Ships and Submarines: Added Resistance in Waves

Understanding Ships and Submarines: Added Resistance in Waves

Added resistance is not simply the force of water hitting the bow. A passing wave changes the water pressure around the whole hull. As the ship climbs a crest, its front rises and the wetted shape changes.

As it drops into a trough, the hull may meet water at a different angle. The ship creates extra waves while responding to the incoming ones. Some wave energy is reflected away from the bow.

Some is turned into ship motion, such as pitching, heaving, and rolling. Energy transferred into these motions is no longer available to keep the ship moving forward.

The size of the effect depends strongly on the match between the waves and the ship. A wave length close to the ship length can produce strong pitching, because the bow and stern are supported differently at different moments. Short, steep waves may strike the bow repeatedly and cause slamming.

Slamming occurs when a part of the hull leaves the water then lands hard on it. It can be uncomfortable for people, damage equipment, and place large repeated loads on the structure. Wave height matters, but wave period matters too.

The period is the time between successive wave crests. Two seas with similar heights can affect a vessel very differently if their periods differ.

The direction of travel changes both resistance and safety. Waves from ahead usually make forward progress hardest because the bow meets each crest directly. Waves from the side can create large rolling motions.

Following waves may reduce resistance in some conditions, yet they can make steering less stable. A ship can surf down a steep following wave, causing rapid changes in speed and heading. Captains therefore do not choose a route only by finding the shortest line on a map.

They consider forecast wave direction, wave period, wind, current, cargo limits, and the comfort of people on board. Slowing down can sometimes reduce severe impacts enough to protect the ship, even though the journey takes longer.

Propulsion adds another important part of the picture. A propeller works best when it receives a steady flow of water. In waves, the stern rises and falls, so the propeller may move closer to the surface.

Air can be drawn into the water around the blades. This is called ventilation. Low pressure around fast-moving blades can form vapour bubbles, a process called cavitation.

When the bubbles collapse, they create noise, vibration, and possible blade damage. Submarines avoid much of the surface disturbance when they operate deeply enough. The depth needed depends on wave length, since long ocean swells affect water farther below the surface than short choppy waves.

When studying this topic, separate the forces that slow a vessel from the motions that make operation difficult. They are linked, but they are not exactly the same thing.