BOW LIFTING BODY WITH DEADRISE

Abstract

A watercraft has a lifting body secured to its bow below the waterline; the lifting body has deadrise on either side of the bow.

Description

SUMMARY OF THE INVENTION

The present invention relates to watercraft having a Bow Lifting Body (BLB) for improved efficiency and seakeeping.

BACKGROUND OF THE INVENTION

A BLB applied at the bow of a ship can introduce numerous positive attributes. First, a BLB provides all the positive attributes of a traditional bulbous bow. Wave cancellation similar to a traditional bulbous bow is provided by a BLB, in an even larger speed range. Also, a BLB can be used for ballast or a sonar dome, similar to traditional bulbous bows.

Second, lifting bodies have a higher lift to drag ratio (L/D, efficiency) than that of a hull alone, most noticeably at high speeds. By adding a component with a higher L/D than that of the original system in the absence of this new addition, it is intuitive that the L/D of the entire system increases.

Bow Lifting Bodies of this type are described in detail in U.S. Pat. Nos. 7,191,725 and 7,004,093, the disclosures of which are incorporated by reference.

These patents disclose the use of a submerged body affixed to the bow of a ship to reduce overall vessel drag and improve overall vessel seakeeping behavior. BLB's have several distinct characteristics which differentiate them from the similarly located conventional bow bulbs. A BLB has both buoyant and dynamic lift components whereas the conventional bow bulb has only buoyant lift. Because the efficiency of a BLB (defined specifically as the ratio of the body's lift to its drag) can be much greater than that of a typical ship hull, at high speeds, a portion of the vessel's weight will be supported by the BLB, thus increasing the vessel's overall efficiency.

A typical bow bulb is designed to be effective at a single design speed. At this speed, the wave generated by the bulb effectively cancels the wave generated by the ship's hull. This wave cancellation reduces the overall drag of the craft at this speed. A BLB, on the other hand, has the ability to cancel the wave generated by the ship over a much larger range of speeds. Test data shows wave cancellation over almost the entire speed range for a specific class of hulls.

A typical bow bulb has an elliptical or teardrop shaped cross section. In plan view, therefore, a bow bulb has very little area to aid in low speed seakeeping. A BLB, on the other hand, has a large amount of plan area giving it a high degree of low speed damping, keeping motions low. Finally, a BLB can have active control surfaces at the extremities or wingtips and edges. This inclusion of active control allows for improvements in seakeeping at higher speeds. A bow bulb has no control surfaces or winglets on which to put them.

Studies have been conducted showing the effect of deadrise angle on slamming loads for prismatic shapes. Such studies show that at higher deadrises (i.e., angle of a surface to the waterline) slamming loads were decreased. This decrease in slamming loads is inversely proportional to the deadrise angle, i.e., as the deadrise increased, the slamming loads were reduced.

Investigation into the added mass effects, in the frequency domain, of different shaped bodies has shown that a cupped shape body will have higher added mass. By increasing added mass over a wide frequency range, the time-domain motions of the body can be reduced due to the increase in effective momentum of the body. Similarly, the drag coefficient of a body in the direction normal to the cup will be higher, causing a lower tendency to move.

In general, the effects of anhedral (defined as a wing whose angle in relation to the groundplane is negative) and dihedral wings (defined as a wing whose angle in relation to the groundplane is positive) are well known. The lift decreases with the cosine of the angle in both instances.

The effects on lifting body performance due to proximity to the free surface are known as well. The lift, in general decreases with increasing proximity to the free surface. Similarly, the likelihood of cavitation (defined as the point when the pressure on the lifting body drops below the vapor pressure of the fluid thereby causing the fluid to boil) and separation increase with increased proximity to the free surface.

The behavior of a lifting surface and hull when in close proximity to each other has not been extensively studied. The increase in pressure on the lifting body and hull due to the reduction in cross sectional area between the two bodies is well known. The overall effect is most likely shape dependent and can either improve or hurt overall performance.

It is an object of the present invention to provide a bow lifting body to improve the overall efficiency of the watercraft or vessel to which it is attached.

It is a further object of the present invention to reduce the slamming loads of a vessel employing a BLB.

Yet another object of this invention is to decrease low speed motions of a vessel employing a BLB.

A still further object of the invention is to reduce the presence of BLB tip vortices.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, an improved BLB is provided which meets these objectives by having either negative or positive deadrise angle relative to the surface of the water.

The addition of deadrise to a BLB attached to the bow of a watercraft improves upon the low speed motions of a typical BLB as described in U.S. Pat. No. 6,263,819. In the specific case of positive deadrise, a BLB according to the invention improves the overall efficiency, reduces the slamming loads, and increases the lift on the BLB.

It has been found that adding deadrise to a BLB attached to the bow of a vessel can improve on the overall design of said vessel. The addition of deadrise as described herein increases the pressure on the hull and BLB due to proximity between the hull and lifting surface and reduces cavitation.

In addition to the use of deadrise for bow lifting bodies, the invention contemplates the use of incremented wing angles or “winglets” on the BLB's. Incremented wing angles for increased performance have been used in airplanes for many years. The advantages of specifically designing the wing angle as a function of span include: reduction of tip vortices and increased wing area for a given overall footprint. This concept has been expanded to allow for large changes in deadrise as well as sweep (defined as the angle of the wing as seen from above) in incremented segments as a function of span. It has been shown that by allowing for large changes in deadrise and sweep, the use of incremented wing angles produces increases in efficiency and reduction in tip vortices.

The above, and other objects, features and advantages of this invention will be apparent those skilled in the art from the following detailed description of illustrative embodiments of the invention which is to be read in connection with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is front view of a watercraft having a BLB as described in U.S. Pat. No. 7,191,725 with no wing angle;

FIG. 2 is a front view similar to FIG. 1 of a BLB having positive deadrise in accordance with this invention;

FIG. 3 is a front view similar to FIG. 1 of a BLB having negative deadrise in accordance with this invention;

FIG. 4 is a plot of the change in lift which occurs due to change in wing angle from −45° negative deadrise to 45° positive deadrise;

FIG. 5 is a plot similar to FIG. 4 comparing the change in lift due to change in deadrise for a watercraft with and without a strut connecting it to the watercraft;

FIG. 6 is a perspective view of a watercraft showing pressure distribution on a BLB and its associated hull for a BLB with no deadrise;

FIG. 7 is a view similar to FIG. 6 showing pressure distribution on a BLB and its associated will for a BLB with positive deadrise; and

FIGS. 8a-c are illustrations similar to FIGS. 1-3 showing the use of incremented deadrise or “winglets” on BLB's with positive, deadrise, no deadrise, and negative deadrise respectively.

DETAILED DESCRIPTION

Referring now to the drawings in detail and initially to FIG. 1, a conventional BLB 10 in accordance with the disclosure of U.S. Pat. No. 7,191,725 is illustrated secured to the bow of a hull 12. The BLB has a thicker central portion and tapers towards its lateral edges 14. FIGS. 2 and 3 show the same BLB but with its lateral sides bent in either positive or negative deadrise. Deadrise is the angle a surface of a vessel makes to the horizontal or to the flat waterplane surface.

A BLB with deadrise as shown in FIG. 2 or 3 provides all of the positive effects of a typical BLB. However by including the proper amount of wing angle, slamming loads are reduced, overall efficiency is increased, low speed seakeeping is improved, and BLB lift can be increased.

The improvements to a vessel's low speed seakeeping due to the addition of wing angle or deadrise (either positive or negative) to a BLB is due to the changes in added mass and drag coefficient consequent to the change in BLB shape. The increase in entrained water due to the “cup” shaped cross section as compared to a BLB with no wing angle is clear. This increase in “added mass” will effectively change the vertical and rotational motions of the attached vessel. Since the equation for acceleration can be stated as the ratio of the force applied to the mass of the system (this mass includes the added mass), any increase in system mass will reduce the accelerations for a given amount of force. Similarly, the rotational acceleration is the ratio of the moment applied to the mass moment of inertia (which includes the added mass component of the moment of inertia). Therefore, as the added mass increases, the amount of rotational acceleration for a given amount of moment will be reduced.

FIGS. 6 and 7 show perspective views of BLBs with no deadrise (FIG. 6) and positive deadrise (FIG. 7). These drawing demonstrate how the deadrise increases the area of reduced pressure on the BLB thereby increasing lift.

The improvements to overall vessel efficiency over a wide speed range due to the inclusion of BLB's at the bow of a vessel is known. This increase in efficiency comes from two sources: the cancellation of the bow wave by the BLB-generated wave and the highly efficient dynamic lift generated by the BLB at speed effectively unloading the vessel's hull. The wave generated by the BLB is almost entirely due to the displaced volume of the BLB and its proximity to the free surface. The actual shape of the BLB has very little effect on the size and shape of the wave being generated. For this reason, the inclusion of deadrise (either positive or negative) will not significantly change the wave cancellation as seen in a typical BLB. However, the second component of the improved efficiency (namely, the ability to unload the hull) will be affected by the inclusion of deadrise (again, both positive and negative). In the case of negative deadrise (anhedral), the lift will reduce with the cosine of the wing angle β, as shown in the chart of FIG. 2. Since no other effects on vessel efficiency will be seen, the overall efficiency of the vessel will drop. In the case of positive deadrise (dihedral), the lift will again reduce with the cosine of the wing angle. However, the proximity of the lifting surfaces to the hull will have a large effect on the pressures of both the hull and the lifting body. Simulations have shown that this decrease in pressure on the top side of the lifting body (and thus lift over the entire lifting body) far outweighs the reduction in lift due to wing angle (this is shown in FIG. 7), thereby producing an increase in lift. Since the efficiency of a system can be defined as the ratio of lift to drag, the overall efficiency of the system can be said to have increased.

The reduction in BLB slamming due to the inclusion of positive deadrise is due to the effective increase of deadrise at the bow of the vessel. As shown in FIG. 1, a typical BLB (center) would have very little or no deadrise in cross-sectional view. As has been shown in several studies, by increasing the effective deadrise on a vessel, the slamming loads are reduced. It is clear that the BLB with dihedral deadrise has a much higher effective deadrise angle.

Simulations in two independent, commercial fluid dynamics codes (USAero and CFX) have shown the proximity of the wings to the hull when positive deadrise is included effectively increases the overall lift of the BLB. As noted above, FIGS. 6 and 7 show a three-dimensional view of the BLB with positive deadrise affixed to the bow a vessel alongside a BLB with no deadrise affixed to the bow of the same vessel. As can be seen from the pressure contours, the area of peak negative pressure on the BLB has increased in the case with positive deadrise. Lift is defined as the integral of the pressure. Therefore the deadrise would have increased lift.

The inclusion of deadrise (either positive or negative) reduces the overall span of the BLB by the cosine of the deadrise angle. For practical reasons, the BLB should not exceed the beam of the boat at the installed longitudinal location. In general, it is better to have as much wing area as possible. This helps increase the low-speed damping and increase the high speed efficiency of the boat. However, if the BLB is made too large, the wingtips will extend beyond the limits of the boat. By using deadrise, the effective width of the BLB can be reduced while keeping wing area large.

The inclusion of positive deadrise can have a negative effect on BLB performance if care is not taken. The inclusion of positive deadrise increases the proximity of the BLB wingtips to the free surface (the interface between the air and water). This increase in free surface proximity raises the likelihood of body cavitation. Cavitation occurs when the water along the body boils due to drop in pressure. As a surface approaches the free surface, it can cavitate at a higher pressure. Since the pressure on the BLB will be dropping due to its proximity to the hull, moving the wing tips closer to the free surface can exacerbate their problem. For this reason, in specific cases, it may be desirable to use negative deadrise in place of positive deadrise. Although the improvements in efficiency and slamming will be lost, the likelihood of cavitation will be reduced.

By including the ability to increment (i.e., bend) the deadrise and sweep in segments along the span of the BLB, the overall efficiency of the system can be further improved. FIGS. 8a-c show examples of BLB's with incremented deadrise, i.e., deadrise in two different sections of the BLB. One of these sections on each side of the BLB is a winglet.

Although the present invention has been described herein in connection with the illustrative embodiments, it is to be understood that the invention is not limited to such embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the invention.

Claims

1. A watercraft comprising a hull, including a bow having an outer surface, and a lifting body attached to said bow below the static waterline of the hull; said hull having a fore and aft longitudinal axis, and said lifting body extending laterally away from the outer surface of the bow and from said longitudinal axis; said lifting body being buoyant and having a first lifting body portion adjacent to the bow and extending away from the adjacent outer surface of the bow and at least a second lifting body portion extending laterally outwardly from each side of the first lifting body portion, said second lifting body portions having dead rise with respect to said first lifting body portion on either side of the bow of the hull, whereby said lifting body provides buoyant lift at the bow, hydrodynamic lift at operational speeds and reduction of vertical acceleration in bow movements.

2. A watercraft as defined in claim 1 wherein said deadrise is positive.

3. A watercraft as defined in claim 1 wherein said deadrise is negative.

4. (canceled)

5. (canceled)

6. (canceled)

7. A watercraft as defined in claim 1 wherein said lifting body is mounted directly on the bow of the hull and produces a surface wave train as a result of its shape, its proximity to the free surface of the water and its displacement lift.

8. A watercraft as defined in claim 1 wherein the second lifting body portions of the lifting body taper towards the lateral edges of the lifting body.

9. A watercraft as defined in any one of claims 1 to 3, 7 and 8 wherein said first lifting body portion is thicker than said second lifting body portion.

10. A watercraft as defined in any one of claims 1 to 3, 7 and 8 wherein the lifting body has a thickness to length ratio of between 10 to 33%.

11. A watercraft hull as defined in any of claims 1 to 3, 7 and 8 wherein said first and second lifting body portions have longitudinal cross-sections extending parallel to the longitudinal axis of the hull with each such cross-section including symmetrically cambered and generally parabolically shaped cross-sectional portions.

12. A watercraft hull as defined in claim 11 wherein the cross-sections of said first lifting body portion adjacent the bow has the maximum thickness of the lifting body and the cross-section of the second lifting body portions being smaller than those of said first section.

13. A watercraft as defined in claim 10 wherein the lifting body has a thickness to length ratio of between 10 to 33%.

14. A watercraft hull as defined in any of claims 1 to 3, 7 and 8 wherein said lifting body has a leading edge portion which conforms, when viewed in plan, to a parabolic form.

15. A watercraft as defined in any one of claims 1 to 3, 7 and 8 including winglets on the outer edges of said lifting body.

16. A watercraft as defined in claim 12 including winglets on the outer edges of said lifting body.

17. A watercraft as defined in claim 13 including winglets on the outer edges of said lifting body.