Hull Speed
The theoretical displacement-mode speed limit — determined by waterline length, not engine or sail power.
5.7 kts
A displacement hull pushes a bow wave whose speed is limited by the waterline length.
With a waterline of 18.3′, the Rk 21 tops out around 5.7 knots in displacement mode — after that, the bow wave outruns the hull and resistance climbs steeply.
1.34 × √18.3′ LWL = 5.7 kts
Performance Ratios
Racing handicap, sail-carrying power, stability and comfort — and what each one actually tells you about a day on the water.
Ballast / Displacement
33.3%
Typical cruising ballast — balanced stability and motion underway.
Displacement / Length
147
Ultralight — responsive and fast, but carries less stores.
Hull Speed
5.7kts
Pounds/Inch Immersion
3lbs
Weight needed to sink the hull one inch — loading sensitivity.
PHRF Fleet Position
Where the Rk 21 sits on the PHRF handicap spectrum — lower numbers mean faster boats.
Venture 21
Macgregor 21
Santana 21
Clipper Marine 21
American 21
How It Compares
Side-by-side with the boats most often cross-shopped against the Rk 21.
| Rk 21 | American 21 | Clipper Marine 21 | Macgregor 21 | Santana 21 | Venture 21 | |
|---|---|---|---|---|---|---|
| Dimensions | ||||||
| LOA | 21.0 | 21.0 | 20.9 | 21.0 | 21.3 | 21.0 |
| LWL | 18.3 | 17.7 | 16.7 | 18.5 | 19.3 | 18.5 |
| Beam | 7.9 | 8.0 | 7.3 | 6.8 | 7.5 | 6.8 |
| Displacement | 2 | 2 | 1 | 1 | 1 | 1 |
| Ballast | 665 | 550 | 410 | 400 | 550 | 400 |
| Sail Area | — | 209 | 192 | — | 189 | — |
| Performance | ||||||
| PHRF | — | 306 | 270 | 263 | 264 | 252 |
| SA/Disp | — | 19.8 | 20.8 | — | 21.3 | — |
| Bal/Disp | 33.3 | — | — | 34.0 | 32.4 | 26.7 |
| Comfort | — | 11.4 | 11.1 | — | 9.0 | — |
| Capsize | — | 2.46 | 2.39 | — | 2.52 | — |
| Hull Speed | — | 5.6 | 5.5 | — | 5.9 | — |