Going astern the rudder sits in the weak suction screw current and can direct none of the propeller's thrust, so it does almost nothing until sternway exists; what moves the stern in the first seconds of a back is propeller side force, and it is controlled by rudder set before the engine goes astern and then by less power, not more.
What the rule requires
A propeller driving the boat ahead draws water in from every direction forward of the blades and forces it aft in a stream. The flow into the propeller's arc of rotation is the suction screw current; the flow out of it is the discharge screw current. Both exist whether the propeller is turning ahead or astern, and the discharge current is always the stronger and more concentrated of the two USCG Boat Crew Seamanship Manual §10-A. Nearly everything that makes a vessel behave differently astern than ahead traces back to that single asymmetry.
Held amidships, a rudder sees roughly equal pressure on both faces and the boat holds a straight track. Put it over and pressure falls on one face and rises on the other, and the resulting force pushes the stern to one side; because the vessel turns about her pivot point, the bow swings the opposite way USCG Boat Crew Seamanship Manual §10-B. Rudder force depends on the speed of water flowing past the rudder, which is why it is far greater ahead than astern.
That difference is the most heavily tested fact in this topic, and it is worth stating in both directions:
- Going ahead — the discharge current increases the flow past the rudder, and putting the rudder over directs about half the propeller's thrust to that side. That is a major additional force on the stern, over and above the flow due to headway.
- Going astern — the rudder lies in the suction current, which is neither as strong nor as concentrated, and it can direct no propeller thrust at all. Steering astern comes almost entirely from water flowing past the rudder because of sternway.
A rudder also loses effectiveness if the propeller cavitates and aerated water flows along it.
Ahead, the rudder can direct about half the propeller's thrust to one side. How much can it direct going astern?
None. Astern the rudder lies in the suction screw current and cannot direct any propeller thrust; whatever steering effect it has comes from sternway alone . This is why an operator who backs at idle with no way on gets no response from the helm.
Where side force comes from
Besides thrust along the shaft axis, propeller rotation produces a side force on the stern. Several effects contribute: the propeller works in the layer of water the hull drags with it (frictional wake), the discharge current acts on the rudder, and the blade at the top of its arc may entrain air or work in aerated water. The largest contribution comes from the angle of the propeller shaft, which gives ascending and descending blades different effective pitch angles — the descending blade has the higher effective pitch and produces more thrust, so blade thrust around the arc is unequal and the stern is pushed to one side .
A waterjet has no propeller side force at all, because its impeller runs fully enclosed in the pump housing. Any answer option that has a waterjet walking her stern is wrong on that ground alone.
Backing
Vessels are designed to go forward and many will not back in a straight line. Higher freeboard and superstructure forward give a large sail area up front, so many boats back into the wind USCG Boat Crew Seamanship Manual §10-C. As sternway develops the apparent pivot point moves aft and the bow may swing through a greater arc, so the coxswain watches the bow as well as the stern and keeps firm control of the helm so the rudder is not swung hard over by the water.
On a single-engine vessel propeller side force is the major obstacle to backing where intended, and the rudder has little effect until sternway exists. Those two facts set the whole method: the rudder is positioned before the engine goes astern, because it will not earn its keep for the first few seconds, and power astern is reduced once the boat is moving, because power astern is what is generating the swing.
Two cautions carry real consequence. Do not back in a way that ships water over the transom; boats with low freeboard aft and weight aft can flood the afterdeck in a chop, and water that does not drain at once threatens stability. And because inboards exhaust through the transom and outboards exhaust astern, backing can draw exhaust fumes over the crew and into cabin spaces, so exposure is limited and the spaces are ventilated afterwards.
Backing out of a berth, the stern begins to swing off the line you want. Do you add power astern to get the rudder biting sooner?
No. More power astern means more side force, which is the thing swinging the stern. Reduce power to lessen side force and steer with the rudder as sternway builds .
The compound maneuver
A single-screw vessel never has asymmetric or opposed propulsion available, so the same ends are reached by combining the basic forces in sequence at low speed: rudder position, short bursts of thrust ahead and astern, and the propeller's own side force USCG Boat Crew Seamanship Manual §10-E. Ahead with the rudder over, the discharge current is directed to one side and the stern moves strongly; astern, the rudder is weak and side force dominates. Alternating short bursts ahead against the rudder with bursts astern walks the boat around with little headway — what most instructors on the waterfront call backing and filling, and what the manual treats as the single-screw substitute for a twin-screw twist. It is learned in calm water with no current before it is attempted against wind and current.
Telling it apart
Candidates lose points confusing the two screw currents, and the criterion that separates them is simply which side of the propeller arc the flow is on — and therefore whether the rudder is standing in it.
- Discharge screw current — the flow out of the propeller's arc of rotation. Always the stronger and more concentrated of the two USCG Boat Crew Seamanship Manual §10-A. Running ahead it flows over the rudder, which is why the rudder can throw about half the propeller's thrust to one side. Most often misfiled by candidates who assume that same current is working on the rudder when the engine is backed.
- Suction screw current — the flow into the arc, drawn from every direction forward of the blades. Neither as strong nor as concentrated. Running astern the rudder lies in this current, which is why it can direct no thrust USCG Boat Crew Seamanship Manual §10-B.
One more distinction decides questions about the first moments of a maneuver: rudder force depends on the speed of water flowing past the rudder, while side force is produced by the rotation of the propeller. With the boat dead in the water and the engine clutched astern, there is rotation but no flow past the rudder — so the stern goes where side force sends it.
Working a question
You are single-screw, lying in a berth, and must back clear into a fairway. There is a fresh breeze, the vessel has a high bow and deckhouse forward, and you know from handling her that her stern walks to one side when you back. The chop in the fairway is short and steep, and you have a load of gear stowed aft.
- Establish which way the stern will swing. Side force on this vessel is set by her propeller and shaft, not by the situation, so the swing direction is known before you touch anything. Wind is the second input: with the sail area forward, expect her to back into the wind USCG Boat Crew Seamanship Manual §10-C.
- Put the rudder over before the engine goes astern. Full rudder toward the side that opposes the expected swing. It will do nothing yet, and that is the point of doing it now rather than later.
- Take a quick burst of power astern to get her moving. Accept the swing the side force produces. You are buying sternway, and sternway is the only thing that will make the rudder work.
- Reduce power as soon as she is moving. Less power astern means less side force, and now there is flow past the rudder to steer with. As sternway builds you need progressively less rudder to hold the line.
- If the rudder alone will not hold the stern, take a short burst ahead. With the rudder shifted, that burst uses side force and the discharge current across the rudder to walk the stern back the other way. Use no more power than is needed: too much kills the sternway you worked for, and sets up a discharge current that makes the swing worse.
- Watch the bow throughout. With sternway on, the apparent pivot point has moved aft and the bow may swing through a greater arc than the stern — that is what puts a bow rail into a piling on the far side of the slip.
- Check the transom and the crew. With weight aft and a chop in the fairway, do not back so hard that you ship water over the transom; water on the afterdeck that does not drain at once is a stability problem . Keep the crew clear of the exhaust and ventilate the cabin afterwards.
Once clear and needing to turn short in the fairway with no room for a normal turning circle, the same forces are used in sequence: rudder over, short burst ahead to throw the stern with the discharge current, then a burst astern before headway builds, repeated until the head is round USCG Boat Crew Seamanship Manual §10-E.
Where candidates lose the point
Answering that the rudder steers a single-screw vessel astern the same way it does ahead. The distractor usually reads "put the rudder over and the stern will follow" with no mention of sternway. Astern the rudder is in the suction current and can direct no propeller thrust; it has little effect until sternway exists USCG Boat Crew Seamanship Manual §10-B.
Attributing side force to the discharge current striking the hull or rudder. That effect is real but minor. The largest contribution is the angle of the propeller shaft, which gives the descending blade a higher effective pitch and more thrust, so blade thrust around the arc is unequal USCG Boat Crew Seamanship Manual §10-A. Options naming frictional wake, the discharge current on the rudder, or aeration at the top of the arc are all contributors — the question is which one is largest.
Choosing "increase power astern" to correct a swing. It looks like more authority. It is more side force, and the swing worsens. The correction is less power astern, and if that will not hold her, a short burst ahead against the shifted rudder USCG Boat Crew Seamanship Manual §10-C.
Answering that a boat backs downwind. The sail area is forward — high freeboard and superstructure — so many boats back into the wind.
Expecting prop walk from a waterjet. A waterjet has no propeller side force, because the impeller is fully enclosed in the pump housing.
Watching only the stern while backing. The apparent pivot point moves aft as sternway develops and the bow may swing through the greater arc. A related trap: letting go of the helm astern, when the rudder must be held firmly so it is not swung hard over.
Check yourself
Your single-screw vessel is dead in the water, rudder hard over, and you clutch astern. What moves the stern first?
Propeller side force. Rudder force depends on the speed of water flowing past the rudder, and with no sternway there is none; astern the rudder is also in the suction current and can direct no propeller thrust USCG Boat Crew Seamanship Manual §10-B. The rudder begins to work only as sternway builds.
Backing down a fairway, the stern is walking off your intended track and full rudder will not hold it. What is the correct correction?
A short burst ahead with the rudder shifted, using side force and the discharge current across the rudder to walk the stern back the other way — with no more power than needed, so sternway is not killed and no discharge current is set up that worsens the swing USCG Boat Crew Seamanship Manual §10-C.
A question asks which effect contributes most to propeller side force. Frictional wake, discharge current on the rudder, aerated water at the top of the blade arc, or shaft angle?
Shaft angle. It gives ascending and descending blades different effective pitch angles, the descending blade producing more thrust, so blade thrust around the arc is unequal and the stern is pushed to one side USCG Boat Crew Seamanship Manual §10-A. The other three are contributors, not the largest.
You are backing a boat with low freeboard aft, gear stowed on the afterdeck, into a short chop. What limits how hard you may back?
Shipping water over the transom. A boat with low freeboard aft and weight aft can flood the afterdeck in a chop, and water that does not drain at once threatens stability .
You must hold station with sternway on in a fresh breeze, and the bow keeps falling off. Why is watching the stern alone insufficient?
As sternway develops the apparent pivot point moves aft, and the bow may swing through a greater arc than the stern. The bow is what strikes the pilings while your eye is on the transom .
You must turn your single-screw vessel short in a narrow basin. What are you using, and in what order?
The compound maneuver: rudder position, short bursts of thrust ahead and astern, and the propeller's side force, combined in sequence at low speed. Ahead with the rudder over the discharge current is directed to one side and the stern moves strongly; astern the rudder is weak and side force dominates, so alternating short bursts walks her around with little headway USCG Boat Crew Seamanship Manual §10-E.
After a long period of backing with the crew on the afterdeck, what is the required follow-up?
Limit the exposure and ventilate the spaces. Inboards exhaust through the transom and outboards exhaust astern, so backing draws exhaust fumes over the crew and into cabin spaces .
Check your understanding
One real exam question on Single-screw maneuvering and prop walk, cited to source. No account.
Single-screw maneuvering and prop walk
When backing a single-screw vessel, which force is the MAJOR obstacle to controlling the direction of movement?
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