Review of Sea Cat 21
Basic specs.
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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 boat is equipped with 3,5 berths.
The Sea Cat 21 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.
Fin keel
The Sea Cat 21 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 even shallow marinas as the draft is just about 1.10 - 1.20 meter (3.61 - 3.91 ft) dependent on the load. See immersion rate below.
An outboard motor is often used on this boat. In that case Sea Cat 21 will typically require a power of 2 - 3 hp, alternatively 40 - 48 lbs thrust if you prefer an electrical motor. Electric outboards are becoming popular for sailboat owners who want clean instant power with less noise and no exhaust fumes.
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.
What is Capsize Screening Formula (CSF)?
The capsize screening value for Sea Cat 21 is 2.04, indicating that this boat would not be accepted to participate in ocean races.
What is Theoretical Maximum Hull Speed?
The theoretical maximal speed of a displacement boat of this length is 5.8 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.
The immersion rate is defined as the weight required to sink the boat a certain level.
The immersion rate for Sea Cat 21 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.
What is Motion Comfort Ratio (MCR)?
The Motion Comfort Ratio for Sea Cat 21 is 12.6 - 13.4.
What is L/B (Length Beam Ratio)?
The l/b ratio for Sea Cat 21 is 3.26.
The ballast ratio for Sea Cat 21 is 33%.
What is Displacement Length Ratio?
The DL-ratio for Sea Cat 21 is 135 which categorizes this boat among 'ultra light racers'.
What is SA/D (Sail Area Displacement ratio)?
The SA/D for Sea Cat 21 with ISO 8666 reference sail is 16.8, with a 135% genua the SA/D is 19.0.
What is Relative Speed Performance?
The Relative Speed Performance for Sea Cat 21 is 31
Maintenance
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.
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.
| Usage | Length | Diameter | ||
| Mainsail halyard | 17.6 m | (57.7 feet) | 8 mm | (5/16 inch) |
| Jib/genoa halyard | 17.6 m | (57.7 feet) | 8 mm | (5/16 inch) |
| Spinnaker halyard | 17.6 m | (57.7 feet) | 8 mm | (5/16 inch) |
| Jib sheet | 6.3 m | (20.8 feet) | 10 mm | (3/8 inch) |
| Genoa sheet | 6.3 m | (20.8 feet) | 10 mm | (3/8 inch) |
| Mainsheet | 15.9 m | (52.1 feet) | 10 mm | (3/8 inch) |
| Spinnaker sheet | 14.0 m | (45.8 feet) | 10 mm | (3/8 inch) |
| Cunningham | 3.0 m | (9.8 feet) | 8 mm | (5/16 inch) |
| Kickingstrap | 6.0 m | (19.7 feet) | 8 mm | (5/16 inch) |
| Clew-outhaul | 6.0 m | (19.7 feet) | 8 mm | (5/16 inch) |
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