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Review of IW 31


IW 31
IW 31 anno 1971.


Basic specs.

sail specifications
The IW 31 is a sailboat designed by the American maritime architect bureau Sparkman & Stephens in the late sixties. Several hundred boats have been produced. The IW 31 is built by the Swedish yard IW-Varvet.

Looking for a new boat? Find an IW 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
Floor plan
Floor plan of IW 31. MATESSO PIANI COSTRUZIONE

The boat is equipped with 2 cabins, 5 berths, a galley and toilet facility.


Cockpit
Cockpit
Cockpit of IW 31 anno 1974.
Rig

The IW 31 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 IW 31 anno 1971. MATESSO A CHIOGGIA 1981

Unknown keel type

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.

The boat can enter most marinas as the draft is just about 1.75 - 1.85 meter (5.74 - 6.04 ft) dependent on the load. See immersion rate below.


Engine

IW 31 may be equipped with an inboard Volvo Penta MD1B diesel engine at 10 hp (7 kW). Calculated max speed is about 3.9 knots


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 IW 31 is 1.77, 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 IW 31 is about 120 kg/cm, alternatively 676 lbs/inch.
Meaning: if you load 120 kg cargo on the boat then it will sink 1 cm. Alternatively, if you load 676 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 IW 31 is 27.2.

Low High 69% 0 50 100
Comparing this ratio with similar sailboats show that it is more comfortable than 69% 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 IW 31 is 3.50.

Wide Slim 82% 0 50 100
Compared with other similar sailboats it is slimmer than 82% 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 IW 31 is 44%.

Low High 66% 0 50 100
This ballast ratio shows a righting moment that is higher than 66% 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 IW 31 is 333 which categorizes this boat among 'heavy cruisers'.

Heavy Light 15% 0 50 100
15% 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 IW 31 with ISO 8666 reference sail is 14.4, with a 135% genua the SA/D is 17.2.

Low High 24% 0 50 100
The SA/D ratio indicates that it is faster than 24% of all similar sailboat designs in light wind.


Over- / underrigged

Low High 27% 0 50 100
The IW 31 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 IW 31 is 38

Slow Fast 38% 0 50 100
The relative speed performance is 38 which means that a it is faster than 38% 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 22m2 (236 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.

mainsail specifications genoa specifications spinnaker specifications

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
UsageLengthDiameter
Mainsail halyard 26.1 m(85.5 feet)10 mm(3/8 inch)
Jib/genoa halyard26.1 m(85.5 feet)10 mm(3/8 inch)
Spinnaker halyard26.1 m(85.5 feet)10 mm(3/8 inch)
Jib sheet 9.4 m(30.9 feet)12 mm(1/2 inch)
Genoa sheet9.4 m(30.9 feet)12 mm(1/2 inch)
Mainsheet 23.5 m(77.2 feet)12 mm(1/2 inch)
Spinnaker sheet20.7 m(67.9 feet)12 mm(1/2 inch)
Cunningham2.9 m(9.5 feet)10 mm(3/8 inch)
Kickingstrap5.8 m(19.0 feet)10 mm(3/8 inch)
Clew-outhaul5.8 m(19.0 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 IW 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