At 30 feet 7 inches on deck, with a 22-foot waterline, the Commander 31 has the proportions of what period analysis classed as a heavy cruiser rather than a light racer.
Design and Construction
The fiberglass hull uses a balsa-cored sandwich layup. This construction improves the indoor climate, but it requires special care wherever fastenings or fittings must penetrate the core.
Bianca offered several keel alternatives, including deep and shoal fin keels. Depending on load, the deeper fin draws about 5.45 to 5.75 feet, allowing the boat into most marinas. The shoaler fin option draws about 3.71 to 4.01 feet, opening up even shallow harbors.
Displacement is near 7,715 pounds, with roughly 3,525 pounds of iron ballast. The resulting ballast ratio is 46 percent. Together with a length-beam ratio of 3.50 and a displacement-length ratio of 317, these figures define a beamy, heavily loaded cruiser rather than a featherweight.
Rig, Handling, and Power
The Commander 31 was delivered with more than one rig: either a masthead or fractional sloop. Buyers could therefore choose between classic overlap and modern fractional power.
The boat’s theoretical displacement-hull speed is 6.3 knots. Under power, a Volvo Penta MD1B diesel driving a shaft gives a calculated maximum of about 4.0 knots.
A capsize screening value of 1.81 and a Motion Comfort Ratio between 25.4 and 26.6 describe a stable, forgiving coastal cruiser. An immersion rate of about 673 pounds per inch and wetted surface near 236 square feet complete the picture of a solid hull that reacts slowly to added load but remains steady on the water.
Accommodations
The Commander 31’s 8-foot-8-inch beam provides enough interior volume to make the six-berth layout practical rather than cramped for a boat of this length. Below, she has a dedicated galley and an enclosed head, with mahogany joinery typical of many contemporaries. These accommodations make her a proper cruising boat rather than a weekend racer with a minimal interior.
Known Issues
Because the hull’s sandwich construction uses a balsa core, any work that penetrates it requires special care to prevent core damage or moisture ingress. Buyers and owners should pay particular attention to sealing the core around through-hulls, chainplate bolts, and added fittings.
The Verdict
The Commander 31 is a thoughtfully specified Danish cruiser from the late 1960s, designed by Hansen and Kjærulff. She offers deep and shoal fin-keel options, a choice of rigs, and a mahogany interior with six berths. Her balsa-cored hull demands care at every penetration, but her heavy-cruiser proportions and modest power profile suit comfortable coastal and inland sailing.
Pros
- Deep and shoal fin-keel options widen her cruising range
- Choice of masthead or fractional rig to suit owner preference
- Six berths, a galley, and an enclosed head in a beamy 31-foot hull
- Stable, comfort-oriented heavy-cruiser proportions
Cons
- Balsa-cored hull requires careful sealing at all penetrations
- Modest calculated engine maximum of about 4.0 knots limits motoring pace
