Fin Material

dllee · Joined: 03 Jul 2009 Posts: 5329 Location: East Bay

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.

sailingjoe · Joined: 06 Aug 2008 Posts: 1087

Sailboarder · Joined: 10 Apr 2011 Posts: 656

dllee · Joined: 03 Jul 2009 Posts: 5329 Location: East Bay

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.

GURGLETROUSERS · Joined: 30 Dec 2009 Posts: 2643

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

dllee · Joined: 03 Jul 2009 Posts: 5329 Location: East Bay

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".

GURGLETROUSERS · Joined: 30 Dec 2009 Posts: 2643

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!

Sailboarder · Joined: 10 Apr 2011 Posts: 656

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.