Side by side structure model testing
August 15, 2008 at 9:19 pm #3586
I called the OSH by my parents and they did not have this large a pipe, but maybe where you are. Thanks for looking.
If I calculate 4 floors at 10 feet each I get 40 feet. So are you looking for 4 floors? At 1:25 this is 19.2 inches, which is not so far off from my 14 inches. I realize that whatever sizes we are looking at now are just starting points, but it would be good to start at our best guess.August 15, 2008 at 11:00 pm #3587
Oops. I meant 5 floors. I actually think your paint bucket is pretty close. I do not think that to get it exactly 20 x 50 is worth the expense.
I just got back from Sunnyvale OSH, and no they do not have 12″ PVC. (They also didn’t have the 3/4″ Polypropylene tubing that I need to fix my sprinkler system either.) I spent a few minutes calling around and the best I could find was $8.30 a foot in 20 foot lengths from Ewing Irrigation (Cambell). I would be unsurprised if your parents found the same supplier in this area.
I will comment that just about anything that is the right shape can be made water proof by sticking it into a garbage bag. Thus, 12″ cardboard tube, with round wood inserts would probably be quite a bit cheaper than than buying 12″ PVC.
It is really clear to me that a truss spar seastead will behave very much like a pendulum. A pendulum has a natural frequency of oscillation that is primarily dependent upon the length from the pivot point to the mass. In our situation, the water viscosity will come into play and probably lower the natural frequency. Whenever such a seastead is in waves with a period that match the natural frequency, they will probably oscillate pretty wildly.
If the truss is long enough, the natural frequency will be low enough that waves of the requisite period can not happen. Alas, this makes the truss long and expensive.
I am kicking around the idea of hanging one or more smaller pendulums at the bottom of a shorter truss spar to “mess up” the natural pendulum frequency. Once we have a structure that is does not have a natural frequency to oscillate at, it should be pretty comfortable. I could be wildly off base on this theory.
Anyhow, that is where my thoughts are at the moment.August 16, 2008 at 1:25 am #3588
Ya, that is the same place I found.
I will probably just try using duct tape to connect two buckets together. If it stays this will be 28 inches and so not far from the 24 inch target. It is worth something to me to have a nice model that lasts, particularly if it is modeling a favorite design of TSI management.. I suspect that 6+ months from now there will be some other idea I want to compare to a previous model. I have a shady out of the way place that I plan to stash all the models so that I can use them again if there is a reason to. If I have a model that lasts I won’t have to rebuild it later.
I believe that a longer hanging ballast can increase your natural frequency. I don’t think you need a truss all the way down. I think the truss and the cable will stay in line. In the same way that there is nothing to cause the cable to bend partway down, I don’t think there will be a bend at the joint between the truss and the cable. In fact, I think you can have a very short truss. I have some tests I plan to do with different length hanging ballasts to try this out.August 16, 2008 at 7:46 am #3589
To handle the pendilum resonanse problem you can use underwater sails. A 10×10 meters sail would push about 1000 tons of water four times during one oscillation. All the energy required to accelerate the water will be dissipated. So, two big cris-crossed sails near the bottom of the truss will effectively angkor the truss to the water (and the 3rd one, the horisontal one, would prevent bobbing). Apart from solving the resonanse problem, the sails may decrease the required truss length. I’m not sure how to test it in models, since it would scale differently.August 17, 2008 at 12:58 am #3592
Adding plates definitely entraps more water and increases the effect mass and increases viscosity. This will change the resonance frequency down.August 17, 2008 at 11:55 am #3593
You still need a long truss. A cable cannot transmit bending forces. As a result the rotational (pendulum) stabilizing effect of the ballast to a large extent goes to waste if it hangs in a cable. Only when the structure tilts at a large angle does the ballast come in to play and starts to act stabilizing. Around the middle point it will be very unstable.August 17, 2008 at 12:04 pm #3594
Ok, thanks for that info. A couple of things:
The legs are very long, and does not seem to be perfectly vertical. Possible collision risk between them?
How about something like this but without ballast? Ie balancing the CG on top of the legs. More space per dollar probably, but perhaps this will be too sensitive too waves?
edit: obviously the legs would have to be rigidly attached to the superstructure for this to work, increasing the cost again…August 18, 2008 at 8:58 am #3601
And would drastically decrease the amplitude.August 19, 2008 at 1:40 pm #3603
I would like to build a model Tension Circle that was large enough for two people to float around in and
then have a kite pull me and someone else downwind alongside Anguilla. http://online.offshore.com.ai/balseros/
Some kind of larger pipe for the outer circle seems like the way to go for this. Probably this is
in the 6-inch to 1 foot diameter range with the whole circle like 30 to 50 feet across. Some surplus metal
pipe might be welded together. Might even be able to get some large threaded pipe. But I think I like some large flexible plastic pipe better (HDPE). For this kind of pipe a 1 foot diameter pipe can be bent with a 20 foot radius, so I could make a 40 foot circle. The two ends are then heated and fused together (really there are probably several pieces joined together). It is a relatively inexpensive type of pipe and you don’t have to buy fittings. So the overall costs should be low. For safety it might be good to have 3 or 7 smaller pipe circles bound together. Or the pipe could be filled with foam. Or both. You can get the pipe in larger sizes, so it could work for a full scale tension circle as well.
The one big problem is after ridding this fun model downwind, how do I get it back to the starting point, or to my home. It would be too big to put on a car or a truck and even for my boat to pull upwind. So now I wonder if I could get an inflatable tube that I could make into a tension circle.
PS This forum software removes newlines but does not insert a space, so two words I wrote can end up smushed together in the post.August 29, 2008 at 1:54 am #3683
So I think the ball stays lined up with a 25 foot rope to the 20 lbs ballast in this first video.
These are big waves, bigger than the ball which is simulating a 30 foot ball. The ball does not tip really.
In the second video the ballast is tied right below the ball and the ball tips plenty.
This ball does not have a truss, so I don’t really think we need one.
Updated Ball House page with these:
I also think we could make a 30 foot ball out of HDPE plastic. Probably a big rotomold is the way to go. If you had a 30 foot mold that you could roll on some wheels, I think you could make a really cheap seastead every couple hours.August 29, 2008 at 1:54 pm #3686
I think the ball shape is able to dissipate up and down motion better than a cylinder or pipe. I really don’t seem to have the “heave” problem with the ball. Don’t see any need for “heave plates” on this.
So without heave plates and without a truss, I think this is the current leading low cost design. Particularly if we can make it out of plastic, as I think we can.September 10, 2008 at 1:34 am #3777
“I will probably just try using duct tape to connect two buckets together.”
Why not cut the bottom off one bucket and use rubber cement or some other adhesive? If you cut a couple inches up from the bottom, it will slide right over the bottom of the other bucket (as they are meant to stack when upright and empty, it gets slightly smaller in diameter towards thebottom). You can put screws through both bucket walls if you need more strength, and seal them with adhesive as well.
With a tight-fitting lid on the bottom bucket, you now have a very slightly dumbbell-shaped cylinder, one bucket upright, the other upside-down, with two compartments. Or you could cut the bottom out of both buckets for one open compartment. Adjust your cut for your proportional length/radius desire.September 10, 2008 at 5:20 pm #3784
Vincent can you do one that three spars rather than 4? Triangles are stronger than squares. Also if you cross brace diagonally between the top of one spar to the bottom of the next does it add stability or not?
PS: I have a salters duck based sea stead design I’m working on but a duck is a shape your cheap pipes can’t do. The design is a flat platform sitting on two salters ducks like a steam roller. Two parallel axels like fred flintstones car. The deck would need to be relativily high.
I think the duck may be useful because its the one shape that alters its bouyancy as a wave passes.
For everyone’s information I was involved with the original Oceania Project in a small way. I’m also in several space organisations. And I have a Degree in sustainable Development, sustainable agriculture and renewable energy,water and sewerage.September 10, 2008 at 5:53 pm #3786
>Vincent can you do one that three spars rather than 4?
The only stability problem my multispar model had was with heave. So while I could do it with 3, I don’t see enough chance for it to be interesting/different that I will do the experiment for free, but for $100 I would.
If you are going to put in diagonal bracing and such you are not really doing a multispar, but a semisubmersible. The fun thing about the multispar is that there are no joints that have to be really strong because each spar has its own ballast and flotation.September 10, 2008 at 6:13 pm #3787
>If you cut a couple inches up from the bottom, it will slide right over the bottom of the other bucket […]
Ya, maybe. The waves are going to be pushing on it, there will be about 35 lbs of ballast pulling on it, and it needs to stay watertight. Wish there was a PVC scrapyard nearby with some 1 foot diameter pipe. Would like a nice strong simple model.
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