Snippets, 8/4/2008by wpengineAugust 5, 2008July was a bit of a slow month as I was wrapping things up at Google, but we now have some key initiatives underway:Share:Tags:newsletter previousCommentsnextConference Wiki Signup 19 thoughts on “Snippets, 8/4/2008” vincecate August 5, 2008 at 6:31 am Reply Is your potential design a spar? Or do you not even want to say the general type of design? Wayne Gramlich August 5, 2008 at 7:43 am Reply My preferred design for small inexpensive seasteads is a mini truss-spar. This has a short stubby cylinder (say 20′ across by 50′ deep) with a truss that extends another 40′ – 50′, with ballast on the end. The cylinder is ballasted to about the mid-point 25′ above water and 25′ below water. The closer the center of buoyancy is to the surface, the less wind loading there is, but the more wave interaction there is. The deeper the center of buoyancy is, the lower wave interaction there is, but the higher the wind loading is. The less the spar sticks up into the wind, the less the wind can push it over, but the easier it is for waves to cap the spar. It is all a bunch of trade-offs. Note that the winds in the open ocean can reach “hurricane level” for scale models very easily. If a seastead tilts during a hurricane, but does not sink, that is probably a reasonable trade-off. A 20/30 degree tilt during a hurricane would be unpleasant, but not as unpleasant as being on a ship that gets sunk by said hurricane. The goal of this design is to cost less per square foot than a new/used boat. If it is cheaper per square foot to purchase a new/used boat, that is what any prudent person would do. While it would be nice if the mini truss spar seastead is more comfortable than a new/used boat, it is not a requirement. If the mini truss-spar is safer, that would be nice too. Wayne Gramlich August 5, 2008 at 7:46 am Reply Vertical spar towing is apparently a solved problem. A vertical spar with smooth sides will apparently oscillate back and forth as it towed. To prevent this behavior, a spiral surface is added to the spar to break up the side to side forces. Do not ask me why it works because I do not know. vincecate August 5, 2008 at 1:06 pm Reply >Vertical spar towing is apparently a solved problem.I was not even thinking of oscillations, just the force needed to tow it. So if we wanted to migrate at 1 MPH, how much force is needed? My worry is that if we use diesel it will just cost too much to move.I think the towing and leaning is more a problem for a 200+ foot long spar, and less an issue for the one you describe. I think a big kite would be reasonable for moving it along at 1 MPH. For the short spar I think the question is stability. From earlier conversations you seem to think the truss will work much better than the hanging ballast that I tried with my short cylinder (paint bucket). I don’t really think so, so I will probably try an experiment or two. vincecate August 5, 2008 at 2:44 pm Reply >Note that the winds in the open ocean can reach “hurricane level” for scale models very easily. Wayne Gramlich August 5, 2008 at 7:55 pm Reply The truss should have a significantly greater righting torque. I’ve worked through the math once before and posted it. I’m busy today, so I don’t have the time to do it again. I’ll try to draw some diagrams and post them later tonight along with the math again. Patri August 5, 2008 at 8:02 pm Reply I don’t want to be too specific, but I will say that I don’t think a single spar is a good design for a large seastead, although I agree with Wayne that it’s good for a small one. vincecate August 5, 2008 at 8:48 pm Reply As before, I agree that the maximum torque is greater with a truss but don’t think my models got to where it makes a difference. To be clear, I am not arguing with your math and you don’t need to work it out again. I think as long as you are in the range where the ropes are all in tension that pulling on ropes is the same as pulling on beams. This means the weight is always under the cylinder. But if you have a 2 foot truss and a 1 foot wide cylinder, you have to tip a lot before you get past the edge and the truss wins. Wayne Gramlich August 6, 2008 at 8:12 am Reply The truss wins because it gets to multiply by the length of the truss as opposed to the cable system where it is the radius of the cylinder. Let me try the math again: Let L be the distance from the center of buoyancy to the ballast mass M. Let r be the radius of the cylinder. For the truss, the righting torque is: T(a) = M × g × L × sin(a) For the wire ballast, there is the critical angle c, which is when the mass is directly under the cylinder edge. I am assuming that the cable is attached to the cylinder horizontal to the center of buoyancy. For this situation: L = sqrt(L2 + r 2) × cos(c) The second equation can be rewritten as: sin(c) = L / sqrt(L2 + r2) which will be reused below. For the cable guy system, the torque for angles between 0 and c is: T(a) = (a/c) × M × g × r For angles greater than C, the maximum torque is: Tmax = M × g × r At the same critical angle for the truss, using the equation for sin(c) above: T (c) = M × g × L &timps; sin(c) = M × g × L × (L / sqrt(L2 + r 2)) If L = 4r (what you used in your example): Tmax = M × g × (4r) x (4r / sqrt(4r2 + r2)) = M × g × 4r × (4r / sqrt(16r2 + r2)) = M × g × 4r × (4r / sqrt(17r2)) = M × g × 4r × (4/sqrt(17)) = M × g × 4r × .97 ~= M × g × 4r Thus, the truss has 4 times the maximum torque of the cable system when L = 4r. After the cable system exceeds a tilt of c, no additional torque is applied to the system. For the truss system, the longer L is, the greater the torque multiplier. For the guy wire system, longer L, reduces the critical angle c. Honestly, by the math, the truss system completely beats the guy wire system. vincecate August 6, 2008 at 8:53 am Reply The ropes don’t go slack: vincecate August 6, 2008 at 8:55 am Reply Before we get to the criticle angle “c” for the hanging ballast, so angles between 0 and c, I think the truss and the cables have the same torque. Do you agree? If not, do you think replacing the straight part of the ropes in my latest video with rods would make any difference? If so, why? Nothing would push on the rods and a pull on a rod does the same thing as a pull on a rope. vincecate August 6, 2008 at 5:44 pm Reply >Thus, the truss has 4 times the maximum torque of the cable system when L = 4r. vincecate August 7, 2008 at 1:12 am Reply I made a page with a bunch of videos from my “Short Cylinder” experiments. I think my best result was with a second bucket underwater. This held the 20 lbs and provided drag against movement in the water (up/down/sideways) and the extra inertia of the water (no lid but still gets inertia). Wayne Gramlich August 7, 2008 at 4:33 am Reply I agree, the double bucket experiment looked pretty good. Assume a 1:50 scale, those waves were pretty substantial – 35′-45′. I suspect that people would be a bit uncomfortable, but in no danger of dying if that model were scaled up by a factor of 50. For a truss system, I would add a heave plate to reduce up/down motion. I have no idea what a heave plate would do to a cable system. I remain pretty convinced that a short cylinder with attached ballast is going to cheaper than a boat and probably more comfortable in heavy seas. It looks safer to boot. (I appreciate the patience it takes to just sit there in the water for 10 minutes and just shoot video.) vincecate August 7, 2008 at 5:30 am Reply The double bucket had about 4 feet of rope between the buckets. Each bucket is about 1.5 feet long, so the total length for the system is about 7 feet. For sure the bucket reduces heave (both because of increased drag and increased inertia). If you ever had heave plates big enough that the ballast could not drop fast enough to keep the ropes from going slack, there could be trouble. I think that is much bigger than the area of the bucket. Wayne Gramlich August 7, 2008 at 6:23 am Reply I couldn’t tell how bit the bucket is (no reference object). I think 1:25 is a more accurate scale. vincecate August 7, 2008 at 1:59 pm Reply So I can slide some 2 foot sections of PVC pipe (have some 3/4 inch around) over the 4 ropes. So I think an experiment to test out rods vs ropes is not very hard and so will do it. I also think there could have been a small amount of stretch in the ropes, or they might not have all been exactly the right length, so this might just help. Also, the extra sideways drag of the pipe vs just a very small rope could make it work better. vincecate August 7, 2008 at 4:25 pm Reply Thorizan August 8, 2008 at 7:49 pm Reply Thank you, Patri, for this update. It’s good to hear that we are getting more media coverage, and that the basics for this plan is progressing nicely. Leave a Reply Cancel replyYour email address will not be published. Required fields are marked *Name * Email * Website Comment Save my name, email, and website in this browser for the next time I comment.