LP stands for luff perpendicular, the shortest distance from the clew of a headsail to its luff, measured at right angles to the luff. It is how sailmakers, rating rules and sailors describe headsail size as a percentage: a 150 percent genoa has an LP that is one and a half times the boat’s J measurement, the distance from the front of the mast to the headstay at deck level. Beginners need to know that “a 135” or “a 150” is a percentage of J, not an area or a length. The experienced sailor adds that LP determines overlap, sheeting position and how the sail behaves partly furled, which is why it is the first number UK Sailmakers asks for when a customer wants a new headsail.
How LP is measured
Lay the headsail flat, or hoist it and hold the clew where it would fly, and take a straight line from the clew to the luff so that it meets the luff at a right angle; the length of that line is the LP. It is not the foot length and not the distance from clew to tack, both of which are longer. Because it is a perpendicular, LP is a clean geometric property that does not depend on how the sail is set or how much luff curve it carries. Measurers under ORC and IRC record LP directly, and it appears on the boat’s certificate.
LP as a percentage of J
Dividing LP by J and multiplying by 100 gives the familiar percentage. A sail with LP equal to J is a 100 percent jib, with its clew roughly at the mast when sheeted hard. A 110 percent jib has a little overlap, a 135 percent genoa is a common all-round cruising size, and 150 to 155 percent was for decades the maximum allowed without penalty under the old IOR and PHRF rules, which is why so many boats still carry a “number one” of that size. Anything over 100 percent overlaps the mainsail, and it is the overlap that the percentage really describes.
Headsail area follows from LP: luff length times LP divided by two gives the triangular area. Because LP sets both area and overlap, going from 100 to 150 percent can add half again to the headsail area and moves the clew well aft, changing the sheeting angle and the sheet lead.
Why LP matters for shape, trim and rating
A big overlapping genoa creates a long slot between its leech and the leeward side of the mainsail, and in light air that slot accelerates the flow and adds power. In a breeze the same overlap becomes a liability: the sail is hard to drag through the foretriangle in a tack, backwinds the main and cannot be flattened as easily. Modern design has moved towards big mainsails and non-overlapping jibs of 100 to 110 percent that are easy to handle, with a Code Zero or reacher taking over when more area is wanted on a free leg.
Rating rules track LP because it drives headsail area. Under IRC and ORC the largest headsail’s LP is a measured input, and a bigger genoa raises the rating; under PHRF a boat is rated with a maximum LP percentage and penalised for exceeding it. Before ordering a new headsail a racing owner should check what the certificate allows, and any owner should look at the sheeting position: a longer LP needs its lead further aft, and if the track is too short the sail will never trim properly.
Choosing an LP for your sailing
- Light-air venues and masthead rigs: a 140 to 155 percent genoa still earns its keep where the mainsail is small relative to the foretriangle.
- Furling cruisers: a 130 to 135 percent genoa with a foam luff is the sensible compromise, because a partly furled sail loses shape quickly and a very long LP furls badly.
- Fractional rigs and modern designs: a 100 to 110 percent jib, often with vertical leech battens, gives most of the performance with none of the wrestling.
- Racing inventories: sailmakers name each headsail by LP – a 155 number one, a 135 number two, a 100 number three – so the crew knows which sail suits which wind range.
Frequently Asked Questions about LP
Is LP the same as the foot length of a genoa?
No. The foot runs from tack to clew along the bottom of the sail and is always longer. LP is the perpendicular from the clew to the luff, the shortest such line.
What does a 150 percent genoa mean?
Its LP is 150 percent of the boat’s J. For a boat with a J of 4.0 m, a 150 percent genoa has an LP of 6.0 m.
Does a longer LP always mean a faster boat?
Only in light air and only if it can be trimmed properly. In breeze a smaller LP is easier to flatten, tacks faster and interferes less with the main, and the rating penalty often outweighs the benefit.
LP is the sailmaker’s shorthand for how much headsail a boat carries. Understand it and you can talk about your sails precisely, read your rating certificate and choose an inventory that suits your rig and your waters. Know your J, know your LP percentages, and every conversation about a new genoa starts on solid ground.