April 4, 2022

18 years ago Calvin H. Markwood, Engineering Analyst, and I began Multihull Dynamics Inc.

In 2004 with Mr. Markwood’s original database of 200 plus multihull sailboats, we developed the framework for multihull assessment used on this website and described in Calvin H Markwood’s “Analysis and Comparisons of Cruising Multihulls “, the analytical evaluation technique we use in detail and why we use it. The database has grown to over 1100 multihulls. Our focus is ‘cruising multihulls’ though the formulas can be applied to most multihull sailboats. Over the years we have fine tuned the formulas we use in evaluating both catamaran and trimaran cruising multihulls.

In this Sample Comparison you will see the 12 points of design analysis that we use when comparing 5 vessels at the same time.

The measurements we use are Mr. Markwood’s assessment tool for Bridgedeck Clearance in multihulls, the Nominal Bridgedeck Clearance (Nom BdCl); he also developed the Center of Effort (CE) formula for multihulls used on this website. In conjunction with Mr. Markwood’s assessment tools we have added formulas of Richard Boehmer’s Base Speed (BSpd), Nico Boon’s Texel Rating (TR), Derek Kelsall’s Kelsall Stability Indicator (KSI) and the Kelsall Sailing Performance (KSP), John Shuttleworth’s Stability Speed (SSpd) and Chris White’s and Derek Harvey’s evaluation of the Bruce Number (BN) developed by Edmond Bruce. Details of all of these formulas and interpretation of the results are in the article “Abbreviations, Definitions, Formulas and Terms in the Technical Section.

12 points of multihull design analysis used in our evaluations and reports:

Base SpeedRichard Boehmer
Beam at Hull Centerlines vs. Waterline lengthBasic calculation from specifications
Bruce Number (light air performance index)Edmund Bruce
Center of Effort – in Appendix ACalvin H. Markwood
Displacement-Length RatioEdmund Bruce and classical marine engineering
Hull length to beam ratioBasic calculation from specifications
Kelsall Sailing PerformanceDerek Kelsall
Kelsall Stability IndicatorDerek Kelsall
Nominal Bridgedeck ClearanceCalvin H. Markwood
Sail area/Displacement RatioBasic calculation from specifications
Stability SpeedJohn Shuttleworth
Texel RatingNico Boon

Like so many of you we enjoy the assessments of these boat’s and comparing them to each other. We believe the comparison results between 2 or more multihulls show an accurate relationship between the boats compared; but the “Formula” performance results my not reflect the actual “real world experience on the water”. However we maintain the relationship between the boats compared is accurate, when the data is accurate.

For example when comparing boat A with boat B. Boat A’s Performance Formulas may show a predicted Base Speed (BSpd) of 11 knots and boat B’s Performance Formulas may show a predicted Base Speed (BSpd) of 9 knots. In the real world, on the water experience, boat A may have a Base Speed 8 knots, instead of 11, in which point then we are predicting that boat B will be 2 knots slower and have a real world experience of 6 knots. Though the actual real world performance numbers may be different than what Multihull Dynamics predicts they will be, the relationship of performance and stability between the two or more vessels we believe will be consistent; excluding (1) hull shape below waterline – with a coefficient of drag; and (2) seamanship. We have no way to factor either of the two into the evaluations.

Boat owners and Buyers need real numbers they can relate too: Empty weight, Cruising weight & Payload weight ( we strive to get those), and the difference between Empty and Cruising weight. Finally they need the Max weight. Max weight being the maximum weight that the boat is safe to sail and will answer the helm. While these may not be designer’s terms, they are terms consumers understand and need.

When possible we contact designers to get the data about multihulls. If we are not able to reach a specific designer then we work with what is available to us. The least reliable data is often what is posted on a website where boats are for sale. We avoid sites such as Yachtworld, BOATS.com, etc as a resource for accurate specifications, particularly when it comes to Displacement weight.

Know the boat you are buying. I encourage buyers to weigh boats at the time of Survey. Then use our Custom Evaluation tool to asses to the boat you are considering purchasing.

Make boat weight measurement part of the Survey. Ensure the boatyard you are using for Survey calibrates their boat hauling equipment before weighing your survey boat. Ask them if they will provide you with written statement of the weight of the boat when arranging the Survey, if not look for another yard. Don’t accept something like “It’s about 14K lbs.” It’s your money and your time on the water. Know what you are buying before you buy.

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Fair Winds,

Pat Ross

Multihull Dynamics, LLC

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