Review of Boström 31 Mk II

Basic specs.

The Boström 31 Mk II also known as B 31 Mk II is a sailboat designed by the Swedish maritime architect Bruno Boström together with Jens Grandinsson also from Sweden in the late seventies. A few thousand boats have been built. The Boström 31 Mk II is built by the Danish yard Star-Boat/Plastbåde ApS.

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Hull

Deck plan of Boström 31 Mk II.

Both the hull and the deck is made of fibreglass. A hull made of fibreglass requires only a minimum of maintenance during the sailing season.

The deck is a sandwich construction which improves the indoor climate. Especially when the water is colder than the air in the cabin, then the double hull insulates against cold water and reduce the condense water in the cabin.

Interior

The boat is equipped with 6 berths, a galley, 100 liters (26 US gallons, 21 imperial gallons) fresh water capacity and toilet facility.

The headroom is above average

Note: the boat has also been sold to be home-completed, which means that the quality of each boat may vary.

Rig

The Boström 31 Mk II is built with a masthead rig. The sail area for the mailsail + jib is 30.0 m² (322.9 ft²). The advantage of a masthead rig is its simplicity and the fact that a given sail area - compared with a fractional rig - can be carried lower and thus with less heeling moment.

Keel

Fin keel

The Boström 31 Mk II is built with a fin keel. A boat with a fin keel is more manoeuvrable but has less directional stability than a similar boat with a full keel.

The keel is made of iron. Many people prefer lead keel in favour of iron. The main argument is that lead is much heavier than iron and a lead keel can therefore be made smaller which again result in less wet surface, i.e. less drag. In fact iron is quite heavy, just 30% less heavy than lead, so the advantage of a lead keel is often overstated. As the surface of a fin type keel is just a fraction of the total wet surface, the additional surface area created by the cast iron keel in comparison to a lead keel of the same weight would be negligible for cruising yachts.

Boström 31 Mk II can only enter major marinas as the draft is about 1.80 - 1.90 meter (5.91 - 6.21 ft) dependent on the load. See immersion rate below.

Sailing characteristics

This section covers widely used rules of thumb to describe the sailing characteristics. Please note that even though the calculations are correct, the interpretation of the results might not be valid for extreme boats.

Stability and Safety

What is Capsize Screening Formula (CSF)?

The capsize screening value for Boström 31 Mk II is 1.87, indicating that this boat could - if evaluated by this formula alone - be accepted to participate in ocean races.

Theoretical Maximum Hull Speed

What is Theoretical Maximum Hull Speed?

The theoretical maximal speed of a displacement boat of this length is 6.8 knots. The term "Theoretical Maximum Hull Speed" is widely used even though a boat can sail faster. The term shall be interpreted as above the theoretical speed a great additional power is necessary for a small gain in speed.

Immersion rate

The immersion rate is defined as the weight required to sink the boat a certain level. The immersion rate for Boström 31 Mk II is about 146 kg/cm, alternatively 820 lbs/inch.
Meaning: if you load 146 kg cargo on the boat then it will sink 1 cm. Alternatively, if you load 820 lbs cargo on the boat it will sink 1 inch.

Sailing statistics

This section is statistical comparison with similar boats of the same category. The basis of the following statistical computations is our unique database with more than 26,000 different boat types and 350,000 data points.

Motion Comfort Ratio

What is Motion Comfort Ratio (MCR)?

The Motion Comfort Ratio for Boström 31 Mk II is 20.4 - 21.7.

Comparing this ratio with similar sailboats show that it is more comfortable than 38% of all similar sailboat designs. This comfort value is just below average.

L/B (Length Beam Ratio)

What is L/B (Length Beam Ratio)?

The l/b ratio for Boström 31 Mk II is 3.38.

Compared with other similar sailboats it is slimmer than 76% of all other designs. It seems that the designer has chosen a significantly more speedy hull design.

Ballast Ratio

What is a Ballast Ratio?

The ballast ratio for Boström 31 Mk II is 45%.

This ballast ratio is higher than 73% of all similar sailboat designs. Since the ballast ratio and the righting moment (i.e. better ability to resist heeling) is strongly correlated, a ballast ratio just above average indicates an ability to resist heeling above average.

D/L (Displacement Length Ratio)

What is Displacement Length Ratio?

The DL-ratio for Boström 31 Mk II is 180 which categorizes this boat among 'light racers'.

82% of all similar sailboat designs are categorized as heavier. A light displacement requires less sailarea and has higher accellerations.

SA/D (Sail Area Displacement ratio)

What is SA/D (Sail Area Displacement ratio)?

The SA/D for Boström 31 Mk II with ISO 8666 reference sail is 15.8, with a 135% genua the SA/D is 19.1.

The SA/D ratio indicates that it is faster than 42% of all similar sailboat designs in light wind.

Over- / underrigged

The Boström 31 Mk II has more rig than 27% of all similar sailboats, which indicates that the boat is slightly underrigged.

RSP (Relative Speed Performance)

What is Relative Speed Performance?

The Relative Speed Performance for Boström 31 Mk II is 21

The relative speed performance is 21 which means that a it is faster than 21% of all similar sailboat designs.

Maintenance

Bottom Paint

When buying anti-fouling bottom paint, it's nice to know how much to buy. The surface of the wet bottom is about 27m² (290 ft²).
Based on this, your favourite maritime shop can tell you the quantity you need.

Note: If you use a paint roller you will need more paint than if you use a paintbrush.

Rig

Dimensions of sail for masthead rig.

Are your sails worn out? You might find your next sail here: Sails for Sale

If you need to renew parts of your running rig and is not quite sure of the dimensions, you may find the estimates computed below useful.

**Guiding dimensions of running rig**
Usage	Length		Diameter
Mainsail halyard	25.6 m	(83.8 feet)	10 mm	(3/8 inch)
Jib/genoa halyard	25.6 m	(83.8 feet)	10 mm	(3/8 inch)
Spinnaker halyard	25.6 m	(83.8 feet)	10 mm	(3/8 inch)
Jib sheet	9.3 m	(30.6 feet)	12 mm	(1/2 inch)
Genoa sheet	9.3 m	(30.6 feet)	12 mm	(1/2 inch)
Mainsheet	23.3 m	(76.5 feet)	12 mm	(1/2 inch)
Spinnaker sheet	20.5 m	(67.3 feet)	12 mm	(1/2 inch)
Cunningham	2.8 m	(9.2 feet)	10 mm	(3/8 inch)
Kickingstrap	5.6 m	(18.4 feet)	10 mm	(3/8 inch)
Clew-outhaul	5.6 m	(18.4 feet)	10 mm	(3/8 inch)

Boat owner's ideas

This section is reserved boat owner's modifications, improvements, etc. Here you might find (or contribute with) inspiration for your boat.

Do you have changes/improvements you would like to share? Upload a photo and describe what you have done.

Finally

We are always looking for new photos. If you can contribute with photos for Boström 31 Mk II it would be a great help.

If you have any comments to the review, improvement suggestions, or the like, feel free to contact us. Criticism helps us to improve.

References

[Ref 1]: ISO 8666 Small craft — Principal data.
[Ref 3]: ISO 12215-5 Small craft — Hull construction and scantlings.