Twin-Screw Handling: Opposed Engines, the Twist, and Close Quarters

What the rule requires

Any difference in thrust between the two engines changes the heading, because the drives sit off the centerline and each applies a turning moment to the hull USCG Boat Crew Seamanship Manual §10-D. That single fact runs the whole topic. The vessel does not need the rudder to turn, and at low speed the rudder is not the primary tool.

The differential runs across a range. At one end is a small RPM difference used to hold a course. At the other is splitting the throttles, one engine ahead and one astern, to turn the vessel through 360° in her own length. Commonly called twisting the boat, and the term is fine as shorthand, but the vessel's pivot point, her propeller side force and her turning characteristics all still apply while she is doing it.

The twist as taught is a low-speed maneuver, first practised at clutch speed in calm water: helm amidships, no way on, one engine clutched ahead and the other astern. Worked that way she turns with little or no advance and transfer. Two conditions in that description decide exam questions — the helm is amidships, not over, and the vessel has no way on before the engines are split.

Where the turning moment reaches the hull

Twin outboards on a bracket apply the moment well aft of the hull's pivot point.
Twin inboards apply most of it at the first thrust-bearing member of the drive train, usually the reduction gear or V-drive, much closer to the pivot point; side force reaches the hull through the strut and stern tube.

The practical consequence is that the same throttle split does not produce the same heading change in both installations, and the outboard boat, with its moment arm set well aft, is the livelier of the two.

The limit on differential

Up to a point, the greater the RPM difference the greater the change in heading. Past that point cavitation or aeration sets in and propulsion efficiency falls off on at least one drive. Where that point lies depends on the boat, the propulsion, the sea state and the speed, so there is no number to memorise — only the fact that adding revolutions past it buys nothing and costs thrust on one side.

Holding a course in a crosswind

With wind pushing the bow to leeward, the answer is not constant helm. Turn the leeward engine at higher RPM than the windward one until the pressure comes off the helm. The vessel then tracks with the rudder near amidships and the helmsman is not fighting her.

Side force has not gone anywhere

A propeller draws water in from every direction forward of the blades and forces it aft in a stream. The flow into the arc of rotation is the suction screw current; the flow out of it is the discharge screw current, and the discharge current is always the stronger and more concentrated of the two, turning ahead or astern USCG Boat Crew Seamanship Manual §10-A. That asymmetry is why any vessel handles differently going astern than going ahead.

Rotation also produces a side force on the stern. Frictional wake, discharge current acting on the rudder, and a blade entraining air at the top of its arc all contribute, but the largest contribution comes from the angle of the propeller shaft: ascending and descending blades work at 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. The one installation with no propeller side force at all is the waterjet, because its impeller runs fully enclosed in the pump housing.

Backing

Vessels are designed to go forward, and many will not back in a straight line USCG Boat Crew Seamanship Manual §10-C. Higher freeboard and superstructure forward give a large sail area up front, so many boats back into the wind. 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 to stop the rudder swinging hard over.

Two cautions attach to backing regardless of how many screws are turning. Do not back in a way that ships 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. And because inboards exhaust through the transom and outboards exhaust astern, backing draws exhaust fumes over the crew and into cabin spaces, so exposure is limited and spaces are ventilated afterwards.

You are twisting a twin-inboard vessel to port at clutch speed. Where is the helm, and what does the rudder contribute?

Helm amidships. The maneuver as taught is helm amidships with no way on, clutching one engine ahead and the other astern; the turn comes from the two off-centerline thrusts, not from the rudder . With no way on there is no flow across the rudder to work with anyway.

Telling it apart

The criterion is whether opposed or asymmetric propulsion is available. Everything else about the two techniques follows from that one answer.

Twin screw — available. The heading change comes from the thrust differential, up to full opposed throttles, with the helm amidships and the vessel turning about her pivot point in her own length USCG Boat Crew Seamanship Manual §10-D.
Single screw — never available. The same end is reached by combining rudder position, short bursts ahead and astern, and the propeller's own side force 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. Alternating those bursts walks the boat around with little headway — the compound maneuver that substitutes for a twist USCG Boat Crew Seamanship Manual §10-E.
The case most often misfiled — the single-engine backing recipe. Full rudder toward the side that opposes the expected swing before starting astern, a quick burst astern to get her moving, then reduced power and rudder as sternway builds, is the answer to a single-screw question USCG Boat Crew Seamanship Manual §10-C. Offered as an option on a twin-screw stem, it is a distractor.

Working a question

You are conning a 55-foot twin-inboard vessel in a fairway a little over two boat-lengths wide. Wind is fresh on the port beam. You need to reverse your heading and then back into a slip on the starboard hand.

Take the way off first. The twist is specified with no way on. Any headway remaining converts the maneuver into a turn with advance and transfer, and the fairway does not have room for it USCG Boat Crew Seamanship Manual §10-D.
Helm amidships. Nothing in the maneuver asks for rudder, and with no way on there is no useful flow across it.
Choose the direction, then clutch. To swing the bow to starboard, clutch the port engine ahead and the starboard engine astern: the port side is driven forward and the starboard side aft about the pivot point. Clutch speed only.
Do not hurry it with throttle. Widening the split past the point where cavitation or aeration begins loses propulsion efficiency on at least one drive, and the swing slows rather than quickens. The point varies with the boat, the propulsion, the sea state and the speed, so it is found by watching the vessel, not by a fixed RPM.
Expect the wind to work on the bow. Sail area forward is the larger USCG Boat Crew Seamanship Manual §10-C, so the swing runs away through one half of the turn and stalls through the other. Meet it by adjusting the differential, not by grabbing helm.
Steady on the new heading with the throttles. Rather than carrying constant helm against the crosswind, run the leeward engine at higher RPM than the windward one until the pressure comes off the helm .
Back into the slip watching the bow. As sternway develops the apparent pivot point moves aft and the bow swings through a greater arc than the stern, so the bow is the end that finds the piling on the far side of the fairway.
Check the transom and the crew. Do not back so hard that the afterdeck ships water, and limit the time the crew spends in the exhaust, ventilating spaces afterwards.

Same vessel, same slip, but she is fitted with twin outboards on a bracket. What changes?

The location of the turning moment. Outboards on a bracket apply it well aft of the hull's pivot point, where inboards apply most of it at the reduction gear or V-drive, much closer to the pivot point . The longer moment arm means a given differential produces a larger heading change, so the same throttle handling that suited the inboard boat will oversteer this one. The sequence is unchanged; the increments are smaller.

Where candidates lose the point

Answering that a twin-screw vessel has no propeller side force, or that side force can be disregarded once the throttles are split. The twist is described in the same breath as the reminder that pivot point, propeller side force and the vessel's turning characteristics all still apply. The installation that genuinely has no propeller side force is the waterjet, whose impeller runs fully enclosed in the pump housing USCG Boat Crew Seamanship Manual §10-A.

Putting the rudder over in the direction of the twist. It looks like helping the turn, and on a single screw going ahead the rudder is exactly what you would use. The twin-screw maneuver is specified with the helm amidships and no way on USCG Boat Crew Seamanship Manual §10-D.

Choosing the option that says the wider the split the faster she comes around, without qualification. True up to a point. Beyond it, cavitation or aeration sets in and efficiency falls off on at least one drive. An option stating the relationship as unlimited is wrong on the strength of that clause alone.

Holding a course in a crosswind with helm. The taught answer is the leeward engine at higher RPM than the windward one until the helm goes light.

Assuming two screws make her back in a straight line. Vessels are designed to go forward, and many will not back straight; with the greater sail area forward, many back into the wind USCG Boat Crew Seamanship Manual §10-C.

Check yourself

You are conning a twin-screw vessel going ahead with rudders amidships. The port screw stops turning. Which way does the bow go?

To port. The starboard screw continues to deliver ahead thrust off the centerline, applying a turning moment that drives the starboard side forward; the bow swings toward the side that has lost thrust USCG Boat Crew Seamanship Manual §10-D. Any difference in thrust between the two engines changes the heading, and a stopped engine is the largest difference short of an opposed split.

Making way ahead in a beam wind, you find yourself carrying several degrees of helm continuously to hold course. What is the preferred correction on a twin-screw vessel?

Increase RPM on the leeward engine relative to the windward one until the pressure comes off the helm, then steer with the rudder near amidships . Carrying constant helm is the condition to be cured, not the cure.

Which contributes most to propeller side force on the stern?

The angle of the propeller shaft. It 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 USCG Boat Crew Seamanship Manual §10-A. Frictional wake, discharge current on the rudder and aeration at the top of the arc all contribute, but none of them is the largest.

You are single screw, alongside, and need to turn short round in your own length. What is the technique, and how does it differ from a twist?

The compound maneuver: at low speed, alternate short bursts ahead with the rudder over — which throws the discharge current to one side and moves the stern strongly — against bursts astern, where the rudder is weak and side force dominates. Repeated, this walks her around with little headway USCG Boat Crew Seamanship Manual §10-E. It differs from a twist in that asymmetric and opposed propulsion are never available to a single screw, so the same forces are applied in sequence rather than simultaneously.

Backing your vessel in a short chop with the crew working aft, what two hazards attach to the maneuver itself?

Shipping water over the transom, which floods the afterdeck on a boat with low freeboard and weight aft and threatens stability if it does not drain at once; and exhaust, since inboards exhaust through the transom and outboards exhaust astern, so backing draws fumes over the crew and into cabin spaces. Limit the exposure and ventilate the spaces afterwards USCG Boat Crew Seamanship Manual §10-C.

Twisting at clutch speed, the swing is slower than you want. You advance the ahead throttle well up. What is the likely result?

Cavitation or aeration on at least one drive and a loss of propulsion efficiency, so the swing does not improve in proportion and may fall off . Excess ahead thrust also puts way on her, and the twist depends on having none — advance and transfer appear, which is what the maneuver exists to avoid.

Backing into a slip, you are watching the stern closely and the bow contacts a piling on the opposite hand. What did you fail to account for?

That the apparent pivot point moves aft as sternway develops, so the bow swings through a greater arc than the stern. The bow is watched along with the stern when backing, and the helm is held firmly so the rudder does not swing hard over .