Boat Buying for Absolute Beginners, Part II

Choose the hull that's right for you

24th July 2001.
By David Brown

All boats have hulls. Look closely at this hull, below the final blue stripe, and you can see the strakes, which are used to create lift. (Photo courtesy Baja Marine Corp.)

All boats have hulls. Look closely at this hull, below the final blue stripe, and you can see the strakes, which are used to create lift. (Photo courtesy Baja Marine Corp.)

From personal watercraft to megayachts, every boat is first and foremost a hull. Without an intact hull, a boat quickly becomes… an artificial reef.

By definition, the hull is the actual body of the boat. It provides the flotation and is usually in contact with the water. (In rough seas, performance boats often take flight.) Powerboats come in two broad categories:

  • Monohull— Most boats have a single hull of the traditional “boat shape” with a pointed bow and a flat transom stern. Variations include barges and punts with flat fronts. The bottom of a monohull may be flat, round, V-shaped or have the multiple sponsons (partial hulls) of a so-called cathedral hull.
  • Multi-hull— These boats have two or more hulls joined by a bridge deck or other structure. Twin-hulled boats are called catamarans, while those with three hulls are trimarans.

There is no “perfect” hull. All boats are compromises among performance, useable space and cost. Most people choose monohull designs because they offer relatively large interior space for the price. Newer catamaran designs are challenging this conventional buying wisdom and are becoming increasingly popular. However, the overwhelming majority of pleasure boats are still monohulls, so we will consider that category first.

Monohull Boats

The deep-V hull and narrow beam of this offshore performance boat enable it to knife through rough water. (Photo courtesy Baja Marine Corp.)

The deep-V hull and narrow beam of this offshore performance boat enable it to knife through rough water. (Photo courtesy Baja Marine Corp.)

A boat floats because it displaces a greater weight of water than its own weight. When a boat is sitting still or moving slowly, the hull is said to be “in the displacement mode.” That is, all of the upward forces keeping it on top of the water come from flotation obtained by displacing water. Increase the boat’s speed beyond a certain point and dynamic lift can be created with certain hulls. (More on that in a moment.) The hull rises up and skims along on top of the water. This is called operating in the “planing mode.” Monohulls generally fall into two categories:

  • Displacement hull— All boats operate in the displacement mode at harbor or “no wake” speeds. Some hulls are designed to operate only at displacement speeds. This type of boat is restricted to relatively slow speeds, but it is extremely fuel efficient. That’s why displacement hulls are often chosen by people making long offshore passages — ocean passages are possible with a modest fuel supply.
Displacment hulls, such as that of this 37-foot Nordic Tug, never actually come on plane. (Photo courtesy Nordic Tug)

Displacment hulls, such as that of this 37-foot Nordic Tug, never actually come on plane. (Photo courtesy Nordic Tug)

Displacement-hull boats are not fast. A typical 30-footer can only make about 7 knots (a bit less than 8 land miles per hour). The only way to increase the maximum displacement speed is to lengthen the boat’s waterline. In other words, if you want to go faster in the displacement mode, you have to buy a longer boat. Most have a gentle motion while underway, although rolling (side-to-side movement) can be a problem.

  • Planing hull— Achieving high speeds on the water requires a hull that easily transitions into the planing mode. Characteristics of a planing hull include flat bottom surfaces from amidships aft (from the middle to the back of the bottom) and flat transom (back of the hull). The transom must meet the bottom at a sharp angle. At certain speeds that vary with each bottom, the boat is supported by the same dynamic forces that keep a water skier on top of the water. Put simply, it planes.
Stepped-bottom hulls reduce wetted running surfaces, which translates to less friction and more speed. Look closely at this offshore performance boat and you can see both steps in its hull. (Photo courtesy Thunderbird Products)

Stepped-bottom hulls reduce wetted running surfaces, which translates to less friction and more speed. Look closely at this offshore performance boat and you can see both steps in its hull. (Photo courtesy Thunderbird Products)

The sharp angle (or “break”) between the transom and the bottom is critical to a planing hull. It allows water flowing beneath the boat to come free of the hull without climbing up the transom and where it would create speed-robbing suction. The smooth, rounded shape of a displacement hull does not allow the water to break free in this manner.

Absolutely flat bottoms, a sub-species of planing hulls can be extremely fast, but are not easy to control and tend to bull their way through waves like a pile driver. Giving the bow a pointed shape allows a hull to slice through waves instead of slam into them. For a boat to operate above displacement speed, the V-shape of the bow must “warp” into flatter planing surfaces near the stern.

Stand at the back of a planing hull boat and you will notice that the bottom angles upward from the keel. This angle is known as the “deadrise.” Prior to the 1950s, designs had little or no deadrise to the bottom at the transom in order to achieve acceptable performance with the smaller engines of the era. As more power became available, designers were able to increase the deadrise angle to give boats a softer ride and better control.

Here are a few more terms you’re likely to encounter in bottom design:

  • Round bottom— These are used mostly on vessels intended for displacement speeds. Round hulls provide a gentle ride with little fuel expenditure, but tend to roll from side to side in certain sea conditions.
  • Low deadrise bottom— Sometimes called shallow-V bottoms, these running surfaces have a deadrise angle of 20 to 25 degrees amidships which reduces to 5 degrees or less at the transom. (Deadrise is the angle measure of the “V” in the bottom. It is generally greater at the bow, or “entry” of the boat, and shallower at the transom, or stern of the boat.) This provides a smoother ride, but it still retains the quick ability to plane of a flat-bottomed boat.
  • Deep-V bottom— This type of hull has a deadrise of 25 degrees amidships with reduces only slightly to 20 degrees at the transom. This shape allows high-speed operation in rough water.

With few exceptions, planing hulls have chines and strakes. Chines are the angles on the hull where the vertical topsides meet the bottom. Older designs often had rounded or “soft” chines. Today’s hulls almost universally have a sharp corner or a “hard” chine. Round-chine boats tend to roll a bit more, so they’re not favorites with anglers who prefer the stiffer characteristics of a hard-chine boat. (A boat that resists rolling is said to be “stiff.”)

Strakes are those little “ridges” you see below the chines. Their primary job is to provide lift, which is important in the planing process and overall running efficiency.

Now and Then

Hull shapes evolved slowly during the era of wooden boats because wood only bends and twists so much without splintering. Designers became free to explore their imaginations when fiberglass became the dominant material for boats. Not all of their ideas were good. Many early attempts at cathedral hulls (more on those at the end of this story) resulted in hard-riding, sluggish hulls that required inordinate amounts of fuel. Other designs like the deep-V proved their worth and remain popular. A couple of hull forms are particularly popular today

  • Ski boats— Hulls of competition water-ski boats are designed to minimize wakes at certain speeds so that slalom skiers can slice through them more easily. Ski-boat builders spent years creating running surfaces that produce the flattest, softest wake. Funny twist? Those same designers have revisited those designs in their wakeboard boats, where the goal is to provide big wakes at certain speeds.
  • Stepped hulls— One of the earliest attempts to increase boat speed involved putting small “steps” in the hull. Steps are, essentially, elevation breaks that create multiple running surfaces. As the boat speed increases, it rides on these surfaces, with area of contact with the water moving progressively further aft. The results? Less hull in contact with the water, less friction and more speed with less power than a conventional deep-V hull. The knock, at least against some stepped-bottom boats? Less wetted surface in the water, less friction and reduced lateral stability and control.
A closer look at a two-step bottom reveals the steps themselves and the three running surfaces they create. The ridges on those surfaces are strakes.

A closer look at a two-step bottom reveals the steps themselves and the three running surfaces they create. The ridges on those surfaces are strakes.

Beam, or the width of the hull at its widest point, is important in determining the boat’s fuel efficiency and ride through choppy waters. The trend in recent years has been toward wider boats because increased beam (width) allows more room for interior accommodations. Wider boats also feel more stable, which makes them popular for dockside entertaining. In addition, they are less likely to roll from side to side while underway.

These benefits come at a price. A wider a boat is more likely to pound (slam into waves) as it moves through choppy water. Plus, it can take more fuel to move a wide boat at a given speed than a narrow hull of similar length.

Narrow deep-V hulls, on the other hand, tend to slice through waves, and because they have less wetted surface and less friction, they can be more fuel efficient than wide-body boats of the same length. On the downside, they tend to roll from side to side more while underway and offer less interior space.

Most boat builders are aware of the relationship of beam to length when it comes to performance. High-speed powerboats tend to have narrow hulls with relatively large deadrise angles that often approach a traditional deep-V design.

Cruising boats are much wider. Today it’s not unusual to see the hulls of a cruiser with a beam approaching one-third of the boat’s overall lengths. Modern, high-horsepower engines allow these wider boats to move at speeds faster than the so-called “speedboats” of a generation ago. The penalty, of course, is more fuel consumption.

Compromise a Constant

While this series isn’t devoted to fishing boats, offshore anglers face a dilemma that illustrates why all boat purchases require accepting compromises between speed, comfort and function.

Offshore fishing often requires long runs through rough seas. For that job, a relatively narrow, high deadrise hull is best. But, once on at fishing grounds, a stable platform with a large cockpit for both anglers and their gear is a must. While drift fishing, a wide boat with hard chines and a relatively flat deadrise is desirable. Obviously, one hull can’t meet both demands. Fishing skippers have to choose a mixture of deadrise, beam, length and hull shape that is best for their needs.

With cruising boats there are fewer conflicting demands on the hull. Maximum interior accommodation is the overriding consideration. Most of today’s cabin boats have wide beams to allow for extensive galleys, large beds and comfortable saloons. A rougher ride is less of a consideration because family cruising is primarily done during fair weather and gentle sea conditions.

Until powerboats were mass-marketed, hull shapes reflected the types of conditions faced on waters surrounding individual boat builder’s shops. Boats for shallow water areas where choppy conditions predominate tended to be narrow, with fine entries (narrow bows) and small planing surfaces aft. Contrast that with the wide-beamed, relatively flat-bottomed boats built in areas where crossing sandbars with following sea (running in the same directions as the prevailing swells) was the major problem.

A variety of hull shapes are still available, even though today a single modern factory now turns out boats for Key West, Long Island Sound, the Great Lakes and Puget Sound. It’s up to boat buyers to determine which shapes best meet their individual needs.

The best way to do that? Ride in as many different boats as you can.

Asking friends for a ride is a perfect way to observe the performance of a fair number of hulls. Boat dealers also offer test rides, although they understandably reserve these rides for serious buyers.

Multi-Hulls

Catamarans have become increasingly popular in recent years. They are particularly attractive in the fishing market because they blend high-speed performance and a good rough-water ride with a stable platform for angling. “Cats,” as they are called, have two major drawbacks: twin engines are required for maximum performance and larger catamarans can be too wide for conventional marina docks.

Another drawback is the less useable interior space than in a monohull of similar overall length.

Catamarans have been around for a long time. What’s really new — if something that’s come on strong in the last five or six years can be considered new — about their current popularity is their appearance as contenders in the offshore performance-boat market. Previously, the major use of catamaran design in powerboats was pontoon boats intended for use in protected waters. Today, it’s not at all uncommon to see fishing catamarans and high-performance catamarans sharing the same offshore water.

Each of a catamaran’s two hulls is called an ama. (The popular term these days is “sponson,” but ama is still correct.) Each ama is quite narrow compared to its overall length. In fact, most are too narrow to remain upright except when used as half of a pair of hulls with a bridge deck between.

The rules of hydrodynamics remain constant, so the narrow amas of a catamaran move easily through the water with minimal power requirements. High speeds are possible even though the amas may not actually be operating in the planing mode. Large pontoon boats often reach astonishing speeds with only a single outboard motor for propulsion.

Trimarans have three distinct and separate hulls. This design has been effectively used by sailboat designers to provide a roomy central hull for the accommodations and two outrigger amas for stability. Little use of the trimaran concept has been made in powerboats, although many so-called “cathedral” hulls are close relatives. Instead of three separate hulls, a cathedral design squishes them together until they often share a common planing surface near the transom.

Editor’s note: In Part III of this series, David Brown takes the mystery out of engines and propulsion systems.


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