Correspondence Received – Minimum speed for going astern

4th September 2020

Correspondence Received – Minimum speed for going astern

Initial Report

With respect to your article in MFB56 regarding the master pilot exchange, it mentions that the main engine failed to start astern because the vessel was travelling at 3.5 knots. When the vessel reduced to 3 knots then there was no problem in getting the engine to go astern.

Is the 3 knots figure an owners, IMO, or similar requirement? I have never experienced any problem going astern at whatever speed the ship or engine has been running and would be grateful if you could advise where this came from and reasons for this arrangement.

Following discussion, the Maritime Advisory Board responded as follows

Assuming the vessel does not have a controllable pitch propeller, then in many cases, with way on the ship, the engine will not be able to go astern until a minimum speed is reached – in this case 3 knots or less.

It is normal in a critical, emergency situation for the master to instruct the engine room to perform a crash astern manoeuvre. This procedure should always be posted in the wheelhouse and all the bridge officers must be familiar with it.

A fully loaded motor ship of about 14,000 tonnes displacement manoeuvring from “sea speed” to standstill from a speed of about 14 knots, will still be moving ahead at approximately 2 to 3 knots around fifteen minutes after the ‘Stop’ order was given. The engine rpm will fall from 110 to 40 in about 7 to 8 minutes and gradually come to rest after about 12 minutes.

If a crash stop is demanded, the engine can usually be reversed after about 3 minutes, while still running ahead at about 30 rpm and can be running at 60% power astern in about 5 minutes. A slower vessel, or one in ballast, would take less time to accomplish this. It should be noted that a crash astern manoeuvre causes very high stress levels within the engine and may cause damage.

A master, and indeed a pilot, needs to know the maximum ahead speed through the water at which he can obtain an astern movement. It varies from ship to ship.

Some modern vessels with “optimised” designs of engine are not able to apply braking air for a substantial period – on one class it was a full 17 minutes at loaded draught – from slow ahead. This was discovered after delivery from the yard and had been designed-in, presumably for fuel efficiency and environmental factors rather than concentrating upon vessel manoeuvrability. The company in question ensured that this “quirk” was made a prominent opening part of every Master / Pilot information exchange – standard practice became that a braking tug with a large bollard pull was attached as soon as possible.

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