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Shaping an offshore hull

Jan 1, 2003

What is the best hull shape to voyage across an ocean? Is it a boat with long overhangs? A boat with a lot of beam or deep draft? What style of stern should it have? A rounded stern like a Valiant, a canoe stern like many Colin Archer-style boats, or a transom like most voyaging boats? What style of bow should it haveplumb, raked, or gently rounded?

To determine the best hull shape for ocean voyaging we need to look at what a long-distance voyager does. The boat usually starts a trip fully loaded and ends a trip with a light load. Under those and all loading conditions in between the boat must handle well. In general, when on a voyage, the boat will sail on a reach rather than driving upwind. It is only near the end of a trip, if the destination is dead to windward, that upwind ability is needed. Ocean voyagers might sail for days on end without stopping. They might face bad weather and large seas, so the hull must be able to ride out a storm without causing too much crew fatigue. Comfort and ease of motion through the water mean that the boat needs to have a reasonable displacement. You only need to watch video of some of the boats in the Sydney-Hobart race to realize that light weight equates with increased motion. Because increased motion increases crew fatigue, it can induce a crew to make bad decisions.

When a boat designer thinks of the shape of a boat, he or she thinks in terms of the hull lines and what they do for the boat. A designer also thinks about the weight of the hull in relation to the lines, the shape of the rig, and other factors, many of which the average boat owner does not consider. For example, most sailors don't realize that the rig affects the shape of the hull lines. A boat with a schooner rig does not sail to windward very well, and if the rig doesn't go to windward well, the hull need not go to windward very well either. Consequently, successful schooner lines plans are often full bowed for reaching. Look at the successful Bluenose-style schooners from Nova Scotia. Because a schooner sails mostly on a reach, the stern lines (buttocks) tend to be fairly relaxed to increase waterline length when heeled. This is a good example of how the rig affects the lines.

Another factor affecting the lines is the anchor-handling system. A boat with a heavy windlass, several hundred feet of chain, and two or more anchors all stowed on the bow will need to have more displacement forward than a boat that does not carry anchor-handling gear. To increase the displacement forward a designer can make the bow deeper or increase the entry angle of the bow.

From these examples we can see that the hull is not designed in isolation but must be carefully integrated into the overall design of the boat. However, to look at the best hull for a voyaging boat we must first look at the lines plan and its components: the bow, the stern, and the midships section. Each plays a part in making the hull suited for its purpose.The lines plan

The lines define the shape of the hull. When lines were drawn by hand, a designer could inspect the buttocks, diagonals, and sections carefully and make decisions that hopefully made the boat comfortable and fast. Today lines are developed on the computer and only the sections are readily visible. Computer design has an advantage in that a velocity prediction program (VPP) can be run from the lines and the boat's potential speed estimated. The Nautilus System software from New Wave Systems, in Jamestown, R.I., has just these features and makes it relatively easy to estimate a boat's performance while it is being designed. When developing the lines, three major items need be taken into account: the overhangs at the bow and stern, the midship section, and the length overall. If each is integrated carefully with the others and with the overall design, the boat will be successful.1. The overhangs: In many older boats the overhangs were unduly influenced by rating rules. For example, J/Boats had extremely long overhangs because waterline length was limited to 90 feet. The largest J/Boat, Ranger, was 135 overall, giving overhangs half the length of the waterline. (Designers use the overhang divided by the overall length to give a non-dimensional length of overhang. In this case 45/135 = 33.3%). The International Rule led to long overhangs as seen on 12-meter boats. These boats have a waterline around 45 feet and an overall length of 65 feet (30% overhang ratio). When one of these boats moved at speed, the bow and stern waves were displaced toward the ends of the boat, increasing waterline length and ultimately increasing speed.

Boats that race in the BOC Race (now known as the Around Alone), on the other hand, have no overhangs and tend to be very fine forward and fat aft. The International Measurement System (IMS) rule has also led to shorter overhangs, less than 10% in many cases. Is this trend a good one? For an ocean voyager, I think not. While short overhangs give a longer waterline length, they also provide very little reserve buoyancy above the waterline. A voyaging boat often has heavy weights forward (anchors, anchor chain, windlass) and aft (dinghy, outboard, propane tanks). Contrary to current trends, I believe that a voyaging boat should have moderate overhangs. The length of overhang that I like to see is between 18 and 24%. This will give the bow and stern some reserve buoyancy to help stop the bow submerging in heavy seas, and the boat from pitching in a seaway.2. The bow: Heavy weights tend to stop the bow from rising quickly in heavy seas where green water might be breaking over it. On a boat with a U-shaped bow, the reserve of buoyancy (in other words, the forces that help to make the bow rise when it is submerged) stay relatively constant as the bow submerges. When sailing in a boat with a U-shaped bow, there is often an appreciable difference in trim and speed when one or two people go forward to change sails. This trend is so apparent that planing boards are fitted on some boats to help stop them from diving.

In the V-shaped bow the reserve of buoyancy increases as the bow is submerged further. When green seas are coming aboard, this hull shape helps to reduce the tendency of the bow to dive or submarine. However, due to the influence of the International Offshore Rule (IOR), bow panels on many V-bowed boats are flat, which promotes the panels' resonating in heavy seas. This causes a lot of noise and might eventually lead to structural failure of the panel. A bow with some curvature in the panels helps to resist these slamming impacts and also helps stop the bow from resonating. For this reason, the best bow shape for a voyager might be one with slightly more curvature (and a less modern look) than the latest trendy designs.

3. The midship section: There appear to be two distinct types of production boat today. One is the production voyager aimed at the inshore sailor, who rarely spends more than a few hours at sea and rarely sails more than 30 miles offshore. Typically, these boat owners want reduced draft to get the boat into a relatively shallow harbor. This leads to a midship section that is flat on the bottom and a shallow-draft keel. The trouble with a flat-bottomed boat is that bilge water gets everywhere. Consequently, the owner of a flat-bottomed boat has to make sure the bilge is dry before going to sea.

The other type of production boat is a boat intended for offshore voyaging. Because it will spend a lot of time in deep water, draft is not so critical. Water may also get inside the boat during the trip, and to facilitate pumping it the hull bottom should not be too flat, or the boat should include a deep sump in the hull over the top of the keel. I prefer the deep sump as it keeps bilge water in a central location.

Another trend for inshore voyagers is to have a relatively high beam-to-length ratio. This gives a lot of room inside the boat, and if the beam is carried down to the waterline it tends to help transverse stability. With beam making up a large part of the stability of a boat, many production boatbuilders are using less lead ballast. This trend is showing up in high-volume inshore voyagers. Unfortunately, it leads to a low angle of vanishing stability. A boat that is to sail in the ocean should have a relatively high angle of vanishing stability (around 135° or higher). Typically, this makes it slightly narrower than an inshore cruiser, with a deeper bilge.

4. The stern: The shape of the stern is governed by the slope of the buttock lines, and the slope of the buttocks will vary according to the boat's displacement. For example, a heavy-displacement boat might have steep buttocks that lock the length of the stern wave in near to the boat's stern. As displacement decreases, the buttock lines should be flatter. A moderate-displacement voyaging boat might have reasonably sloped buttocks that help the stern wave move a little farther aft and increase the sailing length of the boat. Boats that are not designed to a rating rule also vary widely in the shape of their sterns. Sterns from Valiant, Island Packet, and the stern of Suhaili are all different, yet Suhaili sailed around the world, as have a great many Valiants.

Other boats that have sailed around the world are the BOC racers. These boats have minimal rules; one is the requirement that the waterline length and the overall length cannot exceed 60 feet. They also carry water ballast, and, in an effort to get the ballast outboard as far as possible, they have developed into excessively beamy, lightweight vessels with sophisticated hulls, rigs, and deck layouts. (In my opinion, most do not have a very good seagoing hull shape.) The stern shape on a BOC boat shows that the beam has been carried well aft in an effort to increase the boat's power on a reach. Unfortunately, a wide stern like this would lift most of a centerline rudder out of the water, so the boat carries twin rudders.

Putting it together

Having looked briefly at the various parts of the hull, we now put the pieces together to get a feel for the best hull shape for a voyaging sailboat about 44 feet overall. This length is both comfortable for a family to live on and easy for them to handle. The hull shape as developed on our Nautilus design program shows a moderate voyager intended to have an easy motion and a displacement of around 28,000 lbs at half-load. Half-load displacement is the designer's way of approximating the boat in sailing condition. It means that all tanks are half-full, and on board is half of the food and provisions, the light sails, and all the crew. Let's look at two more important aspects of a voyaging boat's design.

1. The keel: The keel on an offshore voyaging boat needs to be forgivingthat is, it needs to be long enough to give the boat some directional stability to help the autopilot steer without a large battery drain. It also needs to be short enough so that the boat does not have a huge amount of wetted surface that would make it slow in lighter winds. Because a voyaging boat will not sail directly to windward unless it is heading directly to a windward destination, windward performance is of lesser importance than it would be if the boat were a racing boat.

Another factor that needs to be taken into account is the keel's performance in heavier winds and seas. Ideally, the keel will have enough lateral area to enable the boat to heave to. But I recommend that boats heave to only in winds up to about Force 6. They should assume a different posture if the wind rises beyond that. I hear the long-keel pundits saying that a desire for adequate lateral area means that the boat should have a long traditional keel. Not necessarily! The keel should be slightly longer than usual, but not a long keel. It should be slightly bulbed to keep the center of gravity low, but not aggressively so. Its sectional shape should also be considered with easy handling in mind. For this reason we'll use a 10% section. A thinner section may make the boat hard to handle in strong winds, while a fatter section may make it slow.

2. The rudder: The rudder is similar to the keel. If designed like many of the high-performance rudders seen on racing boats, it is sensitive, fast, and easy to maneuver. But most offshore voyaging boats use an autopilot. Do they need a sensitive, easy-to-use helm? Probably not. In my opinion, it may be better to take a step back and install a less sensitive skeg-hung rudder so that the autopilot is not continuously working to keep the boat on course. This also helps in that the rudder is supported at top and bottom, a factor that comes into play when the boat is hove to and may be moving backwards at one or two knots.

Figure A in the illustration on page 62 shows a sensitive balanced rudder, and figure B shows a semi-balanced, skeg-hung configuration. Figure C shows a compromise rudder that has both sensitivity and reasonable protection. (The argument has always been that a skeg is needed to protect against tangling the rudder with lobsterpot warps. In nearly 40 years of sailing, I have yet to get a pot warp around the rudder. I find that they most often wrap around the prop or shaft.)

The design elements we have outlined here are not likely to produce the fastest boat on the water, but they are not intended to. These elements should come together as a forgiving offshore voyaging boat with a good turn of speed. Each component of the design has been carefully reviewed for its contribution to the overall effort, to give a comfortable, sea-kindly hull shape that will sail offshore in virtually all kinds of weather.

Roger Marshall is a naval architect and designer who lives in Jamestown, R.I.