The foundation of a perfect fit. We start from your boat record, then confirm the measurements that actually decide the cut.
At SurgeSails, we do not start from a blank quote form. We start from a boat record and then confirm the measurements that matter for fit. While our boat database contains factory specifications for thousands of production yachts, every boat is slightly different. Masts get modified, booms get replaced, and roller furlers change the dynamic length of forestays.
To ensure your custom sail fits correctly, taking accurate measurements of your “I, J, P, and E” dimensions is the most critical step in the configuration process.
1. What Are The “Big Four” Dimensions?
These four letters represent the foundational geometry of your sailboat’s rig.
- I (Foretriangle Height): The vertical distance from the highest point on the mast where the headsail halyard is attached, straight down to the sheer line (the deck) abreast of the mast.
- J (Foretriangle Base): The horizontal distance from the front face of the mast, straight forward to the point where the forestay attaches to the deck (or bowsprit).
- P (Mainsail Hoist): The vertical distance along the aft face of the mast, starting from the top of the boom, up to the highest point the mainsail can be hoisted (usually marked by a black band).
- E (Mainsail Foot): The horizontal distance along the top of the boom, starting from the aft face of the mast, back to the furthest point the mainsail can be outhauled (usually marked by a black band).
2. How to Measure: Step-by-Step
You will need a long fiberglass tape measure (at least 100 ft / 30m), a notepad, someone to assist you, and a calm day at the marina.
Measuring ‘I’
- Attach the tape measure securely to your Genoa halyard shackle.
- Hoist the halyard all the way to the top until it stops.
- Measure straight down to the deck alongside the mast. Do not measure to the cabin top if it is raised; you must project the sheer line horizontally to the mast.
Measuring ‘J’
- Run the tape measure from the front face of the mast horizontally forward to the very base of the forestay pin on the deck.
- If you have a bowsprit and the forestay attaches further forward, measure to that point.
Measuring ‘P’
- Attach the tape measure to your Mainsail halyard shackle.
- Hoist it to the highest point possible, or to the lower edge of the upper black band on the mast.
- Measure straight down to the top edge of the boom, or the lower black band on the mast just above the gooseneck.
Measuring ‘E’
- Hook the tape measure on the aft face of the mast, right inside the gooseneck fitting.
- Run the tape straight back along the top of the boom to the forward edge of the black band at the end of the boom.
3. The Roller Furler Offset (For Headsails)
If you are replacing a Genoa or Jib that utilizes a roller furling system (e.g., Harken, Profurl, Furlex), you cannot simply use the ‘I’ measurement to determine the Luff length.
You must also measure the Max Hoist (Pin-to-Pin):
- Attach your tape measure to the upper halyard swivel shackle (the part that goes up).
- Hoist the swivel all the way to the top until it stops just below the halyard deflector.
- Measure down to the lower tack shackle on the furling drum (where the bottom corner of the sail attaches).
- This is your Max Hoist. We will subtract a few inches from this measurement to ensure your new sail stretches properly when tension is applied.
SurgeSails Custom Advisor: Are you unsure about climbing the mast or relying on your own tape measure? When you configure a sail, start from the boat record and confirm anything that may have changed on your own rig. The sailmaker review uses those notes and measurements before production.