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CruisingWiki

Ratios

I have listed three ratios as well – just to try and get some basis for comparison of these differing yacht designs from many different eras. To find out more there is the internet of course! but I found Ted Brewer’s site very informative.

1. Displacement / Length Ratio (D/L)

The result of this calculation is a general guide to overall performance level, load capacity and relative comfort at sea. The manufacturers web site will sometimes base this figure on something less than typical loaded displacement – you have to be quite ruthless in estimating your real weight when extended cruising or even on that blue water adventure!

This uses LWL for its calculation but LOA can matter too. When a boat is heeled the LWL may increase changing this value significantly. Here is what Ted Brewer says…”STORM, a wonderful 27′ LWL sloop on which I raced with Bill Luders many years ago, had a D/L ratio of 386 so she would be considered very heavy by today’s standards. However STORM was 39′ LOA and when she heeled to a breeze her long ends would increase her sailing LWL, thus reducing her D/L ratio to a more reasonable figure when we were beating to windward. If she picked up 3 feet of WL her D/L ratio dropped to about 281, a significant change, and one that made her a very competitive racer in the 1960s”…

D/L range of typical values:

40 to 50 Light racing multihull
60 to 100 Ultra light ocean racer
100 to 150 Very light ocean racer
150 to 200 Light cruiser/racer
200 to 250 Light cruiser
250 to 300 ** Average cruiser (Optimal)
300 to 350 Heavy cruiser
350 to 400 Very heavy cruiser

2. Sail Area / Displacement Ratio (SA/D)

As with the D/L ratio, the displacement figure used must reflect the real-world weight of the boat in the water. The hardest number to get is the correct sail area figure – it depends on the sail plan, and the sea conditions. Total area has traditionally been considered to be the sum of the foretriangle, plus the triangular area of the main, roach not included. Because of the highly roached mains found on many of the newer boats, it’s becoming common to quote the full area of the main.

SA/D range of typical values:

16 to 18 ** Heavy offshore cruiser (Optimal)
18 to 22 ** Medium cruiser (Optimal)
22 to 26 Inshore cruisers, racing boats
26 to 30+ Extreme racing boats

3. Capsize Screening Formula (CSF)

The technical committee of the Cruising Club of America came up with a simple formula to determine if a boat had blue water capability. The CSF compares beam with displacement since excess beam contributes to capsize and heavy displacement reduces capsize vulnerability.

The formula is the maximum beam divided by the cube root of the displacement in cubic feet; B/Displ.333. The displacement in cubic feet can be found by dividing the displacement in pounds by 64, of course.

The boat is acceptable if the result of the calculation is 2.0 or less but, of course, the lower the better. For example, a 12 meter yacht of 60,000 lbs displacement and 12 foot beam will have a CSF Number of 1.23, so would be considered very safe from capsize.

A contemporary light displacement yacht, such as a Beneteau 311 (7716 lbs, 10’7" beam) has a CSF number of 2.14. Based on the formula, while a fine coastal cruiser, such a yacht may not be the best choice for ocean passages.

What’s puzzling is that a yacht like the Tartan 4100 (CSF 2.02) and the Sabre 402 (CSF 2.01), both of which have pretty well documented track records as a blue water cruisers should have a CSF of over 2.0 – indicating that they fail the test described here…they are quite beamy for their weight…I guess.


Other references:

1. This is a link on BoatDesign.Net that also discusses the question of "seaworthiness"

2. This is a great guide for converting sail area to force in kgs

3. In praise of the versatile staysail

4. Understanding the numbers by Roger Marshall on Scribd.com

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