Gaff rig comparisons

Halda- an early gaff cutter (Finesse sail number 7).  The shrouds are attached to U bolts through the beam shelves and there do not appear to be any backstay runners- the mainsail is up against the aft starboard shroud. Reproduced in Nick Ardley’s The Jottings of a Thames Estuary Ditch-Crawler

A gaff cutter rig was an option on a new Finesse. However, the majority of F24 gaff cutters sailing today are conversions. Since there are no class rules to dictate (or recommend) the rig dimensions, designs can differ significantly. It is also possible to alter any of the individual sails as opportunities arise. The gaff mainsail in particular can vary in the lengths of the luff, leech and head (the foot is pretty constant, as the boom does not protrude aft of the transom) and the gaff angle can also be chosen. I thought that it would be interesting to compare those rig designs that I had access to.

I have had to make a number of assumptions along the way and I am happy to hear of corrections and / or observations.

Gaff rig design for Finesse 24 by Maurice Griffiths (1971). Reproduced in Nick Ardley’s The Jottings of a Thames Estuary Ditch-Crawler on page 53.

Maurice Griffiths drew this gaff rig sail plan in 1971 for a Finesse 24, possibly number 6. The tall mainsail was, at 205 sq ft, 79% of the total area of usual Bermudan sloop rig. The mast is drawn on this original plan with a 2 degree rake aft. The rigs that follow are drawn over the outline of this, with the mast rake removed:

Maurice Griffiths rig re-drawn. The scale on the left is height above the water. Numbers on the sails represent the areas in square feet. Note that the mainsail is shown with a convex leech, but the alternative of hollow curve can look very dashing and requires no battens. The foot of the foresail is drawn to allow for a boom pivoting on the forestay. Aspect ratio of mainsail = 3.0.

(The CLR mark is indicative only as it depends on many factors- fore-aft trim, centre plate, bilge keels etc.) The method used for the determination of the centre of effort of the gaff mainsail is shown later on. To calculate the centre for the whole rig, the overlap of the foresails is ignored.

The Finesse 24 Bermudan sloop rig, shown below over the outline of the Griffith drawing, has the area of 259 sq ft as quoted in magazine articles and used in Alan Platt’s original brochure. A roller furling genoa is implied. This pair of sails has a centre of effort further forward than the Griffiths drawing. It is clear that the un-reefed gaff mainsail could only achieve the same balance as the Bermudan by the use of the cutter’s jib.

Bermudan sloop sketched over the Griffiths drawing.  Aspect ratio of mainsail = 3.9. Centre of effort shown in blue.

The Griffiths sketch shows a boomed foresail, with a pivot on the forestay. This is a  practical design, but it does preclude the use of a genoa. It is interesting that the two large rigs presented below both use an overlapping foresail, whereas the two smaller rigs use a boomed sail. Unlike the Griffiths design, these pivot on the Sampson post, and in this arrangement the loose-footed foresail becomes more full as it is let out.

Large rigs: Mariette (dotted lines, sail plan in 2013) and Serinette (solid lines, sail plan in 2016). Mariette’s mainsail aspect ratio = 2.6; Serinette = 2.9. (Serinette’s sail plan was drawn up by Mark Butler of James Lawrence, Brightlingsea)

The centre of effort of the two larger rigs, on Mariette and Serinette, are a little aft of the Griffiths design. Mariette has the more relaxed gaff angle of the two and carries a slightly larger mainsail, balanced out by a slightly larger jib. The two designs are otherwise similar.

Blue Bonnet and Lady Christina have much more modest mainsails, resulting in centres of effort that are both lower and further forward. Blue Bonnet has a steeper gaff angle and a lower, more conventional boom. The higher boom of Lady Christina avoids the crews’ heads and stays well clear of the water on a leeward roll. However, this is less of an advantage than it might be as she uses fixed rear shrouds rather than backstay runners, and this limits how far the boom can be let out to the lee. The picture of Halda (Finesse 24 number 7) shows the same arrangement.

Blue Bonnet (previously Quo Vardis) with sail plan estimated from information supplied in 2021.  Aspect ratio of mainsail = 2.5. Centre of effort in blue.

The smaller mainsails have another advantage- if the gaff if suddenly dropped then the mainsail doesn’t end up in the water.

Lady Christina sail plan 2021. Aspect ratio = 2.2. Centre of effort shown in blue.

Finally, a recommendation from John Leather in The Gaff Rig Handbook. ‘The best proportions’, he wrote, ‘for a gaff mainsail to be set without a topsail are: luff 1.0, head 0.833, leech 1.73, foot 1.02. The angle of the gaff to the centreline of the mast should be 30 degrees and the rake of the boom should be about 6 degrees…’ With a foot length of 14′ 0″, this gives an area of 201 sq ft and an aspect ratio of 2.7, as shown below.

 A mainsail designed to John Leather’s principles. Calculation of area and centre of effort: The sail is treated as two adjacent triangles; centres are joined; lines are drawn perpendicular to this axis from the centres towards the clew and throat; distance along clew line is marked proportional to upper triangle area, and distance along throat line is marked proportional to lower triangle area (triangle areas calculated using Heron’s Formula); marks are joined; centre of effort is at intersection of new line with triangle centres line (Emiliano Marino, The Sailmaker’s Apprentice).

Comparison of mainsails: