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 31 is a no nonsense double-ender designed by Tom Gillmer, a professor of naval architecture at the US Naval Academy. The design takes influence from shapes dating back to Colin Archer’s work nearly a century earlier, and the hull in many respects is similar the straight transomed Allied Seawind 30, an earlier Gillmer creation notable for being the first fiberglass boat to circumnavigate the globe. (The Allied Seawind 30 was successful enough to be updated to the Allied Seawind II)
She’s a full keeled sailboat with an outboard rudder and tiller combo, a configuration often praised for its simplicity and the cockpit is quite small, suitable for ocean passages. The cutter rig with bowsprit has 447 sq. ft. of canvas, plenty for its displacement, but being a heavy displacement boat performance is hampered in light airs. Looking on the bright side, the weight gains dividends with a comfortable motion while underway.
Two interior layouts were offered, one with a stand up navigation station over a large locker and one with quarter-berth. The overall construction quality was of a good standard, suitably strong for offshore work. The hulls were built in fiberglass with the topsides cored in Airex foam, while the deck and cabin house was cored in balsa.
In total 130 boats were built between 1976 and 1987, of which a number were sold as hull and deck kits. A slightly larger and similar Gillmer design is the Aries 32 (around 20 of these were built).
» Southern Cross Owners Association
» Twenty Small Sailboats to Take You Anywhere by John Vigor, (Ch19, p117-123) an in depth look at the Southern Cross 31. ISBN:978-0939837328
» Ryder Yachts
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