With purposeful proportions and light displacement, the Bostrom 31 Mk II is better understood as a coastal performer than as a pocket cruiser.
Design and Construction
The Bostrom 31 Mk II measures 30.61 feet overall and 25.98 feet on the waterline, with a beam of nine feet. Depending on load, its fin-keel draft ranges from 5.9 to 6.2 feet.
The hull and deck are both fiberglass, but they use different forms of construction. The hull is solid glass, while the deck uses sandwich construction. This combination keeps topside weight down without sacrificing hull stiffness in a seaway.
The fin keel is iron rather than lead, a cost-and-tooling choice typical of volume Danish production. Displacement is 7,275 pounds, including 3,638 pounds of ballast, for a ballast ratio of 45 percent.
Those figures, together with a length-to-beam ratio of 3.38 and a displacement-length ratio of 180, place the boat firmly among what the design data calls light racers. It should accelerate quickly, but it isn’t a heavy-weather Atlantic bulldog. A capsize screening value of 1.87 and a motion comfort ratio of roughly 20 to 22 reinforce that picture: this is a lively, moderate-angle boat designed for speed and easy short-handed sailing rather than heavily loaded passagemaking.
Rig, Performance, and Handling
The boat has a masthead sloop rig carrying 322 square feet of working sail in its mainsail-and-jib combination. A 135-percent genoa raises the sail-area-to-displacement index from 15.8 on the reference rig to 19.1.
Even so, the data shows that the rig is lighter than those of 73 percent of comparable sailboats, making the Bostrom 31 Mk II slightly underrigged for its size. That helps keep it manageable when sailing shorthanded, but it also means the boat benefits from the larger genoa in anything less than a fresh breeze.
The theoretical hull speed is 6.8 knots. Wetted surface is about 290 square feet, while the immersion rate is 820 pounds per inch. Together, those figures suggest that the boat sits sensibly in the water and doesn’t wallow when loaded for a weekend.
Recommended running-rigging dimensions are thoroughly documented. Halyards are specified at 25.6 meters by 10 mm, while sheets range from about 9 to 23 meters by 12 mm. Control lines include a 5.6-meter kicking strap and clew outhaul. A new owner can therefore replace these lines to specification without guesswork.
Accommodations
Below, the Bostrom 31 Mk II has six berths, a galley, and a toilet facility. Headroom is noted as above average for the class—a real advantage in a 30-footer, where many competing boats require the crew to stoop.
The fresh-water tank holds 100 liters, enough for a short cruise without constant conservation. Fitting six berths and a separate head into a hull under 31 feet reflects the Scandinavian emphasis on usable interior volume. The above-average standing room also makes the cabin feel larger than the boat’s length might suggest.
Known Issues
The main structural caveat is that the Bostrom 31 Mk II was also offered as a home-completed kit, so the quality of individual boats may vary. Some hulls were finished by amateur builders rather than by the Danish yard.
That creates real variation from boat to boat. A factory-finished example and a backyard completion may differ in deck-core encapsulation, keel-bolt bedding, and interior joinery. These are differences a survey must identify. Buyers shouldn’t assume uniform build quality across the fleet, because 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 specification and a large surviving population. Its slightly underrigged masthead sloop is easy to handle, while the six-berth interior and above-average headroom make it a practical weekender. Its Scandinavian design pedigree shows in the balance of speed and simplicity.
The only cloud is the home-completion program. Each boat needs careful individual inspection rather than blanket trust in the model name.
Pros
- Large production run by Star-Boat/Plastbåde ApS, with consistent core dimensions
- Above-average headroom and six berths in a boat under 31 feet
- Light-racer proportions and documented rigging specifications that simplify maintenance
Cons
- Home-completed kits create quality variation across the fleet
- Iron fin keel rather than lead, with the associated need for corrosion vigilance
- Slightly underrigged compared with its peers and reliant on the 135-percent genoa for performance
