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Measuring sails for replacement

| Sail & Rigging | November 26, 2016


When you need a new sail, there are lots of options in the used sails market. Measuring sails for replacement will give you the information you need to purchase replacement. You will need to know the dimensions of the sails that you have or the rig dimensions. Either one will work and here we show you how to come up with the dimensions you need. There are basically two ways to get the information, first to find out the measurements from data found in various places. The second is to measure the sails you have or measure the mast and rig of your boat.

When a yacht designer specifies the rig dimension he uses these dimensions.

I             The mast height above deck

J             Distance from bow to mast

P            The mast from the boom to the upper end of the sail track and

E            The end of the boom to the mast

These dimensions are then used by sail makers to design the boats sails.


Discovering the I, J, P and E for your boat.

Here are two tools that you can use to find out the rig dimensions for your boat. Select the boat manufacturer from the drop down menu to discover the dimensions




Measuring a headsailmeasuring a headsail

To figure out how big your headsail is or needs to be you need to measure two basic dimensions. They are the Luff and the LP or Luff Perpendicular. The headsail is a basic triangle and two dimensions is all that’s needed. There are other details but these dimensions define the size.

Here we look at measuring the sail you have and want to replace. If you don’t have an existing sail to measure you will need to measure the rig.

Measuring an existing sail

All three corners of the sail have grommets where you attach the headsail halyard, the tack fitting and the sheets.

Measurements are from these points. So if you already have a sail that you want to replace, measure the luff length between the head and tack grommets and the LP (luff perpendicular) from the clew to the luff perpendicular to it.

Measuring for maximum luff hoist

Measure for Hanked-on Headsails

If you attach a measuring tape to your halyard and hoist it as far as it will go, then take the bottom of the tape and measure to where the sail attaches at the bottom end. The tack fitting maybe on the stem fitting or on the deck or on a furler. In each case measure to the bearing of the tack fitting

This is the maximum length you can use. Make an allowance for measurement accuracy and stretch. You need space for the sail to stretch under halyard tension.headsail on roller furler

Measure for sail on roller furler

The red line is the distance from the head fitting on the top furler and the tack fitting on the bottom drum.

Attach a tape to the top swivel, just as the sail would be attached. Hoist the top swivel up the mast with the halyard. When the top swivel is at its upper most point, lock the halyard. With the tape measure down to the tack fitting on the bottom drum.


Measure the LP of a jib

To measure the LP you can tie the end of a tape measure to the headstay. A loop of line can be tied around the headstay and then attach the halyard to the loop along with the tape measure. Now you can hoist the tape up the headstay. Pull the tape measure to the turning block for the headsail. Hoist the tape measure up the headstay until it lies perpendicular to the headstay. See diagram 1.

Typically LP are designated as a percentage of the J measurement. This is what is called the overlap of the headsail i.e. how far the headsail overlaps the mast.

Often you will see 103% or 130% or 150%. The 150% is generally the largest LP.

Measuring a Mainsail

Mainsails are easier to measure as there are fixed measurements P and E, which are the luff length and the foot length. The diagram courtesy of ORC shows a typical mast and boom with black bands.

















Finding the sail you need

How to find the right size sail for your boat on http://www.usedboatequipment.com/

Type  “luff 36” into the search box and you will get the listings with luff lengths with 36 feet and x inches. Chose the sail that fits.

Used Boat Equipment sail search















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