Boats Building

Boat building, one of the oldest branches of engineering, is concerned with constructing the hulls of boats and, for sailboats, the masts, spars and rigging.


Traditional boat building in South East Maluku, Indonesia

Parts

Bow - the front and generally sharp end of the hull. It is designed to reduce the resistance of the hull cutting through water and should be tall enough to prevent water from easily washing over the top of the hull.

Bulkhead - the internal walls of the hull.

Chines - are long, longitudinal strips on hydroplaning hulls that deflect downwards the spray that is produced by the hull when it travels at speed in the water. The term also refers to distinct changes in angle of the hull sections, where the bottom blends into the sides of a flat bottomed skiff, for instance. A hull may have 2 or more chines to allow an approximation of a round bottomed shape with flat panels. It also refers to the longitudinal members inside the hull which support the edges of these panels.

Deck - the top surface of the hull keeps water and weather out of the hull and allows the crew to stand safely and operate the boat more easily. It stiffens an enclosed hull.

Gunwale - The upper longitudinal structural member of the hull. Cannons were bolted to this in the earliest gun armed warships.

Keel - the main central member along the length of the bottom of the boat. It is an important part of the boat's structure which also has a strong influence on its turning performance and, in sailing boats, resists the sideways pressure of the wind

Keelson - an internal beam fixed to the top of the keel to strengthen the joint of the upper members of the boat to the keel.

Rudder - a steering device at the rear of the hull created by a turnable blade on a vertical axis.

Sheer - the generally curved shape of the top of the hull. The sheer is traditionally lowest amidships to maximize freeboard at the ends of the hull. Sheers can be reverse, higher in the middle, to maximize space inside or straight or a combination of shapes.

Stem - a continuation of the keel upwards at the front of the hull.

Stern - the back of the boat.

Strake - a strip of material running longitudinally along the vessel's side, bilge or bottom.

Transom - a wide, flat, sometimes vertical board at the rear of the hull, which, on small power boats, is often designed to carry an outboard motor. Transoms increase width and also buoyancy at the stern.

Construction materials and methods

Wood
The traditional boat building material that was and is still used for hull and spar construction. It is buoyant, cheap, widely available and easily worked. It is not particularly abrasion resistant and it can deteriorate if fresh water or marine organisms are allowed to penetrate the wood. Rot resistant woods such as cedar and oak are generally selected for wooden boat construction. Glue, screws and/or nails are used to join the wooden components. Some types of wood construction include:

1. Carvel, in which a smooth hull is formed by wooden planks attached to a frame. The planks may be curved in cross section like barrel staves. Carvel planks are generally caulked with oakum or cotton that is driven into the seams between the planks and covered with some waterproof substance. It takes its name from an archaic ship type and is believed to have originated in the Mediterranean.
2. Another method of building wooden boats is lapstrake, a technique originally identified with the Vikings in which wooden planks are fixed to each other with a slight overlap that is beveled for a tight fit. The planks may be mechanically connected to each other with copper rivets, bent over iron nails, screws or with adhesives. Often, steam bent wooden frames are fitted inside the hull. This technique is known as clinker in Britain and also as clench built.
3. Another method uses sheets of plywood panels fixed to a frame. Plywood may be laminated into a round hull or used in single sheets. These hulls generally have one or more chines. A type of the plywood panel boat building method is known as the stitch-and-glue method, where pre-shaped panels of plywood are edge glued and reinforced with fibreglass without the use of a frame. Metal or plastic wires pull curved flat panels into three dimensional curved shapes. These hulls generally have one or more chines.

Steel (and before that iron)
Either used in sheet for all-metal hulls or for isolated structural members. It is strong, but heavy. The material rusts unless protected from water. Modern steel components are welded or bolted together. Until the mid 1900s, steel sheets were riveted together.

Aluminium
Either used in sheet for all-metal hulls or for isolated structural members. Many sailing spars are made of aluminium. The material requires special manufacturing techniques, construction tools and construction skills. While it is easy to cut, aluminium is difficult to weld, and also requires heat treatments such as precipitation strengthening for most applications. Corrosion is a concern with aluminium, particularly below the waterline.

Fiberglass (Glass-reinforced plastic or GRP)
Typically used for production boats because of its ability to reuse a female mold as the foundation for the shape of the boat. The resulting structure is strong in tension but often needs to be either laid up with many heavy layers of resin-saturated fiberglass or reinforced with wood or foam in order to provide stiffness. GRP hulls are largely free of corrosion though not normally fireproof. These can be solid fiberglass or of the sandwich (cored) type, in which a core of balsa, foam or similar material is applied after the outer layer of fiberglass is laid to the mold, but before the inner skin is laid. This is similar to the next type, composite, but is not usually classified as composite, since the core material in this case does not provide much additional strength. It does, however, increase stiffness, which means that less resin and fiberglass cloth can be used in order to save weight. Most fiberglass boats are currently made in an open mold, with fiberglass and resin applied by hand. Some are now constructed by vacuum infusion where the fibers are laid out and resin is pulled into the mold by atmospheric pressure. This can produce stronger parts with more glass and less resin, but takes special materials and more technical knowledge.

Composite
While GRP, wood, and even concrete hulls are technically made of composite materials, the term "composite" is often used for plastics reinforced with fibers other than (or in addition to) glass. Cold-molded refers to a type of building one-off hulls using thin strips of wood applied to a series of forms at 45-degree angles to the centerline. This method is often called double-diagonal because a minimum of two layers is recommended, each occurring at opposing 45-degree angles. "Cold-molding" is now a relatively archaic term because the contrasting "hot-molded" method of building boats, which used ovens to heat and cure the resin, has not been widely used since WWII. Now almost all curing is done at room temperature. Other composite types include sheathed-strip, which uses (usually) a single layer of strips laid up parallel to the sheer line. The composite materials in question are then applied to the mold in the form of a thermosetting plastic (usually epoxy, polyester, or vinylester) and some kind of fiber cloth (fiberglass, kevlar, dynel, carbon fiber, etc), hence the finished hull is a "composite" of fiber and resin. These methods often give strength-to-weight ratios approaching that of aluminum, while requiring less specialized tools and skills.

Steel-reinforced cement (ferrocement)
Strong and long lasting. First developed in the mid 19th Century in France. Used for building warships during the war. Extensively refined in New Zealand shipyards in the 1950s and the material became popular among amateur builders of cruising sailboats in the 1970s and 1980s, because the material cost was cheap although the labour time element was high. The weight of a finished ferrocement boat is comparable to that of a traditionally built wooden boat. As such they are often built for slower, more comfortable sea passages. Hulls built properly of ferrocement are more labor-intensive than steel or fiberglass, so there are few examples of commercial shipyards using this material. The inability to mass produce boats in ferrocement has led there to there being few examples around. Many ferrocement boats built in back yards have a rough, lumpy look, which has helped to give the material a poor reputation. The ferrocement method is easy to do, but it is also easy to do wrong. This has led to some disastrous 'home-built' boats. Properly designed, built and plastered ferrocement boats have smooth hulls with fine lines, and therefore are often mistaken for wooden or fiberglass boats. See also concrete ship, concrete canoe.

Hull types

Round bilge hull - As its name implies, the hulls of these vessels are rounded and don't usually have any chines or corners.

Chined hull - These are hulls made up of flat panels. The flat panels are commonly made of plywood sheets, but some use more traditional methods of planking. Chined hulls range from flat bottomed boats such as banks dories, sharpies and skiffs, the side and bottom are 2 distinct pieces meeting at a sharp angle known as the chine. Flat bottomed boats are simple to build and shallow in draft. Most are intended for use in protected waters, though the banks dory evolved as a fishing boat in the open Atlantic. There are also many multichine boats that have more than the one chine each side of the flat bottomed boat. This can allow a round bilge hull shape to be approximated.

Displacement hulls - These are hulls which have a shape which does not promote planing. They travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always.

Planing hulls - These are hulls with a shape that allows the boat to rise higher and higher out of the water as the speed increases. They are sometmes flat-bottomed, sometimes V-bottomed and sometimes round-bilged. The most common form is to have at least one chine to allow for stability when cornering and for a supportive surface on which to ride while planing. Planing hulls allow higher speeds to be achieved, and are not limited by the waterline length the way displacement hulls are. They do require more energy to achieve these speeds.

Boats and Boatbuilding

Boats and Boatbuilding, types and construction of any small, waterborne vessel that displaces and excludes the water surrounding it. Traditionally, boats were distinguished from ships by size—any vessel small enough to be carried aboard a ship was considered a boat. Today, the boundary between boats and ships is no longer defined with precision. Some larger vessels are called boats, although they are longer than some ships. This article focuses primarily on the design and construction of craft less than 20 m (65 ft) long. For a discussion of the history of all waterborne vessels.



Types of Boats
Boats are used in a number of ways. They can be purely recreational, or they can have more practical uses, such as serving as a home or as a method of transportation. Even boats with similar uses may differ in other respects. For example, methods of propulsion range from oars, to mechanical engines, to wind-catching sails.

Boats are classified primarily by method of propulsion—for example, sailboat, motorboat, and rowboat. They are also classified according to function, method of construction and type of materials used, rigging (in sailboats), and other factors.

Basics of Boat Design

Parts of a Sailboat
Although sailboat design varies widely, all sailboats share a few basic components. The boat’s main body is called the hull. The front of the hull is referred to as the bow, while the rear of the hull is called the stern. The rudder extends from the stern and is used to steer the boat. The centerboard, under the hull, helps sailors maintain a steady course by limiting the boat’s movement from side to side. The mast and the boom support the boat’s sails. The mainsail, the largest sail on a sailboat, is fastened to both the mast and the boom. The triangular sail in front of the mast is called the jib.

Only a few basic components are common to most boats of traditional style. The keel is a timber or other element running the length of the bottom of a boat along the center from the bow, or front, to the stern, or rear. The keel serves as the foundation for the frame, which is covered with a waterproof material to form the hull—the body of the boat. The terms keel, frame, and hull are also used in describing modern boats that are not built but are molded in one piece.

I. Buoyancy and Weight

For an object to float on the water’s surface, it must sink enough to displace a volume of water equal to its own weight. For example, if a boat is to carry three people, their fishing gear, an outboard motor, and a supply of fuel—a total weight of about 500 kg (1,100 lb)—then the boat must be made long and large enough to displace 500 kg (1,100 lb) of water without sinking below the water level. Boat designers also have to take into account the weight of the boat itself. The heavier the material used to build the boat, the larger the boat has to be.

II. Trim and Stability

In addition to considering the total weight of the boat and its contents, boat designers must also consider the distribution of weight. The weight of an outboard motor at the stern of a small boat tends to make the boat sink deeply in the water at that end (to “trim the stern”). If that tendency cannot be offset by placing a similar weight in the bow of the boat, then the offsetting must be obtained by broadening the hull at its aft, or back, end so that a greater part of the displacement occurs near the excess weight.

A balance of weights from side to side must also be arranged. Further, to lessen the danger of capsizing, the combined center of gravity of all weights must be sufficiently low in the boat. If other factors make it necessary that the center of gravity be high, then the chance of capsizing must be offset by increasing the width of the hull.

III. Structure

A boat must maintain its shape in the face of local internal weights, such as an engine or a heavy cargo, and it must be strong enough to resist the force of battering waves. Because a hull of sufficient strength can be built of thin material, the risk of local puncture can be great in a boat that is otherwise quite strong. For example, a traditional boat built of skins or of bark, or a modern inflatable boat, are sufficiently strong and buoyant, but all are vulnerable to puncture or perforation.

IV. Watertightness

Boats must also be watertight—that is, invulnerable to leakage through the joints of adjoining pieces. Boatbuilders made wood plank boats watertight by caulking between planks with fiber threads, pitch, or a combination of these materials. For boats of skin or bark, filling interstices with pitch was common practice. The contemporary practice of molding a hull eliminates the problem entirely. Without seams there is no possibility of leakage.

Skin and Bark Boats

The earliest known boats were constructed from a frame of animal bone or light wood covered by animal skins or bark. Historians believe people used such boats as early as 16,000 bc. Several thousand years later, round skin boats, called coracles, were developed in Asia, Africa, the British Isles, and the plains of North America. Coracles have been built in Ireland in fairly recent times. They typically have a framework of woven willow shoots or other soft wood suitable for basketmaking.


Kayak
A kayaker paddles on San Francisco Bay. First used by the Inuit thousands of years ago for fishing and hunting, kayaks are widely used today for recreation. Kayaks have an enclosed deck and an open cockpit, where the rower sits with a double-bladed paddle.

Another type of skin boat is the kayak, a type of canoe created by the Inuit. The kayak is completely enclosed with animal skins stretched over a rigid frame, except for an open cockpit in the center, where the paddler sits armed with a double-bladed paddle. The kayak is wide at the center and tapers to points at both bow and stern. Kayaks in use today have much the same shape that kayaks had centuries ago, although modern kayaks may be molded from plastic, fiberglass, or Kevlar (a synthetic fiber originally developed to replace the steel in radial tires).

Bark-covered boats came into use about the same time that skin-covered boats did. They typically had a light wood frame with a bark covering made of pieces sewn together with root fiber. The frame was separated from the bark covering by a plank sheathing. Gunwales, or side edges, ran from bow to stern and provided longitudinal strength to the frame. The sheathing was held in place by forcing the ribs of the frame under the gunwales. Birchbark canoes up to 14 m (46 ft) long are known to have been built in North America. Modern canoes still have the basic shape and design of their predecessors, but they are usually molded from aluminum, plastic, fiberglass, or other lightweight, durable material.

Ancient Egyptians fashioned reed skiffs by securely binding together bundles of papyrus stems. The extreme lightness of these reed boats made them ideal for fishing in the marshes along the Nile; moreover, they were easy to carry. Equipped with sails and oars, the reed boats also carried cargo and passengers.

Wooden Boats

The earliest wooden boat, a dugout, dates from about 6000 bc and was discovered in what is now The Netherlands. A dugout consists of a log hollowed out with tools or by controlled burning. Early boat builders also constructed craft of sewn planks. This form of construction was used extensively throughout history. Sewn-timber construction was common among the peoples of the Pacific Islands, whose dugouts often had topsides formed of irregular sewn pieces of timber.


Dugout Canoes
Dugout canoes are made from hollowed-out tree trunks and thus are naturally buoyant. They may be plain or elaborately decorated like the two pictured.

Craft constructed of wood planks appear to have been developed gradually from the modified dugout by about 5000 bc, or even earlier in some regions. Log construction methods were also used for ancient Egyptian boats, in which short planks of timber were bolted edge to edge to form a hull.

Framing and planking are the basic components of modern wooden boats. The frame is used to support and stiffen the hull, including the stem, keel, keelson, ribs, knees, and wales. The planking is the outer shell that is fastened to the framing.

I. Lapstrake Construction

In northern Europe, lapstrake, or overlapping, construction was developed to a high order by the 9th century ad, particularly by the Scandinavians. Lapstrake is also known as clinch-built or clinker-built construction. Planks apparently were split off a log by means of wedges, or they were hewn and assembled on the keel, one after another beginning with the garboard, the lowest. In the Viking ships and boats of the 10th century and somewhat later, lashings were still used to secure the planks to the frames. Such construction resulted in a very flexible hull, yet one of relatively great strength. The lashings eventually were replaced by through fastenings, that is, by pegs, and later were replaced by nails driven into the planking and frames.


Viking Ship
This Viking ship, on display at the Viking Museum in Oslo, Norway, is an example of lapstrake construction. In Viking ships of the 9th century and later, external planks were overlapped and lashed to the ship’s frame, producing a strong, flexible hull.

In lapstrake construction the longitudinal seams of the planking are formed by overlapping the edges enough to allow continuous nailing. Thus, the skin, or exterior planking, adds strength to the boat. Frames are then added to give the required stiffness of form. Originally the frames were hewn of natural-crook timber, including root or limb knees and crooked trunks. Steam-bent frames became popular in the early 19th century.

II. Carvel Construction

The smooth-planked, or carvel-built, boat was developed in the Mediterranean region and was probably a natural evolution of the craft constructed of short planks bolted edge to edge used by the ancient Egyptians. Plank and frame construction was probably fully developed before ancient Greece rose to maritime prominence. As in lapstrake construction, the use of pegs probably preceded the use of metal fastenings between plank and framing.


Constructing a Wooden Whaler
Wooden ships like this 8-m (26-ft) whaler are built in stages. Because the timber must be steamed into the properly curved shapes, a fairly complex skeleton and bracing system is needed. Here, workers place horizontal planks called strakes to form the smooth curve of the hull on the starboard side of the ship.

A framing system made of almost equally spaced transverse frames fastened to a continuous keel and strengthened by a pair of gunwales running from bow to stern was a logical step in the development of smooth-planked boats. The problem of seam leakage was solved by using wood tar and caulking material, as in earlier lapstrake craft.

In so-called smooth-lap construction, developed in the 19th century, plank edges are rabbeted, or shaped, so that they lap and still remain smooth at the seams; the rabbeted laps are nailed in the usual lapstrake manner. See Woodworking: Carpentry.

In the early 20th century the use of lapstrake construction decreased, while carvel construction became more popular. The shift was caused largely by the need for extreme structural strength necessitated by the use of motors in small boats. Carvel construction also better met the demand for fast sailing craft used for racing.

III. Plywood Construction

Plywood boat construction began in the United States about 1918 and developed rapidly; the two basic types are paneled and molded. Panel construction involves securing flat sheets of plywood to transverse frames and to the keel and other units that supply longitudinal strength; it is used only with chine-model boats, that is, those with flat bottoms or V-bottoms. The amount of compound curve that may be used in a plywood panel is extremely limited.


Fishing in Newfoundland and Labrador
Plywood panel boats are flat-bottomed, a hull shape that lends itself well to fishing. Here, workers on a fishing boat haul a fish trap aboard off the coast of Cape Spear, the easternmost point in North America.

In molded-plywood construction the form of the boat is established by temporary transverse frames, or molds, and by longitudinal battens (thin, narrow strips of lumber often used to seal or reinforce joints), over which the planking is placed in two or three layers. The first skin is laid on diagonally and secured to the form by staples. Two or three skins are used, and after stapling, the mold is moved into a heating and pressure stage in which adhesion of the skins is accomplished. After adhesion, stiffeners and joinery are added; these usually include keelson, shoe, gunwale, guards, thwarts, decking, outside stem, transom (in a square-stern boat), and centerboard case and mast step (in a sailing hull).

Canvas-Covered Boats

In the construction of canvas-covered small boats, very thin planking is laid over light steam-bent frames; then canvas is stretched tightly over the hull by clamps and a tackle at bow and stern. After the canvas is secured, it is coated with a thin primer or is first wetted with water. The canvas shrinks as it dries, making a smooth surface. The canvas is then painted, outside guards are fitted at the gunwales, and an external keel and stempost and sternpost are fitted. The result is a light and tight boat suited for easy carrying and occasional use.

Aluminum Boats

The first aluminum boats built in the United States were for the polar expedition of American journalist and explorer Walter Wellman in 1894, although the first known use of aluminum was in Europe in 1891. After World War II (1939-1945), aluminum boats became common, usually in the form of canoes or simple craft used for sport fishing. These boats are built from sheets of aluminum, perhaps given curvature by stamping, and are fastened together with rivets. Sandwich-type constructions, intended to increase both lightness and strength, are under development.

Ferrocement Boats

Ferrocement, a closely spaced wire mesh made watertight by plastering of concrete, has some advantage for boats that must be built in small numbers by unskilled workers without sophisticated tools. Waterproof cloth can be formed into boat-shaped bags, which are then inflated to make small craft that are suitable and popular for some uses.

Fiberglass Boats

The greatest number of boats today are molded from composite plastic materials. The mold is a cavity having the exact shape of the boat.

Construction of the boat involves :
(1) Coating the interior of the cavity with a liquid plastic,
(2) Laying on reinforcing material in the form of a cloth or mat of fibers,
(3) Saturating the reinforcing material with liquid plastic,
(4) Possibly applying a layer of lightweight core material, and
(5) Applying more reinforcing and plastic.

The plastic hardens under the action of a catalyst that has been mixed with it; heat may be applied to hasten the hardening action. The result is a strong and relatively lightweight hull that has been built quickly and without the necessity for great skill on the part of builders.

In many cases, interior structure, such as bulkheads and foundations for an engine, is inserted while the hull is in the mold. The interior structure is then bonded to the hull by the plastic and its reinforcement.

The plastic used is generally a resin of the polyester type. Epoxy and vinyl ester resins are also used. The reinforcing material is often a glass fiber, giving rise to the generic term fiberglass boat. Structures of successively greater strength and stiffness for a given weight can be produced with Kevlar or carbon fiber reinforcement.

A core is required to give the composite structure the thickness needed for adequate stiffness and to make it resistant to puncture. If the part being molded is a flat sheet, such as an interior bulkhead, the core is likely to be a sheet of plywood. If a low-weight bulkhead is required, aluminum formed into a honeycomb is a likely core. Sheets of balsa wood or PVC (polyvinylchloride) plastic can also be used where lightness is important. For the sharp curvature common to boat hulls, the core sheets are provided with the material cut into small squares and attached to a cloth sheet. With balsa, they are arranged with the grain of the wood perpendicular to the sheet; the final structure then has a high resistance to impacts and crushing loads because of the natural strength of wood in the direction of its fibers.

Measuring and Modeling Boats

In primitive boatbuilding, the builder often used width of hand, length of forearm, and length between knuckles as units of measurement. Notched sticks also were used for measuring. More advanced building methods, using molds, required that the keel, the stem, and the sternpost be set up on stakes or blocks and that an arbitrary midship frame be erected in place. The curved members required for this frame were made from molds, used by shifting the butts to produce boats of varying size but similar form. After the midsection frame was put in place, two or more battens on each side were secured, one to form the sheer line, one along the turn of the bilge, and often one along the ends of the floor timbers, or short frame members, that crossed the keel. Sometimes other battens were fitted between the bilge and sheer battens and between the bilge and floorhead battens. Individual frame members were then shaped to fit inside the battens at selected intervals; when all the frames were made, the battens were removed and the hull was planked. All frames were hewn from crooked timber.

I. The Half-Model

The next development in boatbuilding was the use of drawings and the builders' half-model. Using drawings to delineate the shape of the hull apparently evolved slowly, but records are obscure. Similarly, little is known of the evolution of the half-model, but it appears to have developed along with the use of drawings. It is known that the use of such plans was common in 16th-century England, during the reign of Queen Elizabeth I, and Mediterranean builders probably employed crude drawings at least as early as the beginning of the 15th century. According to some authorities, the ancient Greeks and Romans also used drawings in shipbuilding and boatbuilding.

By the middle of the 17th century the art of designing boats by drawing plans was known throughout Europe. The sections were formed by compound curves made with the compass; the longitudinal curves were arbitrarily made according to the same principles that had governed the use of battens. The designers employed shifting molds based on the midsection form, so that one set of frame molds could be used by shifting butts to form all the frames; this system became known as whole molding. The boat design was drawn to reduced scale, and then, in a process known as lofting, a full-size drawing of the ship or boat was made. By 1800 drawing up the plans for ships and boats was fully developed, as was the lofting process.

The use of the half-model in lieu of the reduced-scale drawing appears to have originated in Europe at an early date; by the early 18th century carving a model from a solid block of wood was a popular method of design in commercial boatyards and shipyards. The shape of the model was determined by the judgment and art of its maker. The sections needed to form the mold frames controlling the hull shape of the boat were obtained by sawing the model at the desired transverse mold stations and scaling off these sections by drawing ordinates.

II. Lift Models and Lofting

By the middle of the 18th century, builders had developed another type of model, consisting of a blackboard cut to the shape of the hull profile, with the sections formed of thin plank secured at the desired mold stations. The sections were made fair, or capable of being planked smoothly, by battens at suitable locations. Finally, about 1795, the so-called lift model was developed, made of boards of equal thickness pegged together and shaped to the desired hull form. The shape of the mold frames was obtained by taking the model apart and tracing the shape of the lifts, or planks, in their proper relation to one another. By drawing lines across these shapes at frame stations, the transverse widths at the ordinates formed by the lift thicknesses could be determined. The lift model is still in use in many boatyards and remains practical for small craft.

Lofting consists of drawing the full-size shape of the hull on a suitable platform or floor. The required drawing shows the mold sections, or transverse forms, usually five to seven in number. These sections usually are tested by drawing in some longitudinal curves, such as the sheer, and a few waterlines or level lines duplicating the so-called lift lines of a half-model. In addition, buttock and bow lines, longitudinal vertical sections parallel to the centerline of the hull, may be used. Diagonals, representing the batten curves over the molds, may be drawn in. In making the longitudinal drawing, the profile of the hull usually is drawn full size. Lofting is an important operation in boatbuilding, for it is the only way a design may be reproduced accurately or repeatedly. In the more advanced boatshops the structural details also are drawn full size, the lofting being done in minute detail.

Boat Propulsion

Inboard-Outboard Motorboat
Motorboats are popular recreational vehicles, present on many lakes and oceans throughout the world. Inboard-outboard (I/O) motorboats such as this Donzi Z-25 are capable of reaching speeds of 80 kilometers per hour (50 miles per hour).

Until the 20th century, boats were propelled by oar and sail (see Sailing), just as they had been since ancient times. Paddles and poles (for pushing against the bottom of the waterway) were variations on the oar suitable for small craft. The rapid rise of the steam engine to dominance among ships did not affect boats until the late 19th century, when steam engines and boilers had become compact enough to fit into a small hull. The age of steam-propelled boats was brief, however, for the advent of commercial internal-combustion engines came within a few years of boat-size steam machinery. By the late 20th century, the internal-combustion engine, either in diesel or spark-ignition (gasoline) form, had become almost universal in boat propulsion, save for sail, oar, and paddle that survive with many types of recreational boats (see Motorboat).

I. Inboard Motors

The internal-combustion engine was applied to boats as early as the 1880s, and many two- and four-cycle engines of one cylinder or more were developed. The engine, permanently mounted inside the hull, drives a screw propeller by means of a horizontal shaft. Today many of these engines are automotive or truck engines that have been altered superficially to make them suitable for marine service.


Cabin Cruiser
Powerboats are popular as a leisurely form of yachting. A modern yacht may include a kitchen, sleeping cabins, and other conveniences below decks.

A variation on the inboard engine arrangement is to place the engine horizontally at the extreme stern of the boat, connected through a watertight aperture in the stern to a lower unit that is similar to the housing for the right-angle gears of an outboard motor. Craft with such engines are called I/O (inboard/outboard) boats.

II. Outboard Motors


Outboard Motors
Outboard motors, like the ones mounted on these skiffs, are the most common means of propulsion for boats. Because it is mounted external to the hull, the motor is adaptable to almost any boat.

The outboard motor is probably the most common means of propulsion for boats. This machine is nearly always a two-stroke spark-ignition (gasoline) engine, mounted vertically at the stern of a boat in order to drive a shaft that in turn drives a conventional screw propeller through right-angle gears. It has the great virtue of being mounted external to the hull, thus being easily adaptable to almost any boat. Outboard motors operate on a mixture of gasoline and marine oil.

The development of the outboard motor was rather slow in the early years of the 20th century. After World War I (1914-1918), the popularity of the outboard motor grew steadily, and as a result its power gradually was increased, and it was made more reliable. The popularity of the outboard motor increased tremendously after World War II (1939-1945), and small cruisers, runabouts, utility boats, and various classes of racers became available. By the late 1970s, powerful outboard motors, ranging up to 200 horsepower rating, were available, as was special equipment for handling such engines at the water's edge. The use of two outboard motors on cruisers and runabouts was common.

Because of the great power available and because the low transom required by this type of motor creates the danger of the boat being swamped, the safety of the outboard motorboat has become a matter of public and governmental concern. The popularity of this type of boat also has produced crowded waters at lake and seaside resorts. In many areas measures have been adopted to prevent accidents caused by excessive speed, careless or reckless operation, and poor boat design.

III. Water-Jet Drive

Several innovations in propulsion became important during the late 20th century. One is propulsion by a high-speed jet of water ejected from the stern by a pump within the boat hull. The pump is driven by an engine that is identical to the one that might drive the conventional propeller. Water-jet propulsion is most suitable for high-speed boats, since for good propulsion efficiency the forward speed of the boat must be close in magnitude to the afterward speed of the jet. Regardless of boat speed, water-jet propulsion is often used for craft that must operate in shallow or rocky waters, where a conventional propeller might be damaged.

IV. Surface-Piercing Propeller

An innovation of great interest to designers and users of high-speed boats that was introduced in the 1970s is the surface-piercing propeller, in which the propeller is mounted with its center at the water surface. It is most commonly seen in the form called the Arneson surface drive. The drive shaft projects through the stern of the boat, supported by two hydraulic cylinders that adjust the vertical position of the propeller, a feature of great importance because the angle and level of the boat change as the boat speed changes. The engine is fixed in its position, but two universal joints accommodate changing shaft angles.