Your sailmaker should provide a measurement form and explain the tension, endpoints and hardware details required. If any value is ambiguous, photograph the endpoint and mark it on the form.
Before measuring a yacht for new sails
- Record make, model, year, hull number and rig type
- Note any re-rigging, mast, boom, furler or deck changes
- Photograph masthead, gooseneck, tack, clew, tracks and furling drum
- Identify halyards and verify they run freely
- Use a non-stretch measuring tape and a reliable return line
- Record conditions and whether the rig was at normal sailing tension
Core rig dimensions
| Code | Meaning | Why it matters |
|---|---|---|
| I | Height of foretriangle from deck reference to forestay termination | Headsail luff and area envelope |
| J | Horizontal foretriangle base from mast to forestay | Headsail overlap and LP relationship |
| P | Mainsail luff limit or available hoist | Main luff and head clearance |
| E | Mainsail foot limit or usable boom length | Main foot and clew position |
| LP | Headsail luff-perpendicular dimension | Defines overlap percentage against J |
These dimensions describe the rig envelope, not necessarily the finished sail. The sail needs allowances for headboard, tack hardware, shackles, furling swivels, stretch, mast bend and safe clearance.
Mainsail measurements and interfaces
Hoist and luff
Measure between the working halyard attachment and tack bearing point with the intended hardware in place. Confirm headboard clearance below sheaves, backstay and masthead instruments. Record mast bend and whether the sailmaker expects a straight-line or measured-curve luff.
Foot and clew
Measure the usable outhaul range, not only boom length. Identify whether the foot is loose or captured in a groove. Photograph the gooseneck, tack pin, reef horns, clew car or outhaul shackle and any stack-pack interference.
Luff attachment and battens
Slides and cars need manufacturer, model, spacing and dimensions. If the existing system binds under load, solve the track problem before copying it into a new sail. Full battens introduce compression at the luff; the receptacle and car must handle it.
Reefs
Define reef depth by the desired remaining sail area and practical boom geometry. Check that reef tack and clew hardware pull in useful directions, that the sail clears fittings and that reef lines can be led without crushing cloth.
Headsail measurements and interfaces
Record maximum hoist with the furler’s top swivel and tack hardware installed. Measure drum height, foil length, foil groove, feeder position and tack offset. At the clew, document track length and position, car range, shroud clearance, pulpit contact and desired visibility.
For overlapping sails, LP percentage equals LP divided by J, multiplied by 100. But percentage alone does not define shape or sheeting. Two 120% genoas can have different clew heights and leech profiles, producing very different handling and visibility.
Reaching and downwind sails
Measure tack location, bowsprit projection, halyard exit, masthead clearance, sheet leads and furler compatibility. Confirm whether the sprit is rated for close-reaching luff loads. Record the distance between tack and halyard under realistic rig tension.
How to verify yacht sail measurements
- Measure each critical dimension twice, preferably by two people.
- Compare with the existing sail, class data or design plan only as a reasonableness check.
- Mark units clearly and keep one system throughout the form.
- Send annotated photographs of every endpoint and fitting.
- Ask the sailmaker to return a final dimension schedule for approval.
- At commissioning, hoist in light-to-moderate air and check all clearances before loading fully.
Yacht sail measurements to send the sailmaker
- Rig tension and backstay state during measurement
- Furler make, model and foil groove
- Mast and boom track details
- Existing sail problems you do not want copied
- Photos showing tape endpoints
- Preferred reef, clew and visibility outcomes
Yacht sail measurement FAQs
Can I order new sails from my boat model?
Model dimensions are only a reference. Rig changes, rake, furlers, tracks and hardware alter fit, so the actual boat should be measured before cutting.
Should the old sail be measured?
An old sail helps identify hardware and problems, but stretched edge lengths should not become the new design automatically. Measure the rig and available space as the controlling reference.
Who is responsible for sail measurements?
Agree responsibility in writing. A local loft may measure the boat; remote suppliers should provide a precise protocol and confirm every critical dimension before production.
Reviewed 21 September 2026