# Gemini 105MC

2003 — 2011
Designer
Tony Smith
Builder
Performance Cruising
Associations
?
# Built
?
Hull
Catamaran
Keel
Twin Centerboard
Rudder
?
Construction
FG

### Dimensions

Length Overall
33 5 / 10.2 m
Waterline Length
31 9 / 9.7 m
Beam
14 0 / 4.3 m
Draft
1 6 / 0.5 m 5 6 / 1.7 m
Displacement
8,000 lb / 3,629 kg
Ballast
?
• 1 / 1

### Rig and Sails

Type
Sloop
Reported Sail Area
690′² / 64.1 m²
Total Sail Area
?
Sail Area
?
P
?
E
?
Air Draft
?
Sail Area
?
I
?
J
?
Forestay Length
?

Make
Westerbeke
Model
?
HP
27
Fuel Type
Diesel
Fuel Capacity
36 gal / 136 l

### Accomodations

Water Capacity
60 gal / 227 l
Holding Tank Capacity
?
?
Cabins
?

Hull Speed
10.8 kn
Classic: 7.55 kn

### Hull Speed

The theoretical maximum speed that a displacement hull can move efficiently through the water is determined by it's waterline length and displacement. It may be unable to reach this speed if the boat is underpowered or heavily loaded, though it may exceed this speed given enough power. Read more.

Formula

Classic hull speed formula:

Hull Speed = 1.34 x √LWL

A more accurate formula devised by Dave Gerr in The Propeller Handbook replaces the Speed/Length ratio constant of 1.34 with a calculation based on the Displacement/Length ratio.

Max Speed/Length ratio = 8.26 ÷ Displacement/Length ratio.311
Hull Speed = Max Speed/Length ratio x √LWL

10.75 knots
Classic formula: 7.55 knots
Sail Area/Displacement
27.6
>20: high performance

### Sail Area / Displacement Ratio

A measure of the power of the sails relative to the weight of the boat. The higher the number, the higher the performance, but the harder the boat will be to handle. This ratio is a "non-dimensional" value that facilitates comparisons between boats of different types and sizes. Read more.

Formula

SA/D = SA ÷ (D ÷ 64)2/3

• SA: Sail area in square feet, derived by adding the mainsail area to 100% of the foretriangle area (the lateral area above the deck between the mast and the forestay).
• D: Displacement in pounds.
27.6
<16: under powered
16-20: good performance
>20: high performance
Ballast/Displacement
?

### Ballast / Displacement Ratio

A measure of the stability of a boat's hull that suggests how well a monohull will stand up to its sails. The ballast displacement ratio indicates how much of the weight of a boat is placed for maximum stability against capsizing and is an indicator of stiffness and resistance to capsize.

Formula

Ballast / Displacement * 100

?
<40: less stiff, less powerful
>40: stiffer, more powerful
Displacement/Length
111.5
100-200: light

### Displacement / Length Ratio

A measure of the weight of the boat relative to it's length at the waterline. The higher a boat’s D/L ratio, the more easily it will carry a load and the more comfortable its motion will be. The lower a boat's ratio is, the less power it takes to drive the boat to its nominal hull speed or beyond. Read more.

Formula

D/L = (D ÷ 2240) ÷ (0.01 x LWL)³

• D: Displacement of the boat in pounds.
• LWL: Waterline length in feet
111.5
<100: ultralight
100-200: light
200-300: moderate
300-400: heavy
>400: very heavy
Comfort Ratio
11.3
<20: lightweight racing boat

### Comfort Ratio

This ratio assess how quickly and abruptly a boat’s hull reacts to waves in a significant seaway, these being the elements of a boat’s motion most likely to cause seasickness. Read more.

Formula

Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam1.33)

• D: Displacement of the boat in pounds
• LWL: Waterline length in feet
• LOA: Length overall in feet
• Beam: Width of boat at the widest point in feet
11.3
<20: lightweight racing boat
20-30: coastal cruiser
30-40: moderate bluewater cruising boat
40-50: heavy bluewater boat
>50: extremely heavy bluewater boat
Capsize Screening
2.8
>2.0: better suited for coastal cruising

### Capsize Screening Formula

This formula attempts to indicate whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions. Read more.

Formula

CSV = Beam ÷ ³√(D / 64)

• Beam: Width of boat at the widest point in feet
• D: Displacement of the boat in pounds
2.8
<2: better suited for ocean passages
>2: better suited for coastal cruising

### Notes

GEMINI 105MC is a development of the 105M with a number of interior updates. Most visibly, however, the cockpit of the 105Mc can be temporarily enclosed with either canvas or solid panels.

### For Sale

Have a sailboat to sell?