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Review of Sirocco 31 Mk II


Basic specs.

sail specifications
The Sirocco 31 Mk II aka Sirocco 31 Mk2 is a sailboat designed by the British maritime architect Angus S. Primrose in the early seventies. The Sirocco 31 Mk II is built by the Spanish yard Mistral.

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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.
The hull is a double hull 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 one cabin, 5 berths, a galley, 50.0 liter fresh water capacity and toilet facility.


Rig

The Sirocco 31 Mk II is 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 Sirocco 31 Mk II anno 1973.

Fin keel

The Sirocco 31 Mk II is equipped 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 difference between an iron keel and a lead keel can in reality be ignored for cruising yachts.

The boat 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

Sirocco 31 Mk II may be equipped with an inboard Volvo Penta 2001 diesel engine at 9 hp (6 kW). Max speed is 5.0 knots.

The transmission is a shaft drive. A shaft drive will in the long run require less maintenance 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 Sirocco 31 Mk II is 1.96, indicating that this boat could - if evaluated by this formula alone - be accepted to participate in ocean races.


Immersion rate

The immersion rate is defined as the weight required to sink the boat a certain level. The immersion rate for Sirocco 31 Mk II is about 180 kg/cm, alternatively 1008 lbs/inch.
Meaning: if you load 180 kg cargo on the boat then it will sink 1 cm. Alternatively, if you load 1008 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.

L/B (Length Beam Ratio)

What is L/B (Length Beam Ratio)?

The l/b ratio for Sirocco 31 Mk II is 3.43.

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.


Ballast Ratio

What is a Ballast Ratio?

The ballast ratio for Sirocco 31 Mk II is 46%.

Low High 76% 0 50 100
This ballast ratio shows a righting moment that is higher than 76% of all similar sailboat designs. A righting moment (ability to resist heeling) significantly above average.


SA/D (Sail Area Displacement ratio)

SA/D (Sail Area Displacement ratio)
Indicates how fast the boat is in light wind:
- Cruising Boats have ratios 10-15
- Cruiser-Racers have ratios 16-20
- Racers have ratios above 20
- High-Performance Racers have ratios above 24
Sail-area/displacement ratio (SA/D ratio): 14.27


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 9.6 m(31.5 feet)12 mm(1/2 inch)
Genoa sheet9.6 m(31.5 feet)12 mm(1/2 inch)
Mainsheet 24.0 m(78.7 feet)12 mm(1/2 inch)
Spinnaker sheet21.1 m(69.3 feet)12 mm(1/2 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.

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Finally

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References