Leash plug holding strength

General Discussion
1

It wasn’t until I got a 10’ classic longboard with a drill box leash plug that I questioned where do regular shortboard leash plugs get their holding strength.  I just don’t see how a small cup of plastic with a bar that spans it for a leash string stays attached to the board with just some adhesive, fiberglass weave and resin.  It just seems to me that first wave of any size that pulls on the board and the leash pulls back on the board attached to the surfers leg, the thing would pop right out of the board.  I am hoping someone into shaping and glassing can explain why after decades of surfing I have never had one pull out of a board and I’ve never heard of any surfer having one pulled out of a board except for big wave rider Greg Long on a huge wave at Cortes Bank.

 

2

Because there is not so much loads…

Plus elastic leash cut impact loads.

3

Plug/leash attachment point is at one end of the board so big majority of the force is going be applied parallel to length of the board and not directly perpendicular. (If it were the perpendicular, yes, likely would pop the plug out.)

4

Pretty much what’s been said, but lets take a closer look at it- 

First off, Dr. S is right, the strain on the board is usually along the length of it, not all that much area or drag, so that you really don’t get that much strain on the little round plug. On a longboard, well, there’s a lot more of that, the through-the-finbox leash attachment is a pretty good idea. 

Then there’s the leash itself. As mentioned It soaks up a lot of the pull.** F=kx **where the force is F, the length change in a springy sort of thing  like a leash is **x **and you have a spring constant k . That constant is pretty low for urethane leash cord and the leash stretches accordingly. When ya think about it, if it was pulling that hard, it might lead to knee damage or a dislocated hip. 

The bigger the wave or the bigger the board, the longer the cord and the more it could stretch and the more force it could soak up. Beyond that, typically the leash breaks, as it should,Way easier to tie on a new leash than to repair a leash attachment that’s been ripped out by the roots.  

The very first leashes were straight nylon cord which really didn’t stretch much at all ( ouch) , which were followed by surgical rubber tubing ( as used in the best slingshots) with a much longer nylon cord inside it or plain bungee cord, both of which stretched out nicely and absorbed the strain but then the board came back, **fast **and generally with the sharp edge of the fin coming first.  Plus, both would only stretch so far and then they stopped stretching, which made for a rude tug on the ankle. 

hope that’s of use

doc…

5

I’ve never had a leash plug pull out of one of my boards.  I’ve had a pin pull out of the plug and I’ve had the rail saver pull through the rail.  Never had the plug itself come out though.  More than anything it is because the forces on the leash plug are almost always at 90 degrees to the direction it would take to pull the plug out of the board.

6

Doc,

Thank you so much for your thoughtful reply.  I really appreciate you bringing physics to your explanation.  As a physics minor, I’m actually a bit embarrased I didnt think it all the way through.  My thinking was very shallow just imagining a shock load to the leash plug from the leash pulling back from the attached rider at an angle more than 45 degrees relative to the board.  You thought it all the way through with the spring constant (k) reducing the Impulse = F*(deltat) = F(kdeltax)deltat.  Furthermore, the force of tension in the leash more often than not is being applied close to parallel with the board.  All of these things factor in.  Another thing I didn’t consider that the board and rider generally fall in the same direction, Newtons first law. It’s rarely the case the full force of the wave grabs the board and takes off while the rider is at rest underwater. And also even if that were the case, as you mentioned the elasticity of the leash would factor in and and the leash pulling parallel to the board would factor in.

The first and only time I’ve broken a leash happened with a longboard and the context is very telling.  I was riding a 10’  Takayama Model T glassed to the hilt with a drill box leash plug that went through the whole board and the knot was tied off in the fin box.  I’m not seeing the entire fin box getting yanked through the board as a probable outcome, in contrast with a shortboard with a cup plug   The thing feels like it is 30 lbs carrying it down the beach.   Anyway, I was paddling back out one day and punched through one wave and continued on when a really fast wave right behind it was jacking up and getting ready to pitch right on my head.  The last thing  I wanted was to have a lip detonate on my head with that heavy 10’ board so I threw it away from me (no one was around, surfing alone).  I dove underwater after I threw the board.  That wave grabbed the board and took off like I had my leash tied to a dump truck.  That leash broke like it was made of fishing line.  The shock load of it was way too much since I was basically at rest underwater and the board was accelerated very fast in a short period of time.  So with I = F*(deltat) = (ma)(delta*t), the mass (m) of the 10’ log being way more thank a potato chip short board comparably and probably a larger acceleration due to the larger volume for the waves to act on the board and pull on it, the impulse spikes big time. 

I realize I’m geeking out here, but it’s fun. I’m sure there are several other factors that could be explored, but you captured the essence of it.   Thank you again for the break down. I guess  I was just so fixated on this cup plug being being blasted by a leash pulling back on it, I figured it shouldn’t stand a chance of staying in.  But that is a lame excuse for not thinking it through.  Sorry for being lazy,  I don’t know what the hell is wrong with me.

7

 

Thank you so much for your thoughtful reply.  I really appreciate you bringing physics to your explanation.  As a physics minor, I’m actually a bit embarrased I didnt think it all the way through.

(chuckling) Thaks for the kind words- I was once a tutor, several subjects including physics. And once started, it never leaves ya. In any event-

My thinking was very shallow just imagining a shock load to the leash plug from the leash pulling back from the attached rider at an angle more than 45 degrees relative to the board.  You thought it all the way through with the spring constant (k) reducing the Impulse = F*(deltat) = F(kdeltax)deltat.  Furthermore, the force of tension in the leash more often than not is being applied close to parallel with the board.  All of these things factor in.  Another thing I didn’t consider that the board and rider generally fall in the same direction, Newtons first law. It’s rarely the case the full force of the wave grabs the board and takes off while the rider is at rest underwater. And also even if that were the case, as you mentioned the elasticity of the leash would factor in and and the leash pulling parallel to the board would factor in.

Well, if you think of the leash as a simple spring or something elastic, it kinda jumps at you.

Now, springs are modelled as a straight linear function, but really when you get to the liimit of the stretch it has, elastic limit if you like, the force per unit extension goes up, in fact K for a lot of things is a function of X, consider say a bow or something like that. F=kX^n, perhaps. But you can just go with F=Kx for a given F, work off a table or whatever. 

And, you don’t want to overcomplicate anything either. All you want to consider is the board relative to a fixed point for this particular problem. The water pushing it away makes for a certain force on the board and there ya go. Now- 

The first and only time I’ve broken a leash happened with a longboard and the context is very telling.  I was riding a 10’  Takayama Model T glassed to the hilt with a drill box leash plug that went through the whole board and the knot was tied off in the fin box.  I’m not seeing the entire fin box getting yanked through the board as a probable outcome, in contrast with a shortboard with a cup plug   The thing feels like it is 30 lbs carrying it down the beach.   Anyway, I was paddling back out one day and punched through one wave and continued on when a really fast wave right behind it was jacking up and getting ready to pitch right on my head.  The last thing  I wanted was to have a lip detonate on my head with that heavy 10’ board so I threw it away from me (no one was around, surfing alone).  I dove underwater after I threw the board.  That wave grabbed the board and took off like I had my leash tied to a dump truck.  That leash broke like it was made of fishing line.  The shock load of it was way too much since I was basically at rest underwater and the board was accelerated very fast in a short period of time.  So with I = F*(deltat) = (ma)(delta*t), the mass (m) of the 10’ log being way more thank a potato chip short board comparably and probably a larger acceleration due to the larger volume for the waves to act on the board and pull on it, the impulse spikes big time. 

Umm, I wouldn’t be all that concerned with the mass of the board. The area of it, the water resistance, yeah, definitely. You have, in the case of a 10’ board, a whole lot of area, which the wave takes and whoosh, there ya go. And yes, a lot more area and resistance than a short board. 

On the other hand, lets say you stood on a 10 meter diving board with the very same surfboard and attached the leash to something and threw it down, like a spear, as hard as you could. Film it,or video it, see how much extension of the leash you get and how long it takes to where it’s starting to stretch the leash- v=vnought plus gt. Or, if you like, a bucket of something that weighs the same as the board. Same deal. It’s a momentum problem. f=mv. 

Now, experiment 2. If you take, say, a fishing scale or similar. attach the leash at one end to something solid and start pulling backwards. Start recording what force it takes to stretch the leash a given distance. Give you an idea what it takes to stretch the thing a typical distance. 

I don’t think the leash was snapped, by the way. Stretched to it’s limit fast, yeah. May have had a nick in it too. And it snapped. 

I realize I’m geeking out here, but it’s fun. I’m sure there are several other factors that could be explored, but you captured the essence of it.   Thank you again for the break down. I guess  I was just so fixated on this cup plug being being blasted by a leash pulling back on it, I figured it shouldn’t stand a chance of staying in.  But that is a lame excuse for not thinking it through.  Sorry for being lazy,  I don’t know what the hell is wrong with me.

Heh- it is really easy to overthink something. Among my many faults, it is. But don’t look at it like a physicist, it’s an engineering problem, good approximations are fine, a little testing and measuring and see what you can draw from it all. ‘Surf science’ is a bunch of handwaving, after all. 

Have fun

doc…