Boat hulls

Abstract

Power boat comprises hull having a keel line running from bow to stern which defines an axis of symmetry from which port and starboard hull portions of bottom surface extend to meet side walls. The stern of the boat is defined by a transom. A trim plate is mounted for pivoting about a transverse axis to alter the relative position of the bow in the water and control the wake-making or wake-shaping capability. A flat inclined surface slopes upwardly and sternwards from the transition point to form a water flow modifying part of the hull bottom surface. By rotating trim plate about hinge axis so that its trailing edge is lowered, the water flow encountering plate is deflected down and so the stern of hull can experience lift and the bow will “squat” or lower somewhat, with the keel line assuming a progressively lesser angle of attack to the surface of the water. This is accompanied by a change in shape of the wake (and some increase in drag). The wake shaping can thus be controlled to allow for more entertaining wake boarding or to allow for different skill levels of wake boarders. Conversely, rotating trim plate to raise its trailing edge tends to raise the angle of attack.

Description

FIELD OF THE INVENTION

The present invention relates to boat hulls, particularly of the planing type.

BACKGROUND

Water sports including use of motor boats are popular activities. At different times depending on the activity it is desirable to adjust the trim of a boat so that it travels through the water more smoothly or so that it creates a desirable wake for a sporting activity astern of the boat such as water skiing (including slalom skiing) and wakeboarding.

Water skiing involves the use of one or two skis similar to snow skis by a person being towed on a line behind a planing boat. Wakeboarding is similar except that a small board like a miniature surfboard is used instead of skis, and the aim is to make much more use of the disturbed wake generated by the boat in performing manoeuvres on the water and in the air. Wakeboarders prefer the towing boat to generate comparatively large waves in the wake, by comparison to water skiers.

The present invention provides a new hull form that has been found to be very suitable for adjusting the trim of the boat to suit or create different water conditions. For example where choppy conditions are encountered, the trim of the boat can be adjusted to make the boat travel more smoothly through those conditions. Further the trim may be adjusted to create or modify the wake desired by wakeboarding enthusiasts or water skiers and provide for the degree and type of wake-making to be readily controllable. For example in some sports it is desirable to make the shape of the wake smooth rather than having a roll top. The invention may also address a problem of boats that have projecting components at their sterns (as is the case with many boats used for wakeboarding), namely difficulty encountered by persons in the water in boarding the boat over its stern. A useful improvement in accessibility and aesthetics may be provided in some embodiments of the invention.

SUMMARY OF THE INVENTION

Accordingly the invention provides in one embodiment a boat having an adjustable trim, said boat including: (a) a hull with a bottom surface, side walls, transom, a keel line running from bow to stern defining an axis of symmetry from which port and starboard hull portions extend to meet said side walls, (b) a water flow modifying part of the hull located towards the stern comprising an inclined portion of the hull which extends up relative to said bottom surface in a direction towards the stern from a transition point along said keel line, and (c) located aft of said inclined portion a selectively moveable trim member with a surface for contacting water relatively flowing along said flow modifying part and wherein said moveable trim member can be selectively positioned to increasingly obstruct said water flow and thereby lift the stern and lower the bow in the water when the boat is travelling forward.

When used herein, including in the claims, the expression “transition point” is understood as including not only a discrete point where an abrupt change in the direction of the keel line occurs, but also as including a profiled, curved or stepped region forward and aft of which the direction of the keel line is different.

Preferably the inclined portion of the water flow modifying part progressively widens from the transition point in the direction of the stern, however other shapes may be suitable. For example, although not preferred, it is possible that the water flow modifying part of the hull further includes a flat surface which extends rearwardly in a direction towards the stern from the inclined portion and which is substantially parallel to the port and starboard hull portions on either side thereof.

Preferably said inclined portion progressively widens to each and thereafter remains a predetermined width until reaching the trim member. Preferably also said inclined portion towards the trim member is recessed below the port and starboard hull portions on either side thereof.

Preferably the selectively moveable trim member comprises a trim plate, however any other suitable member having a body which can obstruct water flow may be used.

In one possible embodiment the trim plate is moveable about a first axis transverse to the keel line. The axis may be substantially in the same plane as the trim plate and for example, may be provided by a hinge by which the trim plate is mounted.

In another possible embodiment the axis is displaced above the keel line and out of the plane of the trim plate. Preferably the trim plate extends from a drum or cylinder which is selectively rotatable about the axis transverse to and above said keel line. The trim plate may be mounted so as to be substantially tangential to the drum or cylinder periphery.

The degree to which the trim plate is extended into the water changes the trim of the boat deflecting the water flow along the water flow modifying part thus causing the stern to lift and the bow to lower into the water. As further described in relation to illustrated embodiments, the trim member does not need to be of large dimensions. If a patch of rough water is encountered then by lowering the trim plate perhaps by 10% of its range of motion this can considerably improve the smoothness of the passage of the boat through the water.

Also the size and shape of the wake created by the boat can be changeable by moving the trim plate in the water flow along the water flow modifying part of the hull.

Accordingly, in a second embodiment the invention provides a boat hull of adjustable trim comprising:

a hull body having a bottom surface with first and second portions and extending rearwardly from a bow to a stern of the hull body, the bottom surface being substantially symmetrical on opposite sides of a vertical central plane that extends longitudinally of the hull body; and

a trim plate in the stern region rotatable by an actuator about an axis extending transversely of the hull body,

wherein—

• (a) a keel line being a line of intersection between the said central plane and the bottom surface has a first segment forward of and a second segment rearward of a transition point partway along the keel line, the second segment of said keel line extending upwardly from said transition point and the second segment terminating adjacent to a leading edge of a bottom surface of the trim plate;
• (b) the first portion of the bottom surface of the hull extends outwardly from the first segment of the keel line on each side of the central plane; and
• (c) the second segment of the keel line being a centre line of the second portion of the hull bottom surface, that second portion extending outwardly from the keel line on each side of the central plane.

It is possible to proportion the hull body and trim plate, and to adjust the trim plate to control the shape of the wake suitable for wakeboarding.

Preferably, the said second portion of the hull bottom surface when seen in cross-section transverse to the central plane is substantially flat or planar.

It is also preferred although not essential, that the said second segment of the keel line is substantially straight.

In one possible embodiment, the second portion of the hull bottom surface extends transversely between steps that extend longitudinally and are equally spaced apart on opposite sides of the central plane, port and starboard sections of the first portion of the hull bottom surface being located transversely outboard of the said steps so that the first portion of the hull bottom surface extends from bow to stern, the said steps and the second portion over at least a part of its length defining a recessed section or cutaway “tunnel” in the hull bottom surface. The trim plate leading edge preferably then extends across the width of the second portion of the hull bottom surface and across the gap between the two steps.

In a particularly preferred embodiment, the leading edge of the trim plate bottom surface is positioned forwardly of the transom of the hull body. The trailing edge of the trim plate may be forward or astern of the transom or may be longitudinally level with the transom. Having the trim plate wholly or partially under the hull rather allows for easier access to the hull over its stern by persons in the water, because the trim plate need not extend behind the transom of the hull body, or as far behind the transom, as would otherwise be the case. It is preferred that the trim plate does not extend rearwardly of the transom.

In a further aspect, the invention provides a boat having a hull body and trim plate as described herein, propulsion means, rudder means and actuators for adjusting the position of the trim plate.

BRIEF DESCRIPTION OF THE FIGURES

A more complete appreciation of the invention, and further inventive features thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a side elevation of a planing boat according to the invention;

FIG. 2 is a perspective view from below of a stern portion of the hull and trim plate of the boat shown in FIG. 1;

FIG. 3 is a perspective view from below of a stern portion of a boat hull with trim plate also according to the invention;

FIG. 4 is a perspective view from below of a stern portion of a boat hull with trim plate according to a further embodiment of the invention;

FIG. 5 is a perspective view from below of a stern portion of the boat hull as shown in FIG. 4 without the trim plate;

FIG. 6 is a longitudinal cross-section in the central plane of the stern portion of the hull and trim plate as shown in FIG. 4;

FIG. 7 is a longitudinal cross-section in the same plane as and from the same viewpoint as FIG. 6 of a portion of the hull as shown in that Figure, showing an alternative arrangement of a part of that hull;

FIG. 8 is a longitudinal cross-section in the same plane as and from the same viewpoint as FIG. 6 of a portion of the hull as shown in that Figure, showing a further alternative arrangement of a part of that hull.

FIG. 9 is a perspective view of an alternative possible configuration of a trim plate mounted by a drum or cylinder.

FIG. 10 is a longitudinal cross section showing the trim plate and drum or cylinder of FIG. 9 in a boat hull of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a side elevation of a planing boat 1, such as would be used for wakeboarding, water skiing and the like.

Referring to the drawings generally boat 1 comprises hull 2 having a keel line 14 running from bow to stern which defines an axis of symmetry from which port and starboard hull portions 13a and 13b of bottom surface 13 extend to meet side walls 12a, 12b. The stern of the boat is defined by transom 8.

Boat 1 has a hull 2, a propeller 3 driven by a propeller shaft 4 supported by a skeg 5, and a rudder 6. A trim member shown as a plate 7 is mounted to at the transom 8 of hull 2 for pivoting about a transversely extending hinge axis 9 in the manner shown by arrow “C”, by operation of a linear actuator 10. Actuator 10 may be a hydraulic cylinder, for example, or an electric actuator of motor-and-screw type.

FIG. 2 shows the rear part of hull 2 seen from below, to illustrate how it is shaped to be able (with suitable adjustment of trim plate 7) to alter the relative position of the bow in the water and control the wake-making or wake-shaping capability. In FIG. 2, the skeg 5, propeller 3, shaft 4, and rudder 6 are omitted for clarity. However, the position of skeg 5 is shown by a phantom line 11, and trim plate 7 is shown. Hull 2 is of hard chine type, with chines 12c at the boundaries of side walls 12a, 12b with outer edges of bottom surfaces 13a, 13b that extend outwardly and upwardly from a keel line 14. Immediately aft of the position of skeg 5, a flat inclined or ramp surface 15 is defined which slopes upwardly and sternwards from front edges 16. This is a water flow modifying part in this first embodiment of the invention. At an intersection line 18, surface 15 meets a further flat surface 17 that in side elevation (FIG. 1) is typically substantially parallel to keel line 14, forward of the transition point 5, 6. The term “transition point” is intended herein (including in the claims) to mean the point or region in the hull where the bottom surface turns up to form an inclined segment to form the water flow modifying part. The “point” need not be a sharp or abrupt change in the line of the keel but may be a profiled or curving transition or stepped region along a relatively small length of the keel line. The shape of surfaces 15 and 17 is emphasized by phantom lines 25, which show the line of intersection of the bottom of hull 2 and trim plate 7 with a plane that extends vertically and fore-and-aft in hull 2 on its centerline. In effect, hull 2 is a conventional V-type hull, but with a “cut away” flow modifying part of its bottom surface defined by surfaces 15 and 17.

Trim plate 7 is secured at the transom 8 closely adjacent to the rear edge 19 of surface 17. As shown by arrows C in FIG. 1, trim plate 7 can be selectively rotated so that its lower surface 20 lies in the same plane as surface 17 or can be rotated from that position.

A user of boat 1 may alter its trim by rotating trim plate 7 about hinge axis 9. In particular, if trim plate 7 is rotated so that its trailing edge 21 is lowered, the water flow encountering plate 7 is deflected down and so the stern of hull 2 can experience lift and the bow will “squat” or lower somewhat, with the keel line 14 assuming a progressively lesser angle of attack to the surface of the water (not shown). This is accompanied by a change in shape of the wake (and some increase in drag). The wake shaping can thus be controlled to allow for more entertaining wake boarding or to allow for different skill levels of wake boarders. Conversely, rotating trim plate 7 to raise its trailing edge 21 tends to raise the angle of attack.

Referring now to FIG. 3, there is shown a view, comparable to FIG. 2, of a rear portion of a boat hull 50 that is a second embodiment of the present invention. Although the invention is not limited in its application to hard chine boats, hull 50 is shown as being of this type, with chines 51, bottom surfaces 52, a keel line 53 and a transom 54. A suitable skeg position is shown by phantom line 85. A flat inclined or ramp surface 55 (corresponding to ramp surface 43 of hull 31 in FIG. 4) extends upwardly and rearwardly from keel line 53 from a transition point 56 thereon behind skeg position 85 to a trailing edge 57 at transom 54. Close to trailing edge 57 is the leading edge 58 of a trim plate 59 which can be rotated about leading edge 58, or about a horizontal transverse axis close and parallel to leading edge 58, to adjust the trim of hull 50. Hull 50 has no surface equivalent to surface 17 of hull 2. This continuous extension of the ramp surface 55 to the leading edge of the trim plate 59 is believed preferable to the two part surface (15, 17) in FIG. 1 because the wake height is created by the water flow being “bent” upwardly by the ramp surface, whereas the trailing float surface 17 in FIG. 1 may partially nullify the upflow. The trim plate modulates the effect of the ramp. Phantom lines 60 are shown on surface 55 and bottom surface 61 of trim plate 59 and are on the centreline of hull 50. These show the cutaway nature of hull 50.

Ramp surface 55 extends back from two leading edges 62 defined by its intersection with bottom surfaces 52. The provision of ramp surface 55 amounts to a “bevelling” of the rear part of hull 50. Leading edges 62 could be carried back to transom 54 and to do so would be within the scope of the invention. However, it has been found preferable to limit and maintain at a predetermined extent the width of ramp surface 55 and trim plate 59. This is done by “bevelling” hull 50 so that it becomes recessed below the port and starboard hull portions. That is, the ramp surface 55 extends between two upright fore-and-aft steps or planes 66 at equal distances from the central plane of hull 50, as shown. In effect, two wedges (of which one, 63, is shown by phantom lines 67) are left in hull 50, rather than being “bevelled” off.

It has been found possible with this arrangement to provide for improved trim in certain boating conditions or improved wake control (relative to a boat otherwise similar to boat 1) by rotating trim plate 59 so that its rear edge 68 is moved below the position shown in FIG. 3 so that the stern of the boat is lifted and the bow of boat 30 rides lower in the water and/or the boat 30 is of enhanced usefulness for wake boarding and any other activity where exaggerated wake making or wake control is desirable.

By raising the rear edge 68 of trim plate 59, it is possible, conversely, to trim hull 50 so that its bow is higher than when it is trimmed for wake boarding, and so that drag and wake making is reduced.

Referring now to FIGS. 4, 5 and 6, there is shown a rear portion of a boat hull 31 formed according to another embodiment of the present invention. The view in FIGS. 4 and 5 is from a similar viewpoint as the view of hulls 2 and 50 in FIGS. 2 and 3 respectively, to simplify comparison with those hulls. Hull 31 has chines 32, a keel line 33, main bottom surfaces 34 and a transom 47. A trim plate 39 is mounted to hull 31 for rotation about a transversely extending hinge axis 41 parallel to and close to the leading edge 42 of trim plate 39 so as to be rotatable by an actuator 40 about hinge axis 41, in the manner shown by arrow “D”. Hull 31 may be provided with a propeller, drive shaft, drive shaft support skeg, and a rudder, all of which may be similar to, and similarly located to, the corresponding parts of boat 1. For clarity these items are not shown in FIGS. 4, 5 and 6, although a skeg attachment position is shown by a phantom line 37.

Corresponding to the ramp surface 55 in hull 50, hull 31 is provided with an inclined surface 43. Surface 43 ramps upwardly and backwardly from transition point 73 on keel line 33 behind skeg position 37, its leading edges 79 being defined by the intersection of surface 43 and the hull bottom surfaces 34. Ramp surface 43 extends backward so that its rear edge 44 lies closely adjacent to leading edge 42 of trim plate 39. That is, there is no surface corresponding to surface 17 of hull 2.

FIG. 5 shows, in a view similar to the views of FIGS. 2 and 3, the rear part of hull 31 with propeller 35, shaft 36, skeg 37 and rudder 38 omitted but trim plate 39 shown. FIG. 6 shows the same view of hull 31 but with trim plate 39 now omitted also.

Ramp surface 43 extends upwardly and backwardly from transition point 73 on keel line 33, and over part of its length is recessed between upright fore-and-aft steps or surfaces 70, similar to surfaces 66 of hull 50. Hull 31 differs from hull 50 in that the trailing edge 44 of surface 43 is closely adjacent to leading edge 42 of the bottom surface 74 of its trim plate 39 and lies forward of its transom 47. Having a portion of trim plate 39 forward of transom 47 enables hull 31 to be comparatively easily boarded by a person in the water astern of hull 31 as trim plate 39 extends astern of transom 47 lesser than would otherwise be the case. In fact, the trim plate 39 need not extend astern as far as is shown in FIGS. 4 and 6. Trailing edge 44 could actually be forward of transom 47 or in the same plane as transom 47 if required. Such an arrangement maximises the advantage of ease of boarding over the stern.

Trim plate 39 is shown in FIGS. 4 and 6 as having its trailing edge 44 behind transom 47.

The trailing edge 44 of surface 43 and the leading edge 42 of the bottom surface 74 of trim plate 39 are close enough together over their respective lengths that bottom surface 74 of trim plate 39 extends surface 43 backward with substantially no step at the transition between the two, although of course depending on the angle to which trim plate 39 is adjusted, the surfaces 43 and 74 may not be co-planar. To enable this, and as best seen in FIGS. 5 and 6, a shoulder 75 is provided at the rear of ramp surface 43, and trim plate 39 is positioned behind shoulder 75. Shoulder 75, planes 70 and a surface 76 of hull 31 define a recess 77 in which trim plate 39 is accommodated. Recess 77 is proportioned to accommodate a sufficient degree of upward hinging movement of trim plate 39. A further recess 78 is provided in transom 47 for the actuator 40.

Leading edges 79 of ramp surface 43 and leading edges 62 of ramp surface 55 are shown as straight in FIGS. 3 and 4 because the hulls 50 and 31 are hard-chine types with bottom surfaces 52 and 34 being substantially flat in their rear parts. However, the invention may be applied to boats with rounded bottom surfaces as well. Moreover longitudinal strakes and the like, such as are commonly used on vee-type hulls, may be provided on hulls embodying the invention.

Leading edges 62 and 79 are shown as sharp in the Figures, but may in practice be somewhat faired into the local hull contour as desired.

Ramp surfaces 43 and 55 have been described as flat, but some curvature, as would be seen in a side view or in a longitudinal cross-section such as FIG. 6, is not intended to be strictly precluded. Ramp surfaces 43 and 55 have been described as continuing back to the leading edges of trim plates 39 and 59 respectively, with no intervening separate surface comparable to the surface 17 between surface 15 and trim plate 7 of hull 1 in FIG. 1. However, an intervening surface similar to surface 17 that in a longitudinal direction is small by comparison to the trim plate 39, 59 and ramp surface 43, 55 it adjoins is not intended to be precluded.

FIG. 6 shows a flange 80a (of a gland, not shown in full) where a rudder shaft (not shown) passes through ramp surface 43, flange 80a being flush with surface 43. However, as it may be desirable to avoid a lifting force for the rudder shaft axis 87 to be perpendicular to surface 43. FIGS. 7 and 8 (in which arrows point to the bow of the hull 31) show a flange 80 that is perpendicular to shaft 82 where shaft 82 is passed through the ramp surface 43. It is preferred, so as to reduce purely parasitic drag (i.e. drag that does not contribute significantly to wave making), that the lower front edge 81 of flange 80 not protrude from surface 43. The lower front edge 81 may therefore be made substantially flush with surface 43 as shown in FIG. 7. Generally, depending on the angle of rudder shaft 82 to surface 43, this will mean that the rear lower edge 83 of flange 80 will not be flush with surface 43. An alternative arrangement is shown in FIG. 8, where the rear lower edge 83 of flange 80 is made substantially flush with surface 43. In this case, flange 80 sits in a small recess 84 in surface 43. It is thought that the arrangement in FIG. 8 may be advantageous.

In the embodiment of FIGS. 9 and 10, the trim plate 91 is mounted by a selectively rotatable drum or cylinder 95 so that the transverse axis 96 of the cylinder 95 about which the trim plate 91 moves is displaced above the keel line 14 and out of the plane of the trim plate 91. As shown the trim plate 91 is mounted so that it is substantially tangential to the cylinder 95. As the cylinder 95 rotates about axis 96 by an actuator mechanism (not shown) as shown by arrows E, the trim plate 91 moves from a retracted position shown in FIG. 10 in which it is an extension of the plane of the inclined portion 43 into progressively lower positions 91a, 91b shown in broken line in which it projects into the water flow along the inclined portion 43. The dimension of the trim plate 91 can be relatively modest, e.g. about 300 mm in the transverse dimension and about 10 mm in width (i.e. the extent to which it projects down into the water flow). Although small in dimensions, such a trim plate 91 can be very effective in modifying the trim of the boat and controlling the wake's properties. A suitable manual actuator mechanism can include a cable extending from the boat driver's station where a manually movable handle is provided for the driver to operate the trim mechanism. The cable can operate a linkage coupled to the cylinder 95 for rotating it about its axis 96. With such a simple mechanism, the position of the trim plate 91 can be known by the driver from the position of the movable handle. Only small load is experienced by the mechanism as a result of the water flow encountering the small plate 91 and being deflected thereby, so powered assistance from hydraulics or electric drive is unnecessary.

In use of a boat utilising the present invention the position of the trim member can be adjusted by the boat driver as desired for control at speed and/or for wake control. For example, at high speed (say 60 km per hour or more), a small movement only of the trim member into the water flowing along the flow modifying part of the hull can lift the stern and drop the bow which can be needed upon meeting rough or choppy water. When using the boat towing a wake boarder (typically about 40 km per hour) the trim plate can be positioned at about 45° for optimum shape of the wake.

It is to be understood that various alterations, modifications and/or additions may be made to the features of the possible and preferred embodiment(s) of the invention as herein described without departing from the spirit and scope of the invention.

Claims

1. A boat having an adjustable trim, said boat including: (a) a hull with a bottom surface, side walls, transom, a keel line running from bow to stern defining an axis of symmetry from which port and starboard hull portions extend to meet said side walls, (b) a water flow modifying part of the hull located towards the stern comprising an inclined portion of the hull which extends up relative to said bottom surface in a direction towards the stern from a transition point along said keel line, and (c) located aft of said inclined portion a selectively moveable trim member with a surface for contacting water relatively flowing along said flow modifying part and wherein said moveable trim member can be selectively positioned to increasingly obstruct said water flow and thereby lift the stern and lower the bow in the water when the boat is travelling forward.

2. The boat of claim 1 wherein said inclined portion progressively widens from said transition point in the direction of the stern.

3. The boat of claim 2 wherein said inclined portion progressively widens to each and thereafter remains a predetermined width until reaching the trim member.

4. The boat of claim 1 wherein said inclined portion towards the trim member is recessed below the port and starboard hull portions on either side thereof.

5. The boat of claim 1 wherein the water flow modifying part of the hull further includes a flat surface which extends rearwardly in a direction towards the stern from the inclined portion and which is substantially parallel to the port and starboard hull portions on either side thereof.

6. The boat of claim 1 wherein the selectively moveable trim member comprises a plate.

7. The boat of claim 6 wherein said plate is moveable about an axis transverse to said keel line.

8. The boat of claim 7 wherein the axis is substantially in the same plane of the trim plate.

9. The boat of claim 8 wherein said transverse axis in provided by a hinge by which the trim plate is mounted.

10. The boat of claim 7 wherein the axis is displaced above the keel line and out of the plane of the trim plate.

11. The boat of claim 10 wherein said plate extends from a drum or cylinder which is selectively rotatable about the axis transverse to and above said keel line.

12. The boat of claim 11 wherein said plate is mounted so as to be substantially tangential to the drum or cylinder periphery.

13. A boat hull of adjustable trim comprising: a hull body having a bottom surface with first and second portions and extending rearwardly from a bow to a stern of the hull body, the bottom surface being substantially symmetrical on opposite sides of a vertical central plane that extends longitudinally of the hull body; and a trim plate in the stern region rotatable by an actuator about an axis extending transversely of the hull body, wherein—(a) a keel line being a line of intersection between the said central plane and the bottom surface has a first segment forward of and a second segment rearward of a transition point partway along the keel line, the second segment of said keel line extending upwardly from said transition point and the second segment terminating adjacent to a leading edge of a bottom surface of the trim plate; (b) the first portion of the bottom surface of the hull extends outwardly from the first segment of the keel line on each side of the central plane; and (c) the second segment of the keel line being a centreline of the second portion of the hull bottom surface, that second portion extending outwardly from the keel line on each side of the central plane.

14. The boat of claim 13 wherein said second portion of the bottom surface is substantially flat or planar.

15. The boat of claim 13 wherein the second portion of the hull bottom surface extends transversely between steps that extend longitudinally and are equally spaced apart on opposite sides of the central plane, sections of the first portion of the hull bottom surface being located to port and starboard of the said steps so that the first portion of the hull bottom surface extends from bow to stern, the said steps and the second portion over at least a part of its length defining a recessed surface in the hull bottom surface between the port and starboard sections.

16. The boat of claim 15 wherein the trim plate leading edge extends across the width of the second portion of the hull bottom surface and across the gap between the two steps.

17. The boat of claim 13 wherein the selectively moveable trim a plate is moveable about an axis transverse to said keel line.

18. The boat of claim 17 wherein the axis is substantially in the same plane of the trim plate.

19. The boat of claim 13 wherein said trim plate extends from a drum or cylinder which is selectively rotatable about the axis transverse to and above said keel line.

20. The boat of claim 19 wherein said plate is mounted so as to be substantially tangential to the drum or cylinder periphery.