7
$\begingroup$

Can anyone tell me the official or technical term for the effect of when extremely calm water appears when a ship turns sharply in a full circle? I realise it could be related to wakes, but is not a wake in the traditional sense.

For example:

enter image description here

Has any research or modelling been conducted on this effect?

Thanks

Richard

Improve this question
$\endgroup$
6
  • 1
    $\begingroup$ This is most likely caused by the destructive interference of the wakes produced by the ship. They are waves, so when a peak meets a trough of equal energy, they turn into calm water. $\endgroup$ – willow Aug 17 '15 at 22:06
  • 1
    $\begingroup$ My guess would be upwelling in response to the surface water being pushed outwards by the motion of the ship. $\endgroup$ – Deditos Aug 18 '15 at 16:30
  • 2
    $\begingroup$ It's caused by the turbulence caused by the ship. Turbulent water absorbs and diffuses waves, causing a calm spot in the water. (I don't know what the term would be, though.) $\endgroup$ – Daniel Griscom Aug 19 '15 at 15:17
  • $\begingroup$ @Daniel Griscom Can you make the statement "turbulent water absorbs and diffuses waves" rigorous? In my opinion, the most likely explanation would involve the ship induced surface drift, and the conservation of wave action. A video of this phenomenon would be helpful. $\endgroup$ – Nick P Aug 20 '15 at 17:52
  • $\begingroup$ Although I'm confident of the truth, I only have my intuition to back it up. Sorry. $\endgroup$ – Daniel Griscom Aug 20 '15 at 19:25
2
$\begingroup$

I feel bad answering this one without any research reference, but I can tell you that what you are seeing in the OP's picture is a 'man overboard drill.' In that drill, the objective is to rotate the ship's rear section away from a man who many have fallen overboard and then circle the ship around (in an exact circle) to come up alongside the fallen man.

In order to do this with a ship, you throw the rudder over hard and fire up the engines ahead full. With many warships, the engine's toque is enough to produce massive cavitation (bubble formation) for the duration of the turn. As you end the turn you will usually throw a 2/3 backing bell, which will produce even more cavitation. That is the 'trail' you see behind the ship. Specifically, the ship you see there is HMS Echo, which has a diesel-electric azimuth drive mounted below the ship, which will provide a very high torque.

Lots of cavitation (and you can feel a 10,000 ton ship vibrate during these maneuvers) means lots of bubbles are formed up to 15m below the surface (depending on the size of the ship; only about 6m for the pictured Echo). As the bubble propagate to the surface they disrupt the passage of surface wavelets and form the calm area shown.

One more factor is that navies train to put the man on the leeward side of the ship during recovery. Thus, in this picture the stern was swung to port (as we can see from the 'trail') so the man must have been on starboard, so the man is on the 'starboard' side of the ship during this recovery. That means the wind should be on the port side, so the calm area is being protected from the wind by the mass of the ship.

Improve this answer
$\endgroup$
2
  • $\begingroup$ why isn't this a wake ? en.wikipedia.org/wiki/Wake $\endgroup$ – gansub Jun 23 '19 at 13:20
  • $\begingroup$ For the maneuver seen above, the azimuth drive on the given ship will not be propelling the ship forward through the water. It will be be generating thrust nearly perpendicular to the long axis of the ship, causing the tail of the ship to swing with very little motion on the box. Thus, there is no 'recirculating flow behind' the stern of the ship, in the definition of a wake, because the ship is not moving ahead. Its really about a curtain of bubbles (and turbulence) blocking wavelet propagation. $\endgroup$ – kingledion Jun 23 '19 at 18:06

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.