2.7 Bank Effects, Shallow Water & Wake

Confined Water Is Where Masters Earn the Title

Near-coastal and inland Master 100 work is full of narrow channels, canal cuts, river banks, and skinny approaches to fuel docks. Q160 tests the hydrodynamics by name: bank cushion, bank suction, squat, and shallow-water steering loss. It also tests the legal-professional rule every passenger captain must live: your wake is your responsibility.

Bank Cushion (Bow)

When a vessel with headway runs close to a steep bank or canal wall, water piles between the bow and the bank. That high-pressure wedge pushes the bow away from the near bank. Mariners call this bank cushion (or bow cushion).

Cushion alone might seem helpful ("it keeps me off the mud"), but it rarely acts alone.

Bank Suction (Stern)

Along the same near bank, flow past the hull speeds up in the restricted gap, pressure drops, and the stern is drawn toward the bank. That is bank suction (stern suction).

The dangerous combination

Cushion on the bow plus suction on the stern yaws the vessel so the bow aims toward mid-channel (or the far bank) while the stern digs toward the near bank. The ship can sheer violently across the channel if the master does not reduce speed and correct early.

Correct response pattern (exam and practice):

1. Reduce speed — bank effects rise sharply with speed.
2. Increase distance from the bank when traffic allows.
3. Use opposite rudder early and calmly; avoid a panic hard-over that flings the stern harder into suction.
4. In extreme sheer, be ready to stop engines or use brief astern as trained for that hull—know your vessel.

Do not confuse bank suction with prop walk. Prop walk is propeller transverse thrust; bank effects are pressure fields from confined water even on twin-screw ships.

Shallow Water: Squat and "Smelling the Bottom"

Squat is the bodily sinkage of a vessel underway in water that is shallow relative to draft, often with a change of trim (many ships squat by the stern; some hulls trim differently). As speed increases in shallow water, the hull sits deeper than static draft marks suggest. That can mean unexpected grounding in a channel you "should" have cleared on paper.

Smelling the bottom is the traditional phrase for sensing these hydrodynamic changes—vibration, sluggish helm, or the hull's wave pattern—before you read a new depth on the sounder. Treat it as a command to slow down and verify depth, not as a curiosity.

Under-keel clearance discipline

Master 100 routes include tidal inlets, sandbars, and harbor approaches. Plan transit speeds so squat does not eat your clearance at low tide. A vessel that cleared a bar at dead slow may touch at cruise speed because squat and wave troughs stole the margin.

Reduced Rudder Effectiveness in Shallow Water

In shallow water the rudder works in a constrained flow field. Effects commonly tested and observed:

• Larger turning circles (greater advance and tactical diameter)
• Sluggish response to helm
• More power required for the same heading change
• Interaction with squat and bank effects if you are also near a wall

Rule of thumb for exams: shallow water makes the ship harder to turn and harder to stop in a short distance at a given RPM. The professional answer is almost always reduce speed early, not "add more rudder and hope."

Wake Damage and the Duty of the Passing Vessel

A vessel's wake can part mooring lines, damage small craft, flood open boats, injure people on docks, and set floating restaurants surging. For Coast Guard and civil purposes, the standing professional rule is:

The person in charge of the passing vessel is responsible for damage caused by that vessel's wake.

Operational implications for Master 100

• Slow when passing moored vessels, marinas, construction barges, dredges, and working decks with divers or line handlers.
• Slow for small open boats and human-powered craft that can be capsized or swamped.
• "No wake" and minimum-wake zones are not suggestions; treat posted limits as part of safe speed judgment under the Rules as well as local ordinance.
• Passenger vessels often throw a large wake for their length because of displacement and speed; do not assume "I'm only 65 feet" excuses a damaging stern wave.
• If you must pass close, pass at a speed that leaves the moored vessel quiet, even if that means a long, slow crawl and an adjusted schedule.

Creating a hazardous wake is also poor watchstanding under safe speed and good seamanship themes you already studied in Rules of the Road—Q160 simply asks the damage-responsibility line more bluntly.

Worked channel scenario

You are outbound in a canal cut, starboard side close to a steep wall, at too high a speed. The bow cushions off the wall while the stern sucks in; the vessel starts to sheer to port toward oncoming traffic. Immediate actions: reduce speed, ease away from the wall if room exists, apply controlled starboard rudder to stop the sheer without overcorrecting into the opposite bank, and reassess spacing. Passing a marina row moments later, drop to minimum wake so rafted boats do not surge and part lines—you own that wake as master.

Exam framing for Q160

Lock the pairs: cushion = bow pushed off bank; suction = stern drawn to bank; squat = sinkage/trim in shallow water at speed; smelling the bottom = feel of shallow effects; shallow water = poorer rudder, bigger turns; wake damage = person in charge of the passing vessel; slow for moored vessels. If a stem says the bow is forced away from the bank, the answer is cushion—not suction.