Multihull Analysis

[Calculator]

MULTIHULL ANALYSIS
The analytical basis for the Multihull Dynamics, Inc. website www.multihulldynamics.com is as follows.
Many people in the process of buying cruising multihulls are frustrated when attempting to them in sort fact from fiction in the information about these boats in magazines, brochures and 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. This should be no surprise, since few businesses do so unless it is decidedly to their competitive advantage.
And there is another legitimate complaint that boat magazines do not press their advertisers to provide their claims, 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. Being fascinated by sailboats, especially multihulls, and I 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. Whether it is really better than the other performance indicators 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 very difficult 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 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 I was reluctant to attempt the first order comparative analysis I had 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 an analysis of 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 the website. It’s precision is limited by the accuracy of the specification data available on the boats. Advertised weights and displacements are questionable. Sail area used is for main and jib (100% fore-triangle) as much as possible. 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 for stability index calculations. 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 not uniformly available, so they are not considered in the analyses. And you can frequently find different specification values on several websites for most of the boats.
Despite these qualifications, valid first-order evaluations can certainly be done with the data and analysis techniques available. One powerful feature of the website is that, for a modest price, you can enter your own data to evaluate a variation of one of the boats or an entirely new data set. Good luck.

Cal Markwood
Engineering Analyst
Multihull Dynamics, Inc.

[Talbot]

most multihull analysis falls down cause the comparisons are between boats operating at their minimum weight, rather than at the additional weight required for cruising.

[Calculator]

The site analysis is based on DESIGN displacement, what the designer intended the loaded boat to weigh when loaded to the design waterline.

For the benefit of site users, however, there are provisions to input any weight a user feels is appropriate and determine what effect that has on performance and stability.