TECHNIC AND FUNCTION
WHY A SARO-TUNNEL-PROP-DRIVE ?
All Boats and ships need to connect the engine to external media and the surrounding water. This is normally done by meens of a propeller, or a device pushing water in the opposite direction of the desired running direction. In most cases the propeller is dipped into the water at the rear end of the boat. Since water is not a solid media, years of engineering was necessary to achieve the best efficiency. However, there is more work to be done!
SARO, in Germany has done some homework and designed an outstanding drive for fast boats. The principle are similar to an outdrive or a Z-drive, but far superior.
The SARO drive consists of a transomplate, a propeller shaft with a U- joint, a surface piercing propeller ( dipping less than half of its diameter under the boat ), a couple of bearings and seals, a rubber bellow and a housing. No oil filled gear box hangs in the water.
SURFACE PIERCING PROPELLER
It is, by now, common knowledge that a propeller just dipping into the water line by half or less than 50% will have the same “gripp” in the water as a full propeller, but creates considerably less drag. By looking at the prop, it can be recognized that a large portion is not blades, but hub and shaft, plus housing. All these components create drag, thus the boat goes faster. The slight disadvantage in that the prop sucks and pulls air into the water, instead of blowing a solid column of water out. It pushes air with water and reducing the density of this mixture.
SARO has found a solution !
SARO invented the tunnel drive. This is really a half a tunnel, or hood, over the top of the propeller. The tunnel reaches into the water on both sides and prevent air from being pulled or sucked in. Now only water is filling the tunnel, and a solid stream of water is accelerated to push the boat forward.
This propulsion system is between 10 and 18% more efficient than conventional out-drives or fixed props. The greater part of the improvement is due to the surface piercing and tunneled prop system. Some of the improvement is also due to the lack of beveled gears. These gears, used in typical out-drives, normally create a 5 to 10% loss of power. Tests have revealed increases of speed from 45 to 55 miles per hour without changing engine or gear ratio.
STEERING AND LIFTING
The SARO drive steers hydraulically and lifts like any other out-drive. However it has the tunnel, wich thanks to its shape, also acts as a trim tab. This alows it to level the boat to the position for best performance. The prop and drive lift entirely out of the water just like an outboard motor for beaching the boat.