Twin-Screw Vessel Handling: Bridge Equipment, Communications, and Operational Requirements

TL;DR — A twin-screw vessel with pilothouse control for both engines is exempt from the fixed two-way communications requirement between the operating station and the propulsion machinery control location; the wheelhouse must still be constantly manned by competent persons who direct movement, fix position, and monitor all closing contacts. 46 CFR §184.602 33 CFR §164.11

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What the Rule Says

Internal Communications — The Twin-Screw Exemption

Under the general rule, any vessel equipped with pilothouse control must have a fixed means of two-way communications from the operating station to the location where propulsion machinery is controlled. 46 CFR §184.602 The rationale is straightforward: if the operator cannot directly command the engines from the bridge, there must be a reliable, fixed communications link to whoever can.

However, twin-screw vessels with pilothouse control for both engines are not required to have a fixed communications system for that purpose. 46 CFR §184.602 Because the operator at the pilothouse can independently command both shafts directly, the redundancy of a separate fixed communications link to the machinery space is not required for that function.

Three additional communications requirements apply regardless of propulsion configuration:

1. A vessel equipped with an auxiliary means of steering must have a fixed means of two-way communications from the operating station to the location where that auxiliary steering is controlled. 46 CFR §184.602 2. When propulsion machinery cannot be controlled from the operating station, an efficient communications system must be provided between the operating station and the propulsion machinery space. 46 CFR §184.602 3. When the relevant locations are sufficiently close together, direct voice communications satisfactory to the cognizant Officer in Charge, Marine Inspection (OCMI) may substitute for a fixed system. The OCMI may also accept hand-held portable radios as satisfying the communications requirement. 46 CFR §184.602

Bridge Equipment Required on All Vessels Underway

Regardless of the number of screws, every vessel underway must carry and maintain specific bridge equipment. 33 CFR §164.35 Items directly relevant to twin-screw operations include:

A marine radar system for surface navigation. 33 CFR §164.35
An illuminated magnetic steering compass mounted in a binnacle, readable at the main steering stand, plus a current deviation table or compass comparison record in the wheelhouse. 33 CFR §164.35
A gyrocompass and an illuminated repeater at the main steering stand. 33 CFR §164.35
An illuminated rudder angle indicator in the wheelhouse. 33 CFR §164.35
An echo depth sounding device and, except on the Great Lakes and connecting/tributary waters, a device that continuously records depth readings. 33 CFR §164.35
Equipment on the bridge for plotting relative motion. 33 CFR §164.35
An indicator readable from the centerline conning position showing the rate of revolution of each propeller, except when operating on the Great Lakes and connecting/tributary waters. 33 CFR §164.35 On a twin-screw vessel this means a separate RPM indicator for each shaft.
If fitted with controllable pitch propellers, an indicator readable from the centerline conning position showing the pitch and operational mode of each propeller, except on the Great Lakes and connecting/tributary waters. 33 CFR §164.35
If fitted with lateral thrust propellers (bow or stern thrusters), an indicator readable from the centerline conning position showing the direction and amount of thrust, except on the Great Lakes and connecting/tributary waters. 33 CFR §164.35
A maneuvering information fact sheet prominently displayed in the wheelhouse. 33 CFR §164.35 This sheet must include:

- Turning circle diagrams to port and starboard showing time, distance, advance, and transfer for a 90-degree course alteration at maximum rudder angle. 33 CFR §164.35 - Time and distance to stop from full and half speeds (or full and slow speeds). 33 CFR §164.35 - For fixed-propeller vessels, a table of shaft RPM for a representative range of speeds; for controllable pitch propellers, a table of control settings. 33 CFR §164.35 - For vessels fitted with auxiliary maneuvering devices such as bow thrusters, a table of vessel speeds at which the device is effective. 33 CFR §164.35 - All maneuvering data based on calm weather (wind 10 knots or less), no current, deep water (depth at least twice the vessel's draft), and clean hull. 33 CFR §164.35 - A warning statement at the bottom of the fact sheet that vessel response may differ if any of those baseline conditions are varied, including intermediate drafts or unusual trim. 33 CFR §164.35

Simple operating instructions with a block diagram for change-over procedures for remote steering gear control systems and steering gear power units, permanently displayed on the navigating bridge and in the steering gear compartment. 33 CFR §164.35
A telephone or other means of communication for relaying headings to the emergency steering station; vessels of 500 GT and over constructed on or after June 9, 1995 must also have arrangements for supplying visual compass readings to the emergency steering station. 33 CFR §164.35

The owner, master, or person in charge must ensure the wheelhouse is constantly manned by persons who both direct and control vessel movement and fix the vessel's position, and that each such person is competent to perform those duties. 33 CFR §164.11

Additional mandatory practices include:

Every position fix must be plotted on a chart and the person directing movement must be informed of the vessel's position. 33 CFR §164.11
Electronic and other navigational equipment, fixed aids to navigation, geographic reference points, and hydrographic contours must be used when fixing position. Buoys alone may not be used to fix position. 33 CFR §164.11
The danger of every closing visual or closing radar contact must be evaluated, and the person directing movement must know that evaluation. 33 CFR §164.11
Rudder orders and engine speed and direction orders must be executed as given. 33 CFR §164.11
Magnetic variation, deviation, and gyrocompass errors must be known and correctly applied. 33 CFR §164.11
Current velocity and direction, predicted set and drift, and tidal state for the area to be transited must be known by the person directing movement. 33 CFR §164.11
The vessel's anchors must be ready for letting go while underway. 33 CFR §164.11
Speed must be set with consideration for visibility, proximity to structures, squat and underkeel clearance, channel proportions, traffic density, wake damage, current, and any local speed limit. 33 CFR §164.11
Upon entering U.S. waters, the steering wheel or lever must be operated to verify manual steering is functioning properly, unless the vessel has been steered manually from the bridge within the preceding 2 hours. This requirement does not apply on the Great Lakes and connecting/tributary waters. 33 CFR §164.11
At least two steering-gear power units must be in simultaneous operation when capable of it, except on the Great Lakes and connecting/tributary waters. 33 CFR §164.11
If a pilot not a member of the crew is employed, that pilot must be informed of the vessel's draft, maneuvering characteristics, peculiarities, and any abnormal circumstances affecting safe navigation. 33 CFR §164.11

Electrical Power for Essential Loads

Any vessel that relies on electricity to power essential loads — including the propulsion system and its auxiliaries and controls, steering systems, navigation equipment and navigation lights, bilge pumps, fire protection or detection equipment, interior lighting, communication systems, and the general alarm system — must have at least two electrical generators, each attached to an independent prime mover. 46 CFR §28.355 46 CFR §28.850 On a twin-screw vessel, both propulsion systems and their controls fall within this requirement.

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Why It Matters on the Exam

Exam questions on twin-screw vessel handling frequently test the communications exemption under 46 CFR §184.602. Candidates who memorize only the general rule — that pilothouse-controlled vessels need a fixed two-way communications system — will miss the specific carve-out for twin-screw vessels with pilothouse control of both engines. 46 CFR §184.602

Questions also test the propeller RPM indicator requirement under 33 CFR §164.35(l). On a twin-screw vessel, there must be an indicator for each propeller readable from the centerline conning position. 33 CFR §164.35 The Great Lakes exception is a common distractor.

The maneuvering fact sheet is another high-frequency topic. Candidates must know what conditions the data is based on (calm weather, no current, deep water, clean hull) and that a warning about deviations from those conditions must appear at the bottom of the sheet. 33 CFR §164.35

The prohibition on using buoys alone to fix position appears regularly. Buoys may corroborate a fix but cannot establish one. 33 CFR §164.11

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Common Pitfalls

Pitfall 1 — Misapplying the communications rule. Candidates often select "fixed two-way communications required" for a twin-screw vessel with pilothouse control of both engines. The correct answer is that no fixed communications system is required for that function on such a vessel. 46 CFR §184.602

Pitfall 2 — Forgetting the auxiliary steering communications requirement. Even if the twin-screw exemption eliminates the propulsion communications requirement, a separate fixed two-way communications link is still required between the operating station and wherever auxiliary steering is controlled. 46 CFR §184.602

Pitfall 3 — Omitting the Great Lakes exception for propeller indicators. The RPM indicator requirement for each propeller does not apply when operating on the Great Lakes and their connecting and tributary waters. Applying the requirement universally is incorrect. 33 CFR §164.35

Pitfall 4 — Treating buoys as position-fixing aids. Buoys are placed in approximate positions and can be moved by currents, ice, or vessel collisions. They may corroborate a fix established by other means but cannot be used alone to fix position. 33 CFR §164.11

Pitfall 5 — Confusing the maneuvering data baseline conditions. The fact sheet data applies to calm weather (wind 10 knots or less), no current, deep water (depth at least twice the draft), and clean hull. Intermediate drafts or unusual trim are also listed as conditions that may alter vessel response. 33 CFR §164.35

Pitfall 6 — Missing the manual steering check on entering U.S. waters. The requirement to operate the steering wheel or lever to verify manual operation applies upon entering U.S. waters unless manual steering has been used from the bridge within the preceding 2 hours. The Great Lakes exception applies here as well. 33 CFR §164.11

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Quick Check

Q1 — A twin-screw vessel has pilothouse control for both engines. Is a fixed two-way communications system required between the operating station and the propulsion machinery control location?

No. Twin-screw vessels with pilothouse control for both engines are not required to have a fixed communications system for that purpose. 46 CFR §184.602

Q2 — What indicators must be readable from the centerline conning position on a twin-screw vessel operating in coastal waters (not the Great Lakes)?

An indicator showing the rate of revolution of each propeller must be readable from the centerline conning position. If the vessel is fitted with controllable pitch propellers, an indicator showing the pitch and operational mode of each propeller is also required. If fitted with lateral thrust propellers, an indicator showing direction and amount of thrust is required. 33 CFR §164.35

Q3 — Under what baseline conditions is the maneuvering information on the wheelhouse fact sheet calculated?

Calm weather (wind 10 knots or less, calm sea), no current, deep water (water depth at least twice the vessel's draft), and clean hull, for both normal load and normal ballast conditions. 33 CFR §164.35

Q4 — May a mariner use buoys alone to fix the vessel's position?

No. Buoys alone may not be used to fix position. They may corroborate a position fixed by other means, but because buoys are placed in approximate positions and can be moved or sunk, they cannot establish a fix. 33 CFR §164.11

Q5 — A vessel is entering U.S. waters. The officer of the watch has not used manual steering from the bridge in the past three hours. What action is required?

The steering wheel or lever on the navigating bridge must be operated to determine whether the steering equipment is functioning properly under manual control. This requirement does not apply on the Great Lakes and their connecting and tributary waters. 33 CFR §164.11

Q6 — A twin-screw vessel is fitted with an auxiliary means of steering. Does the twin-screw communications exemption eliminate the need for a fixed communications link to the auxiliary steering location?

No. The twin-screw exemption applies only to the propulsion machinery communications requirement. A fixed means of two-way communications from the operating station to the location where auxiliary steering is controlled is still required. 46 CFR §184.602

Q7 — What is the minimum number of electrical generators required on a vessel that relies on electricity to power its steering system and navigation lights?

At least two electrical generators, each attached to an independent prime mover. 46 CFR §28.355 [46 CFR §28.850](cite://46-cfr