1. Skip to navigation
  2. Skip to content
  3. Skip to sidebar




Side by side structure model testing

Home Forums Archive Structure Designs Side by side structure model testing

This topic contains 74 replies, has 10 voices, and was last updated by Avatar of vincecate vincecate 5 years, 10 months ago.

Viewing 15 posts - 16 through 30 (of 75 total)
  • Author
    Posts
  • #3451
    Avatar of vincecate
    vincecate
    Participant

    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

    #3452
    Avatar of vincecate
    vincecate
    Participant
    #3454
    Avatar of vincecate
    vincecate
    Participant

    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

    #3455
    Avatar of Wayne-Gramlich
    Wayne-Gramlich
    Participant

    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?

    #3460
    Avatar of Wayne-Gramlich
    Wayne-Gramlich
    Participant

    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:

    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.

    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:

    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.

    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.

    #3461
    Avatar of vincecate
    vincecate
    Participant

    >>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

    #3462
    Avatar of vincecate
    vincecate
    Participant

    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.

    #3463
    Avatar of Carl-Pålsson
    Carl-Pålsson
    Participant
    • Very nice work in all these tests, Vince.
    • 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.

    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

    #3466
    Avatar of vincecate
    vincecate
    Participant

    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

    #3467
    Avatar of vincecate
    vincecate
    Participant
    • 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

    #3468
    Avatar of vincecate
    vincecate
    Participant

    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

    #3469
    Avatar of Carl-Pålsson
    Carl-Pålsson
    Participant

    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.

    #3470
    Avatar of Wayne-Gramlich
    Wayne-Gramlich
    Participant

    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’.

    #3471
    Avatar of vincecate
    vincecate
    Participant

    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_for_youtube

    I hope other people test models and make videos too.

    #3472
    Avatar of vincecate
    vincecate
    Participant

    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.

Viewing 15 posts - 16 through 30 (of 75 total)

The forum ‘Structure Designs’ is closed to new topics and replies.



Posted on at

Categories:

Written by

Blog/Newsletter

Donate