How to tether floats in a tidal zone?

August 4, 2008 10:24 PM

How should I tether a bunch of floating objects just off of a beach? This is for a temporary (4 day) installation.

I want to tether a few dozen floating objects in water a couple of meters deep off, near a beach. The tides here vary by about 1.5 meters, I think. My default plan is to tether each float to a brick or rock with a couple meters of fishing line. I'm wondering, though, what the pitfalls might be with this scheme, and if there are any clever tricks I haven't thought of, for example--
1. most usefully, a way to estimate the length of line I need without having to actually wade out there with a long pole.
2. a way to keep the floats from wandering too far and fouling each other when the tide is low.
3. a way to adjust the lengths of the lines after the fact.

I thought of maybe tethering the floats with short lines to a (heavier-than-water) grid of some sort, and then anchoring the grid with longer lines. That would be a lot more work and I haven't convinced myself that it'll help much.

Whatever I end up doing needs to be cheap and simple enough that I can manage to do it for 20 to 30 floats. Yeah, this is for an art project.

posted by moonmilk to grab bag (9 comments total)

I know nothing about the specifics of this, but if I was going to do it, I would go down to the relevant beach, and I would find out what local government is responsible for maintaining it (ie, who besides you gets sued if someone gets tangled in your line and drowns), and find out from them (a) what permits if any are required to do this (it may be easiest to call it a buoy, and it may be easiest to attach it to a non-functional lobster pot, if it's legal at the time and place to put out lobster pots); (b) who is best to ask your questions of, ie experienced local fishermen, lifesavers, boat owners etc.

I suspect that the answer to your questions, especially #2, may be heavily dependent on local tides, water clarity, mud, fish, boats, and other factors neither of us is aware of, but presumably, local fishermen etc will be. (Particularly, "likelihood of interference by local fishermen").
posted by aeschenkarnos at 10:42 PM on August 4

If the body of water has much wave action and/or a sandy bottom, bricks and rocks are going to move out of place very quickly.

You might want to look into the setups duck hunters use to secure decoys. They use a product called "Tangle Free," which is a line that...well, resists tangling. It is so resistant that it will untie itself out of knots, so you have to use metal clips instead. There's also a variety of decoy anchors ranging from simple lead weights to more elaborate contraptions which will grab onto any bottom vegetation and help set your line.
posted by jamaro at 10:47 PM on August 4

The engineer in me is already sketching out ways to do this. I've never experimented with this sort of thing before but it seems like a fun project.

Basic setup: A very heavy brick is the anchor, with a line that is simply attached to the float. But the float will bob up and down and move around a lot.

To keep the buoy from moving around laterally, you could use more than one anchor -- maybe three, in a circle, spread out, with three ropes. If the ropes are taut, then the float can't move.

But you want the float to move up and down with tides. So instead of attaching the ropes to the float, attach a metal ring to the float and loop the ropes through the ring. The other end of the ropes attach to a smaller weight. This weight sits underneath the float and is suspended in the water. When the water is high, the buoyancy of the float makes it lift the weight higher up in the water, effectively lengthening the rope. When the water level falls, the weight sinks and pulls the lines taut, effectively shortening the rope.

Problems: waves will knock it around, I'd be worried about fishing line snapping, especially if it's rubbing against a metal ring. Choosing the right weight would be a pain, would need lots of experimentation. Also if the bricks move around, you're in trouble.
posted by PercussivePaul at 11:27 PM on August 4

How about this: run a line from each float down through an eye bolted to a cinderblock, then back up to a smaller-displacement float with an eye around the line coming down, such that the total line length is less than twice low water but more than high water. If the current isn't extremely strong, this should serve to take up the slack in the line as the tide changes and keep the lighter float nearly above the weight.

(Note that I've been around harbors and moorages all my life and never seen anything like this in operation, so maybe there's some reason it won't work - but it seems plausible to me!)

(On preview: heh, this is just the opposite of Percussive Paul's idea - and it's probably easier to get and tune a weight than a second float!)
posted by nicwolff at 11:41 PM on August 4

In addition to the above, recognize that what anchors a ship to the bottom isn't the weight of the anchor itself, but the weight of the chain the anchor is attached to. The anchor is merely the bottom of the chain. If you anchor something with lightweight fishing line, it is certain to move about. If you anchor the floats with something heavier, like chain or rope, you'll get better results overall.
posted by Cool Papa Bell at 12:51 AM on August 5

When anchoring a boat the rule of thumb is to play out about 4 times more rope than the depth of the water you are in (this might vary somewhat with type of vessel or anchor). As Cool Papa Bell points out the weight of the chain should serve to keep the rope taught at low tide.
posted by rongorongo at 2:20 AM on August 5

Are you planning to do this in New York? Depending on the exact location, this activity could be regulated by the state (NYS DEC), the local government (county, town, or village), the Coast Guard, or the Army Corps of Engineers. I know that the DEC wouldn't bother with single moorings (if they're small and only going to be up for 4 days), but if you have a lot (20-30) you might attract some attention. I would suggest calling around to some of these agencies to get a feel for whether or not you will need a permit.

As for knowing how much line you will need--well, the best way of figuring that out, unfortunately, is to go out with a stick at low tide and high tide so you can get a feel for the tidal range.

For mooring small floats in a sandy substrate, I have used pieces of re-bar with one end bent to form a closed loop (I believe the place we bought the re-bar from had a machine to do this). Then, you tie on your floats to the loop end using fishing line or chain or whatever, and you hammer the straight end of the re-bar into the sand, about a foot deep. It worked pretty well to prevent our marker buoys from drifting away, but whether or not it will work for you will obviously depend on the current strength, size of your floats, etc.
posted by Jemstar at 7:36 AM on August 5

Thanks, everyone, for your input! I'm definitely going to look in to decoy rigging -- whether I buy a rig or make something, it's definitely a similar application to mine. (Though I'm still tempted by the simplicity of the one-brick-per-float approach!) Thanks also for the insight that the weight of the rope contributes to the anchoring effect-- maybe I should experiment with using heavy lines rather than monofilament.

A possible answer I'm considering to my question 2 is "don't bother" -- it's only going to be there for 4 days; maybe it's not worth worrying too much about tangling.

Thanks also for your concerns about the legal issues. Luckily, the festival organizers are dealing with that stuff so I don't have to.
posted by moonmilk at 9:57 AM on August 5

Thanks also for the insight that the weight of the rope contributes to the anchoring effect-- maybe I should experiment with using heavy lines rather than monofilament.

While using something a little beefier than fishing line is probably a good idea, nothing but fairly hefty chain will be heavy enough to sink to the bottom at low tide.
posted by ssg at 12:09 PM on August 5