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 Aquarius Pilot Cutter, launched in the late 1970s, was conceived as a limited production offshore cruiser with focus on easy handling, comfort and safety at sea. She takes inspiration from the working boats of the 19th century which were both fast and able to carry large loads, these boats have the hallmarks of an easily driven hull, good windward performance and an ability to carry large amounts of canvas to make up for heavy displacements and payload. In tune with tradition, the boat has heavy scantlings, lots of bronze hardware and even traditional tanbark coloured sails.
The boat was birthed when Frank Parish, an ex-airforce pilot, penned the Aquarius Pilot Cutter for his own needs and engaged the East Coast boatbuilders at Topsail Yachts in Portsmouth, Rhode Island to construct it. In designing his boat, Parish influenced by his own sailing experiences had the chance to meld his appreciation of traditional designs with modern construction. Being a large guy, he went for a roomy cabin with good headroom which necessitated a beamy hull that was capable of carrying a large amount of canvas to push it. At the time many compared the design to those of Lyle C. Hess, who was influential in remaking the traditional pilot boat design for the late 20th century, examples include the Bristol Channel Cutter, her smaller stablemate the Falmouth Cutter, and famously, Lin and Larry Pardy’s 24 foot Seraffyn and their subsequent 29 foot Teleisin. In particular, the internal layout of the Aquarius is identical to Seraffyn, though Parish says he never heard of Seraffyn until after the Aquarius had already started construction.
The Aquarius has modern sail controls with all lines, including reefing controls, leading back into the cockpit – ideal for single handing. Lazyjacks came standard. Belowdecks is a very practical and spacious setup, designed to maximise ventilation flow with no obstructing bulkheads. There’s berths that are 6’ 7” long and 6’ 3” of headroom, rare to see in boats of this size. She’s also well into heavy displacement territory for her size, but bear in mind her 410 square feet of canvas makes up for this (not to mention a healthy 460 square feet for gaffed rigged versions).
Construction is suitably overbuilt – hand-laid solid fiberglass with polyester resin, with a hull thickness varying from 3/8” at the minimum to 5/8” at the turn of the bilge and as much as 3/4” in the keel sections. The ballast is lead encapsulated. Deck and cabin house are fiberglass cored in 1/2” marine ply, and the hull to deck join is via an overlapping shoe box fit bedded with sealant and through bolted on 12” centers.
In total thirty three boats were constructed between 1979 and 1984, mostly marconi-rigged, but gaff-rigged versions were also produced, better suited to the light air sailing prevalent along the East Coast. At least three layouts were offered though the boat was built to order so a high degree of customisation can be seen between examples. The standard engine on offer was an inboard 18hp Volvo, but owners could optionally go with a transom mounted outboard freeing up internal space.
» The Sailor’s Book of Small Cruising Sailboats by Steve Henkel
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