Bostrom 31 Mk II Information, Review, Specs

Sailing Performance & Handling

The sailing personality of the Bostrom 31 Mk II is defined by its high ballast ratio and relatively heavy displacement, which contribute to a stiff, predictable motion in a seaway. With a ballast-to-displacement ratio often exceeding 40%, the boat carries its sail well into freshening breezes before reefing becomes mandatory. While it may lack the light-air acceleration of modern performance cruisers, the B-31 excels in "Beating the Baltic," a reputation earned by its ability to track straight and maintain momentum through a chop.

The hull features a deep V-section forward, which minimizes slamming when sailing upwind. According to historical technical reviews from the Swedish publication Båtnytt, the B-31 is noted for its exceptional directional stability, largely credited to the long-chord fin keel and the substantial skeg protecting the rudder. This configuration makes it an excellent candidate for windvane self-steering, a common sight on many B-31s that have ventured beyond coastal waters. Owners frequently report that the helm remains light and responsive until the boat is significantly over-canvassed, at which point the deep skeg provides a margin of safety against rounding up.

Interior Comfort & Variations

The interior of the Bostrom 31 Mk II reflects the classic yacht aesthetic of the 1970s, heavily utilizing teak and mahogany joinery. The layout is traditional: a forward V-berth followed by a head compartment (usually to port) and a hanging locker to starboard. The main salon features two longitudinal settees with a central folding table, which can accommodate four to five adults for dining. One notable characteristic of the Mk II is the improved headroom compared to its predecessor, though it remains modest by modern standards at approximately 1.80 meters (5'11").

A significant aspect of the B-31's history is the "half-fabrikat" or kit-built market. Many hulls were sold as bare shells for owner-completion. This has resulted in a wide variance in interior quality and layout details; while factory-finished models follow a strict standard, home-finished versions may feature custom cabinetry or alternative galley arrangements. The Mk II specifically optimized the companionway area and the galley, which typically features a two-burner stove and a deep icebox. The presence of a dedicated navigation station and a generous quarter berth to starboard makes the boat viable for extended coastal passage-making.

Known Issues & Buyer’s Checklist

Prospective buyers of a Bostrom 31 Mk II should focus on several age-related technical areas common to Swedish builds of this vintage.

1. Deck Core Integrity: Like many boats of this era, the B-31 utilizes a balsa-cored deck. Soft spots around the chainplates, stanchion bases, and the mast step are common if the hardware has not been re-bedded periodically.
2. Original Engines: Many units were originally fitted with Volvo Penta MD7A or MD11C engines. These raw-water-cooled units are now at the end of their service life, and buyers should check for internal scaling or consider the value of a vessel that has already undergone a repower with a modern fresh-water-cooled diesel.
3. Rudder Bushings: The skeg-mounted rudder is robust, but the bushings can wear over decades, leading to vibration or "clunking" at the helm. Inspecting the play in the rudder stock while the boat is hauled out is essential.
4. Osmotic Blistering: While Boström Båtar used high-quality resins, 40-year-old hulls are susceptible to osmosis. A moisture meter reading of the hull below the waterline is recommended during a pre-purchase survey.
5. Chainplate Reinforcement: On early Mk II models, it is advisable to inspect the bulkheads where the chainplates are through-bolted for signs of water ingress or structural movement.

Community & Resources

The primary hub for technical data and owner support is the B31-Klubben (B31 Club), a dedicated association based in Sweden. This organization maintains an archive of original drawings, rigging specifications, and a registry of hull numbers. While the club's primary communications are in Swedish, it remains the most authoritative source for verifying whether a specific hull was factory-finished or a kit build.

The Verdict

The Bostrom 31 Mk II remains a highly respected "pocket cruiser" that offers a level of build quality and sea-kindliness often missing in newer, lightweight production boats. It is an ideal choice for a couple or small family seeking a traditional, aesthetic vessel capable of handling spirited offshore conditions.

Pros:

• Exceptional heavy-weather stability and tracking.
• Timeless Scandinavian lines and high-quality wood interiors (on factory models).
• Deep skeg-mounted rudder provides superior protection and control.
• Active owner community providing deep technical knowledge.

Cons:

• Limited headroom for taller sailors.
• Variable interior quality due to the prevalence of kit-built versions.
• Potential for significant maintenance requirements regarding the balsa deck core and aging original engines.