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

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