This article looks at the rig and the areas which you need to be aware of to check as necessary.
GENERAL SHAPE AND WEIGHT:
In the class rules (No 23) you will find all the specifications covering the section of the mast, but this will of course be down to the mast maker. There is a minimum weight of the mast in a stripped condition (no standing or running rigging, spreaders etc) and a minimum centre of gravity above the mast datum. If you buy a new mast, you should ask the builder to verify and certify this as once the mast is rigged it is a big job to strip rigging and some fittings including winches, gooseneck and spinnaker track car to check it.
MAST DATUM MARK:
An important mark which must be in place on the mast is the Datum Mark. This is usually best marked with a couple of centre punch marks or a permanent scribe mark.
The mast datum mark is set at 90mm above the sheerline (this is mentioned in several rules and measurement instructions). One way of establishing this mark is to place a straight edge across the boat on the foreside of the mast resting on two narrow blocks cut exactly to 90mm and placed at the extreme outer edge of the deck where the sheerline would be. This is sometimes difficult when washboards are in place or the deck is heavily cambered. Alternatively, the straight edge can be placed above the obstructions and then the distance down the sheerline measured and a calculation made to establish the correct position for the datum mark.
An alternative is with the mast out of the boat, to again place a straight edge across the boat at the fore edge of the mast position either on the deck or above any obstructions. Measure from the straight edge to the mast step and measure the height of the straight edge above the sheerline, calculate the difference and transfer the measurement to the mast.
Many mast makers will leave the section over length so this is an important measurement to check as often a small amount will need to be cut from the tube for the mast to measure.
The mast datum mark position is also important in that it relates to the rig height and the “I” measurement. (Rules 13 & 15).
“I” MEASUREMENT:
This controls the height of the foretriangle and is measured above the mast datum point. It is explained in Class Measurement Instruction 27 (M27). The maximum height to the intersection of the mast and forestay or spinnaker halyard whichever is higher is 9.750m above mast datum. From this you can see it is important that the datum point is correct.
While we are in the foretriangle, we should also check the “J” dimension.
“J” AND SPINNAKER POLE:
“J” is measured from the foreside of the mast (in its most aft position) to where the aft side of the forestay cuts the deck. This dimension is part of the rating calculation and must match the figure on the boats current certificate.
It relates directly to the spinnaker pole. The spinnaker pole is measured from the foreside of the mast to the outer end pulled out. (Details in M27). It should not be longer then “J”, or the longer figure will be used in the rating calculation. If a new spinnaker pole has been ordered at the “J” figure given on the rating certificate it will be to long and will need to be cut down to include the fittings on the mast.
“A” MEASUREMENT:
This is simple to check when the mast is out of the boat. It is the distance between the upper edge of the band at the top of the boom position to the underside of the band at mast head. This must match the dimension on the rating certificate.
Finally, we can look at the boom.
BOOM AND “B” MEASUREMENT:
The boom dimensions are covered in class rule 24 and this should be read for details, but generally the tube and sail track but without fittings must pass through a 137mm diameter circle.
The “B” measurement is again part of the rating calculation. It is measured (M27) along the top of the boom from the after side of the mast (excluding track or jackstay) to the fore edge of the band. An important point to note is that if the mast has an integral sail grove, the measurement is taken to the foreside of the grove. This may involve measuring this depth separately.
RATING CALCULATIONS:
Although mentioned above, please remember that “I”, “J”, “A” and “B” dimensions form part of the calculations for sail area in the rating certificate and these figures must exactly match those on the boat itself.
D. Chivers
26
th
March 2019
