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DanWeiss



Joined: 24 Jun 2008
Posts: 1889
Location: Connecticut, USA

PostPosted: Fri Aug 17, 2012 9:51 am    Post subject: Reply with quote

sailingjoe wrote:
SNIP When I worked for Herman's World of Sports when it was the largest sporting good's retailer in the country, one of the district managers told me in a very emotional way that you don't make money on the expert skiers because they wait until the end of the season to buy at heavily discounted prices. He was defending the fact that they carried mostly middle and low end goods. I found that very much to be the case as I made my way in the sporting world. I serious doubt if an expert windsurfer would ever buy a fin for $1,200 nor wait that much time for delivery.
SNIP


Ever heard of Kashy fins? Approaching $1,100 from what I hear, with wait times of over six months. Even if one's first name begins with "R" or "B" or "A".

Quote:
Actually, there doesn't seem to be a commonly accepted definition of aspect. However, I see the ratio of cord to span as the best. Of course your cord will change from base to tip which makes for confusion. Low Aspect has a wide cord at the base. High aspect has a wide cord at the tip. A longer span will make for a lower aspect, but it will always require a change in the relative cord measurements. Please correct me if I am wrong.
[SNIP


OK, I will.

The span of a fin is what we would measure as the depth or length of a fin. So, a 70cm FW fin has a span of 70cm.

The chord line of a fin is usually measured at the root, from the trailing edge to the middle of the leading edge's curvature.

Sometimes we talk about chord thickness, which refers in common nomenclature as the thickness of the foil. (Looking at it head on, is it thick from left to right?) Understandable confusion exists because the term, chord thickness is bounced around. This makes sense because the thickness of a foil is only significant as a function of its chord length expressed as a percentage. This is why most windsurfing fins start thick at the root and gradually decrease thickness toward the tip. They become narrower at the tip to reduce tip-induced stall, although fin twist greatly affects the need for tapering.

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isobras



Joined: 17 Jul 2012
Posts: 439

PostPosted: Fri Aug 17, 2012 10:48 am    Post subject: Reply with quote

ramps wrote:
Watch an airplane's wing on a bumpy flight, there are several inches of deflection, easily seen by the naked eye.

Boeing 707: 27 feet.
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zirtaeb



Joined: 03 Jul 2009
Posts: 2144

PostPosted: Fri Aug 17, 2012 11:44 am    Post subject: Reply with quote

One only has to recall the amount of backfoot pressure when planing in the straps. Most of that pressure is held by the fin.
Now think. If you put your body weight, and sometimes more, pushing sideways on the fin, how much pressure IS there down there?
In the world of sailboats, almost every centerboard connection is considered "overbuilt" by the smart/brain/nerds" guys. But it has to be, or the delam occurs around the centerboard. Real world application over computer genius design.
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sailingjoe



Joined: 06 Aug 2008
Posts: 1087

PostPosted: Fri Aug 17, 2012 12:23 pm    Post subject: Reply with quote

DanWeiss wrote:


Ever heard of Kashy fins? Approaching $1,100 from what I hear, with wait times of over six months. Even if one's first name begins with "R" or "B" or "A".



Yes, but do the experts buy them at those prices?
isobras wrote:
ramps wrote:
Watch an airplane's wing on a bumpy flight, there are several inches of deflection, easily seen by the naked eye.

Boeing 707: 27 feet.
Frightening, isn't it? Losing a wing is a nightmare, but losing a wing and engine is pure death. That's why the planes that have their jet engines next to the fusilage are considered safer. Nevertheless, pilot error more than likely will be the reason for your demise as it is for most accidents. Here's one for you. By far the greatest fighter pilot during WWII was a man named Marseilles who flew for the Nazis. He was of French descent, but obviously born and bred a German. He wasn't shot down and in fact never took enemy fire. He died during the war due to engine failure. I have a photo of him with his mechanics.
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Sailboarder



Joined: 10 Apr 2011
Posts: 323

PostPosted: Fri Aug 17, 2012 12:38 pm    Post subject: Reply with quote

zirtaeb wrote:
Now think. If you put your body weight, and sometimes more, pushing sideways on the fin, how much pressure IS there down there?


About 60 pounds on the fin according to Drake. This would be for a 190 sailor in static equilibrum. I guess it can increase by 50% or more in dynamic mode, such as a bad jump on chop.

http://joewindsurfer.blogspot.ca/2008/04/jim-drakes-windsurf-physics.html
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zirtaeb



Joined: 03 Jul 2009
Posts: 2144

PostPosted: Fri Aug 17, 2012 1:02 pm    Post subject: Reply with quote

So, clamp the base of your fin down to a solid vise, and bend the fin with 60lbs of force. That's the AVERAGE deflection.
In another, though related vein.... I was sailing at Kailua a few years ago, and TomStone sailed by yelling at me. He came back around while I was standing on a reef, and told me my universal was stretched out beyond the rails of my board. I sailed in. On shore, nobody could tell what was wrong, as we couldn't simulate the stretched uni. We could displace it about 1" maximum, with one guy holding each end of the base.
The forces of sailing around is more than we know.
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GURGLETROUSERS



Joined: 30 Dec 2009
Posts: 1247

PostPosted: Fri Aug 17, 2012 1:41 pm    Post subject: Reply with quote

I'm not clear how, in the 80's, those old longboards with plastic fin boxes and U.S. fitting fins stood up to the loadings.

A couple of us had great fun charging up against a very fast tide race at the entrance to a Scottish Loch as the water drained over a reef on a falling spring tide. There was a clear 6 to 8 foot drop in water level over about a 100 yard range, complete with violent eddies and very prominent little whirlpools with central holes. (They were moving down with the flow before dying out.)

As you hit one at speed the back end of the board jolted violently first one way then the other. The effect was as if the board was pivoting around the mast foot over which my weight was concentrated, so, in effect, the rider just kept pointing straight ahead (apart from feet and legs) as the board did its front and back end swing. Certainly, my weight couldn't have been punishing the fin and box.

Incidentally, the fun came to an abrupt end when I accidentally dropped the rig when a bit off balance, and the mast and sail tried to force down vertically into one of the whirls, almost pulling the board over. No way was I going to chance falling in after that! 200 litre longboards have their uses after all. Laughing
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zirtaeb



Joined: 03 Jul 2009
Posts: 2144

PostPosted: Fri Aug 17, 2012 2:00 pm    Post subject: Reply with quote

First of all, NONE of the '80's boards with glued on fin boxes and plastic fins held up to active usage for longer than one season. If the user was active, sailed lots, and used them in planing winds, EVERY board failed at the finbox area, centerboard area, and mast track area.
It's when the board was stored for most of the year in a garage that they lasted multiple years, and used in 3 mph breezes.
Long fins those days were low aspect, wide chord, 9" tall fins. Low aspect to increase chord for some semblance of "durability".
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GURGLETROUSERS



Joined: 30 Dec 2009
Posts: 1247

PostPosted: Fri Aug 17, 2012 2:15 pm    Post subject: Reply with quote

My plastic fin boxed and plastic finned Bic Be-Bop long board covered just under 3,000 miles (of my total of over 5,000 miles of sea journeys) between 1986, and early 1990. Fact!!! (Other boards added the rest.)

The plastic fin box did NOT fall apart, and the board was often used in stronger winds. (Unavoidable on journeys in SAcotland where, on any occasion it can vary from force 2 to force 6. As I detailed earlier, I broke the fin tab clouting a rock, but the fin box was undamaged.)

Of course the fins were low aspect broad based. Who ever claimed otherwise?

Edit extra. The mast track was replaced after the first year with a bolted in fixed position one to avoid having to replace the plastic sliders which easily broke. Also, the centre board box was reinforced with longer screws, and I made a wooden dagger board to replace the plastic one. The A.S.A construction was weak, and the board became waterlogged BUT, the fin box did not fail!
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Sailboarder



Joined: 10 Apr 2011
Posts: 323

PostPosted: Fri Aug 17, 2012 2:26 pm    Post subject: Reply with quote

My Sailboard Vario lost its US box last year in Hatteras. It was only glued in a depression molded in the ABS. Not bad after 30 years, but Zirtaeb was right that it was almost never sailed on the plane.

By luck, we found the fin and the box and I glued it back in place. Too bad the board took water at some point since it is still the family favorite. It now stays at the cottage.
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