1
$\begingroup$

When an object (say a boat) moves through water, there certainly is a type of friction. If a cargo ship sailing in the sea is able to heat the front side of water against its motion, can this way lead to an increase in fuel efficiency by moving faster?

I just think heating can reduce the friction.

$\endgroup$
  • $\begingroup$ Why do you believe heating would reduce friction by a substantial amount? I know there would be a very very slight effect due to decreasing the viscosity of the water, but my gut instinct is that the effect would be imperceptable. $\endgroup$ – Cort Ammon Aug 12 '16 at 17:08
3
$\begingroup$

Well, if your idea was tried, you would have to balance the reduction in friction (energy gain) against the fuel used to heat the bow of the ship, (energy loss).

enter image description here

As an alternative to the heating / reducing friction idea, here is how modern large cargo ships and ferries utilize a bulbous bow to reduce drag. The effect of the bulbous bow can be explained using the concept of destructive interference of waves (see graphic):

From Ship Design Wikipedia

A conventionally shaped bow (#2 lower left) causes bow wave forms (#4). A bulb alone (#1 top left) forces the water to flow up and over it forming a trough (#3). Thus, if a bulb is added to a conventional bow at the proper position (#1 right), the bulb trough coincides with the crest of the bow wave, and the two cancel out, reducing the vessel's wake. While inducing another wave stream saps energy from the ship, canceling out the second wave stream at the bow changes the pressure distribution along the hull, thereby reducing wave resistance. The effect that pressure distribution has on a surface is known as the form effect.

This bulbous idea is much more efficient than the idea in your post. Heating the bow would use a large amount of energy, remembering the low temperature sea would constantly cool the bow down, and it would also cause turbulence of the water, on a very small scale. Compared to that energy expenditure, friction reduction by heating would not result in overall energy saving.

In aircraft and such things as golf balls, turbulence can reduce drag, but no heating is involved and obviously water is much denser than air, so you could not apply the same idea to ships without testing it first, and I doubt it would be efficient overall. As Cort says above, the viscosity of the water might reduce, but it would be a tiny saving compared to the saving in drag/friction produced by the bulbous bow idea.

$\endgroup$
  • $\begingroup$ Yes. Reducing the energy pumped into the wave is a major factor in reducing the drag of a large ship. On smaller craft, people have successfully experimented with modifications to the boundary layer - see for example the NASA riblets used on the 1984 US rowing shell that won silver in the Olympics, or the 1987 Stars and Stripes that won the America's cup. $\endgroup$ – Floris Aug 12 '16 at 19:13
  • $\begingroup$ I assumed the OP was talking about large ships, because of the power demand. I do seem to remember, and it might be what you refer to above, that certain yachting teams went to great lengths to hide the hull until the craft was in the water. This sort of thing: boat-duesseldorf.com/cipp/md_boot/custom/pub/content,oid,5062/… $\endgroup$ – user108787 Aug 12 '16 at 19:20
  • $\begingroup$ Thanks for broadening my horizon. I just know the function of the bow. As you said turbulence can reduce the drag, is it possible to install a drill on the bow or other ways for creating a certain degree of turbulence to enhance the speed of cargo ship? $\endgroup$ – layten Aug 14 '16 at 7:22
  • $\begingroup$ A few suggestions: ask a question based on the difference between the importance of laminar flow / turbulence (and is it possible to maintain it) on ships versus aircraft after reading these: en.m.wikipedia.org/wiki/Laminar_flow and this: en.m.wikipedia.org/wiki/Reynolds_number I am pretty sure the "nautical" Reynolds number is very low. Also, if you could actually see large ships in dry docks (or the blueprints) you might get an idea of the tricks used to minimise drag. I might use Google "cargo ships dry dock" and then use the images option on search. $\endgroup$ – user108787 Aug 14 '16 at 10:23
0
$\begingroup$

I think not

If you were able to heat the water at the bow of a ship, would that not make the water less dense and cause the ship to settle deeper into the water, raising the water line, therefore creating more friction

The only way I know of reducing the drag or friction of a ships is to lift it out of the water, such as the hydrofoil, as the Russian Engineer Rostislav Evgenievich Alexeyev created.

Alexeyev was the first person that I know of to create high speed ships on the so-called low submerged underwater wings, They were manufactured from 1957 until today. the most popular ones being passenger ships Raketa, Volga, meteor, Kometa, and Burevestnik, with passenger capacity up to 150 persons and cruising speed up to 100 km/h (62 mph; 54 knots).

Not content with that. Alexeyev revolutionized the shipbuilding industry (though in secrecy) by inventing a craft that used ground effect, whereby a wing traveling close to the water is provided with a better lift-drag ratio - thereby enabling a combination of greater ship weight for less power and/or enhanced fuel economy. This technology is from the sixties and I do not know and cannot understand why this method is not being used today.

The ekranoplan had wingspan of 37.6 m, length – 92 m, maximum take-off weight – 544 tons. cruising speed: 430 km/h (267 mph; 232 kn) Until the Antonov An-225, it was the largest aircraft /ship in the world. It was designed to fly at an altitude of 5-10 meters and use the ground effect.

Data from The Osprey Encyclopedia of Russian Aircraft 1875 - 1995 Russias Ekranoplans:The Caspian Sea Monster and other WiG Craft General characteristics • Length: 92.00 m (301 ft 10 in) • Wingspan: 37.60 m (123 ft 4 in) • Tail stabilizer span: 37 m (121 ft 5 in) • Height: 21.80 m (71 ft 6 in) • Wing area: 662.50 m2 (7,131.1 sq ft) • Empty weight: 240,000 kg (529,109 lb) • Max takeoff weight: 544,000 kg (1,199,315 lb) • Powerplant: 10 × Dobrynin VD-7 turbojet, 127.53 kN
(28,670 lbf)thrust each

Performance • Maximum speed: 500 km/h (311 mph; 270 kn) • Cruising speed: 430 km/h (267 mph; 232 kn) • Range: 1,500 km (932 mi; 810 nmi) • Design altitude: 4–14 m (13 ft 1 in–45 ft 11 in) • Maximum sea state: approx - 2 m

Manufacturer Central Hydrofoil Design Bureau Designer Rostislav Evgenievich Alexeyev First flight October 16, 1966 Status Destroyed in 1980

The Lun-class ekranoplan is a ground effect vehicle designed by Alexeyev and used by the Soviet and Russian navies from 1987 until sometime in the late 1990s. (Lun means Harrier from Russian) It flew using the lift generated by the ground effect of its large wings when close to the surface of the water–about 4 meters (13 ft) or less. Although they might look similar and have related technical characteristics, ekranoplans are not, aircraft, seaplanes, hovercraft nor hydrofoils, ground effect is a separate technology altogether. The International Maritime Organization classifies these vehicles as maritime ships.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.