Little ships and the wrong sort of waves

Why the correct manoeuvres for the sea conditions will reduce hogging and sagging stress on both ship and crew

- Clive Marsh

The problem with waves is that they may be the wrong length for your vessel. For example, a 1,000ft container vessel might find that very long waves cause stress on her keel which is not fully supported evenly. A hogging or sagging stress may occur and fractures caused by this type of wave have been blamed for the sudden disappearance of many large vessels.

However, the same very long waves may cause no problems at all for our little boats while we happily ride up and down them. I say happily, but this may not relate to an unfortunate crew suffering motion or sea sickness.

Hogging is the name given to stress on a ship's keel and hull that can cause the centre of the keel to bend upward when the forward and aft ends of the ship are not supported. Sagging is the stress on a ship's hull or keel when waves are a similar length to the ship's hull causing the ship to be in the trough of two waves. This may cause the middle of the ship to bend down slightly and may also cause the hull to fracture. Smaller but short, steep waves may cause all sorts of problems for a little boat but would go unnoticed by a container vessel.

Stress reduction

My favourite little ship for crossing the northern Atlantic was the Beaverash, a general cargo vessel of just 4,529 gross registered tons with a top speed of 14 knots down wave. One winter we passed Cunard's famous original Queen Elizabeth of 83,673 gross tons, nearly 20 times our size and three times our length. She was crawling along or lying hove to and looking most uncomfortable. She could do an amazing 32.5 knots if she wanted but the waves were just the wrong length for her hull at that time or for her passengers' comfort and the captain had taken this action to reduce stress on both his ship and her passengers.