I is the rating-rule measurement of foretriangle height: the vertical distance from the sheer line abreast the mast up to the point where the headstay meets the forward face of the mast. Together with J, the base of the foretriangle, it defines the triangle inside which the headsails live, and it is the first number a sailmaker asks for when quoting a jib or genoa. The beginner needs to know that I is simply “how tall the foretriangle is”. The experienced sailor adds that the exact measuring points vary by rule – some rules take the upper point at the headstay intersection, others at the top of the highest headsail halyard sheave, and the lower point is at the sheer line, not the deck – so an I quoted under one rule may not match the same boat measured under another.
How I Is Measured
Under most rating and handicap systems the lower measurement point is the sheer line, extended athwartships to the centreline of the mast. On a boat with a raised cabin trunk or a bulwark, that point can sit some way below the deck you actually stand on, which is why a tape dropped from the masthead to the deck usually reads a little short. The upper point is the intersection of the centreline of the headstay with the forward face of the mast. If the headstay attaches on a tang or crane that stands proud of the spar, the measurer projects the stay’s line back to the mast surface.
Why I Matters to the Sailmaker
Headsail luff length is bounded by I. A jib hoisted to the top of the headstay can have a luff only slightly shorter than the distance from the stemhead fitting to the intersection point, less allowance for the halyard shackle, the tack fitting and the stretch of the luff under tension. On a furling boat the sailmaker also deducts the height of the drum and the halyard swivel, which together can eat half a metre of hoist. Ordering a genoa off the rule I without checking the real furler dimensions is one of the classic ways a new sail arrives too long.
Foretriangle area is nominally half of I multiplied by J, and rating rules use this area as the basis for headsail credits and penalties. A genoa’s overlap is expressed through LP as a percentage of J, but its area still scales with I, so a tall foretriangle gives a bigger headsail for the same overlap. In terms of sail shape, a taller, narrower foretriangle produces a higher-aspect headsail that points well and generates less induced drag, but it also needs a straighter headstay and, being taller, is more sensitive to twist. Lower, wider foretriangles suit heavier, more powerful cruising boats and are more forgiving to trim.
Cruising and Racing Perspectives
For the cruiser, I matters mostly at ordering time and when changing hardware. Fitting a furler, replacing a headstay with one of a different length, or adding a staysail stay all change the effective height available to a sail, and a quick re-measure saves a return trip to the loft. Cruisers with a furling genoa should also note the hoist actually usable with the furler’s upper stop, because a luff cut to the full I will not allow the halyard to tension properly.
For the racer, I is a rating variable. Under ORC and IRC the declared I feeds into the rated headsail area and, indirectly, the boat’s stability and rig assumptions. Under-declaring is a measurement infringement; over-declaring costs rating for area you cannot use. Racers with a masthead spinnaker halyard set above the headstay should declare ISP correctly and understand that a bigger gap between I and ISP allows a larger spinnaker without affecting the headsail measurement.
Frequently Asked Questions about I
Is I the same as the length of the headstay?
No. I is a vertical height; the headstay runs diagonally from the stemhead to the mast, so its length is roughly the hypotenuse of I and J. A headstay is always longer than I.
Why does my tape measure reading from the deck not match the rule I?
The rule takes the lower point at the sheer line abreast the mast, which on many boats is below deck level, and the upper point at the headstay intersection, not the top of the mast. Cabin trunks, bulwarks and mast collars all produce a difference of a few centimetres or more.
Does I change when I add a headsail furler?
The rule I does not change, because the headstay geometry is unchanged. The usable luff length does change, because the drum at the bottom and the swivel at the top occupy part of the hoist. Always give your sailmaker the furler make and the actual clearance.
I is one of the four numbers that turn a boat into a sail plan. Know your true I, understand how it was measured, and both your rating and your next headsail will fit the boat you actually sail.