Port of MDI

Abbreviations, Definitions, Formulas and Terms

MDI's "First Order" evaluation of Multihulls - an explanation from the basics to performance and stability.

A Primer - Electric Motor Horsepower Ratings


This old saw applies to both animals and advertising alike. And while the distinctions between a pig and a horse should be painfully obvious, attempting to differentiate the true horsepower of an alternating current (A.C.) electric motor from its advertised power is not easily accomplished. No doubt you have noticed a great deal of hype .....

The Texel rating and its development since 1984

The Texel rating and its development since 1984

Updated 14 February 2011


The Texel rating formula has been used in the Netherlands for the first time for the round the isle of Texel race in 1984. That has been the source of its name too. Before that year yardstick numbers were used to approximate the performance you might expect from catamarans. As these numbers were derived from race statistics, it was not possible to determine handicap numbers for new designs.


As a consequence the Texel Race organization, supported by the national sailing authority in Holland, decided to start using rating numbers. They are numbers calculated from measured data of the designs, by the use of a practical performance formula. This formula is inspired by studies from the British designers John Shuttleworth and Derek Kelsall and supported by Richard Boehmer and others. An article of the American designer Olin Stephens published in 1972 about 20 elements of speed was an extra inspiration. On the places 1, 2 and 3 he mentioned the elements length, sail area, displacement. These three elements were shown to have the greatest influence on the performance potential of a yacht.

The S Number

In the February/March 2011 issue of Professional Boatbuilder magazine, the ROVINGS section by Dan Spurr, there was a summary of a section of The Design Ratios by Eric w. Sponberg of SPONBERG Yacht Design Inc. dealing with the S Number. The S Number is a formula that assesses relative performance of sailing yachts, giving values that range from 1 to 10, with sub-ranges of these values assigned to Racing Machines (5-10), Racer-Cruisers (3-5), Cruisers (2-3) and Lead Sleds (1-2).

The formula is: (read the free article for how this relates to multihulls)

Stem Shape

Several boats that have been built recently or are being built to Bernd Kohlers designs show very blunt stems, with sharp angles where the front joins the sides. This is an undesirable practice that will result in turbulence and thus drag.

Significance of Performance and Stability Indicators

Sail Area/Displacement - SA/D, Bruce Number - BN, Base Speed - BSpd and Stability Speed - SSpd are discussed in more detail.  How one measure of performance can be more beneficial than another.  Stability is discussed in relationship to performance.

Hull Cross Sections for Multihulls

Most multihull design books include a section about the pros and cons of various hull cross sections. They dont always agree with each other, and when that happens, it begs the question of why not. The intent of this piece is to shed a little light on that question.

Bridgedeck Clearance for Catamarans Revised 1-'10

The following standards for Bridgedeck Clearance (BdCl) have been discovered through the literature and direct contacts with the sources:

Six Kinds of Cats and Two Kinds of Tris

By Calvin H. Markwood

"Six Kinds of Cats and Two Kinds of Tris" documents the use of the same analytical methodology to compare the designs of prominent catamaran and trimaran designers.

The catamaran designers are Brett Crowther, Kurt Hughes, Derek Kelsall, the team of Morrelli and Melvin, John Shuttleworth, and Chris White. The trimaran designers are Ian Farrier and Dick Newick.

'Multihull Design Considerations for Seaworthiness'
Multihull Dynamics' incorporates evaluation methods developed and elaborated by John Shuttlesworth.
Bow Bulbs

Most ocean-going cargo vessels are built with a large bulb protruding forward from the hull, below the waterline. The purpose of these bulbs is to reduce or eliminate the bow wave and thereby increase speed or reduce power required. The applicability of these bow bulbs to multihulls is an intriguing topic.

Hull Cross Sections for Multihulls Mostly Catamarans

Most multihull design books include a section about the pros and cons of various hull cross sections. They don't always agree with each other, and when that happens, it begs the question of why not. The intent of this piece is to shed a little light on that question.

Classifying Multihulls

When viewing, shopping, admiring multihull sailboats, it is easy to become enthralled with the prospect of high performance - meaning speed. Sailing past another boat is probably one of the primitive urges of man.

Anti Vortex Panels
Analysis of Multihull Sailboats

Many people in the process of buying cruising multihulls are frustrated when attempting to sort fact from fiction. Here is the background about MDI's "First Order" Evaluations

Database Maintenance

To develop the database for the Multihull Dynamics Inc. website, boat specifications were collected over more than a decade from boating books, magazines, websites and communications directly with designers, builders and brokers of Multihulls.

Stability Speed - SSpd

John Shuttleworth offers a formula for the wind speed in miles per hour at which hull lifting will occur in his design booklet "Multihull Designs".

Texel Rating: Multihull Handicap System

Texel Rating is a system which allows different multihulls to race against each other. This system is originally developed by Nico Boon for the biggest cat-race in the world, the Ronde om Texel (the Texel Round) in The Netherlands. Later this system was extended to sea-going catamarans and trimarans.

Bow and Stern Overhang Effects

Multihull design pioneer Hugo Myers evaluated the effects of bow and stern overhang on boat performance and hence on rating systems. This paper is about how his thoughts relate to certain boats in our Database.