Welcome to Charge: the future of energy
FURTHER CONSIDERATIONS CONCERNING NONAUTOMOTIVE TRANSPORT AND ENERGY USAGE
by Daniel C. Sweeney, PhD
In the previous post I discussed the various modifications in the power plant of marine vessels that might be made to cope with a continued decline in fossil fuel reserves. I should perhaps mention one further possibility before discussing innovations in hull design which might arguably be of greater utility in this regard.
There are advocates, though they are not many, for the use of nuclear reactors to drive steam turbines aboard ships. Nuclear submarines of course have used such propulsion since the nineteen fifties as have some surface naval vessels and one civilian freighter, the ill fated Savannah. There would appear to be almost insurmountable technical problems with the scheme, however, putting aside the apprehensions of the public over more extensive use of nuclear power.
The reactors used in nuclear subs are not heavily shielded all around. Full shielding would be prohibitively heavy. Instead the reactor is placed some distance from the crew, and partial shielding prevents direct emissions into the crew’s quarters. Much radiation is allowed to escape into the surrounding ocean, but since the ship is in a constant rapid motion, the ship’s wake functions as an effective barrier so far as the crew is concerned. Such a mode of operation would scarcely be acceptable on the surface of the ocean in crowded shipping lanes or near port facilities.
Possibly a triggered isomer reactor, also known as an energy amplifier, could be used on shipboard. These devices, developed by Nobel laureate, Carlo Rubia, and still experimental at this time, are incapable of runaway reactions as are conventional reactors and simply don’t emit harmful radiation at the same level. Nevertheless, this is a very dark horse.
We think the more likely changes in ship design will involve the form of the hull. Conventional ships operate in what is known as the displacement mode, pushing aside a volume of water equal to the volume of that portion of the ship’s hull that is submerged. The displaced water creates frictional drag as it slides past the hull and also forms a wave cycle whose period is dictated by the speed of the ship. The length of wave from peak to peak is dictated in turn by the speed of passage, with higher speeds producing longer waves. When the length of the wave is roughly equal to the length of the vessel, the vessel is effectively trapped in a trough between two crests and can only travel at the speed of those crests. Most ships cannot effectively climb their own bow waves, so the length and speed of those waves imposes an upper limit on ship speed known as the hull speed. Once a ship begins to overtake its own bow wave it can effectively go no faster in normal circumstances.
Within the context of conventional hull design there isn’t much one can do to reduce losses from skin friction which normally increases with speed. But there are some unconventional approaches that promise a way around the hull speed impasse.
Unconventional hull designs include super slender hulls, multi-hulls, air cavity types, planing and semi-planing hulls, hydrofoils, SWATHs, SLICEs, hovercraft, and surface effect vessels. Various combinations of the above have also been attempted.
Super slender hulls are just what you think they are—hulls that are very narrow in relationship to length. Such hulls create less resistance and have higher hull speeds than conventional displacement craft of the same length. They also create relatively small bow waves that are fairly easy to climb. Their drawback is that they’re often too long for conventional docking facilities if they’re built with adequate freight carrying capacity. Incidentally, the concept is not new. Viking ships had super slender hulls as did some racing sailboats built in the late nineteenth century, but the concept has not never seen extensive expression in monohull ships and boats in this century. For some current concepts, visit www.guydesigngroup.fi. Guy Lonngren, the president of the firm, is a brilliant industrial designer who is also a naval architect. While you’re on the site take a look as well at the beautiful and innovative One 40 sailing yacht.
Multihulls, our next group, include catamarans and trimarans, the latter having three hulls. Most such vessels are actually two or three super slender hulls joined together by a bridge deck and they have all of the advantages of super slender hull plus superior stability and sea keeping capabilities. Nevertheless, the concept has not caught on in merchant vessels although a few small multi-hull destroyers have been constructed in European navies. Bridge decks are fine for pleasure craft or even for warships, but in a merchant ship such a design entails placement of cargo well above the waterline which tends to destabilize the vessel.
Air cavity ships have the virtue of resembling ordinary displacement hulls in all but one respect. The bottom of the hull is flat rather than V-shaped and is provided with side and rear walls for containing a pressured air mass which is pumped into the resulting cavity. Approximately 5% of the ship’s power is required to maintain the cavity, but the drag reduction is on the order of 50%.
Full sized air cavity merchant ships have been built by the DK Group, the well known European shipyard, but the design has been slow to catch on. Merchant shippers tend to be very conservative.
Planing and semiplaning hulls are ubiquitous in recreational speedboats and enable the vessel to exceed its hull speed by in effect climbing its own bow wave and skimming along on the flattened planing area toward the stern of the vessel. Planing hulls can attain very high speeds, but the hulls are inefficient at low speeds when they operate in displacement mode, and much power is normally required to achieve the transition from displacement to planing mode.
Recently, a number of similar designs from M-Ships, Wally Yachts, and ICE Marine appear to have solved the low speed inefficiency problem in planing vessels, however.
All of these vessels make use of what are known as lifting rails, short wing-like structures jutting out from the sides of single slender displacement hull. Lifting rails themselves are nothing new, but in these designs the tips of the rails curve down to meet the water and serve as outriggers as well as enclosing a volume of air in a tapering tunnel that merges with the hull at the extreme rear of the vessel.
When the vessel gets underway, a waves form on either side of the bow as in any conventional displacement vessel, but, unconventionally, the waves enter the side tunnels formed by the lifting rails. As the speed of the vessel increases, the waves grow in height until they touch the upper surface of the tunnel and begin to lift the vessel so that progressively less of the displacement hull is actually in the water. The top of the tunnel is flattened and forms a planing surface, but since this surface is not engaged until the boat is moving at speed, it doesn’t slow it down as does a conventional planing hull. In other words the V-bottom contacts the water at low speeds where it is appropriate and the flat planing surface comes into play at high speeds where it is appropriate.
Such vessels also derive a certain amount of aerodynamic lift from air rushing under the bridge and into the tapered tunnel. The air becomes pressurized in the tapered cavity and tends to push the central hull out of the water, reducing hull resistance. So far this type of hull has only been used in yachts and fast patrol craft.
Our next type of radical hull design is represented by the SWATH (small water area twin hull) and the SLICE. The SWATH is a catamaran supported by submerged pontoons which communicate with the hulls by means of narrow, wave-piercing struts. The submerged pontoons do not themselves create waves, only the struts do, that and they produce relatively small waves, smaller than would be the case with a super slender hull vessel of similar displacement. Furthermore, the deeply submerged hulls are not subject to slamming by waves, and a SWATH vessel is generally much more stable than a conventional ship of equal displacement. But for all their virtues SWATHS have not won many adherents. Only about fifty of the vessels currently ply the seas.
A SLICE (the term is not an acronym and refers to the tendency of the hulls to slice through the water) is a refinement of the SWATH idea where four torpedo shaped pontoons are used to support the vessel instead of the two used in SWATHs. The four pod scheme reduces hull resistance very significantly and thus improves top speed and fuel economy. The SLICE was invented by Navatek a Hawaiian naval architecture firm. So far only a single experimental vessel has been constructed to our knowledge. Many technical analyses have been published on SLICE designs, however, and they tend to bear out Navatek’s claims.
Recently Navatek has developed what they claim is an improvement on the SLICE which the company calls a midfoil. This combines two very peculiarly shaped SLICE pods up front along with one large underwater wing or hydrofoil near the center of the vessel and one small moveable T-shaped foil at the bow. Navatek has published no technical papers to date on the exact hydrodynamics of the hull, but the company claims that the new hull design produces lower resistance and thus better fuel economy than any other hull shape. Only one experimental vessel has been built to date. One naturally tends to be suspicious of sweeping claims for revolutionary improvements in performance in any area of technology, but Navatek holds a formidable reputation in the field of naval architecture and has landed many lucrative naval contracts. As diesel grows dearer and dearer we think that more people will be looking at the midfoil.
Our next unorthodox hull design, the hydrofoil, has been kicking around for over a century. A hydrofoil is essentially a water wing and it works by lifting the hull free of the surface of the water and thus eliminating all resistance except for the drag of the hydrofoils themselves and their supporting struts.
Many design variants have been developed over the years including T-sections, ladder foils, V-foils, and surface piercing foils to name a few. Hydrofoils certainly work, they’re not a discredited concept, but they have their limitations. Most designs do not perform adequately in rough water, and almost all maintain high efficiency over only a fairly narrow range of speeds. Like planing boats, most hydrofoil types require considerable power to pass the hump speed where the wing begins to generate appreciable lift just as most airplanes expend maximum power during liftoff. Above a certain speed, usually around 40 knots, most hydrofoils begin to cavitate, that is, to generate air bubbles around the foils which sharply reduce lift and can damage the foils as well through the turbulence they create. Some hydrofoils, termed supercavitating hydrofoils, have been designed to continue to produce lift past the point of cavitation, but efficiency is sacrificed, and these designs have not found much of a market. Possibly the hybrid midfoil and a related design known as the HSYCAT will win greater acceptance than more traditional concepts, but it’s too soon to tell.
Hovercraft we can dismiss in a few words. Top speeds are limited, and rough water performance is poor, as is fuel economy. The noise generated by the air propellers tends to be objectionable as well. Hovercraft ferries will continue to operate but we do not expect to see hovercrafts much used in commercial shipping.
Finally we come to surface effect vessels, the strangest and perhaps the most intriguing of all the unconventional hull types.
A surface effect vessel makes use of what is known as the wing-in-ground-effect (WIG) principle. This principle has it that the lift of an airfoil increases greatly as it approaches the ground. A vessel operating at elevations where wing in ground effect is present will require much less energy to maintain itself aloft than an aircraft flying out of ground effect and thus can achieve much greater fuel economy than a conventional airplane—four or five times as much, best case. Economy will be considerably poorer than is the case for a displacement hull traveling below hull speed, however.
Wing-in-ground-effect craft fly fairly close to the surface of the water, how close will depend upon the wingspan. Boeing’s proposed Pelican which has a projected wingspan of 500 feet will fly fifty feet above the water. Personal WIGs may fly less than a meter above the surface.
WIGs are driven by aircraft propellers and use air rudders. Some use ailerons and elevators to trim themselves in flight while others do not.
WIGs get much better fuel economy than planing watercraft, and personal WIGs can achieve speeds well in excess of 100 knots with engines of just slightly over 100hp. Larger WIGs are believed to be capable of speeds in the hundreds of knots. Some WIGs are capable of conventional flight.
Because of their superior weight carrying abilities, WIGs can be scaled up to the size of small ships and can carry the same kind of bulky cargo container as merchant vessels though at considerably greater speeds.
Many different designs are extant. The tandem WIG developed by Gunther Jorg resembles a boat and flies at very low altitudes while the Russian Ekranoplane, the earliest successful design, definitely looks like an airplane. Most current designs look much more like aircraft than boats though they all have relatively short, stubby wings. Most are quite substantially built, for a WIG can come in contact with the waves and needs to be able to stand up to heavy seas. The three point hydroplanes used for ocean racing are close kin to WIGs and operate almost entirely in ground effect with only the small planing surfaces at the stern of vessel remaining in contact with the water.
While a number of companies currently build WIGs on a special order basis, no one has ever launched a production model, and all of the projects for constructing large freight carriers have come to naught.
WIGs occupy a niche between large aircraft and fast ocean going vessels. While they are somewhat slower than jets carrying air freight, they are much more economical to operate and if jet fuel prices become sufficiently elevated WIGs could gain a foothold in commercial transport. Navies have been eyeing WIGs for years, and if Boeing succeeds in building the Pelican, the U.S. military will be a likely customer.
In our next post we will explore air travel in the era of diminishing fossil fuel.