Weather & The Sea

The sea is affected by two things above all else - the moon, and the wind.

The Moon and Tides

The gravitational effect of the moon, as it goes around the earth, pulls the water to-and-fro around the surface of our planet. In certain places the speed of the tide is critical. On trips to France every year the Hardiesse will pass through the Chenal du Four, off the coast of Brittany. In this channel, if the tide is going north at 4 knots, but with a light wind the ship is only making 3 knots southwards, the ship would go backwards - pointless! Understanding the tides can be of critical importance.

Every two weeks, at a "new moon" or "no moon", the tide movement is greater, and these are called Spring tides. In between times the tidal movement is less and these are Neap tides.

	Neap Tides		Spring Tides
When the moon is neither a full moon nor a no-moon (also called new moon), the tidal range is not so great. Compare the water level on the harbour wall at low tide, and later at high tide. If you look at some tide tables, you will see it is about 6 hours between high and low tides. Notice also the height of the tide in the tables, and how it changes.		When it is a full moon, or new moon, the tidal range is greatest. At low tide you may see rocks and sand that are normally underwater. This also means there is more water moving - all that volume of water (red arrow) will go out and back in again. This means that half way between high and low tide, the flow is at its greatest speed and force.

The "Rule of Twelfths" is that after high tide when the water starts to fall, in the first hour 1/12th of the water will flow out. In the second hour 2/12ths will flow out, and in the third hour, 3/12ths is flowing out. In the fourth, fifth and sixth hour after high tide, the amount of water flowing out is 3/12ths 2/12ths and 1/12th, because by now most of the water has gone out, it is nearly low tide and soon the flow reverses and starts coming back in. The pattern is the same for the incoming tide.

Marine charts contain information on tidal flows showing the direction and speed of the tide, and on top of this you have to apply your knowledge of whether it is a spring or neap tide, or in between.

Wind and the Sea

The waves on the surface of the sea are caused by wind, and the appearance of the sea gives you an indication of how strong the wind is. Since you are on the internet, rather than copy it here, it is easier for you to look up the Beaufort Scale (named after Admiral Beaufort who in 1805 started using a standard way of recording the wind and sea state). You may find a landlubbers version of the Beaufort Scale - keep looking for the sea version. Pictures of the sea at the top end of the Beaufort Scale are awesome, and so weather and the sea must be taken seriously, and the ship's preparedness for strong conditions is important.

The Hardiesse will carry more or less sail depending on the wind, and will also use different types of sale. In light winds, and when travelling in roughly the same direction as the wind, the square sail and raffees might be used. In strong winds, sails are taken down and smaller sails are used, or current sails are reefed (tied down to reduce their size).

If there is no wind, this gets a bit dispiriting, and there is always the motor if necessary.

Charts, Nautical Miles and Knots

Maritime charts have latitude and longitude as the measurement along the sides of a chart, and so they show degrees, minutes and seconds. The Hardiesse carries charts for all areas she might sail in, from the south coast of Britain to the North coast of France. On a journey across the Channel you will do some chartwork and write up the log, which is done hourly. The log book is used to record, at every hour, the ship's position, course, the wind speed, barometric pressure, and distance travelled. This has been good practice at sea for centuries, and still applies today. The Nautical Mile is used for aviation and maritime purposes, and is unique because it is 1/60th of one degree going from the north pole to the south pole, so it is also a measure of Latitude. A nautical mile is about 1.15 land miles, or 1.85 kilometres. As a measure of speed, one knot is one nautical mile per hour.