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Side by side structure model testing
Wed, 06/18/2008 - 02:21 — vincecate
I should be getting a waterproof camera in a couple weeks.
I am thinking about making a few different designs each out of the same amount of PVC pipe so they are roughly cost equivalent. Then testing them in the ocean side by side.
I am also thinking of having the camera and a glass full of water on the structure. Maybe some jellow or something else too. Idea is that camera will give a view from the structure that shows the waves, the horizon, glass of water, and other stuff so that you get a feel for how stable it is in the slowed down video.
So far I am thinking of:
1) Tension circle house
2) Pipe Spar
3) Semisubmersible or multi-spar
4) Catamaran
Also thinking of a weight/pulley/string to give a known force and see what GPS shows for speed. This should let us say how much force is needed to move the full scale version at some speed.
Anyone have any advice or comments?
I like the idea about using
Sponsorship for test models?
I am trying to see what designs seem the most cost effective for achieving stability on the open ocean for a single family sized structure. Not very worried about speed through the water, and think that migrating around the Sargasso Sea I would not have to deal with more than 25 foot waves. So I am going to test out several models at 1:25 scale using 1 foot waves in/near a harbor. My idea is that if the models each use a 10 foot piece of 2 inch diameter PVC pipe as their construction material (or maybe the same total weight of PVC) they are all sort of cost comparable. Any advice or comments appreciated.
http://wiki.seasteading.org/index.php/User:Vincecate/Models
http://wiki.seasteading.org/index.php/User:Vincecate/Migration
If anyone would like to help out by sponsoring a model for $100 each I will credit you in all writeups. I will buy the materials, build the model, test it in the ocean and probably a pool also, take videos and publish these in youtube, take pictures and make a web page for the whole thing. You would only have to pay after completion with paypal. Can also request a different model.
Started model testing
Put the multi-spar, the tension circle, and the catamaran in the water today. Had 8 to 12 inch waves which is what I wanted for my 1:25 scale models and worst case of 25 foot waves in real life.
The memory card for my water proof camera has not come yet and the 4 GB card from my other camera does not work in it (can only handle 1 GB). I really wanted to shoot video with the camera on the model but I can't do that yet.
My 2.5 lbs steel weights did not hold the legs of the multi-spar down (they would float out to the sides) and my 5 lbs weights were a bit too much. With 5 lbs on each leg (so 20 lbs total) the structure was only like 5 inches out of the water. So some waves covered over the structure, but it was very stable. Had a glass of water on it and it did not tip much at all. I plan to use some jugs with adjustable amounts of sand to get the right weights for each leg soon. Have not measured force/speed yet but it does take a fare amount of force to move it through the water.
The tension circle really looked stable as well. The catamaran also looks very stable. The circle is easy to push through the water and the catamaran will move fast with a very slight push (even the wind).
Shot some video from the water of the multi-spar but I need to redo the experiment with the right amount of weight on each leg. So I am not posting it yet.
The video of the tension circle was shot from a kayak and there is too much camera motion. I am going to try shooting video from the pier next time so the camera is not moving so much. I think we can have enough time to shoot video as the model drifts past the end of the pier for most of the models.
I put up a picture of the models on the beach and one on the multi-spar but will be putting up more interesting stuff in the next few days.
http://wiki.seasteading.org/index.php/User:Vincecate/Models
http://wiki.seasteading.org/index.php/User:Vincecate/Models/Multispar
Video of multispar
I have put up a video that shows a view of the multispar in the waves and then also the view from the multispar (I got my memory card for the waterproof camera!).
http://wiki.seasteading.org/index.php/User:Vincecate/Models/Multispar
Have video of the tension circle and catamaran that I expect to post tomorrow.
Thanks to Joep for sponsoring this model!
One warning - responses to
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 :).
Model scaling laws
I made a wiki page on model scaling.
http://wiki.seasteading.org/index.php/Scale_models
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.
Tension circle in PVC
Here is some video of the tension circle made from PVC pipe. The side sections are 2 foot each.
http://youtube.com/watch?v=mlsTXb_qyRA
Slowed down by a factor of 4 (should be 5 but don't see how to do that in Windows Movie Maker):
http://youtube.com/watch?v=oOYGjGbfmZI
The waves look better in the slowed down video.
Slowed video of multispar
Youtube limits your bandwidth, and a slowed video does not need as much bandwidth and so looks much better.
Here is the multispar video slowed by 4:
http://youtube.com/watch?v=wCHemZs_WBA
Looked a little bumpy
Great work! It looked a little bumpy to me on the top of the spar. No spillage happened, but I suspect different conditions would have caused some spillage.
The multi-spar looked much calmer.
Again, great work!
Early model results
Thanks.
I will mount the water proof camera on the tension circle so I can get up close video of a cup on it. It will be interesting to see how the close up of the water motion compares among the different models.
Also planning on cutting some metal pipe to get 4 lb lengths and tie one of these along side of each of the 4 legs in the multispar. I think this will be more level and more stable than the bags of sand. Have not done force/speed tests on the first 2 models and still have 5 other models to test.
Lots of fun.
5 models run in under 2 hours
The sun is so strong here that I usually only do outside activities between 4:30 and 6:30. Today during this time we ran the 5 gallon bucked with hanging ballast, sphere with hanging ballast, 3.5 foot long 4-inch diameter spar, multi-spar with re-bar for ballast, and the tension circle. Got good video from the pier and also from the waterproof camera using a "poor man's steady cam" while floating in the water.
Waves were a bit smaller today but still good for testing.
Main results are:
multispar working even better than before (now that all weights are the same and firmly attached, unlike the bags of sand) - Also put the cup in the middle and, as expected, it tips even less. So far this is the stability champ. Will post more video soon.
sphere - moves so much that sometimes water in the cup was at 45 degrees (video coming)
bucket - moves a bit less than the sphere, mabye, but also moves too much
3.5 foot spar - not as bad as bucket or sphere but no where near as good as tension circle or multispar
tension circle - water in the cup is very steady (have close up video now) - still my favorite
We voted off 3 contenders today, the sphere, bucket, and 3.5 foot spar. These models would clearly not be pleasant in a hurricane or even a more modest storm. So we took these home. The other 4 are still on the beach (by Grandpa Smitty's place) for further testing.
The one model we have not run yet is the 10 foot spar. The water is not deep enough right next to the pier to allow us to use the camera on the pier. But using the steady cam while floating in the water worked well (unlike taking video from the kayak), so I can use that method to video the 10 foot spar while further out in deeper water.
Uploads to youtube are going slow. Will post videos soon.
Short Cylinder / Paint Bucked model video / new plan
http://youtube.com/watch?v=ODw9kGQhfZ8
This video was shot from the pier but I think you can tell this model is not stable. The weights are 4 or 5 feet under the center of the bucket but they kind of swing back and forth and the whole thing bobs up and down. In the close up video you will see the water in the cup is not staying level but you should expect that after seeing this.
I am going to try two paint buckets connected by 4 ropes on the sides and like 6 feet apart. The bottom bucket will be face up and holding the weights but not closed. This is the "entraped water" idea. The bottom bucket will have more inertia and more resistance to moving through the water either sideways or up and down. So it should make the top bucket much more stable.
My thinking is that if you had slip form molds to make a full sized short cylinder in concrete then you could make both the living/floating cylinder and the ballast cylinder from the same mold. This would probably still be a very reasonable priced seastead.
Video of sphere and short cylinder from water camera
Two more videos shot from the water and slowed down by a factor of 4:
Sphere (Buoy):
http://youtube.com/watch?v=u6U8tc7wp2s
Short Cylinder (Paint bucket):
http://youtube.com/watch?v=Vyi67NwzS-8
Both of these have a lot of movement.
Another video of the tension circle
This is from Thursday:
http://youtube.com/watch?v=D7ZAeB2lJas
Another multispar video
In this one the legs all have the same weight and the cup of water is in the center. It is very stable. This was shot thursday.
http://www.youtube.com/watch?v=mliXEhUMu-4
Lot's of heave
It looks very stable in pitch an roll. However, it looks like it has a large amount of heave. Perhaps the addition of some heave plates to reduce up and down motion?
Oh, ya, heave
My measure of success is not spilling water, which kind of ignores heave. Yes, it does go up and down. Yes, some plates could reduce that. In general I think heave is easier to fix up than pitch or roll problems, so I am not so worried about it.
If we had cross beams between the spars we could also reduce heave, but then we would be more of a semisubmersible.
Hanging Ballast
Until yesterday, I thought that hanging ballast was a cool idea for moving the center of mass down. Upon talking with the marine consultants yesterday, they pointed out that as far as the rigid structure is concerned, the mass is treated as if it is at the top of the cable, not the bottom. So, what we have here is a system where the center of buoyancy is just a tiny bit above the center of mass and it is really unstable.
The solution is to hang your mass at the bottom of a rigid structure (no cables.) This brings the center of mass way down and helps to really make the system stable. In the oil/gas industry, the truss-spar platforms do this.
Here is a link to the FloatTEC Truss-Spar .
Hanging ballast can be a good trick but wrong on Wiki page
On the wiki the "official" page on Hanging Ballast is currently wrong. I have tried to explain this on the discussion for that page on June 3rd and in email to Patri but never got any response (also asked if I could correct it).
However, there is a way that hanging ballast can be done so that multiple wires stay rigid and you don't need a really rigid structure and that is what I did with the bucket. I will make a video of it out of the water tipping back and forth a small amount so that you can see what we have done. Or maybe my double bucket plan will show how it can be more stable.
-- Vince
http://wiki.seasteading.org/index.php/Talk:HangingBallast
I am sure you do NOT have any greater leverage no matter how much longer the cables are in the picture. Cables can not be used as levers, they bend.
There is a way to use cables to make more efficient use of your ballast (but your picture does not do it). If things were as shown but the cables for the weights were long, say down another 100 feet, and then the weights were pulled together in the center and bound together you could win a bit. Then what happens is all of your weight ends up being lifted by the high side and so doing more to restore the structure to vertical. Vincecate 12:11, 3 June 2008 (UTC)
Hanging Ballast, cont.
First, most hanging ballast pictures are drawn the way they are because I asked the 3D modelers to do it that way. So, mea culpa.
Second, I agree that using multiple cables to a single ballast is better than multiple hanging ballasts, because there is some restoring force.
Third, using a rigid spar to move the ballast down will provide way more restorative force.
Fourth, I no longer advocate hanging ballast. I will advocate a ballast truss instead.
Fifth, if you can figure out how to attach "truss" below the paint can and hang the same amount of weight on it, I predict that it will be much more stable. The longer the truss, the more stable.
Sixth, I love the get out in the water and check things out attitude! Great work!
Marine consultants
What else did you learn from the marine consultants? Did they look at any designs? Any feedback?
Mostly Requirements and Trades
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.
Hanging Ballast Video
Here is a video that shows how we did our 20 lbs hanging ballast. It also shows both a right and a wrong way to do hanging ballast and how well they work and don't work. In some cases the ballast is 2 weights of 10 lbs each for a total of 20 lbs and in the bad case it is 4 weights of 5 lbs each for a total of 20 lbs.
http://youtube.com/watch?v=zqSjDemhT7g
I don't really think a solid structure with the same weights and water resistance would have done any better than this hanging ballast did. The win is really just when you tip by enough that one of the ropes goes slack and I don't think this model ever did that. Sticks that you only pull on act about the same as ropes that you only pull on. If I replaced the 4 ropes with 4 sticks, do you think that would make it work much better? I don't. I do think I can get it to work better with a second bucked holding the ballast and some water and a weight on top of the top bucket. Will try this within the next week.
Thanks. I really think we can learn a lot just putting models in the ocean. It is far cheaper than wave tanks at $1,500/day or $10,000/day. Even cheaper than paying someone to do simulations. So I hope others do it too.
-- Vince
Nice video
As the platform tilts, it shifts the ballast at the bottom by a little and the the amount of tension on each of the lines varies. It is not until one of the lines is completely vertical that all of the weight is on one line and the other lines will start to go slack. I did some quick math on a piece of scrap paper and it looks like the platform can tilt until arcsine(r/L) before lines will start to go slack, where r is the radius of the platform and L is distance from the platform bottom down the centerline to the hanging ballast.
With regards to the following:
Actually, I think it will make quite an enormous difference. The maximum righting torque you get with the hanging ballast is approximately r*m*g, where r is the radius of the platform, m is mass, and g is Earth surface gravitational acceleration. With a rigid spar, the torque is L*m*g*sin(theta), where L is the L is the length of the solid truss. Since L>>r, the righting torque should be much larger. My college physics may be a little rusty, but I do not think I have made any major mistakes here.With regards to the following:
I agree that building models is a good idea. The marine engineering folks have been doing it for decades, so it seems prudent to try and learn from the knowledge that they have acquired over those decades. Putting stuff in wave tanks should only be done once you have a pretty good idea that your design is going to work.Hanging ballast / more video coming
>>If I replaced the 4 ropes with 4 sticks, do you think that would make it work much better? I don't.
>Actually, I think it will make quite an enormous difference. The maximum righting torque you...
Yes, the maxium torque is much better. But in my tests I don't think we are getting to where the ropes go slack.
Tried two buckets today, one with the weights in it. It seems to work better (has inertia of water in lower bucket). Will post video soon.
My harbor is too shallow for a 10 foot spar model. So did it without all the weight.
-- Vince
Spar model video
I had 5 lbs on the bottom of this 10 foot pipe and a 2.5 lbs weight on top, but it touches the bottom of the harbor in the deepest part. So I had to take the 2.5 lbs weight off. Below is the video of the test, with about 2.5 feet out of the water. When it is leaning toward the end of the video it might have hit bottom (not sure).
http://www.youtube.com/watch?v=Je5iAj-pswU
Very nice work in all these
If you do requests I would like to see comparisons between different cable lengths as well as one without cables altogether (the weight attached directly to the bottom of the bucket).
-Carl
model testing service
Very nice work in all these tests, Vince.
Thanks!
Just to point out the obvious the water resistance on the ropes as well as buoyancy on the bucket are missing from this dry test.
The dry test was just to show the difference between single hanging ballast with 4 ropes and 4 separate hanging ballasts.
>If you do requests I would like to see comparisons between different cable lengths as well as one without cables altogether (the weight attached directly to the bottom of the bucket).
Yes, this is a good idea and I wanted to try that too. The problem is I can't really do longer in the harbor I usually go to. There are other harbors here and I should be able to find a spot for testing models that go deeper. I did another test with two buckets that I will post about shortly.
I do requests for $100 where I build the model and $50 is you mail it to me. Just have to pay with paypal.
But I also like suggestions and sometimes use them. :-)
-- Vince
Two videos of double-bucket model
Did test with two buckets. The top bucket is just air and the bottom bucket is face up with 25 lbs and water in it. There are 4 ropes connecting the two buckets. The buckets are about 4 feet apart. In the second test there is also 5 lbs on top of the top bucket.
http://youtube.com/watch?v=vX2vPIygBok
http://youtube.com/watch?v=g3-FLqfP3rw
Steps for making video for youtube
I wrote up the steps I worked out for converting video from my cameras to work on youtube:
http://wiki.seasteading.org/index.php/User:Vincecate/Models#Making_video...
I hope other people test models and make videos too.
Youtube resolution and format
Youtube accepts 640x480 resolution. This is unclear from their own documentation but I found out from other sources on a Google search. The benefit of using 640x480 instead of 320x240 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:
http://www.youtube.com/watch?v=ILCCqY1yUmg
http://www.youtube.com/watch?v=x_MtBOgLTkM
http://www.youtube.com/watch?v=b_YEWGoCcn0
A shorter rope should work
A shorter rope should work just as well as a longer, for testing purposes anyway, no? I´m a little sceptical about the usefulness of the rope in the first place, compared to just putting the mass directly onto the bottom of the bucket.
Hanging ballast has some advantages
Look at this video: http://youtube.com/watch?v=zqSjDemhT7g
If I had shorter ropes when we put the 2.5 or 5 lbs on the side of the bucket it would have tipped more.
The 4 independent weights on 4 strings is the same as putting the weights on the bottom of the bucket. Not nearly as stable.
A seastead with 200 foot draft could not get within about 40 miles of the north side of Anguilla. Don't ever try that kind of distance in a dingy and in any small boat that is a very long trip. It would be nice if your seastead could get closer to land than that. A hanging ballast that you could raise when the waves were small could let you get closer to land. So it is a method at least worth thinking about.
Two videos of ball with different length hanging ballast
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.
Long (25 feet) rope to ballast: http://www.youtube.com/watch?v=HqZY7LYdLHk
Short rope to ballast: http://www.youtube.com/watch?v=8Ad1lzixLfg
Updated Ball House page with these:
http://wiki.seasteading.org/index.php/User:Vincecate/BallHouse
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.
Ball does not seem to heave like cylinder or pipe
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.
It helps a little
Hanging a single mass off multiple ropes helps a little. There is some small amount of righting torque. Not nearly as much righting torque as using a solid truss tho'.
hanging ballast helps more than a little
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.
Spar Buoy in small waves
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.
http://youtube.com/watch?v=Sgll7RjzDoc
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.
Spar Buoy in 8-inch waves
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.
http://youtube.com/watch?v=YRCAehPsNRA
Catamaran Model Video
This catamaran model video was shot awhile back but I finally have it up on youtube:
http://www.youtube.com/watch?v=0v1Z8YAMHEY
Medium Spar Buoy
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.
http://www.youtube.com/watch?v=8AauaD68rCU
stable
The multispar looks pretty stable, I like that! I'm sure it matters what the spar spacing is compared to the wavelength, though.
The multispar looks like it
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?
multispar model details
On this page you can see a picture of it out of the water and more details:
http://wiki.seasteading.org/index.php/User:Vincecate/Models/Multispar
There are also a couple of out of the water pictures on my main model page (which links to above and other model stuff):
http://wiki.seasteading.org/index.php/User:Vincecate/Models
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:
http://www.vbuoy.com/thesystem-testing.html
Ok, thanks for that info. A
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...
12 inch pipe for model
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.
I'll check around
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.
Thanks - and four floors
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.
5 floors
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.
same Ewing1.com place / disposable vs durable models
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.