Some FD Mast Details
A brief review
Jonathan Clapp
A brief review
Jonathan Clapp
THE FD MAST is measured up from the deck. Band 1 is placed with its upper edge at or below deck level. The upper edge of Band 2 is located between 500 and 800 mm above band 1. This is the lower limit of the main sail foot. The rule permits this to be below 800 mm, but I am not aware of any FDs carrying their mainsails lower than allowed. Everyone I have talked to about this agrees that higher is better, because there is better wind higher up. The lower edge of Band 3 is the maximum height of the head of the mainsail. The mainsail must therefor always be entirely between the bands. The actual height of the mast is not limited by the rule, just how high the sail can be hoisted.
Band 4 is measured up from deck level, and indicates how high the genoa can be. Again, I have found no examples where the genoa is rigged lower than the rule allows.
The rules do not define the mast below deck level except to say that the mast butt can not be moved while under way and must be forward of a line 3600 mm forward of the transom.
Rule 69 allows the bearing point of the spinnaker halyard to be placed anywhere within the gray triangle in the illustration, referenced to the lowed edge of band 4. This could be by use of a flush mounted sheave box, or an external block on a strap, or a crane of any description.
This rule came out in 1993 with the larger spinnaker. Most boats in use today using the large spinnaker have modified masts. The location of the spi halyard bearing point should also be at maximum allowed height, but this is not always the case, as opinions differed at the time as to the mechanical effects of flying such a large sail from such a limber mast with so many holes cut and drilled into it. On my mast the flush mounted sheave is located at 400 mm above band 4.
Of the boats surveyed, no two of them had the fitting in the same location.
Dutch practice is generally lower than US practice.
A number of people told me they had not moved the spiro-fitting when they moved the spi halyard sheave after 1993. These masts have widely different spiro-fitting locations which leads to the conclusion that there has never been agreement on the optimum location for the spinnaker pole end.
Maximum forward projection of the pole results from the pole being perpendicular to the mast. In point of fact a little geometry reveals that with a pole 2500 mm long the difference between the highest and lowest pole fitting results in only about 24 mm of difference in forward projection for any given clew height. Therefor I conclude (now that I’ve gone and drilled four new holes in my mast) that it is a moot point, as regards sail trim. Ease of operation should be the only compelling factor in locating the pole fitting location.
Thanks to Dennis van Iersel, who collected the Dutch data, and everyone else who contributed.
| 5 Dutch boats 700 640 630 600 530 |
8 US boats 830 800 770 750 720 700 540 300 |