Technical » Analysis of Multihull Sailboats

Release Date: 6/8/2006

Multihull Analysis - "First-Order"

03/12/06

Many people in the process of buying cruising multihulls are frustrated when attempting to sort fact from fiction.  Information regarding these boats vary in magazines, brochures, marketing material, and now on websites.  There is a legitimate complaint that designers and manufacturers do not disclose information comparing their boats with those of their competitors.  I think there is another legitimate complaint, boat magazines do not press their advertisers to provide this information, nor do they do valid comparisons in sail-offs at boat shows and other gatherings.   Race results are informative, but can be misleading with respect to crew skill, boat preparation and outfitting, etc.   Really valid, controlled comparisons are just not readily available.   So an analytical tool is understandably needed.

Since the 1990s, boating magazines have contained a great deal of facts and figures about modern cruising multihull sailboats - catamarans and trimarans.  I am fascinated by sailboats, especially multihulls, and wondered what I could learn by doing a first-order analysis of the data that was available about them.  After refreshing my memory on principles and equations from several sources, I used the data, augmented with some assumptions where data was missing, and did a comparison of 36 of the catamarans.  This work was published as "Ratios and Cruising Catamarans" in Multihulls World no. 23, April/May 1994.  As time went on, I added data from various magazines for a total of over 100 cruising catamarans, revised the analysis and submitted it for publication as "Theory and Statistics for Cruising Multihulls" in Multihulls Magazine vol. 23, no. 2, March/April 1997.***

I got feedback from the articles on analytical methodology, especially with regard to performance and stability.  For the article in the 1994 Multihulls Magazine,  I derived a performance index that considered sail area, displacement and length.  For the 1997 article, I used a similar index by Derek Kelsall called the Kelsall Sailing Performance Number.    Richard Boehmer wrote to me after that article and offered an index called "Base Speed", an empirically-derived indicator of the distance a given boat could travel in 24 hours under a variety of conditions. It can be used to compare speed potential of one or more boats and has been used for handicapping boat races involving a variety of boat types. Whether it is really better or not, it feels better because the answer comes out in knots!

Another index I derived from first principles was a capsize stability index that considered sail area and arrangement, displacement and beam.  This appeared in the 1997 MM article.  I refined the index to use the spacing between the centerlines of the hulls rather than overall beam for a 1999 MM article, but found that beam centerline data was nearly impossible to obtain.  So I scaled drawings in magazines for an approximation that will allow boat-to-boat comparisons and did a mathematical approximation where that was not possible.  John Shuttleworth in "Multihull Designs" by John Shuttleworth Yacht Designs, Ltd., 1998 presents a similar formula for static stability in flat water and says it gives the wind speed at which a boat has to reduce sail. These formulas are used on the MDI website.

Until the latest revision of my work, I have avoided trimarans.  There is a complexity to trimarans that doesn't exist with catamarans, i.e., the outriggers. There is such a wide variety of approaches to designing the outriggers of trimarans that they had frightened me away from the "First Order" comparative analysis as I have done with catamarans.   After all, the two hulls of a catamaran are always either identical or mirror images of each other.    However, I have proceeded with a first-order analysis for trimarans based on the assumption that the main hull is designed to carry the full displacement of the boat.  Analysis of the outriggers is ignored other than for stability.

A qualifying statement must be made about the analysis used for this website.   Its precision is limited by the precision in the basic data available on the boats.  Advertised weights and displacements can cause one to ponder; you can frequently find different numbers on several websites for most of the boats, though the differences frequently are minor.  

We intentionally use Sail Area  for main and jib (100% fore-triangle) only in our calculations.   This allows us to compare boats fairly.   Height of the center of effort of the sails and distance of hull center of effort below the waterline are rarely available, so they are approximated through mathematical calculations for stability index calculations. More detail on this is available in our Technical section. 

Hull design and the layout of hull lifting devices (keels, centerboards, daggerboards, outriggers of trimarans, etc.) greatly effect many performance matters.  However, details about them are rarely available and so are not considered in the analyses.   All this said, "First Order" evaluations can certainly be done with the data and the analysis available.

To further understand MDI's "First Order" specification Database please read Database Maintenance.

Calvin H. Markwood
Engineering Analyst
Multihull Dynamics, Inc.

Contact Cal: multihull.analysis@comcast.net

***Mr. Markwood has since updated the article, now titled "Analysis and Comparison of Cruising Multihulls 2008". The article is for sale on this website. It is a clear and precise primer for evaluation of sailing multihulls. It provides a complete discussion of the analysis of the database behind this web site.

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