Let’s look at what the rule says:
ON OPPOSITE TACKS
Tack in this context is the relationship between your boat and the direction of the wind. In general, a starboard tack is when the wind is coming from the starboard side of your boat, while a port tack is when the wind is coming from the port side of the vessel.
Another way to remember is to simply look where you sit. In racing, we normally sit on the high (windward) side of the boat. So in general, if we sit on the starboard side, there is a good chance that we are on a starboard tack.
But tack is defined in the definitions within the rules which states:
The rules also define the windward side as follows:
Leeward and Windward A boat’s leeward side is the side that is or, when she is head to wind, was away from the wind. However, when sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies. The other side is her windward side. When two boats on the same tack overlap, the one on the leeward side of the other is the leeward boat. The other is the windward boat.
The definition is a bit convoluted as it defines the leeward side from which you then can deduct the windward side. So rather than chasing our tail with rules and definitions, let’s look at some examples.
Let’s follow this example through by having a look at what "keeping clear" means within the definitions of the rules:
Keep Clear A boat keeps clear of a right-of-way boat
(a) if the right-of-way boat can sail her course with no need to take avoiding action and,
(b) when the boats are overlapped, if the right-of-way boat can also change course in both directions without immediately making contact.
Let’s look at some examples on how to keep clear…
The two most likely ways to do this is by tucking under yellow as green does or tacking as blue is doing. Both green and blue comply with Rule 10.
The second part of the definition of windward clarifies this by stating "…However, when sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies…" So yellow is on a starboard tack and blue should keep clear.
Contrast this with the following example where yellow is slowly overtaking blue and eventually the booms touch without causing injury or damage.
Because both boats are sailing downwind by the lee, their tack is determined by the opposite side of where her mainsail is deployed.
So to sum up Rule 10…
- Extremely common "must-know" rule.
- It applies to boats on opposite tacks (port/starboard).
- A boat is on the tack, starboard or port, corresponding to her windward side.
- On a downwind run a boat’s windward side is the opposite side to which her mainsail lies.
- When you are racing, always ask yourself which tack your boat is on even when no other boats are near. Then when the need arises, you only have to determine the other boat’s tack to make an informed decision.