Hull Designs that Minimize wake

Introduction

Wake is undesirable in boats, with a few exceptions, because wake is inefficient. What is wake? Wake is waves that propagate out behind a boat. As anyone who has studied physics knows, energy cannot be created or destroyed; the relevance here is that a large wake represents a large energy input. Boats with a large wake are essentially shoving aside the water, which means they are dumping energy into the water to create waves rather than to move forward. This is OK for some recreational activities, like wakeboarding, but is generally undesirable. Hull designs that minimize wake can have several advantages. Because these hull designs generate less wake, the boats built on them are more fuel efficient.

In addition to being more fuel efficient, boats with small wakes are faster than boats with larger wake, given the same engine. For this reason, speedboats are typically designed with as small a wake as possible. Small wake may also be advantageous for fishing boats, because wake may scare off fish. Many marinas, bays, and lakes have no-wake policies, as well.

Application of modern fluid mechanics and lightweight materials have enabled engineers to develop hull designs that minimize wake much more than older designs did. This article is a review of some of these new wake minimizing hull designs.

Engineering Analysis of Wake

A wake is created because boats must push their way through water. Because every action has an equal and opposite action, bow waves emanate out from where a boat passes through the water. These waves propagate at an angle of about 19 degrees from the boat. As boats move through water, they build up bow pressure (in front of the boat) which actually pushes the water aside, rather than the boat physically parting the water. One method of improving efficiency and minimizing wake is to design the hull such that the bow pressure front is as far out ahead of the bow as possible. This can be achieved by placing a rounded projection in front of the bow, and increases efficiency by up to 25% at designed speed. Another method is to split the hull into several sections, and design the hulls such that the inner wake is canceled out. This is the idea behind a catamaran.

One useful figure to calculate is the Froude number, named after William Froude, a pioneer in the field of fluid mechanics. The Froude number is a dimensionless number, and can be viewed as a ratio of forces describing resistance to movement through water. The equation defining the Froude number in water is

Fh=V / √gL

where V is the hull velocity, g is the gravitational constant, and L is the water line length of the ship. High Froude values cause high resistance, so to minimize Froude number engineers must increase L.

Wake Minimizing Hull Designs

Weight is a major factor in wake creation, so many of these designs are lightweight. Of course, it is impossible to completely eliminate wake. Some boats will inevitably have a large wake, such as particularly large or fast boats. Any minimization of wake saves fuel and increases safe operation speed, however.

Catamaran Ferries – A low-wake Design

catamaran

This boat, designed by All American Marine and Teknicraft Design, is an example of the newly popular catamaran style for large ferries. It is lightweight, with a hull constructed out of aluminum. The cabin and hydrofoil, meanwhile, are constructed out of a sturdy, lightweight composite material. The low wake wash contributes to a very high fuel efficiency. In fact, tests have shown that the boat achieves the same efficiency at speeds between 17 and 29 knots. The 83 foot boat is also large enough to accommodate 150 passengers. At a speed of 22 knots, the boat burns 75.1 gallons of fuel per hour. Compare that to the fuel usage of a typical 90 foot composite boat with the same 150 passengers: 150 gallons per hour! The difference comes from engine usage: this boat uses only 49.5% of its engine power at 22 knots, while the heavier boat uses 80%.

Image: Flickr – Benjamin Nagel – Stena Explorer

Pentamaran Hull Design

The pentaran is an exciting new design that works on the same principles as the catamaran. However, it has extra stabilizing outriggers, which enable increased speed. This hull design is patented by Nigel Gee, who is working with ADX Express, a business consortium, to bring the technology to fruition. The pentamaran is long and thin, and very fast: the 280 meter design is expected to average 38 knots, with a top speed of 41 knots. Because the boat "pierces through the waves," in the words of designer Gee, it is not strongly affected by adverse weather conditions. Despite its high speed, the boat is expected to maintain a high fuel efficiency. Future applications may bring pentamaran technology to yachts and passenger ferries.

Twin Interceptor hull form

This hull form is designed to decrease wake and allow boats to cut through waves, rather than pushing through them. This hull is particularly popular in naval boats. One widely-used design is the Blade Runner twin intercepter hull, which is developed by Bentley Marine. In addition to reducing fuel consumption, this design helps control pitch and roll motions. The hull is designed with a narrow entry profile, which helps reduce drag and wake creation.

Sources

Understanding Boat Wakes

150 Passenger, 83 foot Catamaran Ferry specifications

Pentamaran Boat Article

Twin Interceptor Motion Control System