Review of B 31

B 31 anno 1977.

Basic specs.

The B 31 aka Boström 31 is a sailboat designed by the Swedish maritime architect Bruno Boström together with Jens Grandinsson also from Sweden in the early seventies. Several hundred boats have been produced. The B 31 is built by the Swedish yard Runn Båt AB.

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

Hull

The hull 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.

Interior

Interior of B 31 anno 1977.

The boat is equipped with 6 berths.
Note: the boat has also been sold to be self-made/-interiored, which means that the quality of each boat may vary.

Cockpit

Cockpit of B 31 anno 1977.

Rig

Rig of B 31 anno 1974.

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

The B 31 has been built with more than one type of keel.

One option is a fin keel. A boat with a fin keel is more manoeuvrable but has less directional stability than a similar boat with a long 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 difference between an iron keel and a lead keel can in reality be ignored for cruising yachts.

B 31 can enter most marinas as the draft is just about 1.70 - 1.80 meter (5.58 - 5.88 ft) dependent on the load. See immersion rate below.

Engine

The boat is typically equipped with an inboard Yanmar diesel engine at 8.0 hp (5 kW), which gives a max speed about 3.6 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.

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 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 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 is 25.6.

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 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 B 31 is 42%.

This ballast ratio shows a righting moment that is higher than 58% of all similar sailboat designs. A righting moment (ability to resist heeling) just above average.

D/L (Displacement Length Ratio)

What is Displacement Length Ratio?

The DL-ratio for B 31 is 314 which categorizes this boat among 'medium weight cruisers'.

24% 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.

SA/D (Sail Area Displacement ratio)

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

The SA/D for B 31 with ISO 8666 reference sail is 15.1, with a 135% genua the SA/D is 18.3.

The SA/D ratio indicates that it is faster than 34% of all similar sailboat designs in light wind.

Over- / underrigged

The B 31 has more rig than 42% of all similar sailboats, which indicates that the boat is slightly underrigged.

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 23m² (247 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 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 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.