## Measuring for a Mainsail

1. Maximum luff length (P dimension) - Measure by attaching the tape to the halyard shackle and pulling it to the top of the mast until it stops. Secure the halyard and measure to the bearing surface of the tack pin. If your mast has black bands, raise the tape until it is just at the lower edge of the band. You will probably have to site the position of the tape from off the boat. Be sure to note that this is a "black band distance" when submitting the data to your sailmaker.

2. Maximum leech length - Set your boom at the height and angle you like to have it when the boat is sailing, and using the tape still hoisted to the top of the mast, measure to the bearing surface of the outhaul car with the outhaul fully extended. Be sure to consider any other features on the boat like boom gallows or bimini to make sure that the boom clears them at its lowest point.

3. Maximum foot length (E dimension) - Measure from the bearing surface of the tack pin to the bearing surface of the outhaul car with the outhaul fully extended. If there are black bands on the boom measure from the back of the mast to the closest edge of the black band, and be sure to note this when submitting the data.

4. Distance to the backstay - With the boom horizontal, measure from the back of the mast to the backstay. This will allow the sailmaker to figure out how much roach he can add to the sail.

5. Mast rake details - If you have a retriever line attached to the mainsail halyard, tie a plum bob (or any weight) to the end of it and let the line hang strait down to the deck. Measure the distance from the back of the mast to the plum bob. This is the amount of rake in the mast.

6. Mast bend details - Take the retriever line and attach it to the gooseneck at the back of the mast. Now sight up the mast. You should be able to estimate how many "mast widths" there are between the line and the back of the mast at its widest point. Measure the fore and aft dimension of the mast and calculate the maximum mast bend. Now, leaving the line attached, walk away from the boat and site the mast bend. Estimate where on the mast the maximum bend falls. This will allow your sailmaker to design the luff of the mainsail to match the prebend curve of the mast. Note that you might be able to manipulate this bend with a hydraulic backstay and/or running backstays, however that should not be taken into account at this point. The sailmaker needs the "at-rest" bend.

7. Spreader heights - For lower spreaders, toss a line over the spreader and mark where both ends meet the deck. Measure the line and divide it by half to get the spreader height. Once you have the first, and/or maybe second spreader, you can then eyeball the mast from a distance (using a pencil at arm's length if necessary) and get the best estimate of the height of the rest of the spreaders off the deck.

8. Spreader widths - If you know the distance from the base of the mast to the edge of the deck, estimate the spreader lengths by sighting them from afar.

9. Spreader angles - Match the shroud base with the base of the mast and extrapolate it up the rig. These spreader dimensions are important to the sail-maker. If your mainsail is to have full-length battens, the sail designers must ensure that he does not design the sail with a batten that falls right at the spreader tip, both when fully hoisted and when reefed. By noting the spreader angle and length the sail designer will be able to reinforce the sail with spreader patches, although it's often best to wait until the sail is on the mast before marking for spreader patches.

10. Mast crane - Using the width of the mast as a gauge, estimate the distance the masthead crane extends beyond the back of the mast. This will let the sail-maker know what size of headboard he can have on the sail.

11. Outhaul track - Measure the length of your outhaul track.

12. Outhaul attachment - Measure the jaw width of the shackle on the outhaul car if there is one. This way the sailmaker can be sure that the shackle at the clew fits over the ring in the sail.

13. Clew set-up - Measure from the top of the boom to the bearing surface of the pin of the outhaul car. This is only necessary if the foot of the sail goes into a bolt rope slot or track on the boom; if the sail is loose footed, the setup does not make a difference.

14. Tack set-back - Measure from the back of the mast to the bearing surface of the tack pin. This will allow the sailmaker to set back the tack of the sail below the first slide or bolt rope slot so that there will not be any wrinkles in the corner of the sail.

15. Tack set-up - Measure from the top of the boom to the bearing surface of the tack pin. This is only necessary if the foot of the sail goes into a bolt rope slot or track on the boom. If the sail is loose footed, the set-up does not make a difference.

16. Reef hook set-back - Measure from the back of the mast to the bearing surface of the reef hook. This will allow the sailmaker to ensure that the reef point is located in the correct place in the sail so that there will not be any wrinkles out of the clew of the sail when it is reefed.

17. Reef hook set-up - Measure from the top of the boom to the bearing surface of the reef hook. This will ensure that the boom is at the correct angle when the sail is reefed.

18. Feeder height - Measure from the top of the boom to the luff groove exit or the slide stop.

19. Boom track details (front end) - Measure from the back of the mast to the front of the bolt rope groove or track on the boom. This will ensure that the slides or bolt rope fit the boom.

X If outhaul car - From bearing point of the pin on outhaul car to the top of the boom Y If outhaul car on separate track - Place car in farthest aft position and measurer to aft end of sail track/groove. Z Width of shackle jaw or clew attachment mechanism mmi

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### Responses

• aatifa
How to measure a mainsail for sail boats?
2 years ago
• grant miller
Is the P measurement luff to the centre of the sheave pin?
11 months ago
• Sarah
HOW TO CALCULATE THE DISTANCE BETWEEN SAIL SALIDES ON MAINSAIL?
6 months ago