Review of J/70

Basic specs.

The J/70 is a sailboat designed by the Australian maritime architect Alan Johnstone.

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Rig

The J/70 is built 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

Fin keel

The J/70 is built with a fin keel. The fin keel is the most common keel and provides splendid manoeuvrability. The downside is that it has less directional stability than a long keel.

The boat can enter most marinas as the draft is just about 1.50 - 1.60 meter (4.92 - 5.22 ft) dependent on the load.

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 J/70 is 2.43, indicating that this boat would not be accepted to participate in ocean races.

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 J/70 is 3.07.

Compared with other similar sailboats it is slimmer than 69% 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 J/70 is 35%.

This ballast ratio shows a righting moment that is higher than 34% of all similar sailboat designs. A righting moment (ability to resist heeling) just below average.

SA/D (Sail Area Displacement ratio)

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

The SA/D for J/70 with ISO 8666 reference sail is 24.4, with a 135% genua the SA/D is 28.3.

The SA/D ratio indicates that it is faster than 92% of all similar sailboat designs in light wind.

Over- / underrigged

The J/70 has more rig than 100% of all similar sailboats, which indicates that the boat is significantly overrigged.

Maintenance

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	21.3 m	(69.9 feet)	8 mm	(5/16 inch)
Jib/genoa halyard	21.3 m	(69.9 feet)	8 mm	(5/16 inch)
Spinnaker halyard	21.3 m	(69.9 feet)	8 mm	(5/16 inch)
Jib sheet	6.9 m	(22.8 feet)	10 mm	(3/8 inch)
Genoa sheet	6.9 m	(22.8 feet)	10 mm	(3/8 inch)
Mainsheet	17.4 m	(56.9 feet)	10 mm	(3/8 inch)
Spinnaker sheet	15.3 m	(50.1 feet)	10 mm	(3/8 inch)
Cunningham	2.9 m	(9.5 feet)	8 mm	(5/16 inch)
Kickingstrap	5.8 m	(19.0 feet)	8 mm	(5/16 inch)
Clew-outhaul	5.8 m	(19.0 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.

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Finally

We are always looking for new photos. If you can contribute with photos for J/70 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.