It is well known that when a power boat accelerates from a standstill, the nose goes high and the stern goes low until a certain speed is obtained, which typically is between 20 and 30 miles per hour. Then the boat levels off and is said to be on plane. It is for this reason that transoms tilt so as to be closer to the bow below the water line, thereby to provide some upward thrust on the stern during initial acceleration.
In U.S. Pat. No. 2,886,462, an outboard motor is on a flexible transom plate which causes the motor to be trimmed forward, thereby providing downward thrust when the boat starts up from a standstill, the base of the transom plate acting as a trim tab. When some speed is attained, the force of the water against the bottom of the transom plate causes the motor to level off. The transom is always in the same trim position as the motor, with no choice, and with no control thereover by the operator.
Modern bass boats and ski boats have power trim which allows the operator to tilt the propeller aft and upwardly once plane has been attained, thereby to keep the bow of the boat from plowing, and reduce drag, so that higher speeds may be attained and less effort of the motor is required to achieve any given speed.
Trim tabs have been provided at the aftmost extreme of the hull, extending downwardly somewhat from the bottom of the hull. Static trim tabs however remain in place at higher speeds and thereby produce significant drag, which reduces speed and wastes fuel. Adjustable trim tabs are complex and require electric or hydraulic mechanisms together with operator controls and communication between the mechanisms and the controls.
Referring to FIG. 1, a power-operated transom jack, commonly and herein referred to as a jack plate 9, known to the prior art, has two sides; each side includes a stationary portion 10 fastened to the transom 11 of a boat, and a moveable portion 12 having slots 13 that allow the moveable portion to slide on pins 14 disposed on the stationary portion 10. Depicted diagrammatically is a hydraulic cylinder 17 which responds to fluid in tubing 18 to raise and lower the moveable portion 12.
A trim plate 21 is fastened, as by bolts 22, to the bottom of the moveable portion 12. The portions 10, 12, do not extend from one side to the other, but rather there are similar portions fastened to the transom at the other side of the jack plate (not shown). The trim plate 21, however, is solid from one side of the moveable portion 12 to the other side of a similar moveable portion (not shown).
In the position shown in FIG. 1, the trim plate 21 has no effect, since the path of even its lagging edge 25 is not through water behind the transom 11. If the moveable portion 12 were moved downwardly until the trim plate 21 extended substantially from the bottom of the transom 11, as shown by the fractional dotted lines 26, the trim plate 21 would provide lift and assist the boat in reaching plane while accelerating from rest. However, it would also, as is the case of fixed trim tabs, provide drag and possibly reduce steerability at higher speeds.
Should the moveable portion 12 be positioned below that indicated by the dotted lines 26, it would act as a scoop and direct water upwardly into the jack plate and actually into the boat itself.
Therefore, the exact positioning of the jack plate is critical when employing a trim plate 21 as in FIG. 1.