Crunching Numbers: Defining Displacement

In the interest of general enlightenment with respect to matters nautical I thought I should explain some of those mysterious numbers that always appear at the end of boat reviews and evaluations, both here on WaveTrain and in the marine press in general. Being able to intelligently interpret a boat’s numbers doesn’t mean you’ll be able to fully appreciate its qualities and deficiencies at a glance, but it does give you a big head start.

One of a boat’s most significant characteristics is its weight. This is called displacement, thanks to a fundamental law of nature (first discovered by Archimedes in the 3rd century B.C.) that states that the weight of an object is exactly equal to the weight of the water it displaces when it is placed in water.

The volume of water displaced by an object is also equal to its immersed volume, which, in the case of a floating boat will be its underwater hull volume. If we know a boat’s displacement, we can therefore also calculate its underwater hull volume. This volume, however, will vary depending on whether the boat is floating in salt or fresh water, because salt water is a tad heavier than fresh water. (It weighs 64 pounds per cubic foot, compared to just 62.4 pounds per cubic foot for fresh water.)

A 12,000-pound boat, for example, has an underwater hull volume of 187.5 cubic feet when floating in salt water (12,000 ÷ 64 = 187.5) and a volume of 192.3 cubic feet when floating in fresh water (12,000 ÷ 62.4 = 192.3). In both cases the boat weighs the same, as does the water it displaces, but the amount of water displaced is different. The end result is that the boat will float slightly higher on its lines in salt water, as slightly less water is actually being displaced by the boat. In fresh water it will float slightly lower on its lines, as slightly more water is being displaced, as per the drawing up top.

But a boat’s displacement tells us more about the boat than just its underwater hull volume. It also influences a boat’s speed, in that lighter boats are generally faster than heavier ones, its load-carrying ability (heavier boats can more easily carry big loads than lighter ones), and how comfortable it is (heavier boats normally have an easier motion). To get a realistic sense of these characteristics, however, you first need to get a realistic displacement number to work with–i.e., what the boat will actually weigh when you are using it. In most cases, you can be certain that the displacement number published by the boat’s builder will be a low-ball light-ship figure of some sort. They do this to make their boats look faster compared to other boats, and because they know other builders will be doing the same thing–a classic case of the power of the lowest denominator.

Unfortunately, however, it’s not exactly a common denominator. Some builders are quite unscrupulous and publish displacement numbers that don’t include the weight of such essential equipment as the boat’s sails, its rigging, its batteries, or its ground tackle. Others may include the weight of all the boat’s most basic working equipment, but will not include the weight of crew, fuel, water, or any special equipment. In that special equipment on cruising boats may include such heavy items as generators, liferafts, enlarged battery banks, and extra anchors and ground tackle, this can be a very significant factor. Also, many builders base their published displacement figure on the designer’s estimate of what the hull will weigh and don’t bother to actually weigh the hull after it is built. In most instances, the weight of the hull as built will in fact exceed the design weight to some extent.

Fortunately, some sort of order is being imposed. In the European Union there is now a displacement standard promulgated by the International Standards Organization (ISO), known as “light-craft condition,” that includes the weight of basic gear, but assumes empty fuel and water tanks. This is now the number most often published for new boats built on both sides of the Atlantic that are CE-certified for sale in the European market.

The bottom line, in any event, is that, unless you have specific information to the contrary, any published displacement number you see will need to be adjusted upward, usually by a significant amount. The best way to get an accurate number is to load the boat with what you want to have aboard and then weigh it yourself. In most cases, however, particularly if you are trying to evaluate a boat you don’t already own, this is not feasible. But whenever you do happen to have your boat hauled by a Travelift with a load cell (or, even better, a boat you are having surveyed), you should be sure to make a note of what its actual weight is.

Absent such a glorious opportunity, you can only estimate how much extra weight you will put aboard and add it to the published figure. Nigel Calder, in his very useful Cruising Handbook (International Marine/McGraw-Hill, 2001), suggests that the best average adjustments to make to arrive at a reasonable half-load number (i.e., the boat is loaded to cruise with tanks half full) are plus 2,500 pounds for a lightly used coastal cruiser and plus 3,750 pounds for a bluewater cruiser. For a more heavily equipped coastal boat, or a long-term liveaboard bluewater boat, he further suggests that these corrections be increased to 3,750 and 5,000 pounds respectively. These estimated average corrections are as good as any I have seen.  Alternatively, if you want to be really anal and try to make a much more detailed itemized estimate of the weight you will put aboard a boat, you can use the handy worksheet that appears on page 13 of Nigel’s book.

Once you have a reasonably reliable displacement figure to work with, you can use it to run various formulae that give you a much clearer idea of how a boat will perform. We’ll discuss these in future Crunching Numbers posts.

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