Marine

Study suggests semi-submersible design may make for more efficient ships

Although the 3D-printed model used in the study is mainly remotely controlled, it can autonomously perform actions such as following preprogrammed routes and pursuing moving targets
Washington State University
Although the 3D-printed model used in the study is mainly remotely controlled, it can autonomously perform actions such as following preprogrammed routes and pursuing moving targets
Washington State University

Many readers will already be familiar with so-called narco subs, used to smuggle narcotics from South America into Florida and area. Well, according to a new study, the design of such watercraft could actually have some practical – and legal – applications.

Although narco subs take various forms, the majority of them aren't actually true submarines. Instead, they're user-built semi-submersibles which sit mostly underwater, but that have a small conning tower/snorkel which protrudes slightly above the surface.

This design makes them very difficult for authorities to spot, while forgoing the expensive and complex requirements of true submarines. For example, they don't have to be capable of diving, withstanding great pressure, or supplying their occupants with an onboard air supply.

Led by Prof. Konstantin Matveev, scientists at Washington State University recently set out to further assess the merits of such semi-submersibles. In order to so, they built a remote-control model semi-sub measuring 45 cm long by 10 cm wide (17.7 by 3.9 in). It was tested in Wawawai Bay on Washington state's Snake River, where it reached a top speed of 1.5 meters (4.9 ft) per second.

The experiments indicated that at higher speeds, full-size semi-submersible vessels should be more energy-efficient than conventional surface-going ships. This is due mainly to the fact that semi-subs have a smaller waterplane area, which is the horizontal cross-section of a vessel that is level with the waterline.

"There are essentially two drag components of a ship, one is due to friction (which is proportional to the wetted area) and the second is the wave drag (depends on several factors including waterplane area and speed)," Matveev explained to us. "Semi-subs would have larger drag at low speeds when friction resistance is dominant. However, at higher speeds […] wave drag becomes dominant. Semi-subs have lower wave resistance due to their smaller waterplane area. Thus, they become more efficient than surface vessels in that higher speed regime."

Matveev also noted that due to their smaller waterplane area, full-size semi-subs should be less affected by wave action. As a result, whereas a surface-going cargo ship would have to slow down in rough waters in order to keep its load stable, a cargo-carrying semi-sub could maintain a higher speed.

A paper on the study, which Matveev co-authored with Washington State graduate Pascal Spino, was recently published in the journal Unmanned Systems. The model semi-sub can be seen in action, in the following video.

Source: Washington State University

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9 comments
1stClassOPP
Doesn’t make sense to me. Obviously water has far more resistance then air. I would guess that a surface model equal in weight, and dimensions, the surface vessel would out perform the sub surface vessel. The only way I see to mitigate resistance, is to cause on air bubble emit from its bow. Surface ships could also benefit from re directing their exhaust through portals at the bow and bottom, thus enveloping the whole sub surface structure with air(exhaust).
Username
I'm not sure that playing with one toy can really lead to sufficient data for a conclusive analysis .
jeronimo
This is theoretical nonsense. If you live near a port, you will see why cargo ships are the way they are.
TechGazer
The relative efficiency probably varies with size, so a gigantic container ship may have less wave losses vs skin losses than the tiny model they used. For smaller vessels, my guess is that hydrofoils would provide even lower losses. There are also techniques for reducing wave losses on large ships, such as those submerged bulb noses, and wave cancellation from side deflectors. There's probably still efficiency gains to be made via those methods, especially if the configuration can be changed while sailing, to match the conditions.

Maybe this study can be applied to bathtub toys.
Catweazle
Many years ago I saw a design for a tanker that was fully submerged around the surface with a "sail" with the bridge on top running clear of the surface, on the principle that the waterline drag was considerably greater than that of a fully submerged streamlined hull.
No idea what became of it.
William Waugh
Maybe it is time to revisit the whalebacks of the Great Lakes from the late 1800s.
One even did an Atlantic crossing. One was built in Everett, WA but the rest were Great Lakes ships.
2Hedz
New Atlas has the best comment section on the internet. Where average Joe's challenge university researchers on a whim with their own physics and logic. Keep it up dudes!
jerryd
To cut bow wave drag all one needs to do is have a hull 8x longer than it is wide which can actually enter a port as more shallow and load, unload because it is above water.
They did the sub versions back then to go under Arctic ice but never made past the proposal stage.

Aermaco
The reason drag is less with a fully immersed hull vs much mass above water-line plane is the induced drag-making wake waves turbulence that sucks energy compared to a smooth FULLY streamlined body below the water-line with no to little wake.