Motorboating

Powerboat types

What type of powerboat should I buy? The first-time buyer is confronted with bewildering choice of shapes and sizes, some of them very specialised and intended for varying conditions and uses. If your interest is fishing on rivers or lakes, then a small flat-bottomed boat that is light, stable and easy to transport could well be the boat for you.

Fishing offshore requires a more rugged design with greater freeboard and a hull shape that will slice through the seas rather than bounce uncomfortably over them.

Should cruising be your prime criteria, then size and number of berths may be your first consideration, but running costs and where to keep her should be a close second.

Hull shapes

Hull forms can be divided into two broad categories: displacement and planing.

Planing hull in slow displacement mode.

Planing hulls obtain their dynamic lift from a combination of hull shape and the speed at which they move through the water. As speed increases, the hydrodynamic forces lift the hull up on top of the water, reducing drag and wave-making resistance to allow relatively high planing speeds. The transition point between displacement and planing is known as ‘hump’ speed’. This is where the hull generates enough ‘lift’ to rise up on top of the water and accelerates forward just as if a turbo has kicked in when driving a car.

Same hull in full planing mode.

Displacement hulls

Traditional, non-planing types are known as displacement hulls. Working on Archimedes’ principle that a floating object displaces its own weight of water at rest as well as at speed, these traditional hull shapes are continuing to push aside their own weight of water, setting up a wave at the bow and stern.

As speed increases, the height and distance between these two waves increases to the point where the hull is supported in the water by the wave generated by the bows, and the other at the stern, with a big dip in between.

At this point, the boat has reached its maximum displacement speed. This figure is in direct proportion to the length of the hull and can be calculated quite accurately using the equation:

Wave pattern of a displacement hull operating above its design speed.

Wave pattern of a displacement hull operating at its design speed.

Thus, a displacement hull with a waterline length of 25ft (7.62m) has an effective top speed of 7knots.

If more power is applied, the hull will try to climb up its own bow wave, and the stern wave will fall back, leaving the transom to sink down in the trough, creating a great deal of wash and a wasteful consumption of fuel. Any further increase in speed can only be achieved by an inordinate and highly inefficient use of extra power.

Semi-displacement hulls

A semi-displacement hull is a hybrid of these two types, combining V-shaped forward sections that merge into flat or rounded profile aft. When pushed above displacement hull speed, this design type operates at the lower end planing mode speeds, providing a comfortable – though wet – ride through heavy seas. It is not as fuel efficient or as fast as a fully planing hull and invariably rolls more.

Nelson semi-displacement hull. Excellent sea boats, but very wet.

Elan 35 deep-V sports cruiser running at speed.

A deep-V hull in displacement mode.

A deep-V hull in planing mode.

Planing hulls

The deep-V hull is the most popular form for fast offshore fishing and cruising powerboats. The sharp entry of the bow and V-shaped bottom, carried all the way back to the transom, minimises slamming and smoothes the boat’s ride over waves. The widely flared bow adds to the forward buoyancy, limiting any tendency for the hull to bury its nose when running through following seas, and the addition of spray rails to knock down spray, also contribute to this ‘lift’. At slow speed, the deep- V hull has more draft than a flat planing hull and behaves much more like a displacement design. The deeper the V or angle of deadrise, (often between 18° and 25°) the better the performance and ride in rough water. The trade-off is less speed in calmer conditions than flatter bottomed designs. Some designs incorporate a ‘step’ in the hull approximately 2/3rds of the way back from the bows. Also known as the ‘vented’ hull, the step is designed to suck air under the aft sections of the hull to lessen wetted area and resistance. The bubbles of air sucked in to the flow act like ball bearings to reduce friction.

Orkney cathedral hulled unsinkable dory.

The vented Intrepid sports fishing hull. Air is sucked down through the step to lessen wetted area and resistance.

Cathedral hulls

The cathedral hull is a triple V-shaped planing monohull originally developed in America. It offers a wide rectangular full-length cockpit coupled with remarkable stability and load carrying capabilities. These open dories make excellent tenders and inshore rescue craft.

Catamarans

Powered catamarans are popular in Australia and parts of America, where they are used as fast fishing boats. These twin-hulled designs combine the good stability characteristics of the cathedral hull with the sea keeping qualities of a deep-V hull. Their performance is, however, more susceptible to weight than a deep-V hull.

Glacier Bay power catamaran.

Orkney Vanguard traditional sports/fishing boat

Orkney Boats have been building traditional go-anywhere sports fishing boats for more than 3 decades. The 19ft Vanguard used in illustrations throughout this book marries the traditions and strength of moulded clinker planking with the get-you-home-safely all-weather performance of pilot boat design thinking.

Built in Britain, the Orkney range of sports fishing boats can be found all over the world. Produced in sizes from 13–24ft (4–7.3m) the larger boats within the Orkney range have fully fitted cabins, while the smaller fishing boats can be purchased as open boats or with the addition of a fixed and folding cuddy.

Orkney Vanguard 190

Length: 19.40ft 5.91m Beam: 7.50ft 2.26m Draft: (engine down): 2.50ft 0.76m (engine tilted): 1.00ft 0.30m Internal freeboard: 2.40ft 0.73m Displacement: 2,359lb 1,070kg Trailed weight: 3,240lb 1,470kg Max engine: 100hp Max speed: 30knots Max payload: 6 persons or 1,323lb Transport: trailer Design stability category: C

Ribtec rigid bottom inflatable – RIB

The concept of marrying a deep-V hull to an inflatable collar around the gunwale was first developed by Atlantic College in Wales back in 1967. The idea was then carried forward by Royal National Lifeboat Institute (RNLI), which put a number of Atlantic 21 inshore lifeboats on station around the British Isles. The concept was then adopted by the military as rapid assault and rescue craft.

During the 1980s the good sea-keeping capabilities of the deep-V hull and inherent safety of the inflatable tubes which keep these boats buoyant even when full of water, has attracted a strong allegiance among the boating public worldwide. The Ribeye 6m RIB featured in this book is produced by Ribtec Ltd in the UK and is a fine example of the genre in terms of handling, performance, sea-keeping qualities and safety.

Rebtec Ribeye 6metre RIB

Length: 19.68ft 6.00m Beam: 7.48ft 2.28m Draft: (engine down): 2.50ft 0.76m (engine tilted): 1.54ft 0.47m Tube diameter 1.50ft 0.46m Air chambers: 5 1,070kg Displacement: 1,962lb 890kg Trailed weight: 2,844lb 1,290kg Max engine: 120hp Max speed: 40knots Max payload: 8 persons or 2,425lb (1,100kg) Transport: trailer Design stability category: B

Galeon 440 Flybridge sports cruiser

The 44ft (13.42m) Galeon 440 Flybridge sports cruiser used in illustrations throughout this book is designed by Tony Castro and built in Poland. She is a particularly good example of the flybridge cruiser design with a large saloon, expansive owners’ suite and two guest cabins. Stairs from the aft cabin lead up to the flybridge to provide a commanding position for the helmsman, and seating and sunbathing areas for guests. There is a second...