Give the Hull a Shape

Two boats can carry the same load and displace the same amount of water while looking entirely different. One may spread its volume across a broad, shallow body. Another may be narrower and deeper. Both satisfy the floating balance. A berth, a place where a boat is secured while afloat, has room for only a certain length and width. Shallow water sets a different limit, below the surface.

To choose between them, we need to describe the actual shape. Imagine a boat ashore, where its underside is visible, and then afloat with a known load.

Locate a part, locate another boat

A thwart crosses the hull from port to starboard. Its direction is athwartships, approximately at right angles to the fore-and-aft centreline. A person sitting on it can move inboard, towards that centreline, or outboard, away from it. The same words can describe whether something is within the hull or outside it. A blade worked outboard of the hull is outside the side; a seat moved outboard can still remain inside. Context tells us which relationship is intended.

Amidships also needs its context. A bag stowed amidships along the boat is near the middle of its length. A handle held amidships across the boat is centred rather than offset to either side. Neither instruction specifies an exact measured point unless one is supplied.

An object abaft the rowing seat lies farther towards the stern than that seat. Unlike aft, which locates a general direction or region, abaft makes the relationship to a stated object explicit. A box can be abaft the forward seat while still lying in the forward half of the boat.

Now place another boat off the side. If it lies roughly at right angles to our centreline, it is abeam. Abeam describes the direction towards that object; athwartships describes a direction across our own hull. Move the other boat towards the stern and it comes onto our quarter, between abeam and directly astern. “On the starboard quarter” gives a direction, not a distance.

A working surface and ways through it

An open boat leaves its occupants and much of its interior exposed. A larger hull can carry a deck, a platform forming an upper working surface and covering space beneath. Its forward part is the foredeck. On a larger vessel, decks can also separate levels inside the hull.

Covering the hull gives the crew a surface to work from, but they still need access to the space below. A companionway is an entrance with steps leading down from deck level. A hatchway is an opening and passage through a deck or similar boundary; its movable closure is a hatch. In ordinary usage, hatch can also mean the opening as a whole. Describing whether it is open or closed tells us more about the actual barrier than the name alone.

A small yacht commonly has a cockpit, an open or recessed working and seating area from which it is steered and its sails are controlled. The deck around it, the recess itself and the entrance below have different jobs. Their names do not guarantee how an opening drains or how securely its closure excludes water. We have already seen why the difference matters when a loaded boat settles low.

Measure the same hull in several ways

Standing beside the boat, measure from its foremost point to its aftermost point. This gives a length overall, or LOA, for the endpoints included. Suppose the hull itself measures six metres from the bow’s leading edge to its rear face, with a separate support projecting one metre beyond the bow. The entire assembly occupies seven metres. A quoted LOA might include that support or, under another convention, describe the six-metre hull. The endpoints tell us which boat length the figure actually represents.

Length on deck, LOD, follows a defined deck-level hull length. In this example, take it from the bow’s leading edge at deck level to the hull’s rear face, excluding the separate projecting support: six metres. Length waterline, LWL, measures between the forward and after points where a specified waterline meets the hull. If those points are five metres apart, this same boat has a five-metre LWL. An end projecting beyond such a meeting point is an overhang. A berth must accommodate the occupied length, while LWL describes the length meeting the water at that load.

For length between perpendiculars, LBP, the endpoints are specified forward and after reference lines standing at right angles to a reference waterline in side view. A common convention puts the forward line where the chosen waterline meets the hull’s leading edge at the bow, and the after line through the axis about which the rudder turns. A rudder is a steering blade attached so that it can turn relative to the hull. Measure the horizontal distance between those lines. Other conventions choose different after references. Use the stated rule to choose the endpoints before measuring.

Across the hull, width is beam. Maximum beam is the greatest width under the stated convention. Waterline beam gives width at a particular waterline, commonly the widest part of that slice. A boat can offer generous width above the water while narrowing considerably where it meets the surface. Maximum beam alone does not describe the immersed shape.

The distance below the water has already given us draught. The height above it, measured to the highest relevant point, is air draught, also written air draft. The top of sail-supporting equipment can matter far more than the hull’s height when passing beneath a bridge. Loading changes the water level from which that height is measured, so the boat’s present condition matters to its clearance.

Choose a boat for a narrow berth and the full occupied length and width matter. Choose a channel of limited depth and its actual draught matters. To compare the shapes meeting the water, use the stated loaded waterline dimensions. One boat legitimately has several different measurements because it must answer several different questions.

Follow the outside from bow to stern

At the bow, the two sides meet at a leading structure called the stem. In a traditionally built wooden boat this can be a distinct member to which the planking, the fitted wooden boards forming the hull’s shell, is joined. In a formed shell, the name can describe the corresponding leading structure without implying the same separate timber. The stemhead is its upper end near the deck edge.

The cutwater is the leading edge where the bow meets and parts the water. Lower down, the forefoot is the forward region where the bow blends into the hull’s bottom, its lower external surface. Bow identifies a region; stem identifies a structure within that region.

The outer sides above the waterline and below the deck edge are the topsides. They are surfaces beside the deck, rather than the deck itself. Below the waterline lies the underbody. A drawing or measurement using that word should make clear whether it includes separate projecting parts as well as the hull body.

At the other end, traditional construction may have a sternpost, an upright or sloping member forming part of the rear structure. Planking and, on some boats, steering fittings connect there. The modern stern need not contain an equivalent separate post. Timber members, joined planks and a moulded shell can produce related outlines through different methods of construction.

Several ways to finish the hull

Keep the forward part of our six-metre hull and compare two ways of finishing its stern, first from above. The sides could remain well apart until they meet a transom, a surface or structural assembly closing the stern across the boat. That leaves width near the back for a working space. The transom itself may be flat or curved, upright or sloping. Transom describes the closure; stern names the whole rear region.

Alternatively, bring the sides progressively together towards the stern. In a double-ended hull, or double-ender, the body narrows towards both ends, though the two ends need not be identical. A canoe stern follows that narrowing into a rounded or pointed finish resembling a canoe’s end when seen from above. Follow its sides aft and the working space becomes narrower too. For the same total mass, the immersed body must still displace the same mass of water.

A cruiser stern carries the sides around a rounded rear. In yacht designer Rob Mazza’s comparison of canoe-stern cruising boats, widening and rounding a narrow after end makes room to extend the cockpit farther aft. He describes that fuller end as a cruiser stern. This gives us a useful physical comparison: the narrower end brings the sides together sooner; the fuller rounded end retains more width for people near the back. The names overlap in yacht descriptions. If an unfamiliar boat is described only as having a rounded canoe or cruiser stern, ask how wide the end remains from above and how it slopes in side view. Those details determine where the working room lies.

Now look from the side. Suppose the stern extends half a metre beyond the rear end of its loaded waterline. The projecting after region is the counter; a stern arranged this way is a counter stern. Its after overhang is that half-metre projection, measured from the waterline endpoint to the hull’s after end. A pointed end and a broad transom can each project beyond the waterline: the overhead outline and the side-view projection describe different properties. Our berth must accommodate the projecting hull as well as the length meeting the water. At the bow, forward overhang measures the corresponding projection ahead of the forward waterline endpoint.

Under a traditional counter, the region where the after ends of the planking finish is called the tuck. Look beneath the projecting stern to locate it. The counter names the projection above; tuck directs attention to the planking’s finish beneath it.

A whaleboat gives the double-ended form a particular working setting. American whaling ships carried these smaller boats and lowered them to pursue whales. Mystic Seaport’s account of an 1880s arrangement describes five rowers and an officer, with hunting gear packed aboard. The space between the tapering ends had to accommodate that crew and equipment, while the boat itself had to be carried by a larger ship. The double-ended outline describes its form; its equipment and service identify its whaling job.

Read a slice through the hull

Imagine cutting across the boat and looking at the exposed shape. That slice is a section. A midship section is taken at a defined middle position along the length; it need not be the largest section. Changing the cutting position changes what the drawing tells us.

In a round-bottom hull, the underwater transverse shape is curved. The region where the side turns towards the bottom is the bilge. A smooth curve through that region is the turn of the bilge, giving a round-bilge form. Inside the boat, bilge also names the low space where unwanted water collects. A rounded outside surface and water lying inside the hull are different senses of the same word.

Panels meeting at a lengthwise change of angle form a chine. A hard chine makes a sharply defined corner. A soft chine rounds the transition; designers do not all draw the same boundary between that description and a rounded bilge. With several changes of angle, a multichine hull can use a series of flatter panels to approximate a curved body. Making broad flat pieces and joining their edges offers different construction choices from bending material around a continuous curve.

A flat-bottom hull has a comparatively flat bottom across much of its width. In a V-bottom hull, the bottom rises on either side of the centreline. Its slope away from horizontal is the deadrise, measured at a stated position. A deep-V has comparatively large deadrise.

The sides can change slope as they rise. Flare carries them outwards, making the hull wider higher up. Tumblehome brings them back inwards above their widest part. A hull may have a flared bow and different side geometry farther aft, so a single label need not apply along its entire length.

Suppose our shallow-water boat must now carry a larger load. Adding width to its immersed body can provide the extra displacement without the same increase in depth. A shallow-working sailing barge, a cargo vessel propelled by sail, uses a broad body for that sort of carrying job. The narrow berth still sets a limit on how far we can widen our own boat.

Other working families combine the surfaces differently. A dory has a relatively narrow flat bottom and flared sides: follow its section upwards and it grows appreciably wider. A sharpie is generally narrow, shallow and flat-bottomed; nineteenth-century examples worked oyster grounds and other fisheries. Flat pieces joined along their edges offer a practical route to making such a hull. Its sail arrangement can vary without changing the basic hull description. Dory, sharpie and sailing barge connect geometry with different working requirements and building practices.

Return to the side of the boat. The fore-and-aft curve of its bottom is rocker. The rise and fall of the deck edge is sheer, and the line tracing that edge is the sheerline. Rocker follows the bottom’s curve along the length; deadrise gives its slope across the bottom at a specified section. A boat can have considerable rocker while remaining flat-bottomed across its width.

Finally, slice horizontally at the water’s surface. The enclosed flat shape is the waterplane; the surface it covers is waterplane area. The waterline marks its boundary on the hull. For a small increase in immersion, a wider waterplane generally adds displaced volume more quickly than a narrow one. This connects the shape to our added box: the same extra mass can produce different changes in draught on different hulls. As the boat settles or inclines, the waterplane itself can change, so one slice does not describe every loading condition.

One hull or several

The intended purpose, conditions and priorities form a boat’s design brief. A narrow river, a shallow landing and a need to carry several people are concrete parts of a brief. They help explain why a designer might distribute the required buoyant volume in different ways.

A monohull has one principal hull. A catamaran has two principal hulls beside one another. A trimaran has a main hull with a smaller side hull or float on each side. Catamarans and trimarans are multihulls: they separate buoyant bodies across the water instead of placing all the volume in one body.

An outrigger is a side float or its supporting assembly. Placing a float away from a narrow main hull lets it help resist a sideways tilt: as it is pushed down into the water, it develops upward support at a distance from the main body. Such an arrangement can have one side float rather than the pair of a trimaran. The spacing changes the support available, while the connecting structure must carry forces between the bodies.

Hull count is one property among several. A narrow double-ended boat can be a monohull; a two-hulled vessel still needs a description of each hull’s ends and sections. Width that helps separate the supports can also prevent a vessel entering a narrow berth.

The hull’s shape gives the crew places to work, boundaries that keep water out and defined structures into which a force can be carried. Those become the foundations for a new load above the hull: a supported sail drawing energy from the wind.