Docking Under Power (ASA 118)

Prerequisites

ASA 101, or the ability to helm a keelboat under power.
Cleats, fenders and the basic dock lines: bow line, stern line and springs.

Learning Objectives

ASA 118 is an endorsement that focuses on manoeuvring and docking under power. Its textbook, Docking and Maneuvering Made Easy, covers boats with a single inboard or outboard engine. By the end of this lesson you will be able to:

Explain prop walk, the pivot point and windage, and use them.
Name the dock lines and use a spring line to come alongside and leave.
Back into a slip.
Med-moor stern-to.
Adapt these techniques to a twin-engine catamaran.

The Forces Acting on the Boat

Prop Walk

A propeller does not only push the boat forward or back. It also pushes the stern sideways, mostly in reverse. With a right-handed propeller (which turns clockwise in forward, seen from astern), reverse kicks the stern to port. A left-handed propeller kicks it to starboard.

Find out which way your boat walks before you go near a dock. In open water, put the engine in reverse from a stop with the wheel centred and watch which way the stern goes. Then use it:

Come alongside on the side the stern walks toward. A burst of reverse to stop then also tucks the stern in.
When backing, expect the stern to go the walking way until the boat has enough speed for the rudder to work.

Pivot Point

A boat does not turn around its middle. Going ahead, it pivots about a point roughly a third of the way back from the bow, so the stern swings out when you turn. Going astern, the pivot point moves aft, and the bow swings. When turning away from a dock under power, remember the stern can swing into it.

Windage and Current

Wind and current often push harder than the engine at docking speed. The bow of a sailboat usually blows off downwind first, because it is high and has little underwater area beneath it. Before every approach:

Watch flags and the water to judge wind and current.
Choose an approach that uses them: come in against the stronger of the two, which gives you control at low speed.
If the wind is blowing you onto the dock, stop parallel to it a boat's width out and let the wind drift you in.
If the wind is blowing you off, approach at a steeper angle and get a line ashore fast, usually a spring.

Dock Lines

Bow line: from the bow forward to the dock.
Stern line: from the stern aft to the dock.
Forward (bow) spring: from the bow running aft to the dock. It stops the boat moving forward.
After (stern) spring: from the stern running forward to the dock. It stops the boat moving backward.

Springs are the docking tool. Because they run at an angle, the engine can work against them to hold the boat on the dock while the crew rigs the other lines.

Coming Alongside

Prepare fenders and lines before you start, with the crew briefed on who handles which line.
Approach at a shallow angle (about 20–30 degrees) and at the slowest speed that keeps steerage.
Shift to neutral early and let the boat carry its way.
Stop with reverse, using prop walk to bring the stern in if it walks toward the dock.
Step ashore, never jump. Get the spring on first, then the bow and stern lines.

Powering Against a Spring

With the after spring made fast from a point near the middle or bow of the boat to the dock behind, put the engine slowly ahead with the wheel turned away from the dock. The boat is pushed forward against the spring, which swings it alongside and holds it there, even with the wind blowing it off. The crew can then tie the other lines at leisure.

Leaving the Dock

When the wind is pushing you onto the dock, motoring straight off rarely works. Use a spring:

Springing the stern out: rig a forward spring from the bow, put a fender at the bow, and motor slowly ahead with the wheel turned toward the dock. The bow presses against the fender, and the stern swings out into clear water. Then slip the line and back away.
Springing the bow out: rig an after spring from the stern with a fender aft, and motor slowly astern. The stern presses on the dock, and the bow swings out. Slip the line and motor ahead.

Do not turn sharply until the stern is clear of the dock, because the stern swings out when you turn.

Backing into a Slip

Many US marinas use slips between finger piers, and backing in gives easier boarding from the stern.

Pass the slip going forward, so you can check its size and any lines in the water.
Turn so the stern points into the slip, allowing for the direction of prop walk. Often it is easier to approach so that prop walk helps the stern turn into the slip.
Back in slowly, with steady speed so the rudder works. In reverse the rudder steers the stern.
Use short bursts of forward to stop or straighten.
Get a stern line or spring on early, then the bow lines.

If it goes wrong, go forward, clear the slip and try again. There is no penalty for a second attempt.

Med-Mooring

In the Mediterranean and many charter destinations, boats lie stern-to a quay with an anchor or a laid mooring line holding the bow.

Rig fenders on both sides and stern lines ready.
Line up several boat lengths out from the quay, stern toward it.
Drop the anchor and back slowly toward the quay, paying out chain.
About two boat lengths from the quay, snub the chain briefly to set the anchor.
Back the last distance and pass the stern lines ashore. Then tighten the chain to hold the stern clear of the quay.

Where the marina uses laid moorings (lazy lines), pick up the line from the quay and walk it forward to a bow cleat instead of using your anchor.

Docking a Twin-Engine Catamaran

A catamaran has an engine in each hull and turns with differential thrust: one engine ahead, the other astern. Its counter-rotating propellers largely cancel prop walk, so it backs straight.

Alongside: make a spring fast from the dockside midships cleat, led aft to the dock. Put the outside engine slow ahead and leave the dockside engine in neutral. The boat drives forward against the spring, and because the thrust is offset from the centreline it pulls the stern in too, so the whole hull lies flat against the dock while you add the other lines.
Backing in: the helm station on most cats is centred or aft, which gives a good view when going astern. Approach forward, turn the sterns toward the slip with differential thrust, then reverse both engines straight in.
Coasting: a cat has little underwater area, so it stops quickly and blows sideways easily. Keep a little power on until the lines are made.

Worked Example: Wind Blowing Off the Dock

You are docking a 35-foot monohull with a right-handed propeller, port side to, in 12 knots blowing off the dock.

Rig fenders and lines on the port side. Prepare a midships spring.
Approach slowly at about 30 degrees, a little steeper than normal, because the wind will push the bow off.
As you near the dock, shift to neutral, then a short burst of reverse. Prop walk pulls the stern to port, toward the dock.
The crew steps off with the spring and makes it fast at a cleat aft of the boat.
Engine slowly ahead with the wheel turned to starboard (away from the dock). The boat lies against the spring and fenders while the crew rigs the bow and stern lines.

Common Mistakes

Approaching too fast. Come in at the speed you would be happy to hit the dock at.
Not knowing which way the boat walks in reverse.
Turning away from the dock too early and swinging the stern into it.
Crew jumping to the dock, or trying to stop the boat with their hands or feet.
Giving up control: going into neutral too early in a strong wind and drifting off.

Summary

Know your prop walk and use it. A right-handed propeller walks the stern to port in reverse.
The boat pivots about a third of the way back going ahead, so the stern swings out.
Approach against the stronger of wind and current.
Spring lines let the engine hold you on the dock and get you off it.
Back into slips slowly. Med-moor by dropping the anchor and backing to the quay.
Cats turn on differential thrust and back straight, but drift quickly.

Check Your Understanding

In reverse, a boat with a right-handed propeller walks its stern which way? To port.
Which line stops the boat moving forward along the dock? The forward (bow) spring, running aft from the bow.
Why does the stern swing toward the dock when you turn away under power? The boat pivots around a point near the bow, so the stern moves the opposite way to the bow.
The wind is pushing you onto the dock. How do you leave? Spring off: use a spring and the engine to swing one end out, then motor away.
In Med-mooring, when do you drop the anchor? Several boat lengths out from the quay, before backing in.

Sources

https://store.americansailing.com/docking-and-maneuvering-made-easy-asa-textbook-118/
https://www.cruisingworld.com/how/how-dock-and-swing-catamaran/
https://americansailing.com/certifications/asa-114-cruising-catamaran/
https://blog.marineverse.com/nauticed/marineverse/2025/12/19/catamaran-docking-marineverse.html
https://powerboating-blog.nauticed.org/mediterranean-mooring/