Professional BoatBuilder - February-March 2018

46 PROFESSIONAL BOATBUILDER

I

became interested in the fractional

rig more than three decades ago

when I was contemplating a fast, low-

cost cruising daysailer. At the time, the

typical cruiser or cruiser/racer would

have had perhaps four genoas and jibs

to change down as the wind increased.

Comparing alternative masthead- and

fractional-sail inventories for the same

hull and neglecting o-wind sails, I

found the fractional rig had about half

the total sailcloth area of the masthead

variant and most likely a correspond-

ingly lower cost.

With these benets in mind, I began

evolving an atypical fractional rig for

the P-32 (9.75m) design. e Petrel is

a modern interpretation of the tradi-

tional keel/centerboarder with a nar-

row, easily driven hullform that I

designed and built as an R&D project,

focusing on a few features I felt were

too risky to evaluate on a client’s boat.

While I have only a limited number of

designs to my credit, most of them

have had signicantly dierent rigs. A

principal aspect of the Petrel research

was the rig.

Fractional Rigs

Reviewing references applicable to

fractional rigs in my extensive library

on yacht and aircra design, I found

some of the best empirical and tech-

nical data in C.A. Marchaj’s Sail Per-

formance (2003). He cites the devel-

opments of the Bembridge Redwings,

a 22' (6.7m) class raced on England’s

south coast. ose one-design hulls

were allowed any sail plan as long as

its actual area did not exceed 200 sq 

(18.6m^2 ). Of all the rigs tried, the

more-or-less conventional sloops

came out on top, and of these the best

was a fractional rig with a jib half the

area of the main. e slowest sloop

was one with an overlapping jib and

reduced mainsail area. Wind tunnel

data presented in the same chapter

show again the advantage of the frac-

tional rig when evaluated in terms of

actual not-rated sail area.

It is interesting to note that in 1969

when Maynard Bray modied the rig

on his keel/centerboard yawl Aida

(Herresho Manufacturing Co., hull

#1,002 of 1,926), the jib and mainsail

areas of 154 sq  and 297 sq  (14.3m^2

and 27.6m^2 ), respectively, are in line

with the Redwing optimums. Studying

the history of the C Class catamarans,

which raced for the Little America’s C u p,

shows the evolution of their sail plans,

restricted to 300 sq  (27.9m^2 ) of actual

sail/mast area. Departing from earlier

racing cats, with their more-or-less nor-

mal sloop rigs, the early C Class designs

had fractional rigs with small, almost

spitre jibs functioning perhaps as lead-

ing-edge slats before the transition to

single-sail cat/una rigs.

An anecdote that supports the e-

ciency of the fractional rig with a non-

overlapping foresail is related by Robert

H. Perry (Yacht Design According to

Perry: My Boats and What Shaped em,

2008): “Years ago in Seattle, a dealer

imported a eet of fractionally rigged

Scandinavian yachts called Aphrodite

101s. ese came with non-overlapping

headsails. It was thought that the boats

would need more horsepower in Seattle’s

light air, so a genoa was built for one

boat, and trials were set up against a boat

with the standard jib to evaluate the

Fractional Update

Designing an atypical fractional rig for

a modern cruising sloop or yawl.

by Jay Paris

RIG

DESIGN

FractionalRig171-ADFinal.indd 46 12/29/17 4:11 PM