question: Why does the water behind a boat become incredibly smooth for a period after the boat passes?
comment: It displaces the water it’s moving through, as the waves move in behind it to fill back up the open space they’re equidistant and when they collide they cancel each other out until the tide naturally takes over again.
passages: Free surface effect
  
  The free surface effect is a mechanism which can cause a watercraft to become unstable and capsize.
   Section::::Usage.
  
  The water transfer printing process is extensively used to decorate items that range from entire all-terrain vehicles and car dashboards, to small items like bike helmets or other automotive trim. Films can be applied to all types of substrates including plastic, fiberglass, wood, ceramics, and metal. For the most part, if the item can be dipped in water and can be painted using traditional techniques then the hydrographic printing process can be used.
  
  Section::::Process.
   If a compartment or tank is either empty or full, there is no change in the craft's center of mass as it rolls from side to side (in strong winds, heavy seas, or on sharp motions or turns). However, if the compartment is only partially full, the liquid in the compartment will respond to the vessel's heave, pitch, roll, surge, sway or yaw. For example, as a vessel rolls to port, liquid will displace to the port side of a compartment, and this will move the vessel's center of mass to port. This has the effect of slowing the vessel's return to vertical.
   BULLET::::- Some external forces are not directly measured. In these cases, the offset force is deduced over a period of time, allowing an average value of compensating thrust to be applied. All forces not attributable to direct measurement are labeled "current", as this is what they are assumed to be, but in reality this is a combination of current, waves, swell, and any errors in the system. As is traditional in the maritime industry, DP "current" is always recorded in the direction that it is flowing towards.
  
  Section::::Control systems.
   In a normally loaded vessel any rolling from perpendicular is countered by a righting moment generated from the increased volume of water displaced by the hull on the lowered side. This assumes the center of gravity of the vessel is relatively constant. If a moving mass inside the vessel moves in the direction of the roll, this counters the righting effect by moving the center of gravity towards the lowered side. The free surface effect can become a problem in a craft with large partially full bulk cargo compartments, fuel tanks, or water tanks (especially if they span the full breadth of the ship), or from accidental flooding, such as has occurred in several accidents involving roll-on/roll-off ferries.
   Parts of the pattern may be obscured by the effects of propeller wash, and tail eddies behind the boat's stern, and by the boat being a large object and not a point source. The water need not be stationary, but may be moving as in a large river, and the important consideration then is the velocity of the water relative to a boat or other object causing a wake.
  
  This pattern follows from the dispersion relation of deep water waves, which is often written as,
  
  where
  
  "Deep" means that the depth is greater than half of the wavelength.
   It refers to the tendency of liquids — and of unbound aggregates of small solid objects, like seeds, gravel, or crushed ore, whose behavior approximates that of liquids — to move in response to changes in the attitude of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea states caused by waves and wind acting upon the craft). When referring to the free surface effect, the condition of a tank that is not full is described as a "slack tank", while a full tank is "pressed up".
  
  Section::::Stability and equilibrium.
   Around 09:30, as about 20 people were playing on the beach or bathing, the sea retreated about 100 meters within the space of about 5 minutes. A few people who were bathing, seeing the water retreating around them were left standing, puzzled. Asked about the phenomenon, Thai vendors on the beach were as puzzled as tourists, having never seen such a thing.
   BULLET::::- In naval architecture and marine safety, the "free surface effect" occurs when liquids or granular materials under a free surface in partially filled tanks or holds shift when the vessel heels.
  
  BULLET::::- In hydraulic engineering a "free-surface jet" is one where the entrainment of the fluid outside the jet is minimal, as opposed to submerged jet where the entrainment effect is significant. A liquid jet in air approximates a free surface jet.
   BULLET::::- Maciel Cicchetti, Argentina. Coxswain. 2nd Volvo Ocean Race.
  
  BULLET::::- Brian Carlin, Ireland. Reporter.
  
  Section::::2014-2015 Volvo Ocean Race.:Grounding.
   BULLET::::- Leeway (or drift on power vessels) - it is very seldom that a craft is pointing in the direction it is going, and on a sailboat the angle of leeway is the difference between the heading of the craft and its actual track through the water. This must be corrected for when converting apparent wind angle to true wind direction. The same effect is found when the craft is altering course.
   Underwater air retaining surfaces are of great interest for technical applications. If a transfer of the effect to a technical surface is successful, ship hulls could be coated with this surface to reduce friction between ship and water resulting in less fuel consumption, fuel costs and reduction of its negative environmental impact (antifouling effect by the air layer). In 2007 first test boats already achieved a ten percent friction reduction and the principle was subsequently patented. By now scientists assume a friction reduction of over 30%.
   The first 19th-century loading recommendations were introduced by Lloyd's Register of British and Foreign Shipping in 1835, following discussions among shipowners, shippers and underwriters. Lloyds recommended freeboards as a function of the depth of the hold (three inches per foot of depth). These recommendations, used extensively until 1880, became known as "Lloyd's Rule".
   A recent theory of DVM, termed the Transparency Regulator Hypothesis, argues that water transparency is the ultimate variable that determines the exogenous factor (or combination of factors) that causes DVM behavior in a given environment. In less transparent waters, where fish are present and more food is available, fish tend to be the main driver of DVM. In more transparent bodies of water, where fish are less numerous and food quality improves in deeper waters, UV light can travel farther, thus functioning as the main driver of DVM in such cases.
  
  Section::::Unusual events.
   Significantly, the resulting wavefront angle with the boat's path, the angle of the tangent from "P" to this smaller circle, obviously has a sine of "TR/PR"=1/3, for any and all , , , , etc.: Strikingly, virtually all parameters of the problem have dropped out, except for the deep-water group-to-phase-velocity relation! Note the (highly notional) effective group disturbance emitter moves slower, at 3"v"/4.
  
  Thus, summing over all relevant and s to flesh out an effective Fig.12.3 shock pattern, the universal Kelvin wake pattern arises: the full visible chevron angle is twice that, 2arcsin(1/3) ≈ 39°. 
   The shape of the ocean floor affects the break greatly. In general there are gradual sloping ocean floors which result in a slower crumpling wave (better for long boarding) and steeper sloping floors which result in a faster, hollower breaking wave (better for short boarding). There are also many different types of breaks:
   The "swivel method", or "fan method" as it is referred to by professionals, uses a series of strokes combined with turns that hold the water away from the leading edge of the squeegee; when the turn is completed in the opposing direction, there is no water and no dirt left isolated. However straight strokes, either horizontally or vertically are normally much more efficient than “fanning”. If a few spots are missed, a chamois leather cloth works better for touch up than a towel of cloth or paper. 
   Section::::Effects of hull form and underwater foils.
   For any vessel, a favourable flow will improve the vessel's speed over the bottom for a given speed in the water. Difficult channels are also more safely navigated during slack water, as any flow may set a vessel out of a channel and into danger.
  
  Section::::Combined tidal stream and current.
   The direct source of inspiration for "Rough Waves" may have been the work of the artist Sesson Shukei (), whose still-extant paintings include a number of dynamic and mysterious images of sea waves, among them "Egret, Moon, and Wave" and "The Wind and the Waves". "Rough Waves" also reflects the influence of Tawaraya Sōtatsu, who specialized in gloomy landscapes, but is nevertheless an independent interpretation by Ogata of the frightening and violent force of the sea.
  
  Section::::Influence.
   The underlying principle is schematically shown in a figure. Two flow profiles of laminar flow in water over a solid surface and water flowing over an air retaining surface are compared here.
  
  If water flows over a smooth solid surface, the velocity at the surface is zero due to the friction between water and surface molecules. If an air layer is situated between the solid surface and the water the velocity is higher than zero. The lower viscosity of air (55 times lower than the viscosity of water) reduces the transmission of friction forces by the same factor.
   Section::::Implications for seafarers.
  
  For scuba divers, the absence of a flow means that less effort is required to swim, and there is less likelihood of drifting away from a vessel or shore. Slack water following high tide can improve underwater visibility, as the previously incoming tide brings clear water with it. Following low tide, visibility can be reduced as the ebb draws silt, mud, and other particulates with it. In areas with potentially dangerous tides and currents, it is standard practice for divers to plan a dive at slack times.
   Section::::Background.
   Section::::Characters in "Sea Swept".
   BULLET::::- Sway: The linear transverse (side-to-side or port-starboard) motion. This motion is generated directly either by the water and wind currents exerting forces against the hull or by the ship's own propulsion; or indirectly by the inertia of the ship while turning. This movement can be compared to the vessel's drift from its course.
  
  BULLET::::- Surge: The linear longitudinal (front/back or bow/stern) motion imparted by maritime conditions.
  
  Section::::Stabilization.
  