looking at a boat where this ratio is not provided it can be calculated with this formula:
SA/D = Sail Area (Displacement / 64)2/3 Displacement is the next number
to look at – it tells you how much the boat weights. Alone this number is interesting but does not say much about performance. This is where the next two ratios come into play. The D/L ratio shows how much the boat weights per unit of length. In general terms it provides a relative number that can be compared between boats to give you a feel as to its ‘heaviness’. Most heavy cruisers will have numbers between 350 and 250, racer/cruisers will run from 250 to 175 and racing boats will be below 175. The formula is: D/L = Displacement (0.01xLWL)3
The next formula is the B/D
ratio – it tells what portion of the total displacement is used as ballast. Generally the higher the number the more sail the boat can carry, allowing it to be faster. The formula is simple in that it is just the ballast divided by displacement. Numbers below .35 are generally found on cruisers and numbers above .45 are racers, in between lie the racer/cruisers. Think of it this way, on cruisers a lot of weight is added in tankage, living quarters, auxiliary equipment, etc. which increases the boat’s total displacement and lowers the percentage of the total represented by ballast. The next number to look at is the
beam. Again, this number alone will tell you something about how roomy the boat is below decks but not a lot about performance. The L/B ratio is a better indicator of the beam’s impact on performance; the higher the number the better the performance. This one is pretty intuitive – it takes more work to push a wider boat thru the water than it does to push a narrower boat of similar size. Draft falls into this same area;
while it has an impact on performance most boats come with two options, deep and shoal. And the waters you plan on sailing will have more to say about which you choose than will performance. In addition, the shape
and type of keel will have more impact than will the draft. The rest of the numbers are useful
to know but have little to do with performance. Fuel and water capacities are the best two examples. Depending on what you intend to do with the boat you may want more (i.e. for cruising) or less (i.e. for racing). Mast height is another nice-to-
know number, mostly for going under bridges. Admittedly it has indirect impact on performance but unless you are hard core racing it is not significant. The total sail area, while being affected by mast height, is much more impactful. Therefore it is possible to have a shorter mast and still perform well. For example, a ketch and sloop with the same total sail area on the same hull would have about the same performance, but the mast height of the ketch will be shorter. Length overall (LOA) falls into this
same category in that a longer boats will generally go faster, but it is the waterline length that directly impacts speed. Here is an example that explains this point: compare an older Islander 36 with a LWL of 28’ 3” to a newer
Beneteau 34 with a LWL of 30’ 8”. Here the shorter boat will be faster under power due to its longer waterline. This is one of the reason many of the modern boats have gone to the more plum bows; sweeping the bow back like the Islander’s may have a ‘Classic’ look but the price is a slower boat. Under sail I am unsure which would be faster as the Islander’s waterline will increase as it heels over. My guess is the waterlines might be pretty close so other factors like sail area and set will be the deciding factors. Realistically, LOA is probably a
better indicator of relative price of the boat and moorage cost than it is anything else. You can find an online calculator
for many of these ratios at US Sailing’s
http://www.sailingcourse.com/ I hope this review of boat specs has
been helpful and add to your enjoyment the boat shows!
Mike Huston teaches sailing for San
Juan Sailing in Bellingham, WA. He has been sailing for over 40 years, racing and cruising. He and his wife own a Jeanneau 43DS, “Ilummine.”
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48° NORTH, JANUARY 2012 PAGE 63
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