Bostrom 31 Mk II Sailboat Review, Specs, and Listings

Design and Construction

The hull and deck are fiberglass, but the two are not built the same way: the hull is solid glass while the deck is a sandwich which means the deck is a sandwich construction, a combination that keeps topside weight down without sacrificing hull stiffness in a seaway. The fin keel itself is iron rather than lead, a cost-and-tooling choice typical of volume Danish production, and it carries a 45 percent ballast ratio against a displacement of 7,275 pounds with 3,638 pounds of ballast. That ballast-to-weight figure, paired with a length-to-beam ratio of 3.38 and a displacement-length ratio of 180, places her firmly among what the design data calls light racers — quick to accelerate, but not a heavy-weather Atlantic bulldog. The capsize screening value of 1.87 and a motion comfort ratio of roughly 20 to 22 tell the same story: a lively, moderate-angle boat built for speed and short-handed ease rather than load-carrying passage-making.

Rig and Handling

She is a masthead sloop carrying 322 square feet of working sail in the mainsail-and-jib combination, with a 135-percent genoa pushing the sail-area-to-displacement index from 15.8 on the reference rig up to 19.1. Despite that, the data shows her rig is lighter than 73 percent of comparable sailboats, so she is slightly underrigged for her size — a trait that keeps her manageable shorthanded but means she benefits from the larger genoa in anything under a fresh breeze. The theoretical hull speed is 6.8 knots, and the wetted surface of about 290 square feet, combined with an immersion rate of 820 pounds per inch, means she sits sensibly in the water and does not wallow when loaded for a weekend. The recommended running rigging is thoroughly documented: halyards of 25.6 meters at 10 mm, sheets around 9 to 23 meters at 12 mm, and control lines such as the 5.6-meter kicking strap and clew outhaul, all of which a new owner can replace to spec without guesswork.

Accommodations

Below, the boat is laid out for six berths with a galley and a toilet facility, and the headroom is noted as above average for the class — a real asset in a 30-footer where most competitors force a stoop. The fresh water tank holds 100 liters, enough for a short cruise without constant conservation. The combination of six berths and a separate head in a hull under 31 feet speaks to a Scandinavian priority on usable interior volume, and the above-average standing room makes the cabin feel larger than the length suggests.

Known Issues

The one structural caveat in the record is that the Bostrom 31 Mk II was also offered as a home-completed kit which means that the quality of each boat may vary, meaning some hulls were finished by amateur builders rather than the Danish yard. This introduces real quality variation between individual boats: a factory-finished example and a backyard completion can differ in deck-core encapsulation, keel-bolt bedding, and interior joinery in ways that a survey must catch. A buyer should not assume uniform build quality across the fleet, since the same design name covers both professional and amateur construction.

The Verdict

The Bostrom 31 Mk II is a well-proportioned light-displacement cruiser-racer with a documented, repeatable spec and a large surviving population. Her slightly underrigged masthead sloop is easy to handle, her above-average headroom and six berths make her a practical weekender, and her Scandinavian design pedigree shows in the balance of speed and simplicity. The only cloud is the home-completion program, which demands careful per-hull inspection rather than blanket trust in the name.

Pros