Collision at sea - the unthinkable, Part IV

1:43 AM Wed 14 Oct 2009 GMT

'Busy Harbour'

In this article, Part IV of a four part series, we discuss the situation of yachts and ships in enclosed waters, a ship's 'close proximity' effects on a yacht, how to tell you're on a collision course, the meaning of horn blasts from a ship, how you don't have right-of-way, and suggest a great gadget that may help.

Whenever you are sailing in a harbour, channel or other enclosed waterway where both ships and yachts share the space, there are dangers for a yacht whose crew does not understand the behaviour of water when a large body, like a ship, is passing through.

Complicating the issue is the fact that, in such enclosed waters, a ship is even less manoeuvrable than in open waters, of which every yacht skipper should be mindful.

There is a famous story told by a ship's captain who was sailing up the east Australian coastline, in the calmer waters between the Great Barrier Reef and the shore. His story exemplifies this issue well:

I well remember a French yacht in a very narrow passage in the Great Barrier Reef on the Australian coast in daylight. This fellow was right in the middle of the deep-water route, which caused me to have to move to starboard. The ship I was in was around 120,000 tonnes. As I moved over, the yacht also altered its course, not to avoid us, but to get closer.

This required me to mover over still further, until I was only about 150 feet from the reef and could go no further. The yacht had tons of room. Due to the entrained water between the reef and the ship I could not get the ship away from the reef after passing the yacht until I had passed the end of the reef. It required a change of underpants for me.

Later on Thursday Island, a Frenchman came to the pilot house to buy charts, and I found out it was the same yacht's owner, who was quite surprised at the trouble he had caused. He said that he only wanted to be friendly and get close enough to wave to us on the ship.

There can be a very real danger of collision when in enclosed waters, because of this increased lack of manoeuvrability of ships, particularly those with very deep drafts such as the one in the story above. This means that at all times a yacht should be very wary of passing too close to large vessels, keeping in mind that one should be able to manoeuvre away from the ship with sails only, in case of a loss of engine power at the wrong moment.

How entrained water can affect a yacht:
The reason large ships navigating near sheer walls or coral reefs on one side or even passing another large vessel close-to can have problems manoeuvring is due to the effect of the entrained water between the reef and the ship.

The water pushed aside by the bow has to go somewhere.

It cannot escape to the side where it is blocked by(say) the reef, so it has to speed up to get between the reef and the ship, causing a large reduction of pressure on one side. This can make steering extremely difficult.

The second aspect to be aware of is this: In a river, the ship pushes water ahead of her onto the bank, but as it passes this water runs off the shallow back into the channel quite rapidly, and this is then followed by a pretty hefty quarter wave which rolls over the exposed flats and returns the water to its original level. This movement of water can make life interesting for small craft.

So, should it be necessary to remain in a channel, such as a narrow river, it is advisable to stay to the starboard side of it as close to the bank as possible, but do not forget the wash that the ship will make.

Windward and leeward effects in close proximity:

There is another factor that the cruising sailor needs to keep in mind about the danger of finding yourself in close proximity to a large ship. Obviously, if the yacht is in the lee of the ship it will find itself deprived of a reasonable wind, and will have to rely on its motor to manoeuvre. This may be okay unless something goes wrong with your engine at the same time.

However, the same effect can occur when the yacht is on the windward side of the ship - particularly if the ship is high sided, such as a container ship with a full load, a passenger ship or a bulk carrier.

What happens here is that the wind hitting the side of the ship causes an area of high pressure, and an area of still or erratic air. The mainstream of wind then blows over the top of the ship. So the unfortunate yacht may find itself in this area of erratic air not be able to sail. It can then be sucked against the side of the ship because of the interaction of the water between the two.

Ship's horn in close proximity:
If you are sharing waters with a ship, it is imperative that the skipper of a yacht understand the meaning of a ship's horn blasts:

One short blast means 'I am changing course to starboard.'
Two short blasts mean 'I am changing course to port.'
Three short blasts mean 'I am operating astern.'
Two prolonged blasts followed by one short blast to mean 'I intend to overtake you on your starboard side';
Two prolonged blasts followed by two short blasts to mean 'I intend to over-take you on your port side.'
A long blast every two minutes is used when operating in fog.

On a collision course or not - How to tell:

(Method is valid regardless of the distance between the boats). Say you see a boat seemingly coming towards yours at an angle, and you see it just forward of your starboard stay (If you have a hand compass, and you want to impress your neophyte crew, you can also take a bearing of the other boat.) Check again 2 to 3 minutes later. Three things can have happened:

The other boat is now much more forward of your starboard stay. In other words, the bearing of the other boat has moved forward. Result: If both boats maintain speed, the other boat will pass yours ahead of your bow. No risk of collision. If the boat is very far, re-check every 5 minutes.

The boat is now much more aft of your starboard stay. In other words, the bearing of the other boat has moved aft. Result: If both boats maintain speed, the other boat will pass yours behind of your stern. No risk of collision. If the boat is very far, re-check every 5 minutes.

The boat is still at the same place relative to your starboard stay. In other words, the bearing of the other boat has not changed. Result: If both boats maintain speed, there is a probable risk of collision.

Closing Speed:

How long you have between the appearance of a ship on the horizon and the moment of collision.

Let us assume that the practical distance that you can see is 7 miles. Let us assume that the ship is travelling at 15 knots, a not unreasonable speed. Let us further assume that the yacht is travelling at 6 knots, a reasonable speed for most cruising yachts.

This means, in the worst case of a head on collision, the closing speed is 21 knots. These are quite conservative estimates, as both yachts and ships frequently travel at much higher speeds. Nevertheless, this means that from the moment of sighting the first tiny speck on the horizon to the point of collision is just 20 minutes. Any yacht that puts its 'time between required horizon checks' at more than 15 minutes is taking a risk all the time they are sailing. The risk is of course higher known shipping transit corridors than it is in, for instance, the Southern Ocean.

Watch Commander -

How to never miss another horizon check:
This gadget is so great that some sailing friends of ours on a circumnavigation with four children on board allowed their teenaged children to keep watch a couple of years earlier than they otherwise would have.

The simple Watch Commander, once set, cannot be turned off unless you actually disconnect the power. It is designed to preset the time between horizon checks.

Once set for the number of minutes that you stipulate, it sounds a soft alarm which will not wake sleeping crew at the end of the period. The crew on watch merely has to hit the button and the time period starts all over again.

If the crew has fallen asleep or whatever, after 45 seconds, the soft alarm turns into a shrieking siren that will even scare away nearby fish.
For more information, go to the website .

Right of Way:
Finally, just in case you, as a sailing boat, think that you have right of way (as in the story of Windhoven and Blackwattle in Part 1 of this series) over the container ship, Rule 18 (b) of the International Rules for the Avoidance of Collision at Sea states that

'A sailing vessel under way shall keep out of the way of a vessel restricted in her ability to manoeuvre'

- which applies to all large ships at sea, and also to such vessels as barges and fishing boats engaged in fishing with nets.

To read Part I of Collision at sea, the unthinkable, click here

To read Part II of Collision at sea, the unthinkable, click here

To read Part III of Collision at sea, the unthinkable click here

by Sail-World Cruising