Beam and Draft

For many voyagers, trying to define of the "ideal" voyaging boat is one of the sport’s greatest debates. It is far easier said than done in that there are a large number of factors to be taken into consideration, many of them contradictory.

As a result, every boat is the result of a series of compromises that will differ according to the priorities of the person driving the decision-making process. At one extreme, performance under sail may be the overriding concern; at another, gunkholing in shallow anchorages may be the primary interest. These differing priorities should (if the yacht designer does his or her job) result in very different boats.

When exploring design choices, we can look at a number of commonly quoted numeric parameters that are often used to compare boats and their implications. One excellent place to start is with beam and draft calculations.Contemporary boat trends

Almost all voyaging boats, including world-girdling boats, spend the majority of their time either anchored out, on a mooring, or secured to a dock. At such times the boat is little more than a floating condominium. It is natural to want to make it as comfortable a floating home as possible. This, in turn, calls for space, and as a result yacht designers and boat builders are always under pressure to create as much volume as possible in any given design.

Volume nowadays typically translates into a wide beam, carried as far aft as possible, with high freeboard. The boat owner is sometimes going to want to be able to take this floating home into relatively shoal anchorages. This requires a shallow draft. To get a beamy boat with little draft, the boat must have a flat bottom. Even though this boat will probably not spend much of its life at sea, the builder and owner are still going to want it to perform reasonably well. A couple of keys to maximizing performance are to keep the overall weight, and thus the displacement, as low as possible (lightweight construction), and to minimize wetted surface area by using the minimum keel area necessary to achieve reasonable upwind performance (a fin keel), together with the minimum rudder size and supporting structure necessary to maintain control (a smallish spade rudder).

The kind of boat that is taking shape should be familiar; it can be seen at every major boat show. There is nothing wrong with this boat; it is built to fit a certain formula that is market driven, and by and large it does an excellent job of fitting this formula.

When it comes to voyaging boats, and indeed any boat that may be used offshore, we have to add at least one more criterion to the mix. This is the ability to safely deliver the crew, together with all stores and belongings, to its chosen destination in the worst conditions that might be encountered, and to do this at an acceptable speed and with as little discomfort as possible.

Among other things, this translates into a boat that is reasonably fast but with an easy motion at sea (a seakindly boat), that tracks well and has a light helm, that is stiff enough to carry sufficient sail area to keep moving to windward in heavy weather, and that has, in an extreme situation, the ability to claw off a lee shore under sail alone in heavy seas and gale-force winds. It must, of course, be built strongly enough to survive the gale.Form stability

Just about any boat can be pushed to windward in smooth water, but when things start to get rough it requires a great deal more power to counteract the boat’s windage and motion. Power requires sail area. Sail area requires a stiff boat- i.e, one that resists heeling: all the sail area in the world won’t do a bit of good if the boat rolls over and lies on its side!

One way to achieve stiffness is to increase beam. As the boat heels, the immersed volume shifts rapidly to leeward, keeping the boat more-or-less upright. This is known as form stability. A lightweight, beamy boat generally has excellent form stability. However, when the going gets tough the wide, flat sections, combined with the relatively light weight, are not only likely to make it pound and roll uncomfortably, but also will have a tendency to cause its keel to stall out. As it stalls out, if the boat has a relatively shallow draft and minimal lateral surface area in the keel and rudder, it will offer little resistance to making leeway. If it also has high freeboard, the windage will simply exacerbate problems. In other words, many of those features designed to improve comfort at the dock or on the hook, and to ensure a sprightly performance in relatively protected waters, can become a handicap. A less extreme design approach is needed. The first thing to reconsider is the wide beam.Length-to-beam ratio

The "beaminess" of a boat can be quantified by calculating its length-to-beam ratio – a number obtained by dividing the length by the beam. Often the length overall (LOA) – although in this case it should not include a protruding bow pulpit – and the maximum beam (Bmax) are used, although I prefer to use the waterline length (abbreviated to LWL) and waterline beam (BWL). Note that the two different formulas produce quite different values, so when making comparisons between boats it is essential to see that the same methodology is used to derive the numbers. For example, our Pacific Seacraft 40 has a LOA (excluding the bow pulpit) of 40.33 feet and a Bmax of 12.42 feet, giving a length-to-beam ratio using these numbers of 40.33/12.42 = 3.25 (note that the inverse ratio is sometimes given by dividing the beam by the length, in which case we get a beam-to-length ratio of 12.42/40.33 = 0.308). But if we use the waterline length (LWL) and waterline beam (BWL), we get a waterline length-to-beam ratio of 31.25/11.33 = 2.76.

As noted, for comparison purposes it is preferable to use the LWL and BWL to derive a waterline length-to-beam ratio, but unfortunately, although the waterline length is commonly published, the waterline beam is almost never published. As a result, yacht designer Roger Marshall, in The Complete Guide To Choosing A Voyaging Sailboat (published by International Marine, 1999) suggests that a way to use available data is to work with the waterline length and Bmax x 0.9, which will approximate the waterline beam on many boats (note, however, that when looking at a range of boats, I found this factor varied from as low as 0.75 to as high as 1.00, so this is a pretty crude approximation). When we apply these numbers to the Pacific Seacraft 40, we get:

LWL/(Bmax x 0.9) = 31.25/(12.42 x 0.9) = 2.80.

This is pretty close to the actual waterline length-to-beam ratio (2.76). Lower length-to-beam ratios indicate proportionately more beam; higher ratios less beam. A higher ratio is desirable both in terms of windward performance in difficult conditions, and also as an indicator of handling characteristics and seakindly behavior.Beam and stability

However, this is not the whole picture. Beam affects stability on a cubic basis, which is to say that any increase in beam has a disproportionate effect on stability. If the length-to-beam ratio is kept constant, as length increases, the increase in beam needed to maintain a constant ratio produces a disproportionate increase in stability. For example, a 36′ LWL boat with a 3:1 ratio will have a 12′ waterline beam while a 48′ boat with the same ratio will have a 16′ beam; the 48′ boat will be considerably stiffer, even though it has the same ratio.

What this means is that if two boats have the same length-to-beam ratios, the one with the longer waterline is likely to have greater stability and sail-carrying ability, and better performance to windward. Or, put another way, as length increases the syme relative sail-carrying ability can be maintained with a proportionately narrower beam and thus a higher length-to-beam ratio. As a result, to improve stability and sail-carrying ability, shorter boats need proportionately more beam, resulting in lower length-to-beam ratios. Consequently, there is no absolute length-to-beam ratio “magic number” that can be used for comparing boats; length must also be taken into account: the shorter a boat’s waterline length, the lower its length-to-beam ratio is likely to be.

Nevertheless, when looking at the 35-foot to 45-foot boat range (the “norm” for offshore voyaging these days), for a comfortable offshore voyager I like to see a waterline length-to-beam ratio of 3.00 or higher (using LWL/[Bmax x 0.9]). Shorter boats may have a lower ratio; longer boats should have a higher ratio. Looking at a sampling of contemporary European and American boats (see table on page 88), we see that the only two boats below 40 feet LOA that have a ratio of over 3.00 are the Alerion Express 38 and the Shannon 39. At 40-feet and above, many of those boats that follow the current fashion of short overhangs, which maximizes the waterline length, have ratios of 3.0 and higher, whereas more traditional voyaging boats, with longer overhangs, for the most part do not. Our Pacific Seacraft 40, for example, has a waterline length-to-beam of 2.80. This is the price that has to be paid for its long overhangs combined with the beam necessary to provide a more spacious interior as compared to voyaging boat designs of a generation ago.

Many older, but nonetheless highly successful, voyaging boats in this same size range have waterline length-to-beam of 3.0 and above (based on LWL/[Bmax x 0.9]). Steve Dashew, the designer of the Deerfoot and Sundeer series of boats, has taken the length-to-beam ratio to extremes. His boats commonly have ratios of 4:1, 5:1 and up. This is all to the good except that, because of the relatively narrow beam, in order to establish a reasonable interior volume, the boat has to get longer and the costs start to soar. He writes in the second edition of the Offshore Voyaging Encyclopedia that he and Linda, his wife and partner, decided to see just how small a boat they could design that would contain what they felt to be their minimum requirements for just the two of them, including accommodating a couple of guests for a week or two a year. They arrived at 56 feet in length! Unfortunately, however desirable it may be, such a boat is beyond the budget of most of us, not only up front but also in terms of mooring or dockage fees, gear replacement costs, maintenance, and so on.Keel types

A narrower beam results in less form stability, which can translate into greater heeling when on the wind. To counteract this tendency to heel it’s necessary to put a lot of weight down low. In its extreme form this results in the 14-foot fin keels, with massive lead bulbs, seen on some narrow racing boats.

Clearly, such a keel is not practical on a voyaging boat, but the principle is the same – to get as much weight as possible as low as possible. How low is primarily a function of where the boat is intended to sail. In general, a six-foot draft is acceptable, still allowing access to most of the world’s finest voyaging grounds. However, a boat specifically intended for voyaging in shoal areas such as the Bahamas might be designed with less draft, whereas one intended for Pacific voyaging might have a deeper draft. A voyager/racer, with an emphasis on the racing side of things, is likely to exceed six feet, trading access to some voyaging grounds for improved performance when racing.

For a given draft, the use of a bulb keel keeps the weight as low as possible. A wing keel does the same, but needs to be carefully designed if it is not to foul lines and seaweed, or get stuck in the mud in a grounding. (A wing keel has a shape much like a Bruce anchor. Wing keels originated as a rule-beating device in the America’s Cup, and have since become something of a fad. I doubt that any advantage over a bulb keel outweighs the disadvantages in a voyaging environment.) On our new boat we chose a bulb-keel option, with a draft of five feet two inches, as opposed to the standard deep-keel of six feet one inch. We get a significantly reduced draft with a small loss of windward performance.

The advent of bulb and wing keel types has pretty much put paid to the old debate as to whether it is preferable to have internal or external ballast: the bulb or wing must be external (it’s hard to mold them into fiberglass). Clearly, lead, with its great density, should always be used as the ballast material (as opposed to iron, which is sometimes used to save cost yet it’s only a little more than 60% of the density of lead). We’ve hardly started looking at the process of choosing an “ideal” voyaging boat, and already we are beginning to sense that there are a complicated series of trade-offs between, for example, beam and draft, interior accommodation and windward ability, and comfort on the hook and at sea.

Based on my own experience, which is primarily bluewater voyaging, if I were to settle on two numbers that provide an acceptable beam and draft middle ground for 35- to 45-foot voyaging boats, it would be a waterline length-to-beam ratio of 3.0 or higher, and a draft of six feet or less. Longer boats should have a higher waterline length-to-beam ratio, and may require more draft.

By Ocean Navigator