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.
Classic hull speed formula:
Hull Speed = 1.34 x √LWLA 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
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.
SA/D = SA ÷ (D ÷ 64)2/3
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.
Ballast / Displacement * 100
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.
D/L = (D ÷ 2240) ÷ (0.01 x LWL)³
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.
Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam1.33)
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.
CSV = Beam ÷ ³√(D / 64)
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.
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.
Original article submitted by Robert Malkin.
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