There are many ratios that can be used to compare boat design dimensions many of which are mainly of interest to professional boat designers. For more information see:
Ted Brewer Primer on Yacht Design
Design Basics

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.

Displacement/Waterline Length - D/L
Sail Area/Displacement - SA/D
Ballast Ratio - BR
Hull Speed - HSPD
Table of Aloha Ratios

Displacement/Waterline Length - D/L - Top
(Displacement/2240)/((Waterline*0.01)^3)
This is probably the commonest of these measures, and indicates whether a particular boat is "heavy" (eg a cruising design) or "light" (eg a racing design) for its length. Because it is "non-dimensional" (achieved using a cube of waterline length) this ratio can be used to compare boats regardless of their length and the resulting ratios can be categorized as follows:

Under 100       Very light
100 - 250       Light
250 - 325       Medium
Over 325        Heavy

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.

Sail Area/Displacement - SA/D - Top
Sail Area with 100% foresail/((Displacement/64)^2/3)
This ratio is also "non-dimensional" and can be used to compare boats of different sizes. Mainly it is a "power to weight" ratio so that a boat with a higher value will accelerate better and be a better light air performer. It will reach hull speed with less wind and need to reduce sail sooner if it is to avoid being over-canvassed in a blow.

Ballast Ratio (%) - BR - Top
(Ballast/Displacement)*100
The ballast ratio shows what proportion of the total displacement is ballast and can give an indication of how "stiff" (i.e. a greater resistance to heeling) or "tender" a particular boat is.

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 - Top
SquareRoot(Waterline)*1.34
Displacement (ie non-planing) hulls theoretical maximum speed is governed by the length of the wave created as the boat moves through the water. Since longer waterline lengths create longer waves they are able to go faster. Keep in mind that hulls are usually designed so that waterline length will increase as the boat heels increasing the maximum speed and also that in some circumstances (eg surfing down large waves) the boat ceases to be a true displacement hull and can exceed the theoretical maximum.

Last updated  12 January, 2006 - © Aloha Owners Association