Rope selection

[Denys]

Hello!
I'm working on the application for off-shore environment. There is a pulley system, were 1 pulley is at sea level. It means that during water level change it stays under the water. It means 12 hours above water, 12 under water. I don't have any experience with such environment, so I'd like to ask for some advise. The question is what sea makes with rope, sheave, bearings. Rope dia 6mm, sheave 80mm, pretension just man force. Are there any solutions known already? now as first idea I choose the Dyneema Dyneestar 605 rope and pulley with POM sheave just on stainless axis without bearing. I can make salt test to check corrosion resistance, but what about algae? Other issues?
Thanks for any advises, thoughts!

[a64pilot]

Do anything you can to keep the block out of the water, I don't think the line will be your issue, yes crap will grow on it, but I've seen many a line submerged for long periods that doesn't seem weakened, looks like crap, but seems to retain it's strength.

[GordMay]

Greetings and welcome aboard the CF, Denys.

[barnakiel]

Yes your challenges will be corrosion and marine growth.

Marine growth does not like copper. Check if this material has use for you.

Corrosion does not affect synthetics and some metals do better than others.

UV will try to eat the components above water too.

Cheers,
b.

[Denys]

Hi everybody.
sorry for my late reaction. Thank you all for the advises. I had an oportunity to make test directly in sea water. After a week of test, the pulley was locked due to sea growth and most probably sand inside of bearing. Rope was fine. As I cannot avoid having pulley under water, so need to try smth different instead of pulley...

[paulajayne]

Look up "frictionless ring" - may do the task for you.

[Suijin]

Let's try something different. Why don't you tell us what it is you're trying to accomplish, the outline of the situation, environment, purpose, etc. and see if we have any ideas that help with a solution. What is the gear lifting? What is the use case? Why does the pully need to be underwater? etc.

Any high-tech sailing pulley and line is going to fail, as you've experienced. In fact, the gear being repeatedly submerged and exposed presents just about the harshest environment possible. You'd be best served by a solution that does not include moving parts for that part of the gear.

[Sailmonkey]

Originally Posted by paulajayne

Look up "frictionless ring" - may do the task for you.

Not quite frictionless, that would defy several laws of thermodynamics!

http://www.apsltd.com/hardware/block...nd-blocks.html

[barnakiel]

The same applies to a spectra rope pulled over a large SS ring or tube. Spectra is friction-less "" and so a movable pulley / bearing can be avoided in some applications.

Have fun designing & building.

b.

[UNCIVILIZED]

In that kind of environment, no type of block will work well. And even low friction rings will rapidly get eaten by seawater, being aluminum. Thus losing their named property, "low friction". You might try a block with a UHMW sheave, or polyethylne. As both plastics should retain much of their lubricity (silpperyness) after a good bit of immersion. And Schaeffer makes some stainless blocks with solid plastic sheaves which will work longer than most other block types. My Searunner centerboard cable used to be routed through one.
Also, as stated, go with a slippery line, like bare Dyneema.

[AiniA]

Could you use a titanium ring?

[Stumble]

Originally Posted by AiniA

Could you use a titanium ring?

You could, but I doubt it would work for long. Titanium has pretty poor wear resistance, so the rope sliding across it would probably eat a groove I need it pretty quickly.

I am thinking of a 316 friction ring, combined with dyneema. And just know the stainless will need to be regularly inspected and replaced.

The other option is an intentionally loose bearing. Basically a sheave with an oversized centerhole compared to the pin. This will be a constant replacement issue since the extra space will cause it to fail prematurely. But the large size will minimize entrapment of crud. The best way to prevent growth is just to use it all the time.

No way around it, this is going to be a short life cycle application.