Get Smart, Go Astern

Get Smart, Go Astern

In a study published in Psychological Science (May 2009), Dutch researchers determined that “Backward locomotion appears to be a very powerful trigger to mobilize cognitive resources.”

The Rules of the Road anticipated this conclusion in Rule 8 by advising that “if a vessel needs more time to avoid collision or assess the situation, she shall slacken her speed or take all way off by stopping or reversing her means of propulsion.”  So in the spirit of COLREGS, Plennie L. Wingo, Raju Muniyappan, and enhanced cognition, this week we’ll look at questions having to do with operating astern propulsion.

We’ll start with definitions and theoretical questions and then move to practical applications.


The distance that a vessel travels from the time that the order is given to put engines full astern until the vessel is dead in the water is known as ___________.
A. advance
B. head reach
C surge
D. transfer

Answer: B

Advance is the distance traveled in a forward direction from the moment the rudder is put over to the point at which she is on her new course.  It is measured along a line parallel to the original direction of motion.  Surge is one of the many components describing a vessel’s motion in seas:  she heaves, surges, pitches, sways, rolls and yaws.  Surging describes the longitudinal motion of a ship as she slides forward and backward. Finally, transfer is the distance traveled from the moment the rudder is put over to the point at which she is on her course.  It is measured along a line perpendicular to the original direction of motion.

You have arrived at your anchorage location.  You have put the engines astern prior to letting go the anchor.  How will you know when the vessel has stopped making way?
A. The ship’s Doppler log reads zero.
B. The backwash of the propeller reaches amidships.
C An azimuth bearing on the beam remains steady.
D. All of the above

Answer: B

When the backwash from the ship’s propeller(s) is roughly at midships, the vessel has caught up with herself and is no longer making way through the water.

You have arrived at your anchorage location. You have put the engines astern prior to letting go the anchor.  How will you know when the vessel has stopped over the ground?
A. The ship’s log reads zero.
B. The backwash of the propeller reaches amidships.
C An azimuth bearing on the beam remains steady.
D. All of the above

Answer: C

To determine that the vessel is not making any progress over the ground, the method of choice here is to take the bearing of a fixed object on the vessel’s beam.  If that bearing remains constant, the vessel’s speed over the ground is zero.  Choice A does not specify what type of log — some logs may detect this condition while others do not.

Titanic's_propellers

Titanic’s Propellers

Your twin-screw vessel is moving ASTERN with rudders amidships.  The starboard screw suddenly stops turning.  Your vessel’s head will _________.
A. go to port
B. go to starboard
C remain stationary
D. suddenly drop down

Answer: A

The twin-screw vessel’s propellers generally turn outboard when going forward.  When going astern, they will both turn inward.  If the starboard screw stops turning, the direction of the vessel’s bow will be dictated by the port screw.  When going astern the port screw spins clockwise, which will claw the vessel’s stern to starboard and thereby cause the vessel’s bow to fall off to port.

With rudders amidships and negligible wind, a twin-screw vessel moving astern with both engines backing will back ________________.
A. to port
B. to starboard
C in a fairly straight line
D. in a circular motion

Answer: C

The inboard turning twin-screws offset each other’s motion and the vessel will back nearly straight.

You are aboard a right-handed single-screw vessel with headway on.  The engine is put full astern and the rudder hard left.  What will the bow do?
A. It will swing to the left, and will swing left faster as the vessel loses way.
B. It will swing to the left, straighten out and then swing to the right as the vessel loses way.
C It will swing to the left without increasing or decreasing its swing.
D. The bow will swing to the right.

Answer: B

In this situation the rudder will initially have a greater effect than the engine because the vessel has headway.  The application of left rudder will cause the vessel’s head to swing to the left because she still has way on, water is still flowing past the rudder.  But as she loses headway, the power of the prop asserts itself and clawing the stern to port will cause the vessel first to straighten up and then the bow to swing to the right.

You are aboard a single-screw vessel (right-hand propeller) going full ahead with good headway.  The engine is put astern and the rudder is placed hard left.  The stern of the vessel will swing to ____________.
A. starboard until headway is lost and then to port
B. port
C port until headway is lost and then may possibly swing to starboard
D. port slowly at first and then quickly to port

Answer: A

This question describes practically the same situation as the previous question, but this one wants to know what happens to the stern. Applying hard left rudder while going full ahead and putting the engines astern will initially cause the bow to fall off to port and the stern to move to the starboard.  When the vessel begins to lose way, the directional force of the propeller will take over and the stern will crawl to port.

You are on a single-screw vessel with a right-handed propeller.  The vessel is going full speed astern with full right rudder.  The bow will swing __________.
A. quickly to port, then more slowly to port
B. probably to port
C slowly to port, then quickly to starboard
D. probably to starboard

Answer: B

There is something about a question that uses the word “probably” in two of its choices that recommends additional cogitation.  Walking backwards from the question and towards the nearest simulator — shopping cart, cardboard box, saluki/pekingese mix — what follows is our best analysis.  When a single-screw right-hand prop goes astern, the stern tends to walk to port with the bow, consequently, falling off to starboard.  But leverage induced by full right rudder when going astern would also cause the stern to want to move to starboard.  There would be no quick action in any case, which makes choices (A) and (C) unacceptable.  And the cumulative forces of rudder (as speed increases) and prop would “probably” cause the vessel’s bow to move from starboard, to less starboard, to straight and then to port.  So (B) is the best answer, particularly if it is understood to mean the bow would move less to starboard than it would if under the influence of the prop alone — so probably more to port, if not port.  Which leads to the follow-up question, “What is ‘probably'”?

You are going astern (single-screw, right-handed propeller) with the anchor down at a scope of twice the depth of the water.  As the anchor dredges, you should expect the _________________________
A. stern to walk to the same side as the anchor being used
B. vessel to back in a straight line
C stern to walk to port but at a reduced rate
D. stern to walk to port at a faster rate than normal

Answer: C

The tug of the anchor will reduce the effect of the propeller rotation.