P is the rig dimension that defines the maximum hoist of the mainsail: the distance along the aft face of the mast from the top of the boom at the gooseneck to the highest point to which the head of the mainsail may be hoisted. On a boat measured for racing, both ends are marked with contrasting bands on the mast, and P is the distance between the lower edge of the upper band and the upper edge of the lower band. The beginner needs to know that P, together with E along the boom, is what a sailmaker uses to size a new mainsail. The experienced sailor adds that P is a rig measurement, not a sail measurement: the finished mainsail’s luff is cut a little shorter than P to allow for halyard tension and stretch, and a sail hoisted past the band is a rating breach on any measured boat.
How P Is Measured
Under the rating rules that grew out of IOR and IMS, and under ORC and IRC today, P is measured between the bands, which are placed by the owner or measurer. The lower band sits at the top of the boom with the boom in its normal sailing position on the gooseneck; the upper band sits below the masthead, usually a little under the halyard sheave so the sail can be fully tensioned without the headboard jamming. If the boat has no bands, hoist a tape on the main halyard until it two-blocks at the sheave, measure down to the top of the boom, and treat that as the absolute maximum rather than the target luff length.
For a purely triangular sail, mainsail area is half of P times E. Real mainsails have roach, so rating rules also measure the sail’s width at several heights up the leech to capture the area outside the straight line from head to clew.
Why P Matters to the Sailmaker
When a sailmaker designs a mainsail, P is the first number on the page. It sets the luff length, and with it the position of every batten pocket, the size of the headboard, the height of the reef points and the geometry of the luff curve. That curve is cut into the luff to match the mast’s bend under sailing loads: a mast that bends more needs more luff curve, and if the relationship is wrong the sail shows either a tight vertical crease along the luff or a slack, over-deep entry. Getting P right to within a few centimetres is essential; getting the mast-bend characteristics right is what separates a good mainsail from a great one.
The luff is designed shorter than P by an allowance that depends on construction. A woven Dacron® sail with a luff rope will stretch several centimetres over its life and needs more allowance; a laminated or load-path sail such as a Tape-Drive® or Titanium™ mainsail, with very little stretch along the luff, can be cut much closer to full hoist and will stay there. Either way, the head should reach the band under proper halyard tension, not before, and never beyond.
Related Rig Dimensions
- E: the mainsail foot length along the boom, from the aft face of the mast to the outer band.
- I: the height of the foretriangle, from the sheer line at the mast to where the headstay meets the mast.
- J: the base of the foretriangle, from the front face of the mast to the headstay at deck level.
- LP: the luff perpendicular of a headsail, used with J to state overlap.
Cruising and Racing Perspectives
For the cruising sailor, P and E matter chiefly when ordering a new mainsail or cover. The temptation is to read them from the builder’s brochure, but masts get shortened, booms replaced and goosenecks moved, so always measure the boat.
For the racer, P is a rule limit. The bands are inspected, the certificate states the dimensions, and a mainsail hoisted above the upper band or a boom below the lower band is a straightforward protest. Racing sailmakers design to the bands exactly and think hard about roach and girths, because every square centimetre of mainsail area is rated.
Frequently Asked Questions about P
Is P the same as the luff length of my mainsail?
No. P is the rig dimension between the bands; the mainsail luff is cut shorter, typically by a few centimetres, so the head reaches the upper band under proper halyard tension. A sail ordered with a luff equal to P will hit the band before it is tensioned.
How do I measure P myself?
Hoist a tape on the main halyard to the lower edge of the upper band, or to the highest point the head can reach, and measure down to the top of the boom at the gooseneck with the boom at its normal sailing height. Measure E at the same time.
P is a single number, but a great deal hangs on it: the size of the mainsail, the shape of its luff, the position of the battens and the boat’s standing under a rating rule. Measure it carefully, give it to your sailmaker with E and a note on mast bend, and the mainsail that comes back will fit the boat as if it grew there.