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 35 is a ruggedly built, double-ended cutter intended for blue water passage-making with safety, comfort, and speed. She was designed by Thomas Gillmer, the professor of naval architecture at the Naval Academy in Annapolis who also penned the first fiberglass boat to circumnavigate the globe, the Allied Seawind 30 Ketch. The Southern Cross 35 is wide-beamed and graced with a sweeping sheerline that keeps her exceptionally dry in a rough seaway. At the same time, a relatively high-aspect rig along with a fin keel and skeg-mounted rudder allow her to combine impressive sea-worthiness with surprisingly lively performance.
Thomas Gillmer’s career was a life-long love affair with traditional boats. He drew the plans for the topsail schooner Pride of Baltimore and, following the tragic sinking of Pride in a microburst squall in 1986, designed her replacement, Pride of Baltimore II. He took part in the recreation of the 17th century Dutch merchant ship Kalmar Nyckel, now the tall-ship ambassador for the state of Delaware. Restoration of USS Constitution was carried out accordance with Gillmer’s studies. That traditional orientation informed his designs of pleasure craft, including his early Privateer series and the Colin Archer-inspired Southern Cross 31, with its full keel, tiller-steered outboard rudder, and bowsprit. For both the Southern Cross 35 and 39, however, Gillmer blended a traditional sheer, canoe stern, and cutter rig with a modern fin keel and skeg-hung rudder. The result was a pair of true sea-going boats with significantly improved speed and handling.
The Southern Cross boats were built by the C. E. Ryder Company of Bristol, Rhode Island, a firm with a long-established reputation for sturdy, high-quality construction with great attention to detail. Ryder also built the Sea Sprite line, including the 23, 28, and 34. The first Southern Cross 35 was launched in 1978, the last in 1990 when the company closed its doors. Sailboatdata.com lists a total of 95 hulls built over that 13 year period. Some of them, sold as Gillmer 35s, were owner finished. The factory complete boats were marked with a circular bronze plaque indicating the hull number.
The hull of the Southern Cross 35 is Airex foam core sandwiched between generous layers of handlaid fiberglass, a design that provides overall structural integrity, thermal and acoustic insulation, as well as an extra measure of security in the event of a collision with a floating object at sea. Decks are fiberglass with molded nonskid areas, cored throughout with end-grained balsa. The relatively shallow draft keel of 4’ 11’’ is fully encapsulated lead. The hull to deck joint is strong and dry, with double ninety degree joints topped with a rugged teak caprail. The sturdy and durable Navtec rod rigging is anchored securely into special aluminum drums, glassed into the hull. A 35 gallon fiberglass fuel tank is located midships beneath the cabin sole while twin 45 gallon plastic water tanks lie beneath each of the main cabin settees. Two cabin interiors were available, one finished in white oak with teak trim and another done completely in teak and teak veneer. The joinery work in the main cabin is exceptionally well finished, with care taken throughout to avoid hard edges and sharp corners.
Beneath the waterline, the Southern Cross 35 boasts an aperture-enclosed, skeg-mounted propeller and a fin keel for speed and ease of handling. Gillmer extended the beam fore and aft for stability in a seaway. Above decks, there are other features that well outfit the boat for off-shore sailing. Her rising sheer, ample bulwarks, and wrap-around cockpit with broad combing keep her very dry even in heavy seas. At the same time, her relatively low freeboard, augmented by a cut-away of the bulwarks on either side of the cockpit, treat the crew to an exciting proximity to the passing water. The wide beam, with shrouds mounted well inboard, allow for easy passage fore and aft. In the crotch of the canoe stern, a specially molded, ventilated locker neatly houses twin propane tanks.
The mast is keel-stepped and is rigged with a club-footed staysail, secured by permanently dedicated supplemental shrouds. Factory finished boats came in two versions, one with a portside quarterberth and the other with an aft-facing navigation station, fronted by a good-sized wetlocker. A U-shaped starboard aft galley and twin facing main cabin setees with a large folding table mounted on the mast trunk were standard. Three cabintop hatches (one in the main cabin, one in the head, and one in the forward V-berth) as well as eight opening ports keep the Southern Cross 35 well ventilated. Ample interior storage space is provided with a very large main cabin closet, a V-berth closet and drawers, and stowage outboard of the port and starboard main cabin setees. The original power plant was a 30 hp Universal diesel, perhaps a bit underpowered for the Southern Cross 35’s 18,000 lb. displacement.
The relatively high aspect rig of the Southern Cross 35 allows her to point reasonably well, tacking within 75 degrees, and gives the boat surprisingly respectable light air performance. PHRF New England rates her at 174. Her sail area to displacement ratio is 14.83. The cutter rig and mounting of the mast almost midships enables the Southern Cross 35 to remain well balanced on all points of sail. As a result, self-steering gear, electric or wind-driven, easily handles the rudder with proper sail trim, even in heavy weather. At the same time, the hull design affords a forgiving motion in a seaway. As Gillmer himself reported with some pride, “one owner told me it was the smoothest boat he had ever sailed in the ocean. That’s quite a lot to say about a 35-foot boat.”
Original article submitted by Richard Boothby.
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