International Measurement System: Sails

The International Measurement System (IMS) is a set of rules that define what and how is measured on the boat. It is used by the Offshore Racing Congress (ORC) as a measurement platform on its ORC Rating Systems (ORC International and ORC Club).

Except otherwise prescribed, the IMS uses the Equipment Rules of Sailing (ERS) nomenclature and definitions published every four years by World Sailing (WS), which is the world governing body for the sport of sailing, officially recognized by the International Olympic Committee (IOC).

1. Brief introduction to Offshore Racing Congress (ORC) handicap

The ORC has been the first body to develop rating rules around the central idea of a VPP. The VPP is a complex computer program that predicts a given boat’s speed potential in a given wind velocity. The ORC makes publicly available the calculation details of the ORC VPP, which means that designers and boat owners are able to design new yachts to maximize performance under the rule with a degree of certainty they don’t enjoy under other rules.

The current ORC handicap system is made up of three separate elements:

  • The IMS measurement procedure.
  • A Velocity Prediction Program, ORC Velocity Prediction Program (ORC VPP), which takes as input the IMS measurements.
  • A race management system which, based on the results of ORC VPP, derives various handicaps and obtains corrected times.

ORC International issues certificates for boats that are completely measured in accordance with the IMS and comply with the requirements of the IMS Rules and Regulations.

ORC Club certificates may be issued with less than complete IMS measurement where measurement data may be declared and/or obtained from other sources. This is due to the fact that the required measurements for ORC Club is reduced by the use of a system of calculated default measurements. This system makes it possible to calculate ORC Club handicaps using the same Velocity Prediction Program, but without the burden of a complex and expensive measurement procedure.

The Organizing Authority of any race or regatta will specify whether ORC International or ORC Club certificates are required for entry, but both certificate types can be mixed in any race, since they are fully compatible.

2. Types of sails

The ERS defines a sail as an item or equipment used to propel a boat, and in its Subsection A of Section G, it focuses on trilateral sales, which are sails with only three edges. In this article, we will only focus on this type of sails.

The ERS defines three types of trilateral sails:

  • Mainsail, which also applies to foremast sail and mizzen.
  • Headsail, which also applies to “jib” and “genoa“.
  • Spinnaker, which also applies to “gennaker“.

3. Sail edges, corners, and measurement points

The ERS defines the following edges and corners for trilateral sails:

– Edges:

  • Foot: the bottom edge of the sail.
  • Leech: the aft edge of the sail.
  • Luff: the fore edge of the sail.

– Corners:

  • Clew: the region where the foot and the leech meet.
  • Head: the region at the top.
  • Tack: the region where the luff and the foot meet.

The ERS also designates the following measurement points:

  • Clew Point: the intersection of the foot and the leech, each extended as necessary.
  • Tack Point: The intersection of the foot and the luff, each extended as necessary.
  • Head Point: in mainsails and headsails, it is the intersection of the luff, extended as necessary, and the line through the highest point of the sail at 90° to the luff. In spinnakers, it is the intersection of the luff and the leech, extended as necessary.
  • Aft Head Point (only for mainsails and headsails): the intersection of the leech extended as necessary and the line through the head point at 90° to the luff.

Other sail measurement points of interest specified are:

  • Half Luff Point: the point on the luff equidistant from the head point and the tack point.
  • Half Leech Point: the point on the leech equidistant from the head point and the clew point.
  • Mid Foot Point: the point on the foot equidistant from the tack point and the clew point.

4. Measuring sails

When measuring any sail, the sail must be:

  • completely dry;
  • on a flat surface;
  • with sufficient tension to remove any wrinkles;
  • at normal room temperature and humidity; and
  • without battens.

Permanent attached devices and extensions or straps must not be included.

When the corners are squared off or rounded, the actual measurement point is the intersection of the extended edges.

The ERS specifies, among others, the following primary sail dimensions:

  • Foot Length: the distance between the clew point and the tack point.
  • Leech Length: the distance between the head point and the clew point.
  • Luff Length: The distance between the head point and the tack point.

5. Mainsails

A mainsail is a sail with the luff attached to the mainmast.

The ERS defines the mainmast as:

  • The only mast in a una rig, sloop rig, or cutter rig.
  • The fore mast in a ketch rig or yawl rig.
  • The aft mast in a schooner rig.

5.1. Mainsail measurements

To physically obtain the points where to measure the distances:

  • Fold D1 over the Clew to get m.
  • Fold D1 over m to get u.
  • Fold D1 over u to get t.
  • Fold the Clew over m to get l.

The luff length and the foot length are not measured since P (distance from the mainsail hoist to the top of the boom), and E (distance between the outer point of the boom and the aft edge of the mast, measured with the boom on the mast centerplane at 90° to the mast) are measured on the spars.

The following measurements shall be taken:

MHBMainsail Top WidthThe distance between the head point and the
aft head point.
MUWMainsail Upper WidthThe shortest distance between the seven-eighths
leech point
and the luff.
MTWMainsail Three Quarter WidthThe shortest distance between the three-quarter
leech point
and the luff.
MHWMainsail Half WidthThe shortest distance between the half leech
point
and the luff.
MQWMainsail Quarter WidthThe shortest distance between the quarter
leech point
and the luff.

If any of the mainsail widths are either not measured following the IMS or are not measured at all, the ORC Rating Systems allow them to be taken as:

  • MHB = 0.05 x E
  • MUW = 0.25 x E
  • MTW = 0.41 x E
  • MHW = 0.66 x E
  • MQW = 0.85 x E

5.2. Mainsail corners

Head points, aft head points, tack points, and clew points on different mainsails.

5.3. Mainsail area

The IMS does not mention how to calculate sail areas. In fact, the way how the mainsail area is calculated is defined by the ORC Rating Systems and taken into consideration on the ORC VPP. It is based on the measurements of the widths specified by the IMS.

The ORC Rating Systems documentation defines the calculation of two different mainsail areas:

  • Measured area: it is calculated using a simplified trapeze formula, dividing the luff into amounts of 1/4, 1/2, 3/4, and 7/8.
  • Rated area: it is calculated using the actual heights on the luff from the tack point to the points where mainsail girths are measured. Mainsail rated area shall be the largest rated area of any mainsail in the sails inventory.

5.4. Roach

Real triangular sails would not have an aft head point, i.e., MHB = 0. However, when a sail has an aft head point, the upper side of the sail would not be triangular anymore. In this case, the sail is said to have a roach.

Nor the IMS or the ERS mention roach. However, this concept is considered in the ORC VPP Documentation and the ORC Rating systems (ORC International and ORC Club). The ORC VPP Documentation defines the sail roach as a ratio where the upper 3/4 area of a sail is compared to the area of a triangle with the same base and height as the upper 3/4 part of the sail considered. The result is also divided by a constant, which normalizes the roach measurement with the roach measured in wind tunnel. A roach value of zero corresponds to a mainsail with a triangular 3/4 upper part.

The value of the roach thus obtained is then considered by the ORC VPP for the calculation of the effective rig height.

6. Mizzen

mizzen is a sail with the luff attached to the mizzenmast.

The ERS defines the mizzenmast as the aft mast in a ketch rig or yawl rig.

6.1. Mizzen measurements

Unlike the mainsail, headsail, and spinnaker cases, the ERS does not define abbreviations for mizzen sail dimensions. However, similarly to a mainsail, the IMS describes them and establishes the following measures:

MHBYMizzen Top WidthThe distance between the head point and the
aft head point.
MUWYMizzen Upper WidthThe shortest distance between the seven-eighths
leech point
and the luff.
MTWYMizzen Three Quarter WidthThe shortest distance between the three-quarter
leech point
and the luff.
MHWYMizzen Half WidthThe shortest distance between the half leech
point
and the luff.
MQWYMizzen Quarter WidthThe shortest distance between the quarter
leech point
and the luff.

The procedure to obtain the different measurements points located on the leech is similar to the method used with mainsails.

If any of the mizzen sail widths are not measured, the ORC Rating Systems allow them to be calculated similarly to the case of the mainsail but with the corresponding mizzen measurements.

6.2. Mizzen area

The IMS does not mention how to calculate sail areas. However, in the ORC Rating Systems it is established that the mizzen rated area shall be calculated as for the mainsail with corresponding measurements.

7. Headsails

The ERS defines a headsail as a sail set forward of the mast, or of the foremast mast if there is more than one mast, where the measurement between the half luff point and the half leech point, known as mid width, is less than 75% of the foot length (if mid width length ≥ 75% of the foot length, the sail is considered a spinnaker).

Additionally, the IMS establishes that the distance between half foot point (or mid foot point in the ERS) and half luff point on the headsail shall be not greater than 55% of the headsail luff length (HLU).

7.1. Headsail measurements

The following measurements shall be taken for headsails:

HHBHeadsail Top WidthThe distance between the head point and the
aft head point.
HUWHeadsail Upper WidthThe shortest distance between the seven-eighths
leech point
and the luff.
HTWHeadsail Three Quarter WidthThe shortest distance between the three-quarter
leech point
and the luff.
HHWHeadsail Half WidthThe shortest distance between the half leech
point and the luff.
HQWHeadsail Quarter WidthThe shortest distance between the quarter
leech point
and the luff.
HLPHeadsail Luff PerpendicularThe shortest distance between the clew
point
and the luff.
HLUHeadsail Luff LengthThe distance between the head point and the tack point.

The folding process necessary to obtain the points from where to measure the distances HUW, HTW, HHW, and HQW is similar to that used in mainsails.

Additionally to the IMS, the ORC Rating Systems states that:

  • Headsails with a leech roach shall be completely measured.
  • Headsails without a leech roach, if any of its widths are not measured, it shall be taken as follows:
    – HHB = 0.020 x HLP
    – HUW = 0.125 x HLP + 0.875 x HHB
    – HTW = 0.250 x HLP + 0.750 x HHB
    – HHW = 0.500 x HLP + 0.500 x HHB
    – HQW = 0.750 x HLP + 0.250 x HHB

7.2. Headsail corners

Head points, aft head points, tack points, and clew points on different headsails.

7.3. Code zero sails (Code 0)

The ERS does not mention the concept of code zero. However, the IMS states that headsails with a mid width length of 55% or more (and less than 75%, since they are considered headsails) of the foot length were formerly known as “code 0“.

Besides, since 2014, the section formerly named “Code Zero” in the ORC VPP Documentation has been renamed to headsail set flying (see below).

7.4. Headsails Set Flying

Headsails may be set on the forestay or set flying. The IMS defines a sail Set Flying as a sail set with no sail edge attached to the forestay (the ERS definition is a sail with no sail edge attached to the rig).

Although the mid width length (SHW in spinnakers) and the foot length (SFL in spinnakers) measurements are generally not required for headsails, for headsails set flying, these measurements are needed to determine if the sail is a headsail (mid width < 75% of foot length) or an asymmetric spinnaker (mid width ≥ 75% of foot length).

Flying headsails are measured similarly to any other headsail:

The ORC VPP Documentation explains that there is a wide variety of flying headsails: they can be conceived for close reaching and upwind sailing or designed to give their maximum performance at wider angles. The documentation also states that the driver of the flying headsails performances with wind angle has been identified in the ratio HHW/HLP, ranging from 50% (jib-like performance) to 85% (spinnaker-like performance).

7.5. Jibs and Genoas

The term headsail also applies to “jib”, “genoa”, “code 0”, and “headsail set flying,” which are all now considered headsails.

However, in order to determine which aerodynamic coefficients must be used, the ORC VPP does split headsails into two different types:

  • jib or genoa; and
  • headsails set flying

The ORC VPP Documentation talks indistinctively about jib and genoas. There is no difference in the aerodynamic coefficients for jibs and genoas with or without battens, and both are considered headsails but not set flying.

7.6. Headsail area

As already stated, the IMS does not mention how to calculate sail areas. However, the way how to calculate the area is specified by the ORC Rating Systems.

Similarly to the case of mainsails, two types of areas are defined:

  • Headsail measured area.
  • Head sail rated area, which shall be the largest measured area for each of headsail set on the forestay and headsail set flying in the sails inventory.

8. Mizzen Staysail

The ERS only mentions that the tack of mizzen staysails shall be inboard the sheerline (the line formed by the intersection of the top of the deck and the outside of the hull shell). Other than that, there is no other reference to mizzen staysails in the ERS document.

8.1. Mizzen staysail measurements

The IMS states that mizzen staysails shall be three-cornered and also defines the following measurements:

YSHFMizzen Staysail Head to FootThe shortest distance that can be measured from head to foot.
YSHWMizzen Staysail Half WidthThe shortest distance between the half leech point and the luff.
YSFLMizzen Staysail Foot LengthThe distance between the clew point and the tack point.

8.2. Mizzen staysail area

The ORC Rating systems establishes how to calculate the mizzen staysail rated area based on the measured values of YSHF, YSHW, and YSFL.

9. Spinnakers

It is a sail set forward of the mast, or of the foremast mast if there is more than one mast, where the measurement between the half luff point and the half leech point is equal or greater than 75% of the foot length.

There are two types of spinnakers:

  • Symmetric: it is symmetric in shape, material, and cut, about a line joining the head to the center of the foot. It shall not have adjustable leech lines.
  • Asymmetric: any spinnaker not qualifying as symmetric.

The ORC VPP Documentation considers that symmetric spinnakers are tacked on a pole, while asymmetric ones can be tacked either on a pole or on the boat’s centerline. The ORC Rating Systems establishes that when the asymmetric spinnaker is tacked on the centerline, tack pennants of whatever length may be used.

The term spinnaker also applies to “gennaker“(which can be considered as an asymmetric spinnaker tacked on the boat’s centerline).

9.1. Spinnaker measurements

Symmetric spinnaker
Asymmetric spinnaker

The following measurements shall be taken on symmetric and asymmetric spinnakers:

SLUSpinnaker Luff LengthThe distance between the head point and the tack point.
SLESpinnaker Leech LengthThe distance between the head point and the clew point.
SHWSpinnaker Half WidthThe distance between the half luff point and the half leech
point
.
SFLSpinnaker Foot LengthThe distance between the clew point and the tack point.

9.2. Spinnaker corners

Head points, tack points, and clew points on spinnakers.

9.3. Spinnaker area

The ORC Rating System defines how to calculate the area of a spinnaker: measured and rated areas of symmetric and asymmetric spinnakers are calculated differently. Also, if there is no spinnaker measured, the boat will be rated with an asymmetric spinnaker which area is equal to 1.064 times the area of the largest headsail set on the forestay.

Do you want to read more articles like this?

More info:

Send us a Message

    Related Articles

    The evolution of hull and keel forms – Part 1

    Until the 19th century, the art of yacht design had been mainly influenced by tradition, economic and commercial motivations, understanding of materials and their availability, and genuine or semi-scientific developments in hydro and aerodynamics. Boats had been evolved through a long but reliable process of trial and error. But now, rating rules, which most of them had nothing to do with real scientific knowledge, started to play a significant role in hull design on both sides of the Atlantic.

    Responses