Dealing with Currents
Current is the water movement generated by tides or the wind, some currents are large and constant such as the Gulf stream.
The movement of water over a course area is rarely uniform. Because of this factor the racing sailor has to have an understanding of where currents are at their maximum, how this helps or obstructs progress and where a current will take you sideways away from the mark, or sideways towards the mark. Where possible, using current in different areas to enhance the speed around the course can be much more rewarding than finding a lot of boat speed through increased wind speed.
Tide related currents are changeable with time and will usually have a 6 hours cycle, meaning the direction of the current will change every 6 hours, it also means the speed of the current will change throughout the day, for example if high tide is at 12:00 and low tide is at 18:00 the current will be the fastest at around 15:00 and the slowest at around one hour either side of the time it changes direction, these type of current involve the movement of the entire water column which means it will be mixing the water and trigger a difference in water temperature as well. The flow of water is dictated by a tidal rule called the rule of twelfths, which shows the maximum water movement in the cycle occurs for one hour either side of mid tide, and minimum movement takes place one hour either side of the tide change.
Wind driven currents are generated by the flow of the wind on the surface of the water, it will usually only move the top layer of the water and not generate deep water mixing so does not affect the temperature of the water, at most parts there will also not be a major difference between shallow and deep water in the speed of the current since only the top layer of water is moving.
Constant currents are generated over a large scale usually in the oceans and will be moving from hot water to cold (so most of them will start around the equator and move north or south towards the poles)
Knowing the type of current we are dealing with will help us to better understand the nature of it and assist us in forming a race plan that will yield us the best outcome.
As discussed in a previous blog it all starts with observations, constantly observing, measuring the current, looking at charts or asking the locals should give us enough information as to the behavior of the current we will be facing on the race course.
After establishing what type of current we are dealing with we can prioritize and make a plan.
Tidal current is the trickiest one as there will be many differences on the race course itself and at different phases of the race, We need to prioritize for different legs and areas on the racecourse weather the current have a big influence or a small one (the lighter the wind even the smallest current might prove important), we need to establish at every point during the race if the current is improving our VMG (which means we are getting closer to the mark faster) or reduce our VMG (which means we are getting closer to the mark slower).
The risk aspect will usually be around lay lines either upwind or downwind in which we need to be aware not to put ourselves in a position where the current will be pushing us over the lay line as we will be forced to sail an extra distance.
During our training in Toplevel Sailing we teach our sailors how to measure current strength and direction using a available objects, and how to differentiate the types of current using observation skills and prioritize in order to be able to make the best possible race plan