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Calculating your Sail Areas

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Lots of equipment onboard is load dependant and apart from length and ballast of the boat the primary driver is sail area. It is therefore unsurprising that a lot of hardware manufacturers use sail areas as a primary specification criterion.

This blog gives you the key rig and sail dimensions plus the formulas you need to calculate your sail areas.

Key Rig data


rig data

Source: North sails

I – Height of Foretriangle
Elevation of Forestay, measured down to elevation of main shrouds at sheer line.

J – Base of Foretriangle
Horizontal distance measured from front face of mast at deck to position of headstay at sheer line.

P – Mainsail Hoist
Elevation of upper mast band or maximum main halyard position, measured down to lower mast band or top of boom.

E- Mainsail Foot
Horizontal distance measured from aft face of mast at top of boom to boom band or maximum outhaul position.

Is – Height of Inner Foretriangle
Elevation of Forestay, measured down to elevation of main shrouds at sheer line.

Js – Base of Inner Foretriangle
Horizontal distance measured from front face of mast at deck to position of inner headstay at sheer line.

Py – Mizzen Mainsail Hoist
Elevation of upper mast band or maximum main halyard position, measured down to lower mast band or top of boom.

Ey – Mizzen Mainsail Foot
Horizontal distance measured from aft face of mizzen mast at top of boom to boom band or maximum outhaul position.

ISP – Elevation of Spinnaker Halyard
Measured down to elevation of main shrouds at sheer line.

SPL – Spinnaker Pole Length

STL – Spinnaker Tack Length
Horizontal distance measured from front face of mast at deck, forward and horizontally to position of spinnaker tack attachment point.

 

Key Sail measurement data

 

sail measurement data

 

 

 

 

 

  • MHB – Main Head Board width
  • MUW – Main Upper Width at 7/8 height
  • MTW – Main Third Width at 3/4 height
  • MHW – main Half Width at 1/2 height
  • MQW – Main Quarter Width at 1/4 height

 

 
 
 
 
 
 
 
 
 
 
 
 
head sail

 

 

 

  • HUW – Headsail Upper Width at 7/8 height
  • HTW – Headsail Third Width at 3/4 height
  • HHW – Headsail Half Width at 1/2 height
  • LL – Luff Length
  • LP – Luff Perpendicular (shortest distance from clew to luff)

 

 

 

 

 

 

symetric spinnaker

 

 

  • SLE – Spinnaker Leech*
  • SLU – Spinnaker Luff*
  • *N.B. both the same on a symmetric spinnaker
  • SHW – Spinnaker Half Width (distance from mid-point SLU to mid-point SLE)
  • SF – Spinnaker Foot

 

 
 
 
 
 
 
 
 
 
 
assymetric spinaker

 

 

 

 

  • SLE – Spinnaker Leech
  • SLU – Spinnaker Luff
  • SHW – Spinnaker Half Width (distance from mid-point SLU to mid-point SLE)
  • SF – Spinnaker Foot

 

 

 

 

Basic Sail Area calculations

Mainsail

P x E / 2 (no roach)

P x E x 0.585 (with roach)

 

Mizzen

Py x Ey / 2 (no roach)

Py x Ey x 0.585 (with roach)

 

Jib / Genoa

(I X J) / 2 x LP%

Example LP%: Full size genoa 1 – 150%, genoa 2 - 130%, genoa 3 – 105%, working jib – 85-95%

Spinnaker

(SL x (SHW + SF) / 2) x 0,95

 

More detailed area calculations

Sail area is a big thing for rating certificates. It is a hot area for debate with different rating organisations having their own small variations. However, despite their subtle differences they have similar principles and listed below are the Rated sail areas for the Offshore Rating Congress (ORC) (source ORC Rating Systems 2019):

Mainsail

Mainsail measured area is calculated by the simplified trapeze formula above, dividing the luff in amounts of 1/4, 1/2, 3/4 and 7/8.

Area  = 
P / 8
 
  (E+2·MQW+2·MHW+1.5·MTW+MUW+0.5·MHB)

 

Headsail

Where HLU = Headsail Luff Length, HLP = Headsail perpendicular (LP) and HHB = Headsail head width

Area  =0.1225·HLU·(1.445·HLP+2·HQW+2·HHW+1.5·HTW+HUW+0.5·HHB)

 

Symmetrical spinnaker

Area    =
SLU·(SFL+4·SHW) / 6

 

Asymmetric spinnaker

First you need to calculate Average Spinnaker Luff (ASL)

ASL =
SLU+SLE / 2

Followed by the area

Area  =
  ASL·(SFL+4·SHW) / 6

 

Summary

Basic main and headsail sail areas can be calculated from simple rig data whereas for a more accurate sail area calculations you need detailed measurements of your sail which you can either measure yourself or ask your sailmaker to supply.

 

If you have any questions about sail areas or the implications for specifying any of your sailing hardware or rigging please contact us at support@upffront.com

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