There are many
ratios that can be used to compare boat design dimensions many of which are
interest to professional boat designers. For more information see:
This Microsoft Excel spreadsheet will allow you to calculate the ratios shown on the Design Basics website. It currently has the figures for the Aloha 28 entered, but the boat model and measurements can be amended and the other values will be re-calculated.
The Image-ination Website includes a great Sailboat Ratios Calculator which includes a large database of existing boats, as well as the ability to enter figures for any boat not already listed, comparison and search tools and a "prop sizing calculator" and more!
Of the available figures, there are four in particular that are fairly commonly used in the yachting press and these are included on the Aloha Specification pages on this site. An explanation of each of these terms is provided below as well as a table summarizing the Aloha values for comparison purposes. In these formulas the waterline length, P, E, I & J (as detailed in the diagram below) are measured in feet and inches, displacement and ballast in pounds and sail area in square feet using a 100% foresail.
Length - D/L - Top
In general, a boat with a lower D/L will have better light air performance for a given sail area but it will be more sensitive to loading, likely to have a less comfortable ride in a sea and will likely need to shorten sail sooner.
Area/Displacement - SA/D -
Ballast Ratio (%) -
BR - Top
However, use this value with care. Keep in mind that one boat may have the ballast at the bottom of a deep keel and another in a shallow keel. As a result, two boats with the same ballast ratio could have very different "righting moments" which is what actually determines how "stiff" (or otherwise) it is, depending on the location of the ballast and hull shape.
Some manufacturers who offer shoal and deep keel versions of the same boats (eg Catalina, Hunter) actually increase the amount of ballast in the shoal keel versions to help maintain a similar righting moment. In some cases this works well but in others it can mean that the shoal draft boat has a higher ballast ratio and yet still has a lower righting moment (so is less stiff) than it's deep keel counterpart.
Hull Speed - HSPD
Last updated 12 January, 2006 - © Aloha Owners Association