kashy Fins (absolutely dominant in formula), Maui Ultra Fins, and Select Hydrofoils. |
|
| Back to top |
|
 |
bericw
Joined: 07 Mar 2006
Posts: 90
|
| Posted: Fri Dec 12, 2014 11:38 am Post subject: |
|
|
I agree with the statements that fins work so who cares, haha! However, the repeated statements contrasting viscous forces and lift are confusing. I don't have Roberson and Crowe's text in front of me right now, but I suggest the definitions of drag and lift in the first (?) paragraph of the referenced Section 11 be made clear. Lift is the force component acting normal to the surface, and can be from pressure, viscous, or both. Likewise drag has both terms, and is the force component acting parallel to the surface. Surely the "viscous vs lift" statements can be conflated; angle of attack such that the viscous term in the lift force is zero and no pressure term, all force on fin is viscous drag.
A more straight forward discussion on lift from a fin would be to focus on whether there are forces, regardless of viscous or pressure, acting normal to the fin surface. To restate, there is no lift force only if there is no normal force component, regardless of source. Certainly the magnitude of lift vs drag varies while sailing. "Correct" sailing? "Correct" balance of forces. The experience of feeling the fin participate in "rotating" the board up? Balance of forces off. An immediate consideration of the potential magnitude of lift, i.e., force normal to the fin surfaces, can be made by the noting the existence of "woodies" or "foamies" on the sides of fin boxes. If there is never significant normal force, why are they there? A simple test might be to put a hinge on your fin so it can fold flat to the bottom of your board, and sail with it on the tack that allows it to fold upwind. Related...and following zirtab's no-fin test...while sailing fin first is fun and funny, I suspect my improved level of effectiveness with the board the right way around is related to having enough normal force component on the fin.
Whatever...Have fun sailing! |
|
| Back to top |
|
|
isobars
Joined: 12 Dec 1999
Posts: 20935
|
| Posted: Fri Dec 12, 2014 12:47 pm Post subject: |
|
|
| boardsurfr wrote: | | Speed is higher on downwind angles, so "viscous forces" would be higher, and spinouts should he harder, not easier. |
My guess? Because of the low AOA, there is almost no lift/lateral force from either viscous or asymmetrical flow, thus giving other forces ... bumpy terrain, ventilation, operator error, erratic air flow in big chopswell, sturgeon, whatever ... more opportunity to interrupt proper tracking.
OTOH, tractus interruptus beats the hell out of pearling at those speeds.
| boardsurfr wrote: | | Typical freestyle fins are so under-dimensioned that they do not provide enough lift or "viscous forces" to let you sail upwind; even a small upwind angle requires digging the rails in. ... Interestingly, the trend among high-level freestylers is towards slightly large fins, compared to just a few years ago. |
I'm glad to hear that, as I always wondered how on earth anyone over four feet tall could play in rough water on one 16-18 cm fin, current or no current. Sure, it spins and slides better, but ya gotta go fast and track well sometimes. I haven't tried the modern, truncated, tiny-area freestyle fins yet for that reason, but I have used what I call the "haystack" fin (dual-elliptical"?) on which Matt took the World FS championship years ago. It looks like a cross between this
and this
with a more rounded tip and cleaner trailing edge.
Because hundreds of elliptical fin images got no closer, I'm guessing I can do better these days in search of a single fin combining early planing power, maximum rail-to-rail transition quickness, upwind ability, solid jumping, and good B&J speed. Suggestions? |
|
| Back to top |
|
|
swchandler
Joined: 08 Nov 1993
Posts: 10588
|
| Posted: Fri Dec 12, 2014 2:41 pm Post subject: |
|
|
"I'm guessing I can do better these days in search of a single fin combining early planing power, maximum rail-to-rail transition quickness, upwind ability, solid jumping, and good B&J speed. Suggestions?"
Look to Dale Cook. Doesn't he use high aspect slalom fins? |
|
| Back to top |
|
|
isobars
Joined: 12 Dec 1999
Posts: 20935
|
| Posted: Fri Dec 12, 2014 3:07 pm Post subject: |
|
|
| I don't know what fins he uses, but he and I sail very differently and on very different styles of boards. |
|
| Back to top |
|
|
U2U2U2
Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
|
| Posted: Fri Dec 12, 2014 4:05 pm Post subject: |
|
|
| swchandler wrote: | "I'm guessing I can do better these days in search of a single fin combining early planing power, maximum rail-to-rail transition quickness, upwind ability, solid jumping, and good B&J speed. Suggestions?"
Look to Dale Cook. Doesn't he use high aspect slalom fins? |
cant compare anything about Dale Cook to me, except when we fall in we both get wet.
Maui Ultra Fin and Black Project Fin , MUF BPF
these both works well for me, in all the characteristics described , kinda being myself on single fin development however
_________________
K4 fins
4Boards....May the fours be with you
http://www.k4fins.com/fins.html
http://4boards.co.uk/ |
|
| Back to top |
|
|
swchandler
Joined: 08 Nov 1993
Posts: 10588
|
| Posted: Fri Dec 12, 2014 4:21 pm Post subject: |
|
|
| From looking at photos of Dale Cook when he's high off the water, my guess is that he's using Tectonics Goldwings. From all my years of using them, they're much more friendly and well rounded in performance than most folks think. Also, the Tectonics F1 Falcon is another sweet high aspect fin. I have a 32cm one that I use on my 80 liter Mike's Lab Wave Slalom, and its super playful in the swell and rough water. |
|
| Back to top |
|
|
isobars
Joined: 12 Dec 1999
Posts: 20935
|
| Posted: Fri Dec 12, 2014 7:34 pm Post subject: |
|
|
I have a couple of fins like the small blade you ride, but I don't use them since their focus is on mph. My favorite fins look more like this:
and run from 18 (plus tiny thrusters) to 24 cm for single fin boards, for sails from 3.2 to 6.2. All my boards but a couple of FSWs are fast (as opposed to rockered-out bananas) wave boards, and virtually all my sailing is in powered-up wave-slashing mode, to the extent the Gorge allows. Anything else is icing on the cake, and I don't even care whether their performance is due to viscous forces, foiling, twist, AOA, or logo. I have a hard time justifying a pile of unknown (to me) fins until I'm convinced they will outperform what I have.
Dale is usually out there for speed as it relates to altitude. I don't need more top speed -- the 25-30-foot jump that took out my knee was on a FSW board with a small and not very fast elliptical freestyle fin -- which is why I modified "speed" with "good" rather than "great" in my post above. My first and foremost demand of a fin is 100% tracking no matter how inexpertly I tell it to go flat out in ANY terrain, in any direction I can pull off from 60 to 200 degrees off the wind (any more and I may as well unhook, flip everything, and call it a jibe), at full power and max g forces, in instantaneous and spontaneous rail-to-rail Xitions ... until I choose to slide.
Failing that, I'm just lookin' for early planing and lots of upwind drive, at which point out comes the dusty old Syncro and its 40 cm blade ... if I can find them. |
|
| Back to top |
|
|
Sailboarder
Joined: 10 Apr 2011
Posts: 656
|
| Posted: Sat Dec 13, 2014 10:05 pm Post subject: |
|
|
| boardsurfr wrote: | [
There are quite a few empirical observations that indicate that lift is the dominant force at planing speeds. isobar's example that spinouts at deep downwind angles are much easier it one of them. I tried to give an explanation that relies of lift and reduced angles of attack, and I think it's reasonable. A "viscous force" explanation would not work, however. Speed is higher on downwind angles, so "viscous forces" would be higher, and spin-outs should he harder, not easier.
. |
Another is the fact that when you spin out, you loose most of the grip. When you loose laminar flow on a side of a foil (either a sail, a wing or a fin), it looses a lot of lift. On a plane, you say you stall it, same happen to your sail if oversheeted. If the dominant force was the so called viscous one, you would not loose it abruptly like you do when you spin out. |
|
| Back to top |
|
|
U2U2U2
Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
|
| Posted: Sat Dec 13, 2014 11:15 pm Post subject: |
|
|
| U2U2U2 wrote: | | swchandler wrote: | "I'm guessing I can do better these days in search of a single fin combining early planing power, maximum rail-to-rail transition quickness, upwind ability, solid jumping, and good B&J speed. Suggestions?"
Look to Dale Cook. Doesn't he use high aspect slalom fins? |
cant compare anything about Dale Cook to me, except when we fall in we both get wet.
Maui Ultra Fin and Black Project Fin , MUF BPF
these both works well for me, in all the characteristics described , kinda being myself on single fin development however |
photo is the BPF and MUFs
_________________
K4 fins
4Boards....May the fours be with you
http://www.k4fins.com/fins.html
http://4boards.co.uk/ |
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You can attach files in this forum
You can download files in this forum
|
|
|
Search
Profile
Log in to check your private messages
Register
