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Technical Manual

Mast And Boom Band

by Don Andrews

While you are setting up your boat for the coming season, may I suggest that you take a few minutes to check your mast and boom bands ?

My suggestion is based on somewhat bitter experience. Of the boats presented at the 1980 Nationals, I would wager that more than half were out of class, with bands that were either inaccurate or missing altogether. Some required major surgery to bring them within the rules and some of the major surgery, obviously performed in less than ideal circumstances, led to gear failure later in the week.


There are three measurements to be considered:

  1. A mast band with its top edge 265-275mm above the deck and not above the top edge of the boom when at right angles to the mast.
  2. A mast band with its bottom edge 5400mm maximum above the top edge of the bottom band.
  3. A boom band with its forward edge not more than 2680mm from the aft edge of the mast.

Now, let us look a little more closely at what this simple set of rules implies and why they cause such trouble.

  1. The bands themselves must be of a contrasting colour, not less than lOmm wide and clearly discernable when racing. The word 'paint' does not appear in the rules, though painted bands are more foolproof than sticky tape. They do chip off and need renewing each season.
  2. The deck above which the top edge of the lower band in measured, is a projection of the curve of the deck itself from sheerline to sheerline at the point at which the mast emerges from the hull. This makes for obvious measurement difficulties for two reasons:
    1. A flush fitting deck step is often horizontal plateau slightly below the projected curve.
    2. Superimposed deck steps, common on older boats, have to be discounted.
  3. An additional, and equally important, constraint on the location of the lower band, when the boom is at right angles to the mast, is that its top edge must not be below the top edge of the lower band, or be capable of being pulled below.
  4. The 'aft edge of the mast' means exactly what it says, and includes the sail track. On many metal masts, the track is rivetted onto the mast after the tube has been drawn and starts about 3OOmm above the boom. When measuring for the boom band, remember to make allowance for the track (usually l5mm).

The whole point of these particular measurements is to define very exactly, the position of the sail on the mast and boom, and of the whole rig in relation to the hull. The sail may not go above the lower edge of the 5400mm band, or below the top edge of the 265- 275mm lower band, nor beyond the forward edge of the 2680mm band. Furthermore, the whole rig must be set up within a l0mm tolerance above the deck at the mast step.

So, how to set it up?

  1. Lay mast and boom on the floor and with a set squaree and pencil, mark the projection of the top edge of the boom on the mast. This is the optimum position for the top edge of the lower band. You can mark it lower than your pencil mark, but remember that your upper band is measured from your lower band.
  2. Set up your mast in your boat and taking into account the definition of 'deck' given earlier, check that your pencil mark is within the 265-275mm range above the deck. MEASURE ALONG THE AFT EDGE OF THE MAST, regardless of rake. Do not drop a perpendicular from the mark to the deck, but MEASURE ALONG THE AFT EDGE OF THE MAST, regardless of rake.

    If necessary, engage in surgery at this stage! Chock the mast up or shorten it, but do make sure that your bearing ring can do it's job in a seaman-like manner. There must be more stress on this particular part of the boat than any other.

    The job is made more tricky by the fact that many boom brackets and bearing rings are now rivetted permanently to the mast and are set at 270mm. This is ideal if your boat is decked in the modern manner with a flush deck bearing, but not so ideal if it is not.
  3. Having got the top of the lower band to obey both rules:
    1. 265-275mm above the deck; and
    2. not above the top edge of the boom,

    you are in business. Take the mast out, lay it on the ground, and mark the upper band 5400mm above the lower one. Paint in both bands a contrasting colour, not less than l0mm wide.
  4. Fix the boom on the mast at right angles to it. Allowing for the sail track if it projects, mark the 2680mm point onto the boom for the forward edge of the boom band, and paint in as above. Fix a stop to ensure that the sail cannot be pulled beyond the band.

All easy enough with a little care and patience. But let me finally introduce you to paragraph 10 of the Manual of Measurement:
"It is not the measurers function to mark or paint measurement bands."

So be warned.



Measurers have more trouble with these measurements than any other. Perhaps this article will help OWNERS, whose responsibility it is, to get them right.

With Carbon masts, the definitions changed, so refer to the Class Rules, not to this article!