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Impulse 21
Designer: William Cook  ·  Builder: Impulse Marine / Johnson Boatworks (Usa)  ·  First built: 1986
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About the Impulse 21

Built from 1986 by Impulse Marine and later Johnson Boatworks, this William Cook-designed 21-footer represents a thoughtful approach to compact coastal cruising and club racing. With 150 hulls produced during its production run, the Impulse 21 developed a reputation as a well-balanced performer that punches above its weight class. The fractional sloop rig and modern fin keel with spade rudder configuration deliver responsive handling and solid upwind performance. At 1,300 pounds with 600 pounds of ballast, the boat maintains excellent stability while remaining light enough for easy trailering. The generous 208 square feet of sail area provides spirited performance in light to moderate conditions, with a sail-area-to-displacement ratio of 28 indicating good acceleration and responsiveness. The fiberglass construction prioritizes durability and low maintenance, while the 8-foot beam maximizes interior space within the compact 21-foot length. A comfort ratio of 6.54 and capsize screening formula of 2.93 suggest this boat is best suited for protected coastal waters and weekend cruising rather than extended offshore passages. For sailors seeking an affordable entry into sailboat ownership or a nimble daysailer with basic overnight capabilities, the Impulse 21 offers proven performance in a manageable package.

Core Dimensions
LOA (Length Overall) 21.00 ft / 6.40 m
LWL (Waterline Length) 18.50 ft / 5.64 m
Beam 8.00 ft / 2.44 m
Max Draft 3.25 ft / 0.99 m
Displacement 1,300.00 lb / 590 kg
Ballast 600.00 lb / 272 kg
Sail Area (Reported) 208.00 ft² / 19.32 m²
Design & Construction
Hull Type Fin w/spade rudder
Rigging Type Fractional Sloop
Construction FG
Designer William Cook
Builder Impulse Marine / Johnson Boatworks (USA)
First Built 1986
Number Built 150
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Performance Ratios
PHRF Rating PHRF (Performance Handicap Racing Fleet) is a national racing handicap measured in seconds per nautical mile. Lower values indicate faster boats. Ratings are sourced from US Sailing's national database (median across 60+ regional fleets).
Under 0 — Ultralight racer0–90 — Performance racer90–150 — Cruiser/racer150–210 — Typical cruiserOver 210 — Heavy/slow cruiser
 183
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
 28
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
 46.15
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
 91.66
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
 6.54
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
 2.93
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. 5.76 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
 6.11
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. 528.82 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. 27.85
Rig Dimensions
I (Foretriangle Height) 23.10 ft / 7.04 m
J (Foretriangle Base) 7.00 ft / 2.13 m
P (Mainsail Luff) 26.00 ft / 7.92 m
E (Mainsail Foot) 9.70 ft / 2.96 m
ISP (Spinnaker Luff) 23.50 ft / 7.16 m
SPL/TPS 8.00 ft / 2.44 m
SA Fore 80.85 ft² / 7.51 m²
SA Main 126.10 ft² / 11.72 m²
SA Total (100%) 206.95 ft² / 19.23 m²
Est. Forestay Length 24.14 ft / 7.36 m
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