Understanding Boat Hull Design

Thinking further after yesterdays post about ‘The Ideal Boat:What the Brochures Tell You’. Maybe I dived into the numbers and formulae too quickly. Brochures carry pictures as well and the shear ‘look’ of a boat can tell you a great deal about what the designer intended.

Hull design is a compromise between conflicting demands. Getting the hull shape right is probably the most demanding of all design tasks.

Every hull is unique but generally they conform to one of three basic shapes. Understand the shape and you’ll know a lot about a boat before you step on board.

The Displacement Hull

Think of a traditional fishing trawler or an off-shore sailing cruiser. These boats have a deep draft. They are heavy, stable and predictable in a seaway. Sailing cruisers of this type will plod on for hours without attention to the helm but they are slow.  They ‘push’ their way through the water building up a bow wave but they are too heavy to climb over it so their speed is severely limited. Try to increase speed and you’ll burn increasing amounts of fuel for very little gain.  

The Planing Hull

Think of a flat stone skimming over the water. These boats have a flat spoon-like underwater shape. They sit on the water rather than in it and are designed to climb over the bow-wave so speed isn’t inhibited. Planing hulls need plenty of volume aft to stop the stern burying as the bow rises so they have wide transoms. Speed is determined by your nerve and the power of engine but there are drawbacks. A planning boat has no ‘grip’ on the water. She’s as happy siding sideways as driving forwards so at low speeds she is difficult to control. At high speed the ‘slamming motion’ can loosen your teeth. If your engine fails in heavy weather you have a serious problem, she’ll bounce like a cork and may ship gallons of water over that low wide stern.

The Semi displacement Hull

This is the designer’s compromise between the two extremes. A semi-displacement hull will rise over the bow wave when enough power is applied but at lower speeds there is still enough hull under the surface to ‘bite’ the water. A typical 22ft semi-displacement motor cruiser will behave as a displacement hull at speeds below 7 knots and then rise over the bow-wave to attain 16 knots when the engines are opened up. Some argue that semi-displacement is neither fish nor fowl. At low speed they don’t behave as well as displacement hulls and at high speed they simply aren’t as fast as planing hulls. They still need that wide transom for buoyancy but without the speed of a planning hull they may slide and broach in following seas.

Here I have described three extremes. In reality a designer looking for a mass market will design a boat which tends towards one of these categories to get some of the advantages of a particular hull form without incurring the disadvantages which come with the out and out extreme examples. A careful look at hull form can tell you a great deal.

By way of example, look at this old Westerly Centaur. Publicity of the day described her as a ‘Gentleman’s Yacht’, but that distinctive bow shape, with the moulding for throwing spray away from her when punching into the waves, AND the fact that she carried a large (for her day) 25hp inboard engine, tends to suggest she’s a motor-sailer in disguise. 

Alternatively look at a modern Cornish Crabber, her retro gaff rig suggests, she’s heavy and slow. The hull lines however, suggest she’ll be surprisingly faster than her looks would suggest.

The more you look, the more you’ll see. The old guys knew this intuitively and could predict a ship’s likely behaviour from a half hull model carved out by the builder. The key message for us is that we have to be clear about what we want our boats to do and we have to develop an understanding of the design elements which deliver those qualities.

Seaward