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Saturday, February 26, 2011

A floating breakwater - but....

Where I worked we grew fish in floating cages and we needed to protect them from the waves that weakened the structure and made work difficult. We found a plan for building a floating breakwater out of discarded tires and built up a breakwater large enough to protect the rafts. It was very effective but.... we never did get the fastening system perfected so there was a fair bit of maintenance needed. If someone managed to improve on the fastening, these breakwaters would be a very practical prospect. I'll try to describe them for anyone who wants to have a try.

To make up a module you must start with 9 tires of the same size. (a larger module of 16 tires can be made but if you are using fairly large tires, these can be pretty heavy to shift around). Start by standing three tires on the ground (in the same orientation that they would be in on your car). Put them in a line with the treads touching. Put two tires in the same orientation on either side of the two. They overlap like bricks in a wall. Put a single tire on the outside of the two. Fasten them together (I will go into the fastening below). You now have a diamond shaped unit of 9 tires. If you start with 4, 3-3, 2-2, 1-1, you will produce a unit of 16 tires. Push these units into the water and fasten them together end to end and side to side. You are aiming for a breakwater that is at least as wide as the length of the wave you are wanting to absorb. The way they work is as follows.

The water in a wave doesn't move forward. If you were to suspend something in the water and watch it as a wave goes by, you would see that it moves in a circular path. This floating breakwater connects the rotation of a peak with the rotation of an adjacent trough. They are moving in opposite directions. What happens is that this motion is turned into turbulence and the wave is absorbed.  The shorter the wave , the more efficient the system so you remove all the chop and short waves and are left with swell with a wave length which is longer than the width of your tire breakwater.

Flotation is provided by the air trapped in the top of the tire. As long as there is some wave action, the turbulence stirs air into the water and it gets trapped in the tire. A very long period of calm weather can result in the breakwater sinking but it is easily raised by going down with SCUBA gear and letting some air into the tires. A single diamond is quite enough to float a couple of people and once the tires are joined in an array, there is plenty of flotation for anything you want to put on the unit.

For anchoring, there are a number of reasonably inexpensive solutions. Simple 45 gallon drums filled with concrete with a large steel eye at both ends is a good solution. They are joined with lengths of chain, one behind the other. They dig into soft bottoms pretty well and as long as you have a good length of chain at the anchor end of the anchor line, they give good anchoring. You can  use polyprop rope for much of the connection to the breakwater. The principle is that the chain, where it meets the first drum in the series, should never be at an angle to the bottom. You need enough chain so that in the worst conditions, it is parallel to the bottom where it connects to the first drum.  The leading edge of the drums digs in and provides very solid anchoring.

Actually, while I have been typing this, a thought just came to me. I bet you could get quite an effect from a vertical wave barrier. Simply take some of your diamonds and drill small holes in the top of each tire so they won't float. Fasten them under floating diamonds. The principle here is slightly different although the same at its core. It depends on the fact that for every ninth of a wave length you go down into the sea, the circle of action of a particle decreases by half. Take a wave of 1 metre high and 9 metres long. At the surface, a particle is describing a circle with a diameter of 1 metre. 1 metre below the surface the particle is describing a circle with a diameter of half a metre and at 5 metres, the circle is one thirty second of a metre. In essence, the deep diamonds should anchor the tires and stop them going up and down with the surface waves. The surface waves should break over the unit and change the wave energy into turbulence. Something to try.

So how did we fasten the tires together. We got old conveyor belts and cut them into palm width strips. We then got a couple of winches and pulled the tires together tightly enough to distort the tires. The straps were measured and cut with some overlap and 4 holes in a square pattern was drilled in each one. Brass bolts and brass plates were used to fasten the ends of the belts together. When the winches were released, the belts were nice and tight. The weak spot was where the bolts went through the conveyor belt. Our fastening was lasting a few weeks to a few months but as the barrier got bigger, the amount of maintenance got pretty time consuming.  Possibly in a commercial system one could use the same eye and pin system that is used to join conveyor belts together.

We were always amazed how much buoyancy these units had. A single 9 tire unit was plenty to float a person or two.  You could make a Kontiki with this sort of raft.

Something else we noticed was the amount of sea life they attracted. They were great protection for little fish and the big fish came along to see if they could get the wee ones.. Lots of organisms settled on them as well.

If you have need of such a breakwater, have a go and let me know how you ended up fastening them together. That is the key to this system.  If you are really adventurous, haul one of these into the deep ocean, build a hut on the top and go drifting around the world


Floating Design Netherlands said...

I understand that you have problems fastening your invented floating breakwater.

With almost 20 years of studying and improving floating breakwaters, we have seen lots of the solutions and their problems.

If you want, we can have a deep look into your issue and give comments. to contact us.
Kind regards.

william said...

I would be delighted to hear any solutions you have to this problem and will certainly add your comments to this blog. These breakwaters are extremely effective and the only major problem is the attachment of the tires to each other. By the by, I didn't invent them. They have been around for a very long time.