Not really a fin question, more of a fluids question…Most of my single fin boards have wide tails and have the fin up 12-13 inches from the tail block. If my weight is well infront of the fin I have no spin problems. If I put by back foot over the fin though, the board will spin out if turned or is extremely unstable in a flat trim (as if there is no fin at all)…why?
YOU, big strong guy.
When you turn from forwards, the rail helps with hold, and the bottom resists your power, muting it some.
When you step back, you have no rail, and all your power goes thru TOO SMALL a fin, and you overpower it with both weight and turning power.
Stability against spinning out increases as the fin moves rearward.
Consider a board moving forward with one fin in front of the center of planing mass. If the board leans a tiny amount to one side, fluid flow creates a force that pushes the fin further to that side. As the front of the board moves further to that side, the force increases. It finishes when you spin out.
If the fin is behind the center of planing mass, and moves a tiny bit to one side, the forces on the fin tend to center the rear of the board.
All surfboards steer from the rear, because you cannot steer from the front with a vertically oriented fin (you can with MTB’s kneeboard though, but that would take some significant explanation that is better left to him).
HTH
LeeD & Blakestah are both spot on. But, to simplify, when you are back you are surfing on a minimum surface area and pivoting about the fin. When you are forward your control comes from submerged rail and fin combined.
Hey Lee,
Not much to add to what’s already been said. All things being equal you can optimize your turning power and speed if you get the right fin configuration on the sweet spot for the board and the conditions. That spot can change and so might the fin choice depending on whether it’s 3 foot tubes, 8 foot and kinda soft or 5 foot and real sectiony. Some boards aren’t very versatile when it comes to conditions and fin set-ups while others have a wide range of conditions they’ll perform quite well in.
Even when I find what I think is the supreme set-up for a board I still keep experimenting. I’m compelled to keep learning about how things work. It’s great to have something that you know is gonna rip when the conditions are right but I know I have so much to learn I just can’t help trying something new almost everytime I go out.
Bombs on the way!!!
Mahalo, Rich
Thanks all. I think I get it…but still confused on why it loses purchase when it is not on a rail. I know “cavitation” is not the proper term but that is what it feels like. There is no traction whatsoever when going straight ahead.
in physics for something to remain stable while in motion the centre of mass will always be in front of the centre of pressure …
its a simple rule that cant be ignored when designing surfboards …
what happens when you throw a dart backwards ???
it correctly orients it self so the centre of mass remains in front of the centre of pressure …
lets get that throwing dart and move the tail piece closer to the center …
lets say the darts mass can be moved so the heavy end can slide up and down the main shaft …
if you threw the dart normally the heavy point would stay in front and remain stable in flight … lest get the heavy part of the dart and slowly move it rearwards while its still in flight , as soon as the centre of mass gets behind the tail (centre of pressure ) it will become unstable then do a reverse so now the centre of mass is back in front of the centre of pressure again…
in simple terms thats your problem …
youve probably got some up front rail design that bogs and catches water as well , which will create a pressure drag zone up front , making the problem worse …
regards
BERT
its not that hard … just think back to simple terms of physics and nothing becomes complicated …
Ok…now I get it. Thanks Bert. The tail slide is not really a problem as these boards are supposed to rail turn anyway (very soft rails and curvey bottom up front). You just have to stand in the middle and they work fine. I use the tail slide characteristic to shorten up the turn radius when needed (coming off the top or rebounding on a cutty). I have found that using a swallow tail effectively fixes yet lets you maintain the straighter outline in the tail needed for good rail turns.