Mast datum point is at the top point of the section at the foreside of the mast
Rigging point is attachment of the forestay to the mast, or
the intersection of the
extended forestay with the mast structure.
Place the cursor over the photo or drawing for enlargement.
Mast datum point and Foretriangle base
BAS and J with SFJ
measurements are defining vertical and longitudinal position of the
mast and sail plan relative to the hull. BAS is the height
from the mast datum point to the boom or lower black band on the
mast. It is used also as a reference point for the halyards hoist
measurements on the mast. J is probably the best-known measurement on
boat, since it is used to define the size of headsails. It is
defined as the horizontal distance between the front side of the mast
and the intersection of the forestay with the deck. SFJ then
determines the distance of the forward point of J to
the actual foremost point on the hull.
P is distance between the black
bands and the mast that are limiting the position of the
mainsail luff - while racing the mainsail luff must be between these
bands. In the absence of the upper black band, P is
measured to the upper part of the main halyard sheave. IG is
defined as the height of the foretriangle and is the vertical distance from the
mast datum point (at the sheerline at the mast) to the forestay attachment
to the mast. Similar to IG, ISP is the
spinnaker halyard height and is the maximum height that the head of
or headsail set flying can be hoisted.
Mast profiles are measured for maximum
dimensions above 0.5 * P transversely (MDT1)
and longitudinally (MDL1) as well as for the
minimum dimensions transversely (MDT2) and
longitudinally (MDTL2). Taper length TL
is the vertical distance from the upper black band to the point where maximum mast profile dimensions
occurs. Thus, TL is 0 if mast is not
If there is a structure element of forestay attachment to
the mast, then its distance from the aft side of the mast is
measured as GO. Mast width MW
at that height provides elements for the calculation of the
actual intersection of the forestay with the front side of
Mast weight (MWT) and mast centre of gravity (MCG) are
important factor determining a boat's stability. It can be
measured by weighing the mast at a single point and finding
the actual centre of gravity, or by two separate weight
measurements at the same points from which total weight and
centre of gravity position are calculated.
The mast is weighed with: forestay, backstay, spreaders,
jumpers, shrouds, chain plates, runners, lights, antennae,
wiring and other permanently fitted components all slack
stretched down and secured with light material at the bottom
of the mast. Excluded are: checkstays, halyards, boom van
and reefing lines.
If the mast is not weighed, then lightweight defaults for aluminum or
carbon mast are used. Actual mast weight measurements will
therefore always give a more favorable rating.
Two points mast
measurement with formulas for MWT and MCG calculations
E is the measured distance from the
aft side of the mast to the black band on the boom that
the position to which mainsail foot may be extended. In the
absence of the black band, it shall be measured to the point
on the boom as aft as the mainsail clew can reach.
Boom diameter BD is measured as a vertical cross
section, and if not measured, its default is set to 0.06 *
E. If BD
measurement exceeds the default, the mainsail rated area is
BAL is distance from the black band on the
boom to the actual end of the boom.
Spinnaker pole and sprit
Spinnaker pole length (SPL) the horizontal distance from the forward face of the mast spar, ignoring any fittings and tracks, measured on or near the centerline of the boat, to the extremity of the spinnaker pole. .
The tack point of spinnaker (TPS) when it is tacked on the
boat's centerline is the distance from the foreside of the mast to the
foremost point on which the asymmetric spinnaker or any headsail set
flying can be tacked. If the
bowsprit is retractable TPS shall be measured with it in
its fully-extended position.
Other rig details
In addition to the measurements set above, the following rig details are
Inner stay, which may be adjustable or fixed
Foresetay tension, which may be controlled by
Forestay, Backstay, Both or be Fixed.
Number of spreaders
Number of runners
Carbon mast and fiber rigging
Use of non-manual power for adjusting running
rigging or spars