When you request a quote for new sails, your sailmaker needs to understand the geometry of your rig in order to size the sails correctly. The way we do that is through a set of standard measurements that describe the key dimensions of your boat’s rigging. These measurements are referred to by the letters I, J, P, E, ISP, and SPL, and they come from the original designer’s specifications for your boat. For most production boats we already have these on file, and for others they’re usually easy to find. Either way, don’t worry about tracking down a tape measure just yet. Getting these numbers sorted is something we handle together as part of the quoting process, and the detailed measurement work that goes into actually building your sails comes later.
What the Letters Mean
I is the height of the foretriangle, measured from the deck up to where the headstay meets the mast. This measurement largely determines the luff length of your headsail.
J is the base of the foretriangle, measured horizontally from the front of the mast to where the forestay attaches to the deck. Combined with I, this tells your sailmaker how large a headsail your rig can carry.
P is the mainsail luff length, measured along the mast from the boom to the highest point the mainsail can be hoisted. This sets the height of your mainsail.
E is the mainsail foot length, measured along the boom from the mast to the end of the boom. Together with P, this defines the overall size of your mainsail.
ISP is the spinnaker hoist height, measured from the deck to where the spinnaker halyard exits the mast. This is used for sizing spinnakers and code zeros.
SPL is the spinnaker pole length, measured from the base of the mast to the tack point. On boats with a bowsprit, this is the length of the pole or prod. This measurement is used for sizing symmetric and asymmetric downwind sails.
How to Find Your Measurements
For most production boats, you may not need to do anything at all. We maintain a database of rig specifications for a wide range of production sailboats, so there’s a good chance we already have your numbers on file. If you’re getting a quote and you know your boat’s make, model, and year, that’s often all we need to get started.
If your boat isn’t in our database, sailboatdata.com is a great first stop. It carries rig specs for an enormous range of production boats and is usually accurate for standard configurations. Your owner’s manual or the original builder’s spec sheet are also reliable sources.
For custom builds, heavily modified rigs, or less common boats where specs aren’t readily available, don’t worry. We can help you track down a sail plan or walk you through getting the right numbers. We do this regularly and it’s not as complicated as it might sound. Our detailed measurement guide is a good place to start: [link to 180 Sails measurement guide].
These Numbers Are for Estimating, Not Building
It’s worth understanding what these measurements are actually used for at this stage. The I, J, P, E, ISP, and SPL dimensions come from the designer’s original specifications for the boat. They give us what we need to size your sails accurately and provide a solid quote. They are not the final measurements used to cut your sails.
Before we build anything, we go through a separate and more detailed measurement process specific to your boat. Even on well-known production boats, there can be variation between hulls, rigs get modified over the years, and equipment gets replaced. A replacement forestay fitted slightly differently, a new boom, a mast with a different sheave position, any of these can change things enough to matter. The building measurement process accounts for all of that and ensures the finished sail fits your boat properly, not just a boat of your model.
Don’t Get Hung Up on Getting It Perfect
If you’re in the early stages of exploring options and you’re not sure your measurements are exactly right, don’t let that stop you from reaching out. Approximate measurements are perfectly fine for getting a quote and starting the conversation. We’ll sort out the precise numbers before anything gets built.
In Article 4, we’ll get into the options that go into each sail, from reef points and battens on the mainsail to furling systems and headsail sizing.
