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Single-spar is a very bad idea
Wed, 06/11/2008 - 01:57 — Anonymous
It's true that a single-sparred platform wouldn't bob up and down much, but I think you're missing the point. Multi-spar platforms are stable because:
- the spars don't bob up and down very much;
- there are at least three spars;
- and there's a significant distance between the spars.
A single-sparred platform, like a one-legged stool, would be very wobbly indeed.
I hope you will give your stool it's full complement of legs but, If you insist on experimenting with a single-spar design, please, please, please don't build it in concrete.
Peter
Single can be stable
There are a number of possible ways to get stability. The reason for looking at one leg is that it is probably the most affordable for a single family house. The stability of one leg can be very good. Look at FLIP or other spars. There are guys spending big bucks on single leg designs and the resulting platforms have been very stable.
http://www.sio.ucsd.edu/voyager/flip/
http://www.globalsecurity.org/military/systems/ship/platform-spar-comp.h...
But I too have my doubts about using concrete for a spar.
I dont have the final
I dont have the final answers with regard to material choices either, but suffice to say, these are very solvable questions that will be solved along the way.
A single spar buoy is has
Not necessarily on point
Not necessarily on point one. It is the case that one of the advantages of a multi-spar platform is probably the ability to use less ballast and a shallower draft. It is also able to carry a heavier platform because of greater displacement and easier construction of a broader platform because it doesn't have to balanace on top of a pole.
However, one could still build a multispar platform on the same model as the sinigle spar, with CG below CD, Yoiu just wouldn't be able to take advantage of the greater above-water weight allowance.
In your second point, you're confusing stability and failure modes. Sure, if a leg of the multi-spar platform fails, it may list to the side. On the other hand if a single spar platform fails, it won't list because it has already sunk.
Likely causes of failure are lost buoyancy in one of the legs of the Multi. It starts listing to the side, probably gradully. If it lists enough in bad weather, wind or wave action could tilt it over more completely.
The Single could lose buoyancy due to loss of hull integrity, or potentially, lose ballast due to the extreme length of the spar, by snapping off somewhere in the middle. In which case it bobs quickly up, falls over, and sinks completely if it hasn't maintained enough internal integrity for some buoyancy. In either case, people (if they were inside the spar) are jelly, are thrown around, off the side, or smashed up in in the platform, the safety hull of the platform may fail becuase it is now tipped on its side, etc.
"Not necessarily on point
Hyperstatic is static
"The triple is trying to turn upside down but the legs balance it and keep it from doing so"
And so is any chair, but that does not stop anyone from sitting.
"When comparing a single spar to a tripod it is reasonable to assume that the single spar will be three times larger than each of the legs of the multi legged version"
More like six times larger, at the very least. In order to have any kind of useful stability a single spar needs a significant ballast weight and a significant bonus in length too in order to leverage that ballast into stability and compensate its surplus weight without compromising the intended reduced wave coupling. This makes a single spar design at least double the buoyancy and mass of an equivalent-sized multileg design.
"As described"
The description was pretty sparse and doesn't explicitly address the points you're disputing.
Cost of materials is not likely to be the determinent factor: cost of labour is. So you can't assume that a single spar is 3 times the size of a multi-spar.. It might be far bigger in terms of materials and actual size, but probably far less in terms of useful cubic and top area. Each leg of the multi having 1/3 the number of water tight compartments is also a complete non-sequitur. There is nothing supporting this statement. Number of water-tight compartments is a design decision, only loosely related to cost of materials (more tightly coupled to cost of labour though.)
Again, you're confusing stability with failure modes. Yes, I got your drift, and I'm asserting that your line of thinking is missing the point. Stability is the ability of the seastead to resist vertical bobbing and lateral tilting when operating properly, within design parameters especially regarding wave height.
Your assertion that multi-spar platforms can be "toppled" assumes a failure. Comparing a multi-spar failure and list with a single-spar's normal operating stability is apples and oranges.
To quote you, let's make "fair comparisons". A multi-spar platform is almost certainly going to be more stable than a single spar, although a single spar probably has very good stability, as demonstrated by the FLIP.
To make the other "fair comparison" it's necessary to compare the failure modes of multi-spar vs single spar. If one spar of four fails, does the whole platform necessarily sink? Maybe, maybe not- the platform probably just lists to that corner.
If the single spar fails, then what saves it? Perhaps the safety hull of the platform. This does require a large reserve buoyancy capacity in the platform itself. An extra exepnse, but probably a necessary one. The nature of the failure is also important: if it just leaks and loses buoyancy, the safety hull is great, but if the spar's structural integrity fails between CG and CD, the safety hull of the platform does little or no good.
Both types have their risks, but the structural integrity risk has greater consequences for the single spar. That risk can probably be mitigated by material type and proper engineering. Failures of buoyancy are probably about the same risk for both platforms- i.e. they both sit lower in the water and are affected more by wave action and wind.
spar vs multi-spar reminds me of monohull vs multihull
<blockquote>Your assertion that multi-spar platforms can be "toppled" assumes a failure. Comparing a multi-spar failure and list with a single-spar's normal operating stability is apples and oranges.</blockquote>
The sailing people have had endless debates about wether a monohull or a multihull was safer. This reminds me of that.
The monohull people point out that in a rough ocean the monohull really is only stable upside right and that the catamarans are just as stable upside down. Theoretically a wave cold turn over a multi--spar like it can a multi-hull. Theoretically a spar can tip back up from any wave, like a monohull.
The catamaran people point out that turning over is not necessarily fatal and very rare for cruising cats. Catamarans are usually faster at getting out of the way of storms too. They also point out that monohulls really have two stable positions just like catamarans. But with all the lead monohulls carry the second stable position is at the bottom of the ocean. Catamarans can have enough foam that they are "unsinkable".
In practice it does seem that the chances of a monohull sinking is greater than the chances of a cruising catamaran tipping over. If someone leaves a hatch/window open, or a hatch/window breaks, enough water can come in to sink the boat. A crew mistake does not lead to sinking on a cruising catamaran.
Some of these points carry over to the spar vs multi-spare debate. I would not like a design where a mistake by the crew or a computer could sink it. People and computers do make mistakes.
That's why you have design
That's why you have design parameters. With a mile-high wave, all bets are off no matter the platform. Even a submarine could be caught up in it. But we're talking about withstanding waves at 30-40 meters outside. That's vicious and scary. If seomething bigger comes along, you may well sink. But you can't make it 100% safe, you can just build it to the nth percentile of known risks.