Introduction to Catamaran Stability
|By James Wharram (2004)|
It is 50 years since I designed my (and Britain's) first offshore Sailing Catamaran. The accepted opinion at the time, expressed in Yacht Magazines, was that the offshore catamaran would break up in high sea waves, that their motion on the high seas would be so violent as to render the crew helpless and that the double canoe/catamaran could not sail to windward.
In the late 1950s, the Prout Brothers were developing a 16ft. racing day boat catamaran. It was fast and outsailed all monohull racing, dinghies of the time.Like racing sailing dinghies, without skilled handling, they capsized frequently. Still, with the attendance of the patrolling Race Guard Boats no one died.
Offshore catamarans began to first develop in the 1960s. From the beginning, there were some designers, like myself, who saw them as Form Stable boats following traditional Form Stable Stability values i.e. boats which 'looked after their crew'.
There were also designers (not many), drunk on the speeds of day racing catamarans, who used the sail areas, mast heights and stability values of the day racing catamarans on Offshore Cruising Catamarans, i.e. at all times the Crew looked after the stability of the boat.
Unfortunately, designers of these low stability catamarans have nearly always tended to imply in yacht magazines, to the public, that they are more skilled designers "Because their boats sail faster"?
Equally unfortunate was that by 1976 many of these low stability catamarans were publicly capsizing, when their trained crew got tired or, particularly, when sold to unsuspecting monohull sailors. Suggestions in England and America were made to 'ban offshore multihulls'. Hanneke Boon and I wrote our first article on cruising catamaran stability ("The Stable Multihull") in 1977, and things settled down again.
However, around the late 1980s another group of young designers from racing background or using racing catamaran concepts moved into Cruising Catamaran design and, once again, capsizes with deaths occurred. So, again we wrote in another article on our observations on safe stability for Cruising Catamarans, which you can now read here.
This article was first published in 'Practical Boat Owner' (UK) in August 1991 and since then in several other countries. The I.S.O. has recently also published formulas for calculating catamaran stability as part of the Recreational Craft Directive. On examination, their formula is the same as the one published by us in 1991 with a slightly smaller safety margin for Dynamic Stability (70% to our 60%). So far, the I.S.O. has not yet given recommendations as to what is a 'safe' stability for offshore sailing.
Read James' Catamaran Stability article