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Rock 20
Designer: Heinz-Jurgen Sass  ·  Builder: Birka Marine (Fin)  ·  First built: 1968
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About the Rock 20

Designed by Heinz-Jurgen Sass and built by Finland's Birka Marine from 1968 to 1975, this compact cruiser represents solid Scandinavian craftsmanship from sailing's fiberglass revolution era. With 200 hulls produced during its seven-year production run, the Rock 20 found favor among sailors seeking a well-proportioned small yacht capable of serious sailing. The boat's fin keel with rudder-on-skeg configuration provides good directional stability while maintaining reasonable performance, making it well-suited for coastal cruising and club racing. At just over 23 feet overall but with nearly 20 feet of waterline length, the Rock 20 maximizes sailing length while remaining trailerable for many owners. The masthead sloop rig delivers 217 square feet of working sail area, providing adequate power for its 3,858-pound displacement. Notable design characteristics include a comfort ratio of 20.07, indicating moderate motion in seaways typical of boats designed for both comfort and performance. The capsize screening value of 1.88 suggests good stability for coastal waters, while the hull speed of nearly 6 knots offers respectable performance for weekend cruising and local racing. This Finnish-built pocket cruiser appeals to sailors wanting traditional sailing qualities in a manageable package.

Core Dimensions
LOA (Length Overall) 23.52 ft / 7.17 m
LWL (Waterline Length) 19.52 ft / 5.95 m
Beam 7.38 ft / 2.25 m
Max Draft 3.94 ft / 1.20 m
Displacement 3,858.00 lb / 1,750 kg
Ballast 1,323.00 lb / 600 kg
Ballast Type Iron
Sail Area (Reported) 217.00 ft² / 20.16 m²
Design & Construction
Hull Type Fin with rudder on skeg
Rigging Type Masthead Sloop
Construction FG
Designer Heinz-Jurgen Sass
Builder Birka Marine (FIN)
First Built 1968
Last Built 1975
Number Built 200
Owner Reviews

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Performance Ratios
SA/Displacement Sail Area to Displacement ratio measures how much sail power a boat has relative to its weight. Higher values mean more performance and speed.
Under 14 — Heavy cruiser, slow14–18 — Cruising sailboat18–22 — Cruiser/racerOver 22 — Performance racer
 14.15
Ballast/Displacement Ballast to Displacement ratio shows what percentage of the boat's total weight is ballast. Higher values generally mean better initial stability and stiffer sail carrying.
30–35% — Typical cruiser35–45% — Stiff performance boatOver 45% — Racing oriented
 34.29
Displacement/Length Displacement to Length ratio measures how heavy a boat is relative to its waterline length. Lower values mean a lighter, faster boat.
Under 100 — Ultralight racer100–200 — Light cruiser/racer200–300 — Moderate cruiser300–400 — Heavy cruiserOver 400 — Very heavy
 231.57
Comfort Ratio Ted Brewer's Comfort Ratio predicts how comfortable a boat will feel in offshore conditions. Higher values mean a steadier, more comfortable motion in a seaway.
Under 20 — Uncomfortably rough20–30 — Acceptable coastal30–40 — Good offshore comfortOver 40 — Very comfortable offshore
 20.07
Capsize Screening Formula The USHS Capsize Screening Formula estimates offshore capsize risk. Lower is safer — values under 2.0 are considered acceptable for offshore sailing.
Under 2.0 — Safe for offshore2.0–2.2 — Borderline offshoreOver 2.2 — Coastal use recommended
 1.88
Hull Speed Hull Speed is the theoretical maximum speed of a displacement hull, calculated from waterline length. Most cruising sailboats reach this speed in moderate conditions. 5.92 kn
S# (Speed Number) The Speed Number (S#) is a quick performance index. Higher values indicate a faster, more powerful design relative to its size.
Under 3.0 — Slow cruiser3.0–5.0 — Average cruiser5.0–7.0 — Performance cruiserOver 7.0 — Racer
 2.04
Pounds/Inch Immersion Pounds per Inch Immersion (PPI) tells you how much weight is needed to sink the boat one inch. Useful for calculating how additional gear and stores affect waterline. 514.73 pounds/inch
SA/Displacement (Calc) Calculated Sail Area to Displacement ratio using actual measured sail dimensions rather than reported values. More precise than the reported SA/Disp figure. 14.14
Rig Dimensions
I (Foretriangle Height) 25.60 ft / 7.80 m
J (Foretriangle Base) 9.02 ft / 2.75 m
P (Mainsail Luff) 23.23 ft / 7.08 m
E (Mainsail Foot) 8.73 ft / 2.66 m
SA Fore 115.46 ft² / 10.73 m²
SA Main 101.40 ft² / 9.42 m²
SA Total (100%) 216.86 ft² / 20.15 m²
Est. Forestay Length 27.14 ft / 8.27 m
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