I Have A Real Life Design Problem on My Lake Marina

[recenv]

Background:
I own a lake marina that rents balls for house boats to tie up to on a permanent basis. The boats range in size and weight for 45' to 65' and weight from 2,000lbs to 15,000lbs. The balls are on a grid pattern 9 balls wide by 10 ball deep and 120' to 140' cabling between balls in all 4 directions. The lake in this area is about 2000' wide and about 650' deep and goes up and down up to 200' each year. One side supports are above the high water mark and the other side supports are below the low water mark to provide less stress during as water raises and lowers. You can view it on google earth, Emerald Cove Marina on Bullards Bar Lake in California. The grid is supported at all four corners to land and in the middle of each side. The cables range from 5/8" to 7/8" wire cable.

Problem:
The cable keep breaking during winter storms. The winds get to about 120+ MPH gusts that push the boats back and stretch the grid then let off for a minute, long enough for the boats to start moving back then another gust hits and cables snap. In the past the boats where smaller and the cables would break at the shackle joints as you would expect because the shackles weakened the cable. We have now gone to shackless cable and last winter we broke a 7/8" cable that was 2-3 years old in the middle of a 140' section. It seemed impossible but it happened. The breaks seem random depending on large boat placement and direction of wind gust. The last break was on a front corner support.

Options:
Adding more side supports to the grid to try and elevate some joint stress.

Break up the grid pattern to allow more movement and confine the stresses to known areas.

My newest idea and why I am writing this for people smarter than me to figure out if this is a viable option. The idea is simple, drop a canvas tarp 100'x20'(+/-) into the water before the storm on the front edge of the grid between every 2 balls. The tarp would have 100lbs plus of weight at the lower two corners to keep the tarp hanging down. The idea would be a shock absorber of sorts for the wind gusts with the trapped water behind the tarp slowing the gust movement of the house boats. a sail under water. To make this work I think I would need to tie the lower end, with the weights, back to grid balls or increase the weights to?

The option to simply drop a anchor is no a viable option because of the complications and expense with dropping a anchor at that depth and the constant adjustment of the length form anchor to grid as water goes up and down. Current system requires no cable adjustment.

Any Help would greatly appreciated.

Thanks,

 

[Simon Bridge]

You have a horizontal grid of floating balls linked by cables - the grid must be tethered to something to stop it just floating away.

The cables break between the balls? i.e. the tethering does not break?

You suspect that some sort of resonant effect is responsible?
Then you want to change the resonant frequency well off whatever the wind gusts tend to come in at.
Damping would do it. Adding heavy springs in strategic places would too, and changing the length of the grid.

To be a resonant effect, I'd expect that the houseboats would just about complete an oscillation when the next gust comes - if it is moving back to equilibrium when the next gust hits then it's momentum would oppose the strength of the gust.

If you are serious about finding a solution, you should consider hiring an expert rather than seeking free engineering advise from an internet forum. Work out what the solution is worth to you.

 

[recenv]

Thanks for the reply.
Yes the tethering does break also, but only once.
We have talked to a lot of expert consultants through the years to no avail. With all the field fixes at joints ect. over the years it ends up being a very complex situation and no one wants the liability. I am registered engineer that has been practicing for over 20 years and I understand all the basic principals at play but the constraints of the lake make a standard solution not practical so i am thinking outside the box. This will be a trial error process to see what will work. Simply calculating force and stress is really difficult because of the changing variables. What we have does work with a few exceptions, we have had 5 breaks in 10 years since we have owned it, but i think it could work a lot better with some a creative mind. ie thinking outside the box hence why i am posting here.
Thanks,

 

[Simon Bridge]

You only need to correctly identify the basic physics and work with ballpark maximal (or upper quartile-ish) situations. (No point anticipating freak conditions.)

This may mean going out in a storm and trying to measure how much the balls move/oscillate for instance.
Then you over-engineer for what you find.

The standard solution down here is to drive poles into the lake-bed and run floating piers to the houseboats like a lot of streets. Such a thing could be implemented incrementally with the moorings at a "street" being worth more rent ;)

But you know all this.

You are correct about getting someone creative to think outside the box. It'll probably cost - but it's presumably worth money to you as well? I'd do it but I'm in the wrong country.

You may want to consider approaching the local college engineering or applied physics people to see if there are students prepared to tackle it as a research project. Part completion of a masters or B-hons.

Note: I don't think the canvas tarp thing you describe will work well ... have you considered using something more like a sea anchor: deploy when bad weather approaches?

 

[CWatters]

It sounds like you have a large mass of boats coming to a sudden stop when the steel cable suddenly tightens? Are there larger versions of these spring shock absorbers available? It might help if individual boat owners also use them as well?

http://www.seascrew.com/browse.cfm?MOORING-LINE-SPRINGS---SHOCK-ABSORBER&l=0000000371

The problem also sounds similar to this one..

http://academics.wellesley.edu/Physics/gcaplan/Publications/String/inertia_demonstration.pdf

When there is a long slow pull the top string fails, when there is a short sharp pull the lower one fails. You say that the cable between the floats/boats fails rather than the bank tether. I believe the cable between the floats is equivalent to the lower string in the problem. Perhaps confirming that it's a short sharp tug that's causing the failure.

 

[CWatters]

Google found a patent for a shock absorber design specifically intended for use on mooring lines for a fleet of barges rather than individual boats..

http://www.google.com/patents/US4967681

Various types of shock absorber structures have been suggested in the past for absorbing shock loads in anchor cables, but those devices have been intended for use with small vessels, and not with large fleets of interconnected barges.

..the shock absorbers disclosed in those patents are intended for small vessels, and would be unsatisfactory for use with heavy fleets of barges, which generate very high impact loads in mooring cables.

Accordingly, it is an object of the present invention to provide an improved shock absorber structure for use with mooring cables for heavy vessels, in order to avoid the imposition on the cables of sudden impact loads that would otherwise break the mooring cables.

I've no idea if it's actually any good or even manufactured.

 

[CWatters]

Seems there are really massive shock absorbers available.

http://supflex.com/Free_Calculation1.html