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post #1 of 11 Old 02-22-2010 Thread Starter
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Securing the floating breakwater

The floating breakwater at our club is in desperate need of replacement. It was scheduled to be replaced next year, but last November our clubhouse burned to the ground. Now the breakwater replacement has been pushed back a bit, and as the new Harbor Master this year I am tasked with nursing it along.

The current breakwater is essentially large dock sections anchored at the cove entrance. The sections are chained together and anchored with multiple anchors on each section. The problem we have is that the wave action causes the chains holding the sections together to wear out. The anchor chains also wear out, and the sections come adrift. If we could pretension the anchors to reduce movement from the waves, the chains would last much longer. Adding winches to the breakwater section would be beyond the budget that I have this year. We also wouldn't want to go out to adjust the winches all the time.

Anyone have a good idea for keeping anchors tensioned through about a 24 foot range between minimum and maximum lake levels? I need the solution to last about 3 years with little annual maintenance.

Thanks for any ideas.


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post #2 of 11 Old 02-22-2010
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how about running the chain over a drum with a weight on the chain, the larger the weight the more tension. if you need more travel than you have depth, you can do a 2 or 3 to run set up.

or maybe add weight in the middle of the chain, kind of like an anchor sentenal. this would in effect shorten the scope of the chains
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post #3 of 11 Old 02-23-2010 Thread Starter
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Originally Posted by scottyt View Post
how about running the chain over a drum with a weight on the chain, the larger the weight the more tension. if you need more travel than you have depth, you can do a 2 or 3 to run set up.

or maybe add weight in the middle of the chain, kind of like an anchor sentenal. this would in effect shorten the scope of the chains
Maybe a picture of your first setup would help me see what you're talking about. I thought about the anchor sentinel type setup. I don't think it would give me the range that I need.

I ran the below design by my friend Scott (P.E. PhD). He gave me a couple of good reasons why this would be difficult to pull off. Not the least of which would be the calculation of the length of the rode. Apparently it's not as easy as it seems. He said he has a program that would do the calculations. Maybe he's trying to get it down to an exact length (like a good engineer should), I would just like it to be within a couple of feet. The other problems would be getting an exact location on each anchor. Easy to plan to put them in on a straight line, not so easy to get them placed and set.

This would be the view of the end of the dock section.


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post #4 of 11 Old 02-23-2010
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Why not bolt some of the sections of dock together. That would reduce the amount that were chained together.

How long is the anchor rode for the dock sections and how deep is the entrance to the cove??

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post #5 of 11 Old 02-23-2010
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john what you drew is what i thought. as for lengths that easy, that is a 2 to one set up. ie 20 feet of depth change would take 10 feet of chain under the "dock" on each side of the weight. you need a in your case for 24 feet of depth change a min depth of 12 feet at the low point, and 24 feet of chain. ( all numbers are straight math but there is some extra needed for angle and stuff ) now there is some extra to be made up from the chain going to the anchor, but the greater the angle to the anchor the better.

you can get more range with less depth by adding a second pass thru another block on the weight like below

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post #6 of 11 Old 02-23-2010 Thread Starter
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Why not bolt some of the sections of dock together. That would reduce the amount that were chained together.

How long is the anchor rode for the dock sections and how deep is the entrance to the cove??
The entrance to the cove is about 15 feet at normal pool, it can go as low as 5 feet and as high as 30. The anchor rode is as long as we make it. We're thinking a 5:1 scope is going to be about right, but with a 24 foot range that gets pretty long. Plus, by the time you get a 5:1 scope out, you're in 35 feet of water at normal pool. The long rode is the problem now. With enough rode on the anchors for maximum lake level, it's too long for normal pool. The result is a breakwater that moves around a lot.

We actually welded a few of the sections together last year. The problem is that the sections are angle iron cages so there isn't a lot of area to weld. The welds lasted a weekend. A little wave action broke the welds off one by one like pulling stitches from a seam.


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post #7 of 11 Old 02-23-2010 Thread Starter
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john what you drew is what i thought. as for lengths that easy, that is a 2 to one set up. ie 20 feet of depth change would take 10 feet of chain under the "dock" on each side of the weight. you need a in your case for 24 feet of depth change a min depth of 12 feet at the low point, and 24 feet of chain. ( all numbers are straight math but there is some extra needed for angle and stuff ) now there is some extra to be made up from the chain going to the anchor, but the greater the angle to the anchor the better.

you can get more range with less depth by adding a second pass thru another block on the weight like below

I think the math problems begin when you start lowering the level. As the breakwater lowers, the rode on the left side gets effectively longer causing the breakwater to move right. As it moves right and drops, the rode to the right gets too long at a different (faster) rate. How much too long depends on the arc created by the original rode length of each side.

There's a reason I'm not an engineer. This stuff makes my head hurt.


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post #8 of 11 Old 02-23-2010
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how far from the break water to the docks?
having a 5 foot min is the real problem, i think you will need to do the sentinel approach.

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how far from the break water to the docks?
having a 5 foot min is the real problem, i think you will need to do the sentinel approach.

This may be the way to go. I'll get with my engineer friend so he can poke holes in the idea. The hardest part may be getting the various anchors and weights deployed evenly across the breakwater.


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post #10 of 11 Old 02-23-2010
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breakwater mooring

We just completed our new breakwater project this past December. It's also a floating one. The mooring design is shown below. The wave side is attached to pilings with chain about every 80 feet and the harbor side is held off of the pilings with large anchor weights and smaller sentinel weights. The drawing is a cross section view. It's working quite well so far.






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Hey, can one of you guys pass me a crab?


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