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1998 Gemini 105M

Naples, Florida, United States
$79,900 USD

Ready to cruise Newer CA, newer main sail cover, newer flooring in cockpit. Refurbished auto pilot, less than 800 hours on diesel engine, some windows replaced, newer helm compass and navigation, newer elec macerator head, 4 batteries, dual charger, solar panels, newer stereo, elecpropane fridge, BBQ grill, newer bilge pumps, and misc equipment. In a slip in a marina just 30 minutes from the Gulf of Mexico. Exterior Upgrades: New side windows Rebedded upper main window Grill & stand Halyard & sheets Window covers Sail cover Rocna anchor 9.5’ dinghy & cover 3hp Johnson outboard Lawrence MFD & AIS Netting perimeter Head hatch solar fan LED stern light LED bow navigation light Auto pilot Raymarine wheel drive - rebuilt Bilge pumps (2) in aft cockpit compartments Elec fuel pump Fuel filter system Rebuilt alternator Steering cable replacement to rudders Cockpit flooring Removed old wind generator Nylon mesh lounge seat aft Interior Upgrades: MPPT Controller Webasto 10K A/C - 110V & water pump 4 - 12 volt batteries Dual battery charger LED lighting SeaFlo electric macerator head 2 bilge pumps.

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Specs

Designer
Tony Smith
Builder
Performance Cruising
Associations
?
# Built
1100
Hull
Catamaran
Keel
Twin Centerboard
Rudder
?
Construction
FG

Dimensions

Length Overall
33 0 / 10.1 m
Waterline Length
31 9 / 9.7 m
Beam
14 0 / 4.3 m
Draft
1 6 / 0.5 m 0 11 / 0.3 m
Displacement
8,600 lb / 3,901 kg
Ballast
?

Rig and Sails

Type
Sloop
Reported Sail Area
510′² / 47.4 m²
Total Sail Area
?
Mainsail
Sail Area
?
P
?
E
?
Air Draft
45 11 / 14 m
Foresail
Sail Area
?
I
?
J
?
Forestay Length
?

Auxilary Power

Make
Westerbeke with Sillette saildrive
Model
?
HP
27
Fuel Type
Outboard
Fuel Capacity
36 gal / 136 l
Engine Hours
?

Accomodations

Water Capacity
60 gal / 227 l
Holding Tank Capacity
18 gal / 68 l
Headroom
?
Cabins
?

Calculations

Hull Speed
10.5 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

10.51 knots
Classic formula: 7.55 knots
Sail Area/Displacement
19.4
16-20: good 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.
19.44
<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
119.9
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
119.86
<100: ultralight
100-200: light
200-300: moderate
300-400: heavy
>400: very heavy
Comfort Ratio
12.2
<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
12.15
<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
2.7
>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
2.74
<2: better suited for ocean passages
>2: better suited for coastal cruising

Notes

From BlueWaterBoats.org:

After a 2008 Pacific crossing, the owner of a Gemini exclaimed, “the catamaran hull performs flawlessly in open blue water as well as the light winds of the Kona coast … the forward stateroom is queen sized .. the main cabin seats six … plenty of height in the full head with shower.” Great sailing and lots of living space – what’s not to like?

The Gemini is the most popular cruising catamaran in the world with more than 1000 hulls manufactured over nearly 30 years. While most of those hulls are in service as coastal cruisers, many have crossed the oceans. In 2001 the owner, designer and builder of the Gemini catamaran series, Tony Smith, sailed a new Gemini 105Mc across the Atlantic Ocean for a delivery to Southampton, England topping out at 18 knots of speed (surfing down the face of heavy seas). Many more of these adventures are chronicled in the Gemini Gems magazine (print and electronic versions) or the very active Gemini Yahoo forum.

A large cockpit with wheel steering and plenty of room for an afternoon picnic greats the sailor ascending the aft stairs. A generous, hard foredeck is surrounded by a stainless pulpit for the entire bow and lifelines running along the somewhat narrow side decks. With only about 8000 pounds of displacement, anchor chain and rode need not be overly heavy. Sailing a boat with such light displacement across the ocean requires careful consideration of heeling angles and reefing. Anything above seven degrees of heel means it’s time to reduce sail.

Returning to the cockpit allows the sailor access to the interior. Starting at the same level as the cockpit sole, immediately one is greeted by a salon table that comfortably seats four and even eight in a squeeze. Descending a few steps to the left and moving forward is found the head and shower. Hot water is provided by a propane driven instant heater, like an RV or many European homes. The big advantage is that if you have water in the two 35 gallon fresh water tanks, then you can have hot water.

In the aft part of both hulls are two births. The master cabin is in the starboard hull forward, featuring a queen sized bed. There is a tremendous amount of storage for dry goods in the galley and a propane driven refrigerator (using the ammonia cycle instead of a compressor).

The Gemini is mainly driven by the roller furled genoa and somewhat small main, though many sailors add screechers, asymmetrical spinnakers and jibs to their sail lockers. Each hull has a cable driven, hand-cranked centerboard that can be completely tucked into a trunk in the hull, reducing the draft (assuming the rudders are likewise raised), or dropped three feet into the water, dramatically improving upwind performance. While most sailing and motoring can be accomplished with both boards let half-way down, the ability to adjust the boards to wind, current, sailing or anchoring conditions is one of the Gem’s great strengths.

The 105 and 105MC are driven by a 27 HP Westerbeke diesel engine driving a Sillette tilting saildrive. The saildrive is often a source of concern and maintenance is critical on the bellows.

History

There are three models of the Gemini, the classic (1981-1990), the 3200 and 3400 (1990-1996) and the most popular, the 105M and 105MC (1996-2012). This article focuses on the most popular model, 105M and 105MC, which differ only in details of the interior construction. The classic, 3200 and 3400 were all manufactured in Annapolis, Maryland. Recently, Performance Cruising has partnered with Hunter Marine to manufacture the 105Mc in Alacua, Florida. There are nearly always pre-owned examples of all three types on the market.

The model most likely to be used in blue water is the 105M or 105MC discussed here. For this model, the hull lines were redrawn moving the maximum beam further aft to reduce wave drag. These new hulls have a 9:1 length:beam ratio. The new hulls also have a somewhat higher static stability. The newer hull design is stiffer, increasing seaworthiness for trans-ocean cruising.

The hull is solid GRP, laminated from woven roving and mat with isophthalic polyester resin. An outer layer of polyester resin and gel coat protects against osmosis. The decks and cockpit sole are GRP sandwich with balsa wood core.

Links, References and Further Reading

» A review of the Gemini 105M by John Kretschmer, Sailing Magazine, Aug 1999
» Chesapeake Bay Magazine’s review of the Gemini 105Mc

Credits

Original article submitted by Robert Malkin.


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