Collision Risk in a Narrow Tidal Channel
A 22 m workboat is proceeding up a dredged channel 90 m wide at 8 knots with a 2-knot foul tide, giving 6 knots over the ground. Visibility is 1.5 nm in drizzle. Radar is working but the operator is a deckhand with limited training. A 45 m tug with a tow is coming down the channel at 10 knots over the ground, showing the appropriate shapes. A small yacht is crossing from the port side of the workboat at about 5 knots, aiming for a marina entrance 0.6 nm ahead. The workboat's skipper is also monitoring a VHF call from a third vessel asking for the channel.
1. Identify the vessels and the applicable rules, and state who must give way in each interaction.
Read the worked answer
Workboat versus tug with tow. Both are power-driven and meeting nearly head-on in a narrow channel. The collision regulations require each to alter to starboard and pass port-to-port. A tug with a tow is severely restricted in ability to deviate; the workboat should treat it as the stand-on vessel in practice and make an early, obvious alteration to starboard, ideally confirmed on VHF with the tug. The workboat should keep to the starboard edge of the channel as far as is safe.
Workboat versus crossing yacht. The yacht is crossing from the workboat's port side and is the give-way vessel. The workboat is stand-on and should hold course and speed, unless the yacht fails to act, in which case the workboat must take action to avoid collision. In a narrow channel the yacht should not impede the workboat's safe passage, and the workboat should not be forced to leave the channel.
Priority of concerns. The head-on tug is the immediate hazard because closing speed is about 16 knots over the ground and the channel is only 90 m wide. The crossing yacht is secondary but could foul the workboat's starboard alteration. The VHF request is a distraction and should be answered briefly or deferred.
Key point. The rules give a hierarchy, but the skipper's job is to sequence actions: deal with the highest closing-speed threat first, then the crossing traffic, then the radio.
2. Assess the risks created by limited visibility, an untrained radar operator and the narrow channel, and recommend a course of action.
Read the worked answer
Visibility and detection. At 1.5 nm in drizzle, visual detection of a small yacht is marginal. Radar should extend detection range, but an untrained operator may mis-tune gain, misread a small echo, or fail to plot relative motion. The radar is only as good as the person interpreting it.
Narrow channel constraints. With 90 m of width and a tug-and-tow occupying much of it, the workboat's ability to alter to starboard is limited. Grounding or bank effect is a real hazard, especially at 8 knots through the water.
Speed and stopping distance. At 6 knots over the ground, the workboat covers 0.1 nm per minute. A 0.6 nm closing distance to the yacht gives about 6 minutes - enough time if action is early, not enough if the crew waits for certainty.
Recommendations.
- Reduce speed to the minimum consistent with steering, perhaps 5–6 knots through the water, to buy time.
- Sound the appropriate fog signals and switch on navigation lights; make the vessel conspicuous.
- Put a second person on the radar and have the deckhand report contacts aloud; the skipper should verify.
- Call the tug early on VHF, agree port-to-port, and confirm the tow's length and any trailing gear.
- Treat the yacht as unpredictable; if it does not alter, take early action to starboard but keep clear of the bank.
- Defer the third vessel's request until the immediate situation is resolved.
Judgement. The dominant risk is not the yacht but the combination of constrained water and degraded detection. Slowing down is the single most effective action because it reduces closing speed, increases decision time and limits the consequences of any error.