Professional BoatBuilder - February-March 2018

FEBRUARY/MARCH 2018 49

RIG DESIGN: Fractional Update

original Swedish suffix “kryssare”

refers to this ability, but today, in most

of the English-speaking world it has

been taken to mean “cruiser.”

 e historical record does not pro-

vide much to distinguish fashion from

science, and I regret not discussing

the subject decades ago at Cowes

Week when I was sailing with Knud

Reimers, a leading designer of the

Square-Meter boats.

Once again, Marchaj provides some

technical insight into the subject of

hull and rig dimensions—for exam-

ple, restricting them to fractional

foretriangles. The most common

classes of these Skärgårdskryssare/

Skerry Cruisers are the 22- and

30-Square-Meter boats.  ey were

originally designed speci cally for

sailing amongst the Skerrys, small,

rocky islands of the Stockholm Archi-

pelago, where their excellent ability to

perform hard on the wind was neces-

sary for beating through the narrow

passages between the islands.  e

fractional foretriangle back down to

the mast slightly above the partners;

and the single lowers led a from the

spreader bases down to the chain-

plates.  e compressive loads of the

diamonds are therefore contained

within the spar, and only the lowers’

loads are seen by the hull. Lateral sti -

ness for the top 6' (1.8m) of the mast is

provided by the extra carbon layup

engineered by the mast’s designers at

Seldén.

 e rig is set up as a “permanent”

bent mast.  e mast is vertical at the

partners, and the diamond upper

shrouds are permanently tensioned to

bring the masthead a 1.4' (43cm).

 en the backstay is tensioned by the

six-part tackle on the starboard leg of

the split lower backstay, resulting in

some 2' (61cm) total curved “rake.”

 is degree of bend—about 75% of

Seldén’s recommended maximum—is

not altered when sailing.

I decided on this setup a er study-

ing Herresho ’s S-boat, the 30-Square-

Meters, and the writings of Manfred

Curry (Yacht Racing:  e Aerodynam-

ics of Sails, 1949), and envisioning

advantages for my particular applica-

tion.  e S-boat Herresho Manufac-

turing Co. #828 of 1,918 was Nathanael

Greene Herresho ’s only design with

a curved mast.  e Herresho Marine

Museum and various Herreshoff

experts I asked could not explain this

anomaly other than suggest that this

was his  rst marconi-rigged boat and

perhaps he was seeking a planform

analogous to a highly peaked ga

main. Curry’s work,  rst published in

1925, shows how curved masts were

inspired by his studies of birds’ wings.

Curry was also an early advocate of

full-length battens. Because his book

was published in German, French,

English, and Turkish, one might infer

that it influenced the rigs of the

Square-Meter boats.

Dating from 1908 and with the last

major revision in 1925, the Square-

Meter Rule is perhaps the oldest of all

the current rating rules.  e ratings

are simply the actual mainsail areas

accompanied by a few limitations on

Until the modern era, one-design keelboats almost always had fractional rigs.

Nathanael G. Herreshoff’s S-boat of 1919 is an example, although its club jib is rela-

tively smaller than normal with respect to the main. The 27.5-footer (8.4m) is Herres-

hoff’s only design with a curved mast and has somewhat complex rigging supporting

its lightweight spar.

M.A. COOPER DESIGN

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