The present invention is directed to an improved apparatus and method for controlling the angle of attack of planing-type hulls. More specifically, the present invention provides a dual rudder steering system that both steers the boat and also controls the attack angle or attitude of the planning-type hull.
An understanding of the contribution made by the present invention requires an understanding of planing-type boat hulls. Boats having planing hulls are designed to operate most efficiently at speeds where the hull "planes" on top of the water with a minimum area of the hull in contact with the surface of the water. The percentage or portion of the hull surface area that comes in contact with the water during optimum planing operation of the boat depends largely upon hull design. Typically, more than half of the hull surface is in contact with the surface at planing speeds. However, with hulls having a transom dead rise, or an upward angle from the keel to the chine, less than one-half of the hull may be wetted or in contact with the surface at planing speeds. Further, as little as one-fourth of the surface area of high performance planing-type hulls may be in contact with the surface during optimum high speed operation.
In order to efficiently operate a boat with a planing-type hull at planing conditions, the attack angle or trim angle of the hull must be carefully controlled. If the bow is too high and the stern is too low, the attack angle is too high or too positive (i.e. +5.degree.) and the hull will operate inefficiently and even oscillate. If the bow is too low and the stern is too high, the attack angle is too low or too negative (i.e. -3.degree.) and the hull operates inefficiently from excessive wetted surface. It is therefore important to operate the boat with the hull in a near horizontal position or optimal attack angle. The specific optimal attack angle will depend upon the boat design, hull design and the speed at which the boat is travelling.
One way to control the attack angle of the boat with a planing-type hull is the use of hydrofoils. Hydrofoils can be used to both raise or lower the transom and therefore adjust the attack angle in the positive direction (i.e. lowering the stern) or the negative direction (i.e. raising the stern). However, the use of hydrofoils is not without its drawbacks. Primarily, the addition of two hydrofoils increases the drag of the boat in the water and decreases efficiency. Further, the hydrofoils are manually controlled and the effective operation of hydrofoils can be a difficult skill to learn. Further, the cost of two hydrofoils (one for each chine) along with the equipment used to control the angle of the hydrofoils in the water is expensive and can add substantially to the cost of the boat.
In inboard motorboats, as shown in U.S. Pat. No. 5,249,994 issued to applicant, it has been found that it is advantageous to pass the propeller shaft through the hull of the boat as opposed to the transom. Specifically, when the propeller shaft is passed through the hull of the boat, a conventional shaft log and conventional packing gland may be used to seal the portion of the hull where the propeller passes through. In contrast, when the propeller shaft is passed through the transom of the boat, special shaft ports, radial thrust bearings and packing glands are required thereby increasing the cost of manufacture.
However, in the economical design where the propeller shaft is passed through the hull of the boat, the larger propeller shaft angle in relation to the keel increases the vertical lift at or near the transom/hull junction thereby causing the transom to rise and lowering the attack angle. The lift generated by the propeller shaft angle and action of the propeller can be quite large and further cause the transom to be raised past an optimum point whereby the hull may reach a negative attack angle which causes the boat to operate inefficiently.
Accordingly, it would be highly desirable to provide an alternative to hydrofoil systems or an alternative means for controlling the attack angle of the boat that does not substantially add to the cost of the boat or substantially contribute to hull drag. The present invention proposes to do this by employing a dual rudder mechanism that both steers the boat and also controls the attack angle of the boat.
Dual rudders are known in the art. Specifically, a rudder disposed on either side of the propeller and preferably directly behind the propeller is an effective steering mechanism because the propellers receive the maximum thrust of the high velocity water emanating from the rotating propeller.
The present invention provides a specially designed dual rudder steering system that is capable of controlling the attitude of the hull. The specially designed dual rudder steering system of the present invention is especially adaptable to boats with planing-type hulls and inboard motor boats where the propeller shaft angle is relatively steep due to the passing of the propeller shaft through the rear of the hull as opposed to the transom.