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 Southern Cross 39 is a no nonsense double-ender following the theme of her two smaller siblings the Southern Cross 31 and Southern Cross 35. Designed by Thomas Gillmer, a professor of naval architecture at the US Naval Academy, she has the same canoe stern and sharply rising sheer line, but underneath Gillmer gave her a fin keel and skeg hung rudder arrangement to increase the performance of this passage making machine. Her cutter rig with bowsprit has 774 sq. ft. of canvas, plenty for her displacement of 21,000 lbs.
Launched in 1981, the first 13 hulls were factory built and bathed in fine light teak. About half of the subsequent models were sold as kits and owner finished so interior layouts can vary. The most common configuration has a V-berth forward, quarter berth portside, and superb seagoing galley amidships. The galley is easy to access from the companionway with a U-shaped orientation for security offshore. The hulls were built in fiberglass with Airex foam coring throughout while the deck and cabin house were balsa cored. The ballast is internally set cast iron. Water tanks are set in the keel while the fuel tank is underneath the aft berth.
These boats were built until 1990 when the Ryder factory closed.
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