Thoughts on Dyneema for guard rails?

josephmoore said:
Sounds like a big number, but:

F = 1/2 m v2 / s

Where s is the slow down distance of the impact (citation here: http://www.engineeringtoolbox.com/impact-force-d_1780.html)

So... rough and ready, but for an 80kg man (at least, he'll be wearing clothes, you'd hope!) falling across a foredeck (say an vertical actual height of 1m?) with a guardrail deflection of what, 20cm? Far enough to bend a stanchion but not snap it off or stop it working?

1/2 * 80 * 4.43^2 / 0.2 = 3924.98N

Now that number will obviously be spread across the tension in the guardwires and several stanchions, but it seems to me like if someone had a pretty big fall against one (certainly on your average size yacht) the chances are they'd crumple and absorb some of the impact. Certainly if someone fell hard against my stanchion I'd expect it to stay attached and the guardwires not to snap, but I'd be replacing it when I got home.
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Wouldn't F = m g h / s be more appropriate?
 
Ok :D

Well then. I would say probably not as we're not talking about an immovable object in the stanchions and guardwires, but one which has some elasticity in the system which is why I opted for F = 1/2 m v^2 / s which takes the distance to slow the object into account (the further the distance, the less shock load there is)

The velocity squared part takes into account gravity. Instantaneous velocity at the end of the fall is calculated with this: v = sqrt(2gd) (http://en.wikipedia.org/wiki/Equations_for_a_falling_body) where g is gravity at 9.8ms^2 and d is the vertical distance in metres) - not perfect here as you're not falling down onto the wires, but probably sliding across the deck, or tumbling head over heels after tripping over so I assumed a pretty small distance of 1 metre which gave the velocity of 4.43m/s.

Seems right to me, but I'm no physicist or mathematician. I could be wrong!
 
On our old 6 ton falmouth pilot we had wire rails I changed them to 8mm braided line. On a trial for man over board I found that when trying to get on board from the water getting under the wire was almost impossible. But with the rope I just cut it and rolled onto the deck. 8mm braid is as strong as 4mm wire and easier to attach as you don't need any special tools.
 
theguerns said:
On our old 6 ton falmouth pilot we had wire rails I changed them to 8mm braided line. On a trial for man over board I found that when trying to get on board from the water getting under the wire was almost impossible. But with the rope I just cut it and rolled onto the deck. 8mm braid is as strong as 4mm wire and easier to attach as you don't need any special tools.
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You can do the same with wire rails if you use lashings at one end to connect to the pushpit. You just cut the lashings and create 3 or 4 ft of slack. You can be back in business in 5 minutes with a new lashing.
 
skipper_stu said:
The height is such!
I see them as a phsychological thing and fender line anchors only!
Stu
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Who does this and do they like the idea of having to reseal their stanchions every other year?
LadyInBed said:
I''ve been thinking about using bungee cord!
It will stop people trying to move my boat by pulling and pushing on it :mad::D
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Oafs and morons.
 
KellysEye said:
Don't do it, no other yacht has Dynema guard lines, stainless wire is used for good reasons, strength and no abrasion.
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No so, boats do use dyneema, it's stranger than steel wire rope and chafe Isn't an issue with just a tiny bit of care.
It's used in logging, oil rig mooring, trawling, bullet prof vest and many other applications.
Amazing stuff, shame it's so expensive.
 
vyv_cox said:
Parafil was mentioned earlier. Not sure what the material is but it is a man-made fibre, not metal, plastic coated. My first boat had guard wires(!!!) of it, no problems on a very widely travelled vessel.
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Never heard of it, so looked it up :)

http://linearcomposites.net/?pageid=Parafil.xml

Seems to need their particular end fittings rather than being spliced as per a standard 12-braid dyneema, but you probably end up with a more reliable/testable/certifiable system at the end? They show case studies for all sorts of industrial applications so I guess it will have to have met plenty of Standards! I like the way it can be combined with various different outers to produce a product which is particularly flexible, abrasion resistant or flame retardant depending on the specific application.
 
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