Current and tide

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The IOM Worlds in Croatia (IOM World Champs 2001) had modest current which was more down-course, while the IOM Worlds in Vancouver (IOM World Champs 2003) showed significant cross-current or tide.  In Croatia I thought I had got to grips with current, and discovered in Vancouver I really didn't have a clue!  It's been bugging me ever since, but it was only at a recent Race Officer's training seminar held by the RYA in Lymington in 2010 that I began to understand what happens to something like an IOM sailing on a course with cross-current or cross-tide.  The following diagrams are my attempt to document this understanding.

In the following diagrams, I assume a modest current of 0.5 m/sec (1 knot), a boat top speed of 1.3 m/sec (possibly a little generous, maybe 1 m/sec would be better representative of an IOM), and a wind of 3 m/sec (6 knots).

Start line

We begin by asking what to do with the start line if there is a cross-current, and the first step is to ask what kind of wind the boats experience as they mill about the start area.

 

 

We imagine a boat that is not sailing, but is drifting happily with the current.  The wind will appear to come, not from dead upwind, but slightly to the right or left by about 10 degrees.  And, as the boat starts to sail, this is the wind which drives her.  Hence, the starting line should be set square to the driving wind, about 10 degrees off, as illustrated above, by dropping the pin end or the committee boat end as required.  Drop the pin end if starboard tack is down current, or drop the committee boat end if starboard tack is up current.

Course over ground (CoG)

Then, we ask about the boat's course, in this case about the boat's course over the ground, or CoG.  This is useful because the windward mark is anchored to the ground, and we want to know how to get there as quickly as possible, and how to set a racing course which is as fair as possible.

 

 

If the boat is beating against the current, although she makes 1.3 m/s through the water, she only makes 1.1 m/s over the ground, and seems to have no leeway at all.  If beating with the current, while still making 1.3 m/s through the water, she makes 1.7 m/s over the ground, and seems to have much higher leeway that 'usual'.  She's around 50% faster over the ground, but her leeway or lack of it is completely deceptive...

Apparent wind

What does this feel like to the boat?

 

 

If beating against the current, the apparent wind seems to be around 40 degrees off the bows (left part of diagram above).  If beating with the current, the apparent wind seems to be around 26 degrees off the bows (right part of diagram above).  I now know what was so confusing in the conditions at Vancouver.  When beating against the current, even though the boat looks like it has no leeway and looks like it is pointing terrifically, it is going a little slowly and it is well off the wind;  an IOM should be able to hold its apparent wind at around 30 degrees off the bows.  And, when beating with the current, even though the boat looks like it has huge leeway and looks like it is sailing low, it is zooming along and yet it is too high on the wind;  25 degrees apparent on an IOM is not normally fast...

Pointing or footing

Given the apparent wind for the boat when sailing up- or down-current, the following diagram suggests the adjustments you might need to make when sailing.

 

 

In theory, beating against the current, you should be able to bring the boat up another 10 degrees or so.  I know, it looks impossible from the breakwater...  And, again in theory, beating with the current, even though you are zooming along, you are sailing too high, and should foot by 5 degrees or so.  I know, it looks utterly stupid from the breakwater...

The other highly significant issue to notice is what has happened to the apparent wind speed.  On the breakwater, I measured wind speed as 3 m/s, well within the wind range for my No.1 rig.  I got onto the water, wondering why everyone else was in No.2 rig, and soon discovered the reason -- I was way overpowered!  How did that happen?  Well, beating against the current, I was just able to hold on to No.1.  But beating with the current, the apparent wind was out of No.1 range, and inside No.2 range, just because the current had added extra zip to the apparent wind.

Setting the windward mark

The following diagram shows the position of the windward mark if you are laying a course in a tidal racing area.

 

 

If you didn't realise what was happening with course over ground, you might set the windward mark a little, maybe 10 degrees, to one or other side of the course, at right angles to the start line which you knew needed to be biased 10 degrees itself to account for the tide.  But with course over ground factored in, the windward mark needs to be in a completely different place, over 35 degrees to one side, if you want starboard and port beats to take approximately the same time.

Setting the beat

The following diagram shows, on the left, how the beat would be set when there was no current, and on the right how it would be set when starboard tack is up-current.

 

 

On the run

From the point of view of racing, and from course setting, the run is much less dramatic, accords much more with 'common' sense, and the adjustments to helm and course fairly obvious.

 

 

2010-05-03


2011 Lester Gilbert