Side by side structure model testing
July 21, 2008 at 2:44 pm #3473
In a hanging ballast with ropes that don’t stretch the restoring force can be as if all your ballast weight was on the side of your structure that the waves are trying to lift up. This means the waves have to try to put more force than this, which I don’t think is easy for waves to do for reasonable sized ballast. So while it is true that when tilted at 45 degrees a truss can put far more righting force, I don’t think you need ever tip more than the range where a hanging ballast works as well as a truss.
In my dry hanging ballast video I think the reason the 10 lbs does not work is that the ropes stretch and they are not exactly the right lengths. So some of the weight is taken by the side ropes and not all is taken by the high side. I think with wires I could have put almost as much on the side of the bucket as I used as ballast.
Cost is a really important factor in the success of seasteading. A few cables will be much cheaper than a truss. If it works as well in the range we are really in and costs less, then it is better than a truss.
Anyway, I don’t think you should write off the hanging ballast idea just yet. It really can help more than a little.
Could also use cables with a central pole, like a ships mast upside down and below the water. Could have spreaders and cables so that the pole stayed straight and the cables could take most of the weight of the ballast. The pole could make sure the cables never went slack. This should be cheaper than a truss.
I could model this with a broomstick between the bucket and the ballast with the same 4 ropes around. But I don’t think it is much different (except for the drag of the stick in the water) and so don’t think I will. I could also have shot video underwater to see if the cables went slack and might do that if I go out again. But I really don’t think they went slack so probably won’t bother with that either.July 21, 2008 at 2:53 pm #3474
What else did you learn from the marine consultants? Did they look at any designs? Any feedback?July 23, 2008 at 3:03 am #3485
We spent most of our time talking about requirements and trade-offs. A few specific designs were discussed. We really hammered home that we can not afford to build things using the exact same model as the oil/gas exploration companies; their requirements are quite a bit different from ours.
I believe the requirements document is somewhere on this web site, but I sure can’t find it right now. Sigh.
We did mention the possibility of having a seastead community that floats in an annual cycle in the north Pacific or north Atlantic.
They should come back with their initial proposals next week on the 29th.July 24, 2008 at 10:35 pm #3498
Youtube accepts 640×480 resolution. This is unclear from their own documentation but I found out from other sources on a Google search. The benefit of using 640×480 instead of 320×240 is that Youtube adds a “watch in high quality” option with much higher quality playback. It is also best to use mp4 format.
for examples see:July 27, 2008 at 5:56 pm #3511
This catamaran model video was shot awhile back but I finally have it up on youtube:July 27, 2008 at 6:59 pm #3509
Yesterday I took my 10 foot Spar Buoy model to a harbor with deep enough water. Problem is the waves were unusually small, around 3 inches. This is around 6 feet for full scale, which is a common sized wave in the ocean around here. The wind was typical for Anguilla, like 15 to 20 MPH, so scaled up this is 75 to 100 MPH simulated wind. The model leans downwind significantly with just a cup on top. The pipe is 2 inches wide, or scaled up 50 inches in real size. The cup is around the same width but a real house would be much wider. So I suspect that a scale house on this model would catch so much wind that it would tilt too much to be safe. I think this is interesting and may test it. If we are able to always avoid hurricanes or big storms using migration this may not be a show stopper for the spar buoy, but it seems worth understanding.
This model also heaves up and down. The individual 3-inch waves don’t tip the model. There were some very small swells that came in at times and might have contributed to the heave.
A related issue is pulling this through the water. To move a full scale one at 1 MPH is like moving a 1:25 scale model at 1/5 MPH. Moving at this speed without tipping it way over seem to require not just pulling at the top, but maybe half way down. With fins on my feet and pulling on the top of the model I can pull this through the water so fast that it lays nearly on its side. I still want to measure the force needed to pull the model at 1/5 MPH while vertical.July 30, 2008 at 2:21 pm #3513
Took my 10 foot model to Shoal Bay on Sunday. I estimate the waves were 8 inches. With fins on it just took like 3 minutes to get past the reef to deep water (from where I parked by the beach) and about 7 minutes to get back (drifted downwind some). This is the beach closest to my house. My office is along the ridge between the two rightmost palm trees in the video between 2 and 4 minute marks on the video. The wind, leaning, and heave were about like Saturday.August 6, 2008 at 6:43 pm #3549
I have previously posted video for a “Short Cylinder” and a “10 foot Pipe Spar” but had not gotten around to the middle spar tested. This is a 4-inch diameter pipe that is 3.5 feet long. The inside diameter is 4 inches, so the outside is about 4.5 inches. At 1:25 scale this is like 9.4 feet wide and 87 feet long. The waves are like 6 to 8 inches, so like 12 to 17 feet. You can see me in the water with the waterproof camera, but I don’t have video from that camera for some reason. This was shot from a tripod on the pier.August 10, 2008 at 3:40 pm #3567
The multispar looks pretty stable, I like that! I’m sure it matters what the spar spacing is compared to the wavelength, though.August 10, 2008 at 3:43 pm #3568
One warning – responses to waves do not scale linearly, as you suggest with 1:25 and 1ft / 25 ft waves. Different motion characteristics scale differently. The scaling laws are well known, which is why they can do model testing, but they are more complicated than just linear. This is still a great first test, just be aware of the limitations of your methodology :).August 10, 2008 at 10:51 pm #3571
I made a wiki page on model scaling.
At 1:25 scale I don’t think my models and speeds are such that surface tension or surface friction are an issue, So I don’t think it is very complicated really.
>One warning – responses to waves do not scale linearly, as you suggest with 1:25 and 1ft / 25 ft waves.
If the vessel scales linearly, and the wave height scales linearly, and the wavelength scales linearly, and the speed scales with the square-root, and you scale time with the square-root, the motion is said to be in “similitude”. In practice what this means for us is that if you slow down a video of a model by the square root of the scaling factor then things should look like they would for a full scale version. Really, it is kind of simple. And it is really very accurate. If you think this is not so, please check with your marine consultants.
My main problem is that with Windows Movie Maker I can only slow down the video by a factor of 2, so doing it twice I can get 4. However, I can’t get 5 which is what I really need for a 1:25 scale model. Probably some other software can do it, just have not found it yet.August 11, 2008 at 11:23 am #3577
The multispar looks like it oscillates vertically (bobs up and down) quite a lot. I wonder whether this is just a temporary effect or if this movement is sustained by the waves. Perhaps if the harmonics with wave frequency is just right it would bob almost to where it is completely submerged? Or maybe the spars are just too short. It would be cool to see the structure out of the water so you know how it looks and how long the spars are.
Now I just saw that this structure is anchored. So this is actually a tension leg platform? I guess that would explain the “bobbing” and the fact that it never seems to tilt/lean/list (whatever the correct term is).
Is the anchor cable the elastic cord? If so, is this correct? Should this not be a rigid wire?August 11, 2008 at 12:02 pm #3578
On this page you can see a picture of it out of the water and more details:
There are also a couple of out of the water pictures on my main model page (which links to above and other model stuff):
Going up and down like that is called “heave”. The model has very little resistance to up and down movement, if you put in some it would not do that. Such resistance might be “heave plates”.
It was not anchored. No, it is not a tension leg platform. If it was it would not bob up and down. Each leg has its own ballast. In the end I used two pieces of 2 foot long 1/2-inch re-bar on each leg for ballast.
The elastic cords let each of the 4 spar legs move relative to the main platform. This is kind of like:August 15, 2008 at 7:33 pm #3584
My current “short cylinder” model is only 14 inches tall. I would really like to have one that was 24 inches to match Wayne’s proposed 50 foot high cylinder at 1:25 scale. I could do this with a 2 foot section of 12 inch diameter PVC pipe, but nobody in Anguilla sells pipe that large. I found some in San Jose, where my parents live, and it is $166 for 20 feet (ewing1.com – not really PVC). Not sure I really want to spend that. Since I only need 2 feet, I was wondering if there is anyone else in the Bay Area who would like some pipe for making a model? Or anyone want to sponsor this model for $100? My father can get it, cut off 2 feet, and mail that to me. Or is there anyone who just happens to have 2 feet of 12-inch pipe laying around?
Once I have a good model for Wayne’s proposal I can compare it side to side with the others at the same 1:25 scale.August 15, 2008 at 8:19 pm #3585
I (Wayne) obviously live in the San Fransisco bay area. I’ll look around for some 12 inch pipe today. Orchard Supply Hardware (OSH) usually sells shorter sections. I’m going to be in that section of OSH today, since my sprinkler system needs some upgrading. If I can find it, I’ll send it to you gratis.
Please do not fixate on 20′ diameter x 50′ long. These are numbers I pulled out of thin air. The 20′ is sort of the smallest diameter room that I would want to live in. The 50′ is to give me 4 floors with 10′ between the floors.
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