Experiment with 9-foot floating marina model

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• #542

vincecate
Participant

Today I did a test of a 9-foot model of a floating marina. The model structure seems strong (left it tied up in the waves, will see later). However, the wave reduction was not as much as I had hoped. This is a larger version along the same lines of ealier tests.

I am the guy with the 2 boys, not the cameraman.

Expect to do more tests over the next few days in different wave conditions. Will post more info.

#2737

Anonymous
• Nice work.
• Perhaps it would be useful to place a couple of similar floating objects (boat, seastead etc), one inside and one outside the breakwater, to get some kind of reading on the difference in heaving.
#2739

vincecate
Participant

Yes, that would be good. I had a little radio controlled boat that I planned to drive inside and out. However, it died. It is actually in the picture next to the model floating slip inside the wavebreak – it is small like 1 foot by 3 inches.

Will try to do that in the next few days. Problem is the wind will push it along.

I put together another video that shows us getting the sections out of the car and also carrying the assembled model down to the water, as well as the first video of it in the water.

#2741

Participant

According to what I recently learned on breakwaters and wave mechanics, such a low scale is not accurate: attenuation of waves depends on wavelength and the location and height of the breakwater: the energy of a wave is spread on a height of water equal to half its wavelength, so obstacles in this zone are hit by the moving water and brake the wave. In your case a lot of the energy was simply moving below the model because the wavelength is too big relative to the height of the waves and height of the model.

#2742

vincecate
Participant

I think that if you scale down the model and scale down the waves that the motion you get is accurate, just scaled down. But you are right that much of the energy just goes under the model. And the model does ride up and down on the longer waves. I think if we built one 30 times bigger and tested it in waves 30 times bigger, we would get a similar motion.

I suspect that “wavebreak” or “breakwater” is not the best way to think about this. In my “floating marina” the desire is to be able to protect boats inside. If we could have slips that held small boats it might be ok if the whole marina went up and down and tilted a bit. So the marina rides on the big wave like a raft and blocks the small waves. If the marina is 300 feet in diameter the motion of the marina would be like a 300 food diameter raft. But the water inside is going up and down. Still, it is much less violent than what a 30 foot boat would normally experience. However, it is moving and regular marina slips are not designed to handle this. I even wonder about suspending the small boats in the air sometimes.

The other use I like for this basic design is for a single family house on one that is like 100 feet in diameter. You put the house in the middle. Cover the wires with some kind of webbing, material, some dirt, and plant grass. Then you have a house on a floating island. The house won’t hold exactly still, but I think the motion would be gentle enough for me to hack, unless there was a big storm outside. The water going up and down under the grass/house hould be ok (probably need vent).

#2746

Anonymous

It needn´t be as advanced as a R/C boat. A couple of rubber ducks loosely tethered to stop them drifting away should do it. Or just some blocks of wood.

#2756

vincecate
Participant

Couple more ideas that I think are fun:

If the wall sections are big enough they could be living quarters. They could even be independently owned. If part of a 1000 foot diameter structure they would be plenty stable enough to live on and work in. Slips inside the marina might rent to boats for \$500/month and maybe wall sections earn a share of the gross, possibly over \$2,000/month.

If the ropes to the center are removed but the wall sections were still connected together in a flexibly way, it would be possible to move the wall through the water like a big snake. So now instead of a 1000 foot diameter circle it would be like a 3141 foot line. In this shape it would take very little energy to move through the water. The thrust could even come as a little push from each of the sections (some central control of all so that direction and shape were as desired). All the sections together should be able to go much faster than one section alone, and have a smoother ride. Solar power should be sufficient to make a decent speed.

#2766

Participant

I really don’t think the scaling can be so simple. Wavelength scales linearly with scale, wave energy to the square of scale, inertia of the model rises to the cube of scale, dynamic drag to the square, and so on… The result you get with a 1/100 scale model will inevitably be wildly inaccurate. I’d suggest using this study of floating breakwater mechanics to predict motion of designs for seasteads in waves, because that’s pretty much what they will be: floating rigid blocks impacting the waves.

#2776

vincecate
Participant

That looks like an interesting paper. However, my design for a wavebreak is different than those he analyzes.

In “4.1 Prediction Techniques” he says, “There are a number of techniques that were employed by designers to predict performance of numerous breakwater types. The primary approaches include previous experience, analytical methods, numerical methods, field trials, and laboratory testing.”

Under “4.1.5 Laboratory Testing” he concludes “Despite these shortcomings, it is this approach which holds the most promise for performance prediction.”

This laboratory testing means scale models. Scale model testing of boats in wave-tanks is very common and I believe very accurate. Water mass/inertia/waves scale down well. I think you have to scale time as well if you are trying to interpret some things. If you get too small surface tension and other things can cause “scaling errors” but at 9-feet these should not be a problem.

The other thing is it is easy and fun. Building models and boys and dad both playing at the beach. I don’t get to control the waves like a guy with a wave-tank, but I can come back to the beach different times to check under different conditions. For me it is a 5 minute drive to my father-in-laws place at the harbor. We put it in Friday morning and I went back late Friday and early Saturday (it made it through the night!) There is no wave-tank rental fee. I also test the strength of the structure at the same time I am testing the wave breaking performance of the structure. And I very quickly get a feel for the results.

#2783

thebastidge
Participant

Some things just have to be done empirically. Goddard had to test rockets which failed, as well as rockets which worked.

#2785

vincecate
Participant

The waves this morning were smaller. Took some video with a toy boat inside and outside the marina, and in a model slip.

#2786

vincecate
Participant

Interesting book:

Physical Models and Laboratory Techniques in Coastal Engineering (Advanced Series on Ocean Engineering)

http://www.amazon.com/exec/obidos/ASIN/981021541X

Talks about issues of viscosity, friction, and surface tension in scale models, or “physical models”. Still looks like a 9-foot model can probably ignore these and not be far off.

#2787

thebastidge
Participant

Copying this book info over into the Library thread in research.

#2793

vincecate
Participant

>The result you get with a 1/100 scale model will inevitably be wildly inaccurate.

A 112 foot diameter circle would be 1/4 acre and a very comfortable lot size for a house on a floating island. A catamaran 60 feet long and 30 feet wide is usually viewed as stable enough for going far out to sea. So 112 foot diameter should be very stable. We are putting the flotation on a very wide base. This is only 112/9 or scaling about 12 times larger than my current model. Not so very far away (though by mass it would be around x1000).

But before even a factor of 12 scaling I would make test models like 30 or 40 feet in diameter. These would probably use ICF/foam-block/concrete or maybe aluminum hurricane shutters to make the wall sections. Any 1000 foot sized ones are further off and I will learn from other versions before then. If I scale by a factor of 3 or 4 every 3 or 4 years I will be happy soon enough.

— Vince

#2826

Anonymous

It definitely looks calmer inside the breakwater. The boat is more still and the water surface looks more smooth. It´s hard to tell wether the level of protection is “enough” for this to be worthwhile though. I suppose the effect will increase with increased draft of the structure? Also what is that coloured stuff with the serrated edges on the sides?

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