Are you safe to be sleeping aboard?

A screaming horn breaks through the night. Startled awake the crews of a bank of moored yachts blearily being to think. There’s a wild, deep thrashing and screaming resonating in the hull. Sinister and so very different from the utterly regular Thump, Flump, Thump, of a freighter in the channel.

Before sea-boots can be reached for, a sudden rending crash begins. One single eye blink later half the yacht has vanished. In another, only ruined shards remain, swallowed in the foaming water, glowing strangely green in the actinic glare of a searchlight.

Are you safe to be sleeping aboard on this mooring?

Collisions between craft moored near fairways in busy harbours and heavy commercial craft are uncommon but high consequence events for small craft which typically lead to catastrophic damage. The hazard to life for a crew crew asleep aboard, especially if post passage or pub, when alertness is reduced are obvious.

Large vessels can lose control in shallow or narrow channels due to hydrodynamic interactions which are usually speed related. The energy involved is such that few commercial craft have the power available to break free from the effects once they begin.

The master or crew of a small craft can assume that such effects might occur when the draft of commercial craft approaches that of the waterway (or dredged channel) or her beam is a meaningful proportion of the channel width.

Boating folk are already accustomed to informally risk assessing a mooring: will the anchor drag, when was this buoy last maintained, does that pontoon look like it deserves the name – are all usual parts of deciding on a berth. Adding consideration of whether the berth is protected from intrusion by an errant merchant vessel is an easy step that could save a life. Ask yourself: Am I safe to be sleeping on this mooring?

Some investigation reports

I got to thinking about this question after reading Case 15 in the MAIB Safety digest 2020 Edition 2 and reflecting on a past experiences including moorings on the East coast rivers that are often in proximity to commercial traffic.

Lest you doubt me, Case 9 in the MAIB Safety Digest 2019 Edition 1 illustrates how badly small craft come off in a collision with an Offshore Support Vessel. Relatively speaking, this is not a large merchant craft. For clarity, the collision in this case did not arise from the effects discussed here.

The NTSB has an impressively thorough report on collision between the tanker Chembulk Houston collision with M/V Monte Alegre a Container Carrier / Boxship caused by inter-ship hydrodynamic interactions in narrow channels:. What is striking here is both how the bridge teams struggled to understand the effects the ships were experiencing and the energy involved in the interactions that prevented straightforward escape from the situation

Finally barge strings, commonly found on major river and canal systems in the US and Europe can be associated with loss of control that results in catastrophic damage to bankside moorings and infrastructure. A good example is the NTSB investigation into the ‘Contact of Leviticus Tow with Plaquemine Point Shipyard