ISAF Case 15.

In that case, the outside boat was entitled to mark room and is prevented from tacking by a windward boat.

CASE 21
Definitions, Mark-Room
Definitions, Room
When a right-of-way boat is obligated to give mark-room to
a boat overlapped inside her, there is no maximum or
minimum amount of space that she must give. The amount
of space that she must give depends significantly on the
existing conditions including wind and sea conditions, the
speed of the inside boat, the sails she has set and her design
characteristics.
Question
When rule 18 requires a right-of-way boat to give mark-room to an inside
boat that overlaps her, what is the maximum amount of space that she
must give? What is the minimum amount of space that she must give?
Answer
In this situation, the definition Mark-Room states that the inside boat is
entitled to room for four manoeuvres:
 Room to leave the mark on the required side.
 Room to sail to the mark, but only if the inside boat’s proper
course is to sail close to the mark.
 Room to round the mark as necessary to sail the course.
 Room to tack, but only if these additional conditions are met: the
inside boat is overlapped to windward of the outside boat, the tack
is part of the rounding necessary to sail the course, and the inside
boat would be fetching the mark after her tack.
The definitions Room and Mark-Room do not include any reference to a
maximum or minimum amount of space, and no rule implies that the rightof-
way outside boat must give a maximum or minimum amount of space.
She must give the inside boat the space she needs in the existing
conditions to carry out those manoeuvres promptly in a seamanlike way.
In addition, the inside boat is entitled to space to avoid touching the mark
and space for her to comply with her obligations under the rules of Part 2
with respect to the outside boat as well as any other nearby boats.
The term ‘existing conditions’ deserves consideration. For example, the
inside one of two dinghies approaching a mark on a placid lake in light air
will need relatively little space beyond that required for her hull and
properly trimmed sails. At the other extreme, when two keel boats, on
open water with steep seas, are approaching a mark that is being tossed
about widely and unpredictably, the inside boat may need a full hull length
of space or even more to ensure safety. A boat with a spinnaker flying
often needs more space than one with her spinnaker stowed. A boat that is
planing or surfing may require less space to turn than a boat that is
climbing a steep wave. The ‘existing conditions’ also include
characteristics of the inside boat. For example, a boat with a long keel or a
multihull may require more space to round a mark than a more easily
turned monohull. A boat with a large rudder may need less space to turn
than a boat with a small rudder.
The phrase ‘manoeuvring promptly in a seamanlike way’ has implications
for both boats. First, it addresses the inside boat, saying she is not entitled
to complain of insufficient space if she fails to execute with reasonable
efficiency the handling of her helm, sheets and sails while manoeuvring. It
also implies that the outside boat must provide enough space so that the
inside boat need not manoeuvre in an extraordinary or abnormal manner
(see also Case 103).