Irwin 31
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Photo: Ahunt / CC0
About the Irwin 31
Launched in 1968, this Ted Irwin design represents one of the earlier entries in the production cruising sailboat market that would flourish throughout the following decades. The Irwin 31 combines traditional proportions with practical features that made it an attractive choice for coastal cruising and weekend sailing. With a displacement of 9,600 pounds and substantial ballast of 3,800 pounds, this masthead sloop offers reassuring stability for cruising sailors. The 7.5-foot draft provides good upwind performance while remaining manageable for most harbors and anchorages. The fiberglass construction with balsa-cored decking reflects quality building practices of the era, delivering both strength and reduced weight aloft. The boat's sail area of 448 square feet and SA/Displacement ratio of 15.92 indicate moderate performance characteristics well-suited to comfortable cruising rather than racing. The comfort ratio of 29.37 suggests a motion that favors seaworthiness over speed, while the capsize screening value of 1.81 falls within acceptable ranges for coastal sailing. Built by Irwin Yachts in the USA, this design helped establish the builder's reputation for producing affordable, well-constructed cruising boats. The 25-horsepower auxiliary engine provides adequate power for maneuvering and charging systems during extended cruising.
Core Dimensions
LOA (Length Overall)
31.08 ft / 9.47 m
LWL (Waterline Length)
22.25 ft / 6.78 m
Beam
9.58 ft / 2.92 m
Max Draft
7.50 ft / 2.29 m
Min Draft
3.33 ft / 1.01 m
Displacement
9,600.00 lb / 4,354 kg
Ballast
3,800.00 lb / 1,724 kg
Ballast Type
Lead
Sail Area (Reported)
448.00 ft² / 41.62 m²
Design & Construction
Hull Type
Keel/Cbrd.
Rigging Type
Masthead Sloop
Construction
FG w/balsa cored deck
Designer
Ted Irwin
Builder
Irwin Yachts (USA)
First Built
1968
Associations
Irwin Facebook Group
Related Sailboats
Seafarer 31 Mkii · Hughes 31 · Catalina 310 · Island Packet 31 · Offshore 31 Cheoy Lee
Owner Reviews
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Known Issues & What to Watch For
The balsa-cored deck is prone to water intrusion around deck hardware, chainplates, and stanchion bases, leading to core rot. Irwin's deck hardware installation practices of this era were not consistently well-bedded, making wet or punky core a very common finding on surveys.
The deck-to-hull joint on early Irwin boats used an outward-turning flange secured with through-bolts and typically covered by an aluminum toe rail. This joint is known to leak and should be carefully inspected for separation, water intrusion, and fastener corrosion.
1968–1975 models
The centerboard trunk on the keel/centerboard configuration is prone to leaking where it meets the hull. The pivot pin and pendant hardware for the centerboard are also subject to corrosion and wear, and replacement parts can be difficult to source.
The centerboard pendant and its attachment hardware inside the trunk are difficult to inspect and frequently deteriorate undetected. Failure of the pendant can result in the board dropping or becoming inoperable at sea.
Irwin used relatively thin fiberglass layups in the hull on early 31s, and stress cracking around the keel trunk area and chainplate attachment points is commonly reported. These cracks should be assessed carefully to rule out structural flexing.
1968–1978 models
Chainplate attachment on the Irwin 31 is often through the deck or to interior bulkheads that are not fully bonded to the hull. Bulkhead delamination or fastener pull-through at chainplate locations has been reported and warrants close inspection.
Interior joinerwork and bulkhead bonding on Irwin boats of this era is often done with minimal or deteriorated tabbing. Bulkheads may have separated from the hull liner over time, reducing structural integrity and causing cosmetic damage.
The rudder on the Irwin 31 is a fiberglass shell construction that can develop internal voids and water intrusion. Waterlogged rudders are a known issue and can affect steering feel; sounding the rudder during survey is essential.
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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
192
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
15.92
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
39.58
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
389.07
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
29.37
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
1.81
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.
6.32 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
1.13
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.
761.63 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.
15.92
Rig Dimensions
I (Foretriangle Height)
37.00 ft / 11.28 m
J (Foretriangle Base)
12.10 ft / 3.69 m
P (Mainsail Luff)
32.00 ft / 9.75 m
E (Mainsail Foot)
14.00 ft / 4.27 m
SA Fore
223.85 ft² / 20.80 m²
SA Main
224.00 ft² / 20.81 m²
SA Total (100%)
447.85 ft² / 41.61 m²
Est. Forestay Length
38.93 ft / 11.87 m
Engine & Accommodations
Engine Make
Universal
Engine Model
Atomic 4
Engine Type
Gas
Horsepower
25
Fuel Capacity
25 gals / 95 L
Water Capacity
50 gals / 189 L
How It Compares
vs. Catalina 310, Hughes 31, Island Packet 31, Offshore 31 Cheoy Lee, Seafarer 31 Mkii
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Dimensions
LOA
Irwin 31 ★
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LWL
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Beam
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Displacement
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Ballast
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Sail Area
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Performance
PHRF Rating
(lower = faster)
Irwin 31 ★
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SA / Displacement
(higher = more powerful)
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Ballast / Disp
(higher = stiffer)
Irwin 31 ★
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Comfort Ratio
(higher = more comfortable offshore)
Irwin 31 ★
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Capsize Screening
(lower = safer)
Irwin 31 ★
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Island Packet 31
Offshore 31 Cheoy Lee
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Displacement / Length
(lower = lighter)
Irwin 31 ★
Catalina 310
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Hull Speed
Irwin 31 ★
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Speed Number
(higher = faster)
Irwin 31 ★
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