Review of Rock 20

Rock 20 anno 1974.

Basic specs.

The Rock 20 aka Rock 24 is a sailboat designed by the Swedish maritime architect Heinz-Jurgen Sass in the late sixties. A few hundred boats have been produced. The Rock 20 is built by the Finnish yard Birka Marin.

Looking for a new boat? Find a Rock 20 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.

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 of Rock 20 anno 1974.

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 even shallow marinas as the draft is just about 1.20 - 1.30 meter (3.94 - 4.24 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 Rock 20 is 1.94, 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 5.9 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 Rock 20 is about 92 kg/cm, alternatively 517 lbs/inch.
Meaning: if you load 92 kg cargo on the boat then it will sink 1 cm. Alternatively, if you load 517 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 Rock 20 is 19.0.

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 Rock 20 is 3.15.

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 Rock 20 is 34%.

This ballast ratio shows a righting moment that is higher than 27% of all similar sailboat designs. A righting moment (ability to resist heeling) just below average.

D/L (Displacement Length Ratio)

What is Displacement Length Ratio?

The DL-ratio for Rock 20 is 231 which categorizes this boat among 'light crusers & offshore racers'.

42% 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 Rock 20 with ISO 8666 reference sail is 14.1, with a 135% genua the SA/D is 16.7.

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

Over- / underrigged

The Rock 20 has more rig than 76% of all similar sailboats, which indicates that the boat is significantly overrigged.

RSP (Relative Speed Performance)

What is Relative Speed Performance?

The Relative Speed Performance for Rock 20 is 31

The relative speed performance is 31 which means that a it is faster than 31% 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 16m² (172 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	19.0 m	(62.3 feet)	8 mm	(5/16 inch)
Jib/genoa halyard	19.0 m	(62.3 feet)	8 mm	(5/16 inch)
Spinnaker halyard	19.0 m	(62.3 feet)	8 mm	(5/16 inch)
Jib sheet	7.3 m	(24.0 feet)	10 mm	(3/8 inch)
Genoa sheet	7.3 m	(24.0 feet)	10 mm	(3/8 inch)
Mainsheet	18.2 m	(59.9 feet)	10 mm	(3/8 inch)
Spinnaker sheet	16.1 m	(52.7 feet)	10 mm	(3/8 inch)
Cunningham	2.7 m	(8.7 feet)	8 mm	(5/16 inch)
Kickingstrap	5.3 m	(17.5 feet)	8 mm	(5/16 inch)
Clew-outhaul	5.3 m	(17.5 feet)	8 mm	(5/16 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 Rock 20 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.