weather - page 4
8.4 Sea condition - Electric thunderstorms
Waves are the product of wind blowing across water. Wave height and wavelength metrics are shown below. Another important metric is the Period. The Period of a wave is deemed as the time it takes for two consecutive crests to pass a stationary point. The frequency, although not used much in ocean wave metrics is the number of waves to pass a stationary point in a certain amount of time. e.g. 10 waves per minute is a measure of the frequency whereas the Period then would be stated as 1 wave per 6 seconds.
Since waves are primarily the result of surface wind action, they can be accurately predicted. Waves have troughs and crests. Sailing in moderate seas is safe and easy, but as waves grow, their capacity for doing harm is greatly increased and requires expert sailing skills. As waves get steeper and steeper the tops become tenably unstable. When propelled further by wind, the tops fall off creating breaking waves. Take away the wind and the tops return to being stable but the waves themselves continue because there is nothing to stop them. This resultant wave is known as swell.
Where the depth of water decreases to 0 - 20 ft (6m) a wave height will suddenly increase. You're familiar with this at your local beach. While sailing, when you see waves of unusual localized height or breaking you should be aware that shallow water exists at that point.
Swells are the result of waves from distant storms sometimes hundred of miles away. Their wavelength is long and generally are not a problem; with one exception: many sailors are subject to seasickness because of swells.
Electric storms are frightening. On inland waters, they can arrive very quickly, especially on a hot sultry day. Electric storms are frequently accompanied by high and very erratic winds. Huge wind bullets which are sudden wind gusts can come seemingly out of nowhere. While you should always be prepared for a storm, when you observe a heavy cloud formation rising rapidly with huge white clouds, called “thunderheads”, it is time to make further preparations. Get to a safe harbor if time permits. If not, prepare for a storm: reef the mainsail, lower the jib sail, douse spinnakers, close hatches, keep away from metal objects if possible, put on life jackets, turn off electronic gear. Crew safety is paramount.If a violent thunderstorm is approaching take all sails down and turn on engines. Reduce the apparent wind velocity on your boat by running downwind.Nowadays, thunderstorms are decently predictable even if you can not see the cloud formations.
When encountering an electric storm, it's best to leave the area and get off the water. If this is not possible, then get everyone below decks. Do not touch metal objects. While theoretically, your mast is insulated from the water in some vessels, at the voltage levels generated in lightning the electrical discharge can jump across the insulating materials. Much of this can be avoided by checking the weather before you go out to see if there is storming activity and high Cape.
There is an old myth that says to wrap your anchor chain around the mast and put it into the water to allow lightning to travel overboard and not into the boat. This is certainly a myth and every lightning engineer will tell you that lightning will not travel around the right angle turn from the mast to the chain.The best protection against a lightning strike is prevention in the form of an ion dissipater which is a hairy looking thing resembling Cousin It from the Addams Family. It sits atop the mast.