Rope

[Paul Arden]

Hi engineers!

So rope gets tighter when wet. I know this because I have rope around the terrace and it's raining! Why is this? Is this because the fibres expand widening the diameter and therefore shortening the length? I'm guessing this doesn't apply to nylon ropes.

Thanks,
Paul

[gordonjudd]

So rope gets tighter when wet. Why is this? Is this because the fibres expand widening the diameter and therefore shortening the length?

Paul,
It depends on what you mean by "tighter."

If you mean a natural fiber rope gets shorter when wet, then your explanation makes sense to me.

As you say it probably depends on the type of rope. The Young's modulus of some rope goes down (and thus will stretch for given load when it gets wet.) A quick check on the web found this information:

Nylon rope has a lot of stretch (up to 40%) and is very strong for its size, allowing it to absorb shock loads well. However, when it is wet it can lose up to 25% of its strength. It wears well, resists mildew and rot, and does not float. Nylon three-strand is the preferred line for dock lines since it stretches sufficiently to dampen the shock of wave action and wind against your cleats. Just make sure it does not stretch too much for the situation in which you use it.

Polyester rope wears better than polypropylene, is almost as strong as Nylon, and retains its strength when wet. It does not stretch as much as Nylon and does not float. Polyester (such as Dacron) is used for sailboat running rigging, anchor rode, towing lines and other applications where you don't want line stretch to interfere. It will, however, chafe easily so check it often and protect as necessary.

Natural fibers such as manila, sisal, hemp and cotton will shrink when they get wet and also tend to rot or become brittle. Manila is still used today on large ships and is the best natural fiber for mooring lines, anchor lines and as running rigging. Manila has a minimum of stretch and is very strong. However, it has only about one-half the strength of a comparable-sized synthetic line.

Gordy

[Paul Arden]

Right, thanks Gordy. I had it wrong then. The fibre is simply shrinking! It's some kind of natural fibre.

Thanks,
Paul

[gordonjudd]

I had it wrong then. The fibre is simply shrinking!

Paul,
No I think you had it right. Its diameter will increase while its length gets shorter.

Also if it was made taut between two posts when dry, then manila rope will increase in tension when it gets wet as described here: Thus your characterization of "tighter" (assuming it was strung-up taut while dry) was correct as well.

When ropes like manila and sisal get wet they will shrink in length and at the same time they will get fatter.

There really isn’t anything you can do to stop this and its important you take it into account because the pull the rope can exert is substantial. I know of a Garden Designer who installed some 18mm Manila taut between 3” posts around a deck and the first time it rained the shrinkage of the rope tore one of the posts off its fixing. I have also heard this was used as an ancient method of torture –

Gordy

[Paul Arden]

Interesting. It's still the properties of the fibre itself however. I had concluded that it was the construction of the rope that caused it to shorten as its diameter expanded.

Cheers, Paul