a semi-submersible is a boat in which the majority of its mass is under the waterline. this results in a craft with exteremly good stability because most of the ship is under the surface of the water. Spar and muti-spar seastead designs are basically semi-submersibles with a poor use of space. about 90% of the mass is under water as a ballast and 10% is used above the surface for a seastead. with a semi-submersible you have the 90% underwater, but it is in the general shape of a sub, not a spar and is used as living space, resulting in a cheaper more mobile seastead.
It seems like semi-submersible refers to a method of operation and not a particular shape. Semi-submersible oil platforms have large underwater horizontal pontoons. They also have vertical cylinders. Seadrome, ClubStead and Spar Buoys have only vertical cylinders. All generally have living spaces above the water.
Semi-sub mostly refers to shape, although the historical origin of the word probably reflects a mode of operation (towing vs stationary deployment). Indeed, the classic semi-sub has horizontal submerged pontoons.
If you try and break down the physical principles on which a semi-sub is based, its real-estate above water, air-gap with minimal structure, flotation below the water. This has the effect of staying clear of the waterline, where most wave-action is going on. Thats essentially the same thing as multi-spars are doing though, modulus the horizontal pontoons.
Clubstead can be regarded as a multi-spar, but given its lack of slenderness, its more of a semi-sub id say. Same thing for seadrome.
It seems nearly any design that floats, and has basically stable characteristics, will be semi-submerged. However, this description misses the point and could be misleading.
Submersible seems to imply deliberately changing state between above water and below. If you’re not designing for dual-purpose operation, it would seem that semi-submerged is a better term.
So for proposed definitions:
Submerged: Fully underwater, all the time. Probably requires a submersible vessel to reach it.
Semi-submerged: a design that has significant usable space underwater, all the time. Key point: The focus of the design is the underwater portion of the structure. Personally, I would not count simlpe displacement requirements as semi-submerged- for example in a conventional ship. Otherwise, everything that wasn’t a raft would be semi-submerged. That’s a change from the generally accepted usage that will be confusing to neophytes and possibly experts as well.
Submersible: a vessel which is able to change state at will from surface operations to completely underwater operations.
Semi-submersible: a vessel designed to change state from surface operations to mostly underwater, but maintains access/ a portion above the surface.
Look-up “Flip ship” and “SeaOrbiter.” Both designs run counter to your description, although the purpose isn’t to be self-sufficient, they are efficiently designed, with rooms below the water-line, to efficiently provide for the specific missions they perform/will perform(SeaOrbiter is still in the works and design finalizaton stage).
If you can’t swim with the big fish, stick to the reef
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