Article by Greg Chisholm, diagrams by Paul Pascoe, October 2001 Article courtesy of Victorian Sabot Sailing Association

Transcription

1 Sailing in the Tide - A Few Basic Principles Article by Greg Chisholm, diagrams by Paul Pascoe, October 2001 Article courtesy of Victorian Sabot Sailing Association Too much tide and not enough room Introduction Sailing in the tide can have a dramatic effect on the tactics used in a race or, alternatively it can have minimal effect. How much effect will depend upon a number of factors including the relative amount of wind and tide. Often the tide does not run at a uniform pace across the course. It usually runs fastest in the deepest part of any channel and it will run at its slowest near the edges. It usually runs slower inshore. Sometimes, however, there is a sand bar that runs at right angles to the flow of the water which will produce a fast tidal spot. Check the speed of the tidal flow at any fixed mark, the start mark is a good one. The green path shows the normal laylines to the weather mark, while the black lines show the path that you would actually travel if affected by a cross tide as shown. Judging marks becomes a lot more difficult if there is a tide running. This article will help you get it right! Sailing in the Tide These rules are generally true but each circumstance requires some thought to see what applies. Sometimes there is conflict eg tide to the left and wind to the right. That is what makes sailing interesting. That is why sailing is a thinking man s (woman s) sport.

2 1. When sailing in a direction against the tide keep out of the tide as much as possible In the diagram, the tide is a lot weaker along the shore (see Rules About the Tide 5 below), so try and sail in this weaker tide on the upwind leg.

3 2. When sailing in a direction with the tide keep in the tide as much as possible. In the diagram, stay out in the strong current on the downwind leg. 3. When sailing in a direction across the tide never sail down-tide of the rhumb line (the straight line between the two marks). In the diagram, the green boat steers the boat as if it is going to pass higher than the next mark, but with the effect of the tide it sails a straight line. The red boat steers directly to the mark, but gets pushed down below the rhumb line and towards the end of the leg may struggle to get back up to the

4 mark, especially if the wind is light. It is usually better to stay even a little high of the rhumb line as it is always easier to come down to the mark, but much, much harder to come up to the mark if you misjudged it a bit and fell below the rhumb line, or if the wind drops a little. 4. Allow for the tide at the start and finish - will it push you back from or forward across the line? In this example, the tide would be pushing you away from the line, and you may end up late for the start. If the tide was going the other way, you would expect a lot of people to be over early as the tide pushes them closer the line, especially if the boat is luffing and not moving through the water very fast.

5 5. All things being equal start at the end nearest to the direction the tide is coming from. The green boat will spend less time on starboard tack fighting against the tide. 6. All things being equal finish at the end farthest to the direction the tide is coming from. The green boat has spent less time fighting against the tide.

6 7. Allow for the tide when rounding marks - marks are like magnets to a Sabot! 8. When sailing upwind across the tide - sail to the side of the course where the tide is coming from first (as long as all other things are equal) Rules about the tide 1. The stronger the tide the more effect it has on the fleet - basic statement. 2. The weaker the wind the more effect the tide has on the fleet - ie the more wind the less the tide has an effect. 3. The strength of the tide will be determined by the phase of the moon - this can be seen on the high and low water mark on any tide chart. The bigger the difference the stronger the tide. 4. Tide will generally flow fastest in a deep channel and slower in a shallow patch, particularly if the channel runs in the same direction as the tidal flow. 5. Tidal flow will be least at the edges of a channel or in a shallow area ie close to shore. 6. A rule that does not apply to one class racing (ie Sabots) is the lower the power weight ratio of a boat the more the tide has an effect eg a heavy ocean racer would be more effected by tide than a centre-boarded catamaran. 7. The closer the boat is to the shore line the more likely the tidal flow will follow the shoreline.

7 8. The tide will generally run fastest if the wind is pushing in the same direction as the tide. 9. When the wind is pushing against the tide a short sharp chop will be produced (bad for sabots) 10. When reading a tide chart the time of maximum flow is the most important information - this is often given in a stream chart. (to learn more go to the web site - and follow the prompts under the discussion on tides and streams (ps Dr Boat is Will Phillip s dad))