Side by side structure model testing
June 18, 2008 at 2:21 am #613
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
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?June 18, 2008 at 8:39 pm #3259
June 30, 2008 at 2:11 pm #3402
- I like the idea about using the same amount of material for cost equivalence. For extra realism one could perhaps time the manufacturing and include that in the equation as labor costs.
- A regular mono hull ship would probably be interesting to compare with. Determining the relative size of this for a fair comparison might be tricky though.
- Another way to measure cruise speed if the force gauge doesn´t work or proves too inexact could be to tow the structure behind a R/C boat, or just mount an “outboard” of sorts on the structure.
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.
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.July 16, 2008 at 3:39 am #3434
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.July 17, 2008 at 3:43 am #3435
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!).
Have video of the tension circle and catamaran that I expect to post tomorrow.
Thanks to Joep for sponsoring this model!July 17, 2008 at 6:04 am #3437
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!July 17, 2008 at 10:53 am #3436
Here is some video of the tension circle made from PVC pipe. The side sections are 2 foot each.
Slowed down by a factor of 4 (should be 5 but don’t see how to do that in Windows Movie Maker):
The waves look better in the slowed down video.July 17, 2008 at 2:21 pm #3438
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:July 17, 2008 at 8:09 pm #3439
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.July 18, 2008 at 3:12 am #3440
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.July 18, 2008 at 10:24 am #3441
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.July 18, 2008 at 1:53 pm #3442July 18, 2008 at 6:55 pm #3443
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 .July 18, 2008 at 8:10 pm #3444
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.
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)July 18, 2008 at 9:18 pm #3446
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!
The forum ‘Structure Designs’ is closed to new topics and replies.
Posted on at