Friday, September 11, 2009

Coastal Navigation and Tidal Planning

In an effort to continue my training and increase my abilities, I registered for a symposium in Maine with the hopes of getting more on the water experience and learn how to better handle myself, my boat and deal with tides, and navigation. When I saw the Coastal Navigation and Tidal Planning course listed on the curriculum I thought I had found the perfect class. The class is part of the new BCU 4 Star Sea Kayak Leader Award. Although I was not yet heading towards that award level, I thought it would still come in handy. I was more then a little surprised and slightly disappointed to discover (after showing up) that the class was not an on water class. It was actually a classroom class. But it was still well worth it. I feel that I got a lot out of the class and I just have to remember how to use what I learned. The class was broken down into four major topics.

We learned about tides and the various types. There are spring tides which are the biggest and neap tides which are the smallest. Spring tides occur when the moon and sun line up, so basically at new moon and full moon. Neap tides occur when the moon is halfway through waxing or waning. Spring tides can be affected even more in the spring when the earth is closer to the sun causing an even stronger gravitational pull. Tides are caused by the water sliding across the surface of the earth. Water is not lifted, it is moved. Based on the continental shelf, tides may be only a few feet in height or they may be tens of feet in height. Charts with depths marked show lowest mean depth. As this is a mean average, it is possible to have a negative tidal difference. Recently, the UK switched over to lowest astronomical depth meaning that when everything is in alignment, that is the lowest possible depth. While it is still possible for a negative tide, the tidal difference is almost trivial.

The period of the tide is just over 6 hours from low to high or high to low. The moon travels around the earth in the same direction that the earth is spinning. The moon has a 28 day orbit. Over the 24 hour rotation of the earth, the moon has moved 1/28th of an orbit. So the earth is constantly playing catch up with the moon. This causes the tides to slowly move forward each day.

Wind and Waves
Wind causes waves but where does the wind come from? Wind is caused by changing temperatures in the air. The air near the equator heats up and rises heading towards the cooler air at the poles. The cold air at the poles, moves down towards the equator to head up. This circular motion of air moves both on a grand scale and at smaller scales. As the air moves towards the poles, some of the air loses it heat and sinks back down to the equator causing smaller circular currents. All this is what happens in an ideal world. But once land is involved, air heats up faster over land during the day and cools off faster in the night. Water is a great temperature regulator. The air over land will shift with day and night causing on-shore and off-shore breezes. Add in the rotation of the earth and the wind now twists around.

Waves are formed by off-shore weather patterns. The wind from the weather patterns stirs up the water and the waves travel out from that point until they meet the shore. Waves can be generated miles away from the shore. There tends to be a period or pattern to the waves. While no two waves are the same, there is a period to the wave sets. Watching waves on shore, waves will start small, then build up to larger waves then decrease back down to small waves again. The breaking of waves occurs when the water has no where to go but up, generally caused by the beach shelf, sand bars, reefs and other obstacles on the ocean floor.

Just as air can be heated and moved, so to can water. The ocean is similarly heated at the equator and travels up to the poles in a great heat exchange. The continents provide some resistance as well as more water from rivers. As water flows between islands, stronger currents are formed as water is pinched and forced through narrower gaps. With the tide and movement of the water, there is various types of current. A slack current is when the tide is at its slowest, typically at the high or low portion of the tide, just before it changes direction. The current can ebb or flow, ebb meaning it is going out and flow meaning it is coming in. The fun part about currents is the cool things that are created that make for great play spots. Swells and waves moving along rocky shores are fun to paddle around and through. Swells moving over ledges create waves which can be surfed. Tidal races are standing waves formed when water moves through a constriction.

Weather affects the tides and even affects the tide height. With a high pressure system, the pressure of the air pushing down on the water causes the tide height to diminish. The opposite is true with a low pressure system, less pressure allows the tide to become higher. High pressure systems do not have fronts associated with them while low pressure systems do. The clashing of fronts causes weather changes either in the form of thunderstorms, or rain.

This can be broken down into three sections; bearings, courses, and headings. Bearing are the straight line direction to any object whether you intend to travel there or not. Bearings can be used to triangulate a position in the ocean when several markers or land features can be seen. Headings are the direction of travel, whether that takes current and wind into account or not. Courses are the path actually taken, as in how the wind and current actually affected the path of travel. Bearings, courses, and headings can all be the same or they can all be different. Knowing how strong the current or wind is in a particular area can allow for compensation.

So much more
This is really only the barest amount that I learned from the class. There was so much more. It was a very good class and I found it useful. We actually walked down to the beach and took a few bearings and discussed how to triangulate and plan a trip. We learned how to read charts and obtain locations from a chart. Most charts are in both true north and magnetic north so we learned how to correct between the two. The rule of thumb was when going from the small world (chart or map) to the big world (real world) you add the deviation. When going from the real world to the small world, the deviation is subtracted. The compass rose has all the needed information about deviations and annual change.

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