Understanding Overlap Rules
A river-specific explanation
If you’re new to racing—or even just sailing in company—few things cause more head-scratching than the phrase:
“You had overlap!”
On open water, overlap rules already take some getting used to. On a narrow, stream-affected river like the Thames, they can feel positively mysterious. Boats appear suddenly from odd angles, the river bends, the stream pushes everyone sideways, and before you know it someone is explaining the rules very confidently from the wrong end of the conversation.
This article explains overlap rules simply, with a particular eye on how they work on a river.
1. What Is an Overlap? (Plain English Version)
Two boats are overlapped when they are close enough side-by-side that one is not completely clear ahead or completely clear astern of the other.
In practice:
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If you can draw a line across the back of the boat in front and the other boat’s bow has crossed it → overlap exists
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If the boat behind is completely behind that line → clear astern
Overlap can be:
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Leeward overlap (inside, closer to the wind’s downwind side)
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Windward overlap (outside, closer to the windward side)
This distinction matters a lot.
2. The Core Rule (Upwind and Downwind)
When two boats are overlapped:
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The windward boat must keep clear of the leeward boat
That’s the backbone of overlap rules and it applies everywhere—sea, lake or river.
However… the river adds complications.
3. Why the Thames Makes Overlaps Tricky
On the Thames:
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Boats are closer together
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Courses bend
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Stream slides boats sideways
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Wind shadows force sudden course changes
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There is often no “room to spare”
So overlaps appear quickly and disappear just as fast.
A boat can gain overlap simply because the stream pushed it sideways—not because it deliberately sailed there.
This doesn’t make the overlap invalid.
It just means anticipation matters more than intention.
4. Inside and Outside at Marks (The Big One)
This is where most river arguments begin.
At a mark:
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If boats are overlapped when the first boat enters the zone (usually three boat lengths),
This applies even if:
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The overlap feels very late
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The river bend compresses boats
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The outside boat suddenly realises there’s no space
On the Thames, marks are often close to banks, trees or moored boats, so giving mark room early is essential.
5. River-Specific Reality: The Bank Is Not “Room”
A common misconception on rivers is:
“There wasn’t room because of the bank.”
Unfortunately, the rules don’t care about geography.
If:
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An inside overlap exists
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And you are the outside boat
You must still give room—even if that means you have to slow down earlier than you’d like.
On a river, good sailors defend early to avoid being squeezed later.
6. You Can’t Create an Overlap From Nowhere
A key protection in the rules:
If a boat establishes an inside overlap from clear astern, the outside boat must be given time and space to respond.
In other words:
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No barging in at the last second
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No “I appeared, therefore I’m entitled”
But on the Thames, timing is tight—so if you leave it late, you’re gambling.
7. Overlap vs Common Sense
River sailing rewards sailors who combine rules knowledge with courtesy.
Good habits include:
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Backing off slightly rather than forcing a marginal situation
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Remembering that everyone is managing wind shifts and stream at the same time
Winning an argument but damaging a boat (or a friendship) is never a good trade.
8. Typical Thames Overlap Scenarios
You’ll often see overlaps arise:
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Approaching a downstream mark with stream pushing boats together
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On tight reaches near the bank
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When one boat exits a wind shadow faster than another
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When tacking in confined space
Being alert to these moments prevents panic manoeuvres.
9. The Golden Rule for River Racing
If you’re unsure who’s right:
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Ease early
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Keep the boat under control
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Avoid contact
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Sort it out calmly afterwards
The Thames is narrow. Pride should be narrower.
Final Thoughts
Overlap rules aren’t there to trip people up—they exist to keep boats predictable and safe. On a river like the Thames, where space is limited and conditions shift constantly, understanding overlaps is as much about anticipation and restraint as it is about rule-reading.
Learn the basics, expect overlaps to happen quickly, and sail with enough margin that nobody needs to shout.
That’s good racing—and good river manners.
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