The operating efficiency of any boat is directly related to its weight; the lighter the weight, the better the performance. The speed of a boat is determined by its weight to power ratio, and a reduction in weight is the functional equivalent of increasing the motor power, but without the increase in electrical power usage that this would require.
The Reservoir Runner 500 is a six horsepower electric outboard motor that weighs less than 90 pounds, including the controls (but not including the batteries). A comparably powered inboard motor system weighs much more because a low RPM electric motor that may be direct coupled to a propeller shaft is physically larger and heavier than a motor that runs through a gearbox. (Outboard motors have reduction gears in the lower unit). A separate gearbox that would allow the use of a higher speed motor in an inboard installation weighs enough that it offsets the weight advantage of a high speed motor. The weight of the propeller shaft, shaft log, driveshaft, thrust bearing and propeller also add up to much more weight than the mechanism of an outboard motor, and in addition, a rudder mechanism is needed for steering.
The propeller and rudder on a boat with an inboard motor are fixed in position and are subject to damage if they contact the bottom or an obstruction. An outboard motor is designed to tip up if an obstruction is encountered while in operation, and may also be manually tipped up if the boat is to be beached or to traverse shallow water. The Reservoir Runner motors have a shallow water drive feature that allows them to be tipped up part way to run at low speed in shallow water. This gives the motor 5 inches more clearance over the bottom.
The overall efficiency of the propulsion system on a boat is also determined by the efficiency of the propeller itself, and how its design relates to the particular application. At low (displacement) boat speeds, the larger the diameter of a propeller, the greater will be its efficiency. Large diameter propellers must be turned at lower speeds, however, and also extend deeper below the boat and thus need deeper water for operation.
Reservoir Runner outboard motors achieve very high propeller efficiencies by using a Kort Nozzle or ducted propeller. The nozzle is a hydrodynamically shaped, cast aluminum, ring that surrounds the propeller and increases the volume and flow of water through it, much like a fan jet on an airliner. This results in an increase of thrust and efficiency of 50 to 60 percent, at boat speeds of from 0 to 12 knots, even without using a large propeller. Tugboats have used Kort Nozzles for over 50 years because of the fuel savings and increased towing power they afford.
A Kort Nozzle also surrounds and protects the propeller from damage and renders it safe around swimmers and wildlife.
The control and maneuverability of a boat operating at low speed is also much better with an outboard motor. A boat with an inboard motor uses a rudder for steering, and a rudder needs water to be flowing against it to be able to push the stern of the boat around. An outboard, on the other hand, uses the thrust of the propeller for steering. At low boat speeds a quick pulse of power will "kick" the boat in the desired direction. The Kort Nozzle used on Reservoir Runner electric outboards provides even more steering control due to its directed stream of water; the turning radius of a boat with an outboard motor incorporating a Kort Nozzle is 20% smaller than one with an open propeller.
A final advantage of an outboard motor is its ease of installation and maintenance. An inboard motor must be incorporated into the design of a boat while an outboard motor is attached to a boat. This allows any boat with a suitable design to become an electric boat without major surgery.
An inboard installation is quite complex, requiring a precisely aligned propeller shaft with its bearings, struts, and shaft log through the hull of the boat. In addition, a rudder mechanism is required for steering.
An outboard installation, on the other hand, consists of merely attatching the motor to the stern of the boat. Steering may be done either with a tiller handle on the motor, or by attaching a standard steering mechanism to the motor and steering from a console. The ease of installation, with the elimination of potential hull leaks at the shaft log and rudder, makes an outboard motor a particularly attractive choise for the craftsman who wishes to build his own boat.
Maintenance on an inboard system requires working in/on the boat itself. A small boat often has a cramped engine compartment and adjustments and maintenance are awkward at best. In particular, the shaft log must be carefully maintained to prevent leaks. An outboard motor is fully ascessable and the only maintenance normally required is to change the lubricant in the gearcase. If any repairs need to be made, it is a simple matter to remove the motor from the boat so that the work can be done "on the bench" in the shop.
© 1996, James M. Graham III
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