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)
The Stevens 47 is a fast, three stateroom cruiser designed in 1981 by Rod Stephens of the legendary Sparkman and Stephens firm. The Stevens 47 is arguably the most well respected bluewater, three stateroom sailboat produced and is apparently highly prized on the brokerage market. Her soft motion in a seaway, swift 200 nautical mile per day passages, and luxurious accommodations are unrivaled by yachts of a comparable vintage.
It’s said the design was inspired by the Gulfstar 50 which was produced during the late 1970’s by Vince Lazzara, one of the pioneers of fiberglass construction. The boat builders and owners, Queen Long Marine of Taiwan, gave Sparkman and Stephens complete design freedom, hoping for a lucrative vessel to produce.
Although the design was aimed at the charter trade she was really much more. Bill Stevens, the Caribbean Charter king, immediately realized the vessel’s potential and purchased as many as he could for his British Virgin Islands charter fleet as well as lending his name to the vessel. All told, 138 boats were built, 56 under the Stevens name before Queen Long Marine rebranded under the Hylas 47 name, building a further 82 boats. Production ended in 1991 with the introduction of a modified 49-foot version, the Hylas 49, that is still being built today.
The Stevens 47 has a modern raked bow for easy anchor clearance, slight sheer, and a “rocket ship” stern. The long keel is paired with a separate skeg-hung rudder arrangement that leads to a soft and balanced motion offshore. Her inboard mounted chainplates allow for ease of movement along the side decks as well as tight sheeting angles. She comes with a powerful cutter rig and the main mast is a bridge-clearing, Intracoastal-friendly 63 feet. Her beamy hull means plenty of room down below for her three-stateroom accommodations. Some Stevens 47’s had split berths aft though most feature a centerline queen. The interior is in fine light teak with fantastic joinery work.
The hull is solid hand laid fiberglass while the deck is balsa or Airex cored and joined with the traditional inner flange joint. A grid-like network of nine full length stringers, twelve transverse timbers, and a stainless steel I-beam to support the keel stepped mast stiffen the molded hull. The ballast is a lead insert of 14,500 pounds encapsulated in the fiberglass keel.
The 47′s greatest strength is her sailing capabilities. The performance of these Stevens is legendary. The hull shape and keel-rudder configuration make them surprisingly stiff and fast boats, well-balanced in any condition with a smooth motion offshore. They are known to reel off 200 nautical miles days with ease.
While the Stevens and Hylas versions were both built to good standards at Queen Long Marine, the Stevens versions have beefier deck hardware from their commissioning at Bill Stevens’ North Carolina yard. These days any version has likely been seriously refitted and loaded with extra equipment. Watch for water damage to the veneer as the port lights can be prone to leaks.
» Capria, Mary Ann and John Macevoy, Cruising World, Stevens Custom 47: Swift and Secure
» Jordan, Richard, Waves, Hylas 47: The Original Hylas
» Kretschmer, John, Used Boat Notebook, Stevens-Hylas 47
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