Vessel  I  Measurement;

The "I" measurement is from deck to halyard sheave. It is used to cross
check other measurements and to determine the size of the foretriangle
so that sail area for overlapping genoas can be determined correctly.

The I measurement is also used to help detemine hank on sail luff
lengths. As smaller headsails are not full hoist.

Please note! - I is NOT the sails luff length

For instance, the IOR rules for sail design state; (Height of the
foretriangle is the "I" measurment).

Storm Jib: Its area is limited to five percent of the height of the
foretriangle squared. The rule states that the luff of the storm jib
must be shorter than 65 percent of the height of the foretriangle.

Heavy Weather Jib: Its area is limited to 13.5 percent of the height of
the foretriangle squared. The ORC rules state that this sail cannot have
reef points. If either the storm jib or heavy weather jib are made to
fit a luff-groove device, the sail must have an alternative means of
being attached to the stay. The most common alternative method is to
have grommets along the luff so that you can tie the sail to the stay.

In Genoa's for hank on sails, sail design is based on these rules;

Correct sail design uses the following luff length formulas as a \
percentage of the vessel's I measurement;

110 Genoa 84%
120 Genoa 90%
135 Genoa 97%
150 Genoa 100%

In reality, once a roller furler is installed, many sailors set their
luff length to be their foil maximum hoist. Which then tends to alter
the designers sailplan design specifications.