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Review of B 31 Mark II


B 31 Mark II
B 31 Mark II anno 1981. B 31 Mk II


Basic specs.

sail specifications
The B 31 Mark II aka Boström 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. Several thousand boats have been produced. The B 31 Mark II is built by the Danish yard Star Boat.

Looking for a new boat? Find a B 31 Mark II or similar boat for sale

Hull
Both the hull and the deck is made of fibreglass. Generally, a hull made of fibreglass requires only a minimum of maintenance during the sailing season. And outside the sailing season, just bottom cleaning and perhaps anti-fouling painting once a year - a few hours of work, that's all.
The deck is a double deck which improves the indoor climate.

Interior
The boat is equipped with 6 berths, a galley and toilet facility.
The headroom is above average
Note: the boat has also been sold to be self-made/-interiored, which means that the quality of each boat may vary.

Rig
The boat equipped with a masthead rig. 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
Keel
Keel of B 31 Mark II anno 1982.

Engine

B 31 Mark II is typically equipped with an inboard Yanmar 2GM diesel engine at 15.0 hp (11 kW), which gives a max speed about 4.9 knots.

The transmission is a shaft drive. A shaft drive will in the long run require less maintenence than other types of drive e.g. a sail drive.

The fuel tank which is made of stainless steel has a capacity of 30.0 liters (7 US gallons, 6 imperial gallons).


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 B 31 Mark II is 1.83, 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 6.3 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 B 31 Mark II is about 125 kg/cm, alternatively 703 lbs/inch.
Meaning: if you load 125 kg cargo on the boat then it will sink 1 cm. Alternatively, if you load 703 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 B 31 Mark II is 22.5 - 25.6.

Low High 62% 0 50 100
Comparing this ratio with similar sailboats show that it is more comfortable than 62% of all similar sailboat designs. This comfort value is just above average.


L/B (Length Beam Ratio)

What is L/B (Length Beam Ratio)?

The l/b ratio for B 31 Mark II is 3.38.

Wide Slim 76% 0 50 100
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.


D/L (Displacement Length Ratio)

What is Displacement Length Ratio?

The DL-ratio for B 31 Mark II is 295 which categorizes this boat among 'medium weight cruisers'.

Heavy Light 31% 0 50 100
31% of all similar sailboat designs are categorized as heavier. A heavy displacement combined with smaller water plane area has lower acceleration and is more comfortable.


RSP (Relative Speed Performance)

What is Relative Speed Performance?

The Relative Speed Performance for B 31 Mark II is 21

Slow Fast 21% 0 50 100
The relative speed performance is 21 which means that a it is faster than 21% of all similar sailboat designs.


Maintenance

Rig

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
UsageLengthDiameter
Jib sheet 9.3 m(30.6 feet)12 mm(1/2 inch)
Genoa sheet9.3 m(30.6 feet)12 mm(1/2 inch)
Mainsheet 23.3 m(76.5 feet)12 mm(1/2 inch)
Spinnaker sheet20.5 m(67.3 feet)12 mm(1/2 inch)

Boat owner's ideas

This section shown boat owner's changes, improvements, etc. Here you might find inspiration for your boat.

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



Finally

We are always looking for new photos. If you can contribute with photos for B 31 Mark 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