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 Island Packet 31, designed by Bob Johnson, founder of Island Packet Yachts, was the most risky and the most successful of the Island Packet lineage. Styled as a traditional cruiser but with more than a hint of broad hulled ‘catboat’ in her appearance, around 262 were produced between 1983 and 1989. She has the simplicity, roomy interior and shallow draft of the catboats, used for transport and fishing around the waters of the New England coast, as well as the wide ‘codhead’ hull.
The Island Packet 31 doesn’t come cheap for a 31 footer. Her huge interior and solid construction are a big part of her allure, notwithstanding the good name that the Island Packet Yachts brand has built. Not everyone is a fan of her unusual design but she’s considered to be a comfortable classic liveaboard yacht, particularly in shoal waters. Opinion is divided on whether she’s truly an offshore cruiser.
In 1979 in Largo, Florida, naval architect Bob Johnson, having designed boats for a number of years for other outfits such as Irwin and Endeavour, decided to set up on his own and began building small boats under the company name Traditional Watercraft Inc. In 1980, modifying old molds from an out-of-business Bombay Yachts, he created the Island Packet 26. The boat was marketed as the Mark I, Mark II and eventually the Island Packet 27 and was quite a success. In 1983 Bob took a leap of faith and put the company on the line essentially, in a time when many yacht builders were struggling to stay in business by designing the Island Packet 31 from scratch. His leap ended gracefully in 14 orders for the IP31 at the U.S. Sailboat show in Annapolis and saw the beginning of a seven year production run which only ended in 1989 with the advent of the Island Packet 32.
The excellent resale value of the Island Packet 31 on the used boat market reflects not only the popularity of the boat but also that of Island Packet Yachts. Over the years Island Packet have earned their place amongst the top producers of cruising yachts, not only for their proven designs but for the apparently outstanding level of customer service and support which they provide. It appears that no man with an Island Packet yacht is an island.
Most Island Packet 31’s come with a double-headed sloop rig but 10% have the plain sloop rig that apparently works well with a 150% genoa. The quirky looks are bestowed by an almost perpendicular stern, short overhangs, sweeping sheer and a stubby bowsprit as well as the broad beam she carries almost throughout, with her maximum 11′ 6″ beam forward of amidships. On deck the flat expanse of cabin roof, full length hand rails and wide side decks make working safe and easy. The cockpit is generously sized at over 7ft but really too big for serious offshore sailing.
Inside, she’s light and airy and has the accommodation of a much bigger boat. There are some clever features such as the folding door/fold down chart table combo which can be used to close off the double quarter berth aft. Plenty of headroom above, a generous sized head, several hanging lockers and a full size wrap around galley fill out the plentiful available space.
Island Packet market traditional designs married with modern construction techniques and although not a heavy boat, time has proven the IP31 solidly built. Island Packet use their own unique product, Polycore, for coring the deck and it appears to have stood the test of time with no reports of delamination. Although liners are used for the interior opinion has it that they are sensibly installed, leaving access to the bilges and other critical areas.
Below the water she carries Bob Johnson’s full foiled keel, a hallmark of Island Packets. It’s essentially a fin keel but stretched lengthways fore and aft into a long keel in a nod to enhancing performance while preserving a shallow draft. As a result draft is only 4′ and around 10% of 31’s were built with centerboards, reducing the draft to 3′ feet. The keel is not fastened to the hull but an integral part of it and according to Bob Johnson is something that’s at the heart of an Island Packet.
Like the catboat, the Island Packet 31 is not renowned for her windward performance. Not unexpected with her unusual volume distribution and large wetted surface area. However, thanks to her short bowsprit she can carry a decent amount of sail and with her long waterline she’s apparently capable of reaching at 6 knots. She’s well balanced with good directional stability but not very responsive. Owner’s report that she’s easy to single hand and is stiff enough to carry maximum sail in up to 20 knots. Light air performance is disappointing. Other owners report an unpleasant slow rolling motion under sail, particularly in chop, thanks to her large roll angles.
Island Packet 31’s are not cheap and their excellent resale value on the used boat market means they usually sell fairly quickly. A current search of the used boat market reveals prices from 45,500 to 67,800 US dollars depending on age and condition. Buyers should note that the Island Packet 31 is challenging to manoeuvre under power, particularly in reverse. A few incidents of blistering have arisen but there are no major problems with this boat.
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