Buy Boats, Sell Boats, Review Boats

Posted by on Jul 23, 2013 in Carousel Feature, Multihull, US |

What’s the Difference Between Boat Hulls?

It's difficult to talk boats until you understand their parts. And the biggest (and most important) part is the hull.

This blog is part of a series for new boaters. To read more from the full article, see Boat Buying for Absolute Beginners, Part II.

Boat hulls — the actual bodies of boats — provide flotation and are usually in contact with the water. Boats come in two broad categories:

  • Monohull — Most boats have a single hull with a pointed bow and a flat transom stern. The bottom of a monohull may be flat, round, or V-shaped.

  • Multihull — 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.

boat hulls

Boat hulls come in all sorts of different types. Look closely at this hull, below the blue stripes, to find the strakes, which are used to create lift. (Photo courtesy Baja Marine Corp.)

Most people choose traditional monohull designs, although catamarans are becoming increasingly popular for both power and sail.

Monohull Boats

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 “in 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 with certain hulls and the hull rises up and skims along on top of the water. This is called “planing.” Monohulls generally fall into two categories:

  • Displacement hull— Some hulls are designed to operate only at displacement speeds. This type of boat is restricted to relatively slow speeds but is extremely efficient to run. 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 onto a plane. Characteristics of a planing hull include flat bottom surfaces from amidships aft (from the middle to the back of the bottom) and a flat transom (back of the hull). The transom must meet the bottom at a sharp angle.

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. Stand at the back of a planing hull boat and you will notice that the bottom angles upward from the keel, which is known as “deadrise.”

A few more terms you might hear:

  • Round bottom — used mostly on boats intended for displacement speeds. This shape provides a gentle efficient ride, but tends to roll from side to side in certain sea conditions.

  • Low deadrise — 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. This provides a smoother ride, but it still retains the quick ability of a flat-bottomed boat to plane.

  • Deep-V bottom — This type of hull has a deadrise of 25 degrees amidships which 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. 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  characteristics of a hard-chine boat. (A boat that resists rolling is said to be “stiff.”)

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

Other hulls

Some more popular hull forms:

  • 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.

  • Stepped hulls — Steps are, essentially, elevation breaks in the hull 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, resulting more speed with less power than a conventional deep-V hull.

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 allows more room for interior accommodations. Wider boats also feel more stable, which makes them popular for dockside entertaining.

A wider 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.

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 length. Modern, high-horsepower engines allow these wider boats to move at speeds faster than the so-called “speedboats” of a generation ago. The drawback, of course, is more fuel consumption.

Multihulls

Catamarans have become increasingly popular. 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” have two major drawbacks: twin engines are required, and larger catamarans can be too wide for conventional marina docks. Another drawback is less useable interior space than in a monohull of similar length.

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. The narrow amas of a catamaran move easily through the water with minimal power requirements, so high speeds are possible even though the amas might not actually be planing.

Trimarans have three separate hulls. This design has been effectively used by sailboat designers to provide a roomy central hull for 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.

Other installments in this series: