Technical » Bow and Stern Overhang Effects

Release Date: 3/2/2006

BOW AND STERN OVERHANG EFFECTS

In 1964 a paper written for presentation to the Society of Naval Architects and Marine Engineers, in San Diego, CA, multihull design pioneer Hugo Myers evaluated the effects of bow and stern overhang on boat performance and hence on rating systems. Myers was dealing with just catamarans, not trimarans. The leeward hull of a catamaran transfers more and more of the boat weight and sinks deeper as the boat heels. If there is overhang in the bow and stern, boat length at the waterline will increase as the hull sinks. This is not the case with a trimaran.

He concluded that catamaran performance measurements should take into account 10% of the overhang in light wind conditions and 30% in medium. In other words, rather than using the waterline length for calculations of PI-C, Base Speed and Texel Rating as done on this website, it would be the waterline length plus 10 to 30% of length overall minus waterline length or Lwl + .1(Loa-Lwl) for light air, or Lwl + .3(Loa-Lwl) for medium. As an exercise, this was applied to the catamaran database for this website and it was found that the average change was less than 1% and the greatest was about 4% for one of James Wharram's Narai Mk IV with an incredible overhang of nine feet on a 41 foot boat.

The boats Myers considered were five elegant Choy/Seaman/Kumali catamarans from the early 1960s, Allez Cat, Imi Loa, Imua, Makia and Patty Cat II plus his own Eunike. The CSK boats had extensive overhang - Myers' boat had very little.

Thus, the performance indicators for catamarans on this website are potentially understated for boats which have large bow and/or stern overhangs.

Calvin H. Markwood
Engineering Analyst
Multihull Dynamics, Inc.

Contact Cal: multihull.analysis@comcast.net