Review of San Juan 21 Mk II

Basic specs.

The San Juan 21 Mk II is a small sailboat designed by the American maritime architect Don Clark in the mid seventies. The San Juan 21 Mk II is built by the American yard Clark Boat Company.

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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.

Rig

The San Juan 21 Mk II is equipped with a fractional rig. A fractional rig has smaller headsails which make tacking easier, which is an advantage for cruisers and racers, of course. The downside is that having the wind from behind often requires a gennaker or a spinnaker for optimal speed.

Keel

CentreBoard

The San Juan 21 Mk II is equipped with a centreboard keel. A centreboard keel is a pivoting lifting keel, allowing to sail both coastal and inland waters.

The boat can sail close to the beach as the draft is just 0.21 - 0.31 meter (0.69 - 0.99 ft) dependent of 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 San Juan 21 Mk II is 2.59, indicating that this boat would not 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 5.5 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 San Juan 21 Mk II is about 73 kg/cm, alternatively 413 lbs/inch.
Meaning: if you load 73 kg cargo on the boat then it will sink 1 cm. Alternatively, if you load 413 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 San Juan 21 Mk II is 8.0.

Comparing this ratio with similar sailboats show that it is more comfortable than 14% of all similar sailboat designs. This is a comfort value significantly below average.

L/B (Length Beam Ratio)

What is L/B (Length Beam Ratio)?

The l/b ratio for San Juan 21 Mk II is 2.93.

Compared with other similar sailboats it is slimmer than 73% of all other designs. It seems that the designer has chosen a slightly more speedy hull design.

Ballast Ratio

What is a Ballast Ratio?

The ballast ratio for San Juan 21 Mk II is 32%.

This ballast ratio shows a righting moment that is higher than 31% 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 San Juan 21 Mk II is 113 which categorizes this boat among 'ultra light racers'.

85% of all similar sailboat designs are categorized as heavier. A light displacement requires less sailarea and has higher accellerations.

SA/D (Sail Area Displacement ratio)

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

The SA/D for San Juan 21 Mk II with ISO 8666 reference sail is 26.6, with a 135% genua the SA/D is 31.1.

The SA/D ratio indicates that it is faster than 98% of all similar sailboat designs in light wind.

Over- / underrigged

The San Juan 21 Mk II has more rig than 100% of all similar sailboats, which indicates that the boat is significantly overrigged.

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 13m² (139 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 fractional 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	18.8 m	(61.8 feet)	8 mm	(5/16 inch)
Jib/genoa halyard	18.8 m	(61.8 feet)	8 mm	(5/16 inch)
Spinnaker halyard	18.8 m	(61.8 feet)	8 mm	(5/16 inch)
Jib sheet	6.2 m	(20.5 feet)	10 mm	(3/8 inch)
Genoa sheet	6.2 m	(20.5 feet)	10 mm	(3/8 inch)
Mainsheet	15.6 m	(51.3 feet)	10 mm	(3/8 inch)
Spinnaker sheet	13.8 m	(45.1 feet)	10 mm	(3/8 inch)
Cunningham	2.7 m	(8.8 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 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 San Juan 21 Mk 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

[Ref 1]: ISO 8666 Small craft — Principal data.
[Ref 3]: ISO 12215-5 Small craft — Hull construction and scantlings.