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)
Touted “the most expensive boat of its size” at her introduction in 1977, the diminutive Mariah 31 helped build Pacific Seacraft’s reputation for making quality boats. She was designed by the original co-founder of Pacific Seacraft, Henry Mohrschladt who only two years earlier kicked off the business with Mike Howarth building boats out of Howarth’s garage.
The Mariah 31 is a ridiculously sturdy boat, with hull thicknesses seldom seen in boats twice her length. We’re talking one inch at the topsides extending to 3 inches at the bilge, and deck thicknesses of an inch and a half (where you can hear owners complain they can’t readily find through-deck bolts long enough). As testament to her strength, Paul Lutus during his solo circumnavigation in Selene writes of surviving a blow with a semi-submerged shipping container without taking on any water, the impact had enough force to throw him clean off from his berth while he slept.
Not surprisingly she is heavy, requiring a lot of canvas hung from her 4ft bowsprit to keep her moving. Later MkII versions introduced a 5ft bowsprit. The interiors have a nice layout, loads of headroom and are finished in high quality teak.
Under sail she’s generally considered a slow boat, expect to clock regular 100 mile days in the trades with a well set up rig. She’s at her best on a reach, with 14-18 knots on the beam – expect a solid 6 knots. However with the wind from behind, her tub-like underbody and shoal keel doesn’t do much to reduce rolling motion which can get uncomfortably large.
Production ceased in 1983, the rumor was that the boat was too expensive to keep going. Before production ended a number of boats were sold as hull and deck kits and finished by their owners.
» Mariah 31 Yahoo Group, owner discussions.
» Confessions of a Long Distance Sailor by Paul Lotus, a solo circumnavigation in a Mariah 31.
» Mariah 31 Sea Trial by Earl R. Hinz, Sea Magazine, Aug 1978.
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