Boats of many shapes and sizes, from small launches to fishing boats to flat-bottom punts, use outboard motors for propulsion. An outboard has the engine mounted in the same unit as the propeller, avoiding the mechanical complexity of a driveshaft running from an inboard motor to the prop. It is common to see twin, medium-sized outboard motors mounted in place of a single large one. This arrangement confers a number of advantages.
Having two motors can serve as an insurance policy; should one of the motors fail, the other one can bring the boat home. Unfortunately, if the two motors are twins designed to work together, the prop on each is too large for a motor to work effectively alone. In this situation, the second motor can indeed bring the boat home, but only with the throttle barely open and moving the boat at 5 to 7 knots. An alternative is to have a second small motor that serves only as a backup and can be adjusted to allow full-throttle use.
Adding a second motor will generally (but not universally) result in a speed boost. The exact boost depends on how much power the motor provides relative to the weight gain it imposes on the boat. The change in speed can range from a gain of 30 percent to a loss of 5 percent, depending on that ratio.
A twin motor has a definite advantage in power over a single outboard. Leaving the boat and load unchanged, using a dual-outboard setup distributes the load over more propeller blades. Since these blades have a maximum load they can push, adding blades adds pushing power. Dual-motor boats also handle changes in load from empty to full with less over-revving than a single-motor boat would experience.
Using two smaller engines instead of one large one is unlikely to save you much fuel; with the right motor-hull combination, though, a dual-motor boat can get equivalent fuel economy to a single-motor model. This means that the other advantages of a dual motor can be weighed without considering a great increase in fuel consumption.
Maneuverability and Planing
Dual outboard motors can provide extra maneuverability in tight spaces if each motor can be throttled separately from the controls. They can also get the boat hull out of the water more quickly as it accelerates. This is called "planing." Since planing is the best mode of travel for saving fuel, this is also a fuel economy benefit.
- Photo Credit outboard motor boat image by Wimbledon from Fotolia.com
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