Cal's Evals » GEMINI 105Mc

Release Date: 1/7/2011

Cals Eval - Gemini 105MC

The Gemini 105MC is the latest in a family of Gemini catamaran sail boats in the lower-30 foot length range. It is characterized by relatively narrow beam, full length bridgedeck and significant popularity. The following information from the Multihull Dynamics, Inc. website database shows how this 31.75 foot waterline length boat compares with 49 other cruising catamarans with waterline lengths from 31 to 32.5 feet. Averages and ranges for the comparison boats are in brackets.

Displacement: 3.26 tons [3.42 tons (range for 49 boats is 1.72 to 6.5tons)]

Length Overall: 33.5 feet [34.07 feet (range is 31.17 to 41 feet)]

Length at Waterline: 31.75 feet [31.6 feet ]

Beam Overall: 14 feet [18.49 feet]

Sail Area (Main plus 100% fore-triangle): 510 sq. ft. [587 sq. ft. (range is 342 to 1001)]

Sail Area/Displacement Ratio: 21.68 [24.31]

Beam between hull centerlines: 10.84 feet [14.93 feet]

Bcl/Lwl ratio: 0.34 [0.47]

Bridgedeck Clearance 18 in. [24 in. is nominal for 32 ft. LWL catamarans]

Lwl/Bh ratio: 10.06 [9.88]

Base Speed: 9.06 knots [9.36 (range is 7.73 to 10.97)]

Texel Rating: 130 [126]

Bruce Number: 1.17 [1.23]

Stability Speed: 16.86 knots [18.43 (range is 14.16 to 24.9)]

Kelsall Stability Index 28.67 knots

The Gemini is lighter than average and carries less sail area than average. The SA/D ratio is 12% lower. This shows up in the base speed being on the low side of the mid-range. Its most visible characteristic in comparison, is the significantly narrower beam. This accounts for the lower than average stability speed. Furthermore, the very low Bcl/Lwl ratio of the Gemini cats suggest wave interaction between the hulls which might adversely affect top speeds, although the hull waterline length to beam ratio of over nine should limit the size of its bow wave.

The full-length bridgedeck is the result of putting a lot of living space on a relatively short boat. Unless loading is controlled very carefully to keep weight out of the ends, this can result in significant pitching in waves. Further, when this is combined with the narrow and low tunnel between the hulls, slamming of waves on the undersides of the bridgedeck should be expected.

The Gemini has pivoting centerboards which reportedly give it excellent tacking ability.

Calvin H. Markwood
Engineering Analyst
Multihull Dynamics, Inc.

Contact Cal: multihull.analysis@comcast.net