Rule 17 – On the same tack, Proper Course
“If a boat clear astern becomes overlapped within two of her hull lengths to leeward of a boat on the same tack, she shall not sail above her proper course while they remain on the same tack and overlapped within that distance, unless in doing so she promptly sails astern of the other boat. This rule does not apply if the overlap begins while the windward boat is required by rule 13 to keep clear.”
Proper course. A course a boat would sail to finish as soon as possible in the absence of the other boats
One of the more challenging rules within our sport to judge and enforce, rule 17 comes into play usually on a downwind leg between two boats. This rule provides some ‘protection’ to the leadingboat as it will stop the trailing boat from holding you off course. When a trailing boat does become overlapped to leeward of their competitor from clear astern, they must sail their proper course at all times, including not luffing their opponent, nor sailing beyond their layline to the next mark.
As the leading, windward boat in a rule 17 scenario, it is vital to remember that:
- It is their proper course, not yours.
- You must still keep clear and avoid contact should they break this rule
The key point to this rule is to realise other boats TWA (true wind angles) and gybing angles to their next mark as these may be different to yours. When sailing in a one-design fleet, and you believe a boat has broken this rule, it is vital to note down your proper course wind angle, versus the TWA you had to alter to because of another boat.
It is also important to remember that if an overlap was created through a tack/gybe, then the leeward boat has all the rights under Rule 11 (which you can read about here).