Bottom Shapes

This is a collection of several vessel-bottom shapes. Attempts are made to keep this entry about stability over hydrodynamics, though many hull shapes have their distinctive hydrodynamic properties. Most bottoms have more then one of these shapes over the length of the vessel. The cross section at the beam of the vessel is considered the main hull shape.

Irregular Bottom

A raft is most often made with what is available, though often rectangular there can be no definition of the outline or bottom. We imagine a raft as made with many similar shaped objects, and basically rectangular in shape. The ideal raft has much of its buoyancy on the outline this would increase the stability. A raft will have an irregular bottom, even the most irregular bottom has properties. The vessel will not be blown easy by the wind, but will follow the fastest water current. These properties make it a good vessel to follow non white water rivers with the flow. The tendency to follow the fastest flow can make it hard to control. Poling may be the best propulsion option, however only course corrections will be possible as the current can not be beaten. Any propulsion may be possible though the drag in any direction will result in low speeds. High powered propulsion most likely will destroy the structure.

Round Bottom

A round bottom is relative easy to make, jet rather hard to use. Only used for small vessels. The sailor has to know the motions of this type of vessel to know how to move around aboard. Any durable instability will make the vessel tipple over and it will capsize. The rolling stability is limited to a small area above the keel-line, to either side of this line the center of gravity should stay below the boards.

An advantage is the sharp curves this kind of vessel will be able to make. The lack of bilges will easy yawing, turning on the spot is possible, though the original direction of motion is not stopped. This property makes a round bottom ideal for tugboats. Just consider the attachement point for the tug hawsers to be as low and as centered as possible. At high speeds this hull will tend to climb out of the water, though the slightest wave will make it roll and spin out¹.

The disadvantage of rolling can be partly undone by the use of bilge keels. These can be as simple as some steel plates attached at the fortyfive degree angles of the hull. These will not only reduce rolling but also make it possible to ground the vessel without high risk of tipping over at low tide.

Flat Bottom

A flat bottom and near square bilges will have much better stability, when you move your weight to the board, also the displaced volume of water on that side will increase. This is the self righting property of flat bottomed vessels. The increase in stability reduces the ease of turning. Though as sharp turns are usually only made at low speeds short after or before mooring this is of little disadvantage. Another advantage is the relative low draft and the limited damage on grounding². A disadvantage is that at higher speeds and low ground clearance the vessel is sucked toward the ground. The relative high velocity of the waterflow underneath the vessel reduces the local pressure and sucks³ the vessel down.

Chine Bottom

A relative easy way to build a vessel using flat sheets of plywood, steel or aluminium is by making a chine hull. You only have to bend the sheets in one direction. By making a little V-shape for the bottom plates, near fortyfive degrees for the bilges and near ninety degrees for the boards you combine ease of build and rather good sailing properties in one go. These vessels do not tipple as long as you use the bottom for the main load. The chine hull has reasonable stability over a wide range of speed. There is not much draft by the hull. For sailing leeboards or an inboard daggerboard and a dedicated skeg blade will be needed.

V-Shaped Bottom

The V-shaped bottom is popular for motor yachts. By placing the engines, fuel bunkers and water tanks as low and aft as possible the vessel will stay upright and hang into a curve. Whithout the engine and bunkers the vessel may be top heavy and capsize at launch. The V-shape also tends to lift the vessel up from the water at speed while creating impressive waves. A more flat bottom under the stern will make planing possible. The surface of the V-shape often has ridges higher on the bow and going down to the stern, this is another feature encouraging the hull to go planing without loss of control.

S-Shaped Bottom

The S-shaped bottom is used for sailing yachts. The S-shape is for port side from the bow, for starboard as seen from the stern. The shape is partially created by the need for the smooth surfaced sailing vessels of some leeway drag. This is why these vessels have their fin like keel. Another added feature is the bulb at the bottom end of the keel, this is for ballast. No heavy motor and fuel bunker but high rigging give sailing vessel a relative high center of gravity. Using a bulb under the keel adds the lever effect to the mass of the ballast, reducing the required mass substantially. The only drawback is some increased draft.

Cavity Bottom

Tunnel shaped bottom or multi V-shaped bottom, perhaps W-shaped bottom, they all have the goal to achieve planing at moderate speeds. Due to the relative wide total body this vessel will have the same stability as flat bottomed. The cavity will concentrate the wavefront to help the vessel lift up from the water. When not in the water but on the surface these vessels can go very fast. This hull design is therefore used mainly for speed boats or power boats. The major drawback is that besides water also air tends to build up pressure under the hull. A spinout is inevitable above certain speeds or when waves hit the vessel at critical amplitude and direction.

Bow Shapes

The bow of a vessel has a major influence on the behaviour of the vessel regarding to waves and turning. Ferries may have complex bows to accomodate the vehicles in boarding and disembarking. Some ferries use a mounth like mechanism to open, on others the bow or stern splice to the sides.

Sharp Bow

A sharp overhanging bow above the waterline is ideal to deflect high waves With a sharper angle the force pushing the vessel back is significantly reduced. The planes of the sharp bow where the waves are deflected are called the flares. The overhang of the bow increases the pitching motion of the vessel. Last year ocean race the vessels had an inclined bow, this is not only an estethic consideration, the reduction in pitching also decreases the stress on mast and length of the hull. A sharp bow extending under the waterline gives some reduction in resistance while ploughing through the water. There will be less pitching and yawning though less manoeuvre ability.

Bulbuos Bow

The bulb on modern sea freighters is to break waves and to increase the waterline length. Unlike in ancient times this is not a ram. Sitting on the beach and having a small party with the waves after a vessel has passed one will be a little disappointed. Only some less impressive waves drop on the beach. The bulb makes the vessel slide through the water with apparent ease.

Spoon Bow

Sometimes seen with wooden sailing dinghies, the rounded spoonshape will deflect higher waves the extended deck actually enables a firm mounting base for the fore stay as well as increasing the length of the vessel when heeling firm.

Stern Shapes

The stern has the least importance of the hull shape, though there are several functional aspects.

Inclining Stern

A flat inclining transom is very popular on yachts, when anchored it is an ideal swimming and mooring platform, often with dedicated stairs to go up or down. The dinghy hanging on a davit over the transom under sail can be launched and used for leisure, shopping or fishing. The square stern does give a calm wake but does not react good on incoming waves. Either moored or sailing incoming waves from the stern will make the vessel yawn.

Overhanging Stern

Many vessels make use of the deck space above the stern. This is often for cabin space, on cargo vessels, or for mast support, on sailing vessels. The wave deflecting properties are also a reasonable argument.

Bulbous Stern

Cargo vessels not bothering about their looks may have a bulbous stern, this reduces the wake and increases the waterline length. Most often this is combined with an overhanging stern shape.

The wedged surface behind powerboats sometimes named antiventilation board also contributes in this way.

Multi Sterns

On dual engine powerboats the planing part of the bottom is often split as like a multihull. These vessels tend to have a skeg like structure before each propeller. The stern gives a dual wake but relative good planing control compared to a flat bottomed stern.