Experiments with GSP Biminis.


[Bulletin Board]


Posted by GES on 2005-08-07 17:06:53

The Bimini Twist is simply a very complex slip knot. You fold the end of the line over to make a loop (say around a pin), and you press the tag end up against the main line to create friction between the lines.
When you pull the main line away from the pin, the friction between the two lines applies tension to the tag end, which tries to stop the line from slipping around the pin. If you create enough friction, the line doesn't slip around the pin, so you get half of the tension in each line; at the loop end.
As you move along the tag end from the loop, the tension in the tag end drops, while that in the main line increases, until it reaches the full tension ahead of the 'knot'. The transfer of tension between the two lines is the result of the friction. There is also a requirement that the main line stretches more than the tag end, along the contact area.

This makes common sense, since in the tag end part of the 'knot', the tension goes from zero (0) up to T/2, so it averages T/4, while in the main line the tension goes from T down to T/2, so it averages 3T/4; or three times the average tension in the tag end.

So assuming the materials stay within the elastic range, the main line part of the 'friction pair', will stretch three times as much as the tag end stretches. It is that differential stretch, that accomplishes the tension transfer between the two lines by friction.

Now to push the two lines together, you don't really have to twist them; suppose they simply remain straight and parallel, and you simply overwrap them with the tag end of the line, with some tension in it.

Now in monofilament under tension, there is considerable stretch, and this length extension is accompanied with diameter contraction. The ratio of the diameter contraction, to the length extension, is called 'Poisson's Ratio, and it is a characteristic property of elastic materials. If the ratio is exactly 0.5 (half) then for small extensions, the volume of the material remains exactly constant; the extra length exactly compensating the reduced crossectional area. So the density of the material would not change when stretched.

A seat of the pants feel would suggest that when we stretch something, the energy we are applying should pull the molecules or atoms apart, so we might expect the volume to increase, and the density to drop; and typically it does.

So most materials have a Poisson's ratio less than 0.5, and 1/3 is pretty common.

In the Bimini Twist, the stretch as we tie the knot, reduces the line diameters, and the overwraps go on under some tension so the overwrap line also stretches and thins down.

The end result is that when we terminate the 'knot', with zero, one or two half hitches, and the final cap (maybe Rizzuto style), and stop applying tension, the mono springs back, and the two parallel lines grow fatter, as does the wraps, so the wraps push the two lines together creating the friction that we seek.

Actually the wrapping stops the lines from going back to their original diameter, and that prevents the lines from returning to their original length. Poisson's ratio works backwards as well. This is the same process that creates spool bursting side pressure when we wind mono on a reel under tension.

Now in the actual Bimini, we complicate the picture and increase the friction by twisting the two lines together instead of leaving them straight and parallel, as I described above.

By twisting the two lines together, we in effect created a threaded bolt and nut combination. If you take two kid's slinkies, you can screw them into each other just like an ordinary threaded bolt and nut. And that is what we have created when we twisted the lines in the Bimini.

Now when you screw a nut onto a bolt, or two slinkies together, you quickly find out than you can't simply pull them apart lengthwise. There is so much friction between the shallow sloping walls of the thread that you can't slide them against each other.

So that effect is added to the friction in the Bimini slip knot. Except the line is not a rigid material, so when we pull on it the two lines do in fact try to unravel around each other like a couple of intertwined snakes, but creating a lot of friction in the process.

But what of GSP. It's easy to see how the Bimini works in squishy nylon, but GSP is much harder material, and it doesn't stretch anything like nylon, so the Poisson diameter thinning process is going to be much less in GSP, even if both materials had exactly the same value for Poisson's ratio. I don't have any idea what the value is for either material, but I doubt they are much different.

So GSP Biminis don't generate the same sideways pressure that mono ones do for the same number of turns. Also GSP has a significantly lower coefficient of friction than nylon, so with a lower side pressure, and lower coefficient, the friction generated is a lot less, which is why they tell you to do 70 twists for a GSP Bimini.

Now I believe Bill Nash says that you never really reach 100% strength with a GSP Bimini, even with 70 turns.

Now I don't kow about you all, but I have never tied a 70 turn Bimini in my life.

I can afford to get less than 100% with GSP backing if I use 50# stuff, so what is of more interest to me, is the following question. If the Bimini twist is a slip knot, just how many turns do you need in GSP so that it doesn't slip before it finally breaks.

Russ and I talked about this in Loreto, so I decided to do some tests with 50# Australian Bubonic Braid to see what happens.

So I started with a classical 20 twist Bimini, finished off with a single half hitch and a six turn Rizzuto-ized cap.

I put the loop around my metal pliers handle so it could slide, and then I pulled like hell on the line.

And eventually, it slipped. Man was that ever beautiful; it was smoother than any reel drag I have ever felt, but the darn knot stayed intact and simply slipped down towards the loop end shortening up the loop. Absolutely no line damage occurred, but on close inspection the wrapped twisted portion had taken on a slightly wavy profile instead of the dead straight that I started with. Very subtle but it is there; the two snakes didn't exactly co-operate completely as they were slid along each other.

So then I tied a 30 twist Bimini, and so far I haven't got it to slip yet, and I didn't break the line yet either.

But remember that in the water, those two snakes are going to be well lubricated, so they may not protest as much.

I guess I will play with this some more, and see just how many twists does it take to stop the world's most complex slip knot from slipping in a GSP line.

I'm betting it is a lot less than 70 turns. I'm likely to stop at whatever that number is.

Close enough for me.

George


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