US 3688723 A
A hydrofoil system for water-craft consisting of a bow hydrofoil and a stern hydrofoil of a surface-piercing transversely stabilizing type is described which does not require adjustment or trimming means when the craft is running. Each hydrofoil is approximately V-formed in both front and top views such that the pointed ends of the two V-forms face away from each other, pointing downward ahead and downward astern, respectively.
Description (OCR text may contain errors)
United States Patent- Ulvesand et al.
[ Sept. 5, 1972  HYDROFOIL SYSTEM FOR WATER CRAFT  Inventors: Sture Ulvesand, Fregattvagen 8, Lidingo; Bo Bengt Urban Bonthelius, Norr Malarstrand 78, Stockholm, both of Sweden  Filed: Dec. 9, 1970  Appl. No.: 96,432
52]- US. Cl. ..1l4/66.5 H 51 Int. Cl. ..B63b 1/26  Field of Search ..1l4/66.5 H
 References Cited UNITED STATES PATENTS 4/1960 'Hebrank 14/665 H 3,199,484 8/1965 Wiberg ..l14/66.5 H
Primary Examiner-Andrew H. Farrell Attorney-John A. Jeffries ABSTRACT A hydrofoil system for water-craft consisting of a bow hydrofoil and a stern hydrofoil of a surface-piercing transversely stabilizing type is described which does not require adjustment or trimming means when the craft is running. Each hydrofoil is approximately V- formed in both front and top views such that the pointed ends of the two V-fonns face away from each other, pointing downward ahead and downward astern, respectively.
8 Claims, 6 Drawing Figures IIYDROFOIL SYSTEM FOR WATER CRAFT DESCRIPTION OF THE INVENTION The present invention relates to a hydrofoil system adapted for use on various types of water craft to raise the hull at least substantially above the water surface at cruising speed firstly by means of a surface-piercing stern hydrofoil of a transversely stabilizing type, which carries the main part of the weight of the craft and is located abaft the center of gravity of the craft and extends uninterrupted from the ends downward aft to the longitudinal vertical center plane of the craft, and secondly by means of a surface-piercing bow hydrofoil of a transversely stabilizing type, which extends uninterrupted from its ends downward afore to said center plane of the craft and carries the remainder of the weight of the craft.
The carrying surface of a surface-piercing, i.e., never entirely submerged, self-reefing hydrofoil and, thus, also the specific load of the hydrofoil varies with the speed, weight and waves. For this and other reasons, the known hydrofoil systems formed by only two rigid surface-piercing hydrofoils cannot ensure a good longitudinal stability which also requires trimming means to provide smooth longitudinal movements and stable running.
A hydrofoil system constructed in accordance with the invention is auto-stabilizing both transversely andlongitudinally and automatically moderates any pitching and does not require movable trimming means such as trimming-tanks, trimming fins or the like. Though the angles of attack of the hydrofoils are nonvariable in operation, apart from possibly being adjustable when not in use, the hull is prevented from being raised too high above the water level whereby the ride would become instable and not under full control, particularly when the load of the craft is low or negligible at top speed. Such high speed stability at low load will be further increased by other features of the invention.
In the accompanying drawings:
FIG. 1 is a side view of the craft equipped with a hydrofoil system according to the invention shown by way of example.
FIG. 2 is a front view thereof.
FIG. 3 is a plan view of the starboard half of the bow hydrofoil of the hydrofoil system of FIG. 1.
FIG. 4 is a front view thereof.
FIG. 5 is a plan view of the starboard half of the stern hydrofoil of FIG. 1.
FIG. 6 is a front view thereof.
FIGS. 1 and 2 are a side view and a front view, respectively, of a boat 1 provided with a hydrofoil system constituted only by a bow hydrofoil 2 and a stern hydrofoil 3. FIG. 2 shows the starboard half of the bow hydrofoil and the port half of the stern hydrofoil. When the boat is dead in the water, the water surface is that shown at 5 (FIG. 1). When the boat is running at cruising speed, it will be entirely above the water level then being at 4 relative to the boat.
FIGS. 3 and 4 are a plan view and front view, respectively, of the starboard half of the bow hydrofoil 2, whereas FIGS. 5 and 6 show the starboard half of the stern hydrofoil 3 in the same way. The port halves of the two hydrofoils are reversally identical and the starboard and port halves of the respective hydrofoils are symmetrical with respect to the longitudinal vertical I plane through the center line of the boat as indicated by a chain line in FIGS. 3-6. The relative fractional values indicated in FIGS. 3-6 are fractions of the straight spanL of each respective hydrofoil. Furthermore, certain absolute angular dimensions, i.e., degrees, are indicated. All indicated dimensions, though being typical, are by way of example and are approximate. In the top view, the stern hydrofoil 3 has the form of a truncated arrow head or V pointing astern but may even form a pointed arrow head. In the present example, the width of the stern hydrofoil 3 as measured in the longitudinal direction of the boat is the same everywhere, whereas the corresponding width of the bow hydrofoil is diverging, i.e., increasing, toward the v outboard ends of the bow hydrofoil. Even in front view, i.e., when seen in the longitudinal direction of the boat, the two hydrofoils are V-formed, the bow hydrofoil having a broken V-form, for example having the angles indicated in FIG. 4, and the stern hydrofoil is formed as a truncated V (both in top view and front view) the legs of which are inclined 30 with respect to the horizontal plane such as the dihedral indicated in FIG. 6. The two hydrofoils should preferably have a cross-sectional form similar to that of a segment of a circle so that the planing bottom surface is flat and the top surface is curved to be convex, the maximum thickness of this profile being approximately one tenth of the width. Besides, the hydrofoils should be provided with upper projecting anti-ventilation ribs or strips 10 which are known per se and with supports 11 to be mounted on the hull of the boat. The bottom side of the hydrofoils is preferably provided with conventional splashguard ribs 9 increasing the lateral stability of the craft and reducing splash and splatter when running. Furthermore, vertical stabilizing fins 12 are provided at, or on either side of, the center line of the boat to improve the stability of the course and to reduce the danger of veering. The vertical longitudinal center plane of the boat is also that of the hydrofoils.
The center part of the exemplary bow hydrofoil 2 shown in FIGS. 3 and 4 extends between the two break points 13 of the broken V and subtends about 55 from said vertical center plane whereas the two side parts extending beyond the points 13 subtend about from the vertical plane. The angle of attack to the water of said center part, or at least the central area thereof, is zero or slightly positive. The angle of attack of said side parts extending beyond the points 13 increases from about 03 at said points 13 to about 5-l0 at the free ends, preferably from 0 to about 7. The width of the hydrofoil measured perpendicular to the leading edge of the hydrofoil diverges linearly from said center plane of the boat to the ends from L/20 to L/9. The total moulded length of the bow hydrofoil 2 is 2L/ 3.
The width of the stern hydrofoil in the longitudinal direction of the boat is approximately U9, and the center part 14, the ends of which carry vertical fins 12, is preferably wider in said direction than the side parts. The total dimension of the center part 14 measured in the transverse direction of the boat is preferably chosen between L/6 and L/3 in dependence on general conditions and requirements. The center part 14 may be linear in the lateral direction of the boat as shown in FIG. 6 but may be curved such as to be convex on the upper side and concave on the bottom side in which case the center of the central part 14 may be higher than the ends thereof by an amount of the order of one tenth of the lateral dimension of the whole center part 14. The center of the curved central part 14 may be dimensioned and located such that it emerges above the water level, i.e., pierces the water surface, under top speed whereby the stabilization is improved, particularly when the total weight of the boat inclusive of the load is very low. The inclination of the side parts on either side of the center part 14 is about 30 with respect to the horizontal plane. If the dimension of the center part 14 in the lateral horizontal direction of the boat is L/3, i.e., one third of the span of the stern hydrofoil, each side part has, of course, likewise this dimension L/3 in said direction.
As the bow hydrofoil 2 seen in top view is V-shaped and formed as an arrow head (without shaft) pointing downward ahead (see FIGS. 1 and 2) and has an apex angle of approximately 80, the carrying capacity of the hydrofoil is effective across a long range in the longitudinal direction of the boat and far ahead, without requiring a submerged carrying area of a size as large as that of a corresponding hydrofoil which is rectangular (in top view) instead of V-shaped. A conventional, surface-piercing bow hydrofoil extending transversely to the center line of the craft is subjected to a fairly considerable loss of carrying capacity when running in rough water, particularly in a following sea of a certain wave crest amplitude so that the boat is lowering and tends to dive down to strike the water level, resulting in a considerable reduction of speed. The more the boat rises above the water level, the more forward the point or center of support of the bow hydrofoil will be positioned. When the boat lowers or when waves are moving in the direction ahead of the center of gravity, the point of support will move in the same direction so that the resulting lift force is displaced ahead as the submerged effective surface of the hydrofoil decreases and, thus, is reefed.
In the present example, the V-form or arrow head shape of the stern hydrofoil seen in top view is less marked and has an obtuse apex angle of about 125 whereas the apex angle of the bow hydrofoil is acute. The stem hydrofoil extends downward astem. A combination effect resulting in a number of advantages partly mentioned more above is obtained in that the two hydrofoils seen in top view form two arrow heads, the rear sides of which face one another, and in that the said arrow heads are inclined downward and away from the craft. Also the stern hydrofoil has a similar effect so that the center of support will move astem when the stern of the boat rises excessively far above the water surface or passes a wave trough. If, however, the present stern hydrofoil would be replaced by a conventional one wholly extending at right angles to the center plane, such conventional hydrofoil must be located far more aft (and this would not always be possible) to make possible stable running conditions at top speed when the boat carries no load. Otherwise, the stem of the boat would rise to a level at which operating would be fairly instable and unsafe.
If, when a wave is passing, a greater lift is required at one of the two hydrofoils, the resultant lift force travels in a direction to the center of gravity of the boat so that the lever of this force is reduced and the movements become smoother.
What we claim is:
1. A hydrofoil system for water craft comprising a bow hydrofoil and a stern hydrofoil, each hydrofoil being of a surface-piercing and laterally stabilizing type uninterrupted between its ends and extending spaced from and below the bottom of the craft, characterized in that the bow hydrofoil extends ahead and downward and the stern hydrofoil extends astem and downward whereby the resultant lifting force from forward motion of the craft tends to move longitudinally relative to the longitudinal axis of the craft as the submerged surface of each respective hydrofoil increases or decreases according to relative submergence at bow and stem.
2. A hydrofoil system as defined in claim 1 wherein each hydrofoil seen in top view is generally V-shaped, with the planes of each V being inclined downward ahead and downward astern, respectively.
3. A hydrofoil system as defined in claim 2 wherein the bow hydrofoil seen in front view has the form of a broken V symetrical to the longitudinal center plane of the water craft, the center portion of the V having an apex angle of 1 10 and an angle of attack to the water which is substantially zero, whereas the two side por tions of the hydrofoil, extending from either side of said center portion, are inclined by a dihedral angle of 15 with respect to the horizontal plane and have a positive angle of attack which successively increases in the direction of. the ends of the side portions from between 0 and 3 to between 5 and 10, with the dimension of the side portions measured in the longitudinal direction of the craft exceeding that of the center portion and increasing from the ends of the center portion to the ends of .the side portions.
4. A hydrofoil system as defined in claim 2 wherein the stern hydrofoil seen in front view has the form of a truncated V being symetrical to the longitudinal center plane of the water craft, having a center truncated portion being horizontal and having two side portions extending from the center portion being inclined 30 with respect to the horizontal plane and having a positive angle of attack which successively increases to the ends of the side portions from 0 to 3, and wherein the width of each side portion measured perpendicular to its leading edge is at least equal to its dimension adjacent the center portion.
5. A hydrofoil system as defined in claim 2 wherein the hydrofoils have a plane bottom and a curved top surface, the maximum profile thickness being substantially 10 percent of the profile width, measured at right angles to the hydrofoil in the plane thereof.
6. A hydrofoil system as defined in claim 3 wherein the bow hydrofoil has a span of L as seen in front view with each of said side portions of the bow hydrofoil having a span of substantially L/3.
7. A hydrofoil system as defined in claim 4 wherein the stern hydrofoil has a span of L as seen in front view with the center portion of the stern hydrofoil having a dimension in the span direction of between U3 and U6.
8. A hydrofoil system as defined in claim 1 wherein each of the hydrofoils is provided with anti-ventilation ribs on the upper surfaces of the hydrofoils to prevent sucking down of air along said upper surfaces, and
splash-guards on the bottom surfaces of the hydrofoils to increase lateral stability and reduce splash, and vertical stabilizers to increase the stability of course and reduce the veering tendency during banking.