GB2028725A - Improvements in or relating to motor sailers and fast motor boats - Google Patents

Abstract

The hull (a) of a fast motor boat has a step (h) which is located between 10% and 25% of the waterline length (Q) from the stern and curves upwardly aft of this step. The hull operates as a displacement hull with minimum drag at slow speeds and as a planing hull at fast speeds. Partial or full width hinged flaps (i1 and i2) may be provided at the step and may be retracted to fair into the hull and lowered to extend the planing lines of the bottom. <IMAGE>

Description

SPECIFICATION Improvements in or relating to motor sailers and fast motor boats This invention is concerned with motor sailers and fast motor boats by which term is meant boats having a characteristic in terms of: VVL greater than 3, where V is the velocity in feet per sec. and L is the static water line length in feet.

An object of the invention is to improve the performance of such boats. Basically the shape of boat hulls can be divided into two main categories; viz. the displacement hull, which will be referred to in this context as the "slow" category and the planing hull which will be referred to as the "fast" category.

The slower displacement hull throughout its relatively slow speed range displaces as the same suggests, a volume of water which is equivalent to its own weight, since its forward speed is insufficient to generate any dynamic lift. To reduce the fluid dynamic drag to a minimum, the underwater hull design of these boats is double ended when seen in plan and in profile the afterbody run is curved upwards in relation to the horizontal or waterline.

Skegs or fin keels are often located aft for directional stability or other reasons but these do not play any part in reducing the basic hull drag. Present day motor sailers for example fall into this category.

In the fast or planing category of hull on the other hand, the underwater (static) hull design resembles a rounded wedge when seen in plan, the blunt end being the stern. In profile the afterbody run is straight, running parallel or nearly so to the waterline, when the hull is at rest. Such a boat operates efficiently at the higher speeds ( greater than 3) when the forward motion produces dynamic lift and eventually permits the hull to skim over the water. In this condition the hull no longer displaces a volume of water equivalent to its own weight since a portion of this weight is supported by dynamic lift. As the speed increases so does the lift until at very high speeds almost all of the weight is supported dyna mically.

Most displacement hulls are incapable of planing speeds even if sufficient power were intalled to permit them, in theory, to do so. Planing hulls on the other hand run inefficiently at slow speeds and require more power than displacement hulls to operate at these speeds. This results in increased fuel consumption, which can be significant if long periods of slow operation are necessary due to bad weather or other reasons. Where the power avail able is limited, such as in the case of propulsion by sail, the performance (speed) of this type of hull would be very poor.

From one aspect, the invention provides a fast motor boat in which a step is located at between 10% and 25% of the static waterline length from the stern and the hull lines aft of the step curve upward to reduce eddy formation at displacement speeds.

From another aspect, the invention provides a motor boat having a conventionally shaped planing hull on the aft end of which is an extension providing a step at its forward end and curved upwardly from the step to reduce eddy formation at displacement speeds, said step being located at between 10% and 25% of the static waterline length of the extended hull from the stern.

The present invention thus provides a hull which can operate efficiently with a minimum of drag at displacement speeds and which can also run in a fully planing condition. As in the case of the displacement boat, the underwater hull design is double ended when seen in plan and in profile the afterbody run is curved upward. So as to permit such a shape to plane, a transverse step is located across the entire bottom of the hull from chine to chine. The bottom surface of this step is a continuation of the straight running lines of the hull. The upright portion of the step should be deep enough to allow proper ventilation so that the wake may detach itself from the portion of the hull aft of the step when the hull starts to plane. The same basic hull shape can be achieved by building an extension onto the transom of a conventional planing hull.In one form, the step on a 39 foot (11.88 m) planing motor sailer, is located at 20% if the static waterline length from the stern and the depth of the step is 1.7% of the chine beam at the step. The depth of the step is not critical although it is essential that adequate ventilation does occur otherwise planing is not possible. The depth of the step may be extremely small, for instance if full width (from chine to keel) retractable flaps are used, the depth need only be the thickness of the flaps themselves.

The fore and aft location of the step can vary to suit different design requirements but should be located between 10% and 25% of the waterline length from the stern.

Other parts of the invention are embodied in the preferred form which will now be described in some detail with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a typical displacement hull design with hard chine sections, although round bilge sections are commonly used; Figure2 is a plan view of Figure 1; Figure 3 is a rear elevation of Figure 1; Figure 4 is a side elevation of a typical planing hull design with hard chine sections, although round bilge sections are sometimes used; Figure 5 is a plan view of Figure 4; Figure 6 is a rear elevation of Figure 4; Figure 7 is a side elevation of a planing motor sailer according to the invention; Figure 8 is a plan view of Figure 7; Figure 9 is a rear elevation of Figure 7.

In the drawings similar references are employed for similar items in the various figures.

A conventional shape of a displacement hull is shown in Figures 1 to 3. The hull a is double ended as shown in Figure 2 and is curved upwards both at the bows d and at the stern c in relation to the static waterline 0. The conventional shape of a planing hull is shown in Figures 4 to 6. The hull a is roughly wedge shape in plan with a blunt stern c which enables the boat to plane at fast speeds on the lower edge of the stern.

The boat of the present invention is shown in Figures 7 to 9 and comprises a hull a with sheer b, transom c, bow d, keel e and chine f. At between 10% and 25% of the length of the static waterline Q from the stern, a step h is provided.

The boat also has a retractable centre plate g, a propeller shaft j, shaft bracket k, propeller i, rudder m, mast n, boom o and main sail and jib p. The static waterline is indicated at 0 and planing wake between keel and chine is indicated at r.

Although Figures 7 to 9 show a single propellerand rudder, twin rudders or twin propellers may be used.

The rudder m is of near triangular shape which enables an automatic reduction in area as the speed of the craft increases. This is desirable since a lot of rudder area is required under sail when the wake aft is at Q. When fully planing, when the wake is at r, only the hatched portion of the rudder is in contact with the water.

Partial or full width hinged flaps i1 and i2, extending from chine to keel, may be fitted so that when retracted as shown at i1 they fair in the step with the aftbody of the craft to reduce drag and when lowered as shown at i2 extend the planing bottom surfaces of the hull. The trailing edges of these flaps may be lowered below the horizontal to generate lift and thus adjust trim.

Claims (10)

1. Afast motor boat in which a step is located at between 10% and 25% of the static waterline length from the stern and the hull lines aft of the step curve upward to reduce eddy formation at displacement speeds.

2. A boat according to Claim 1 having, at the step, partial or full width hinged flaps which when retracted, fair into the aft body of the hull and when lowered extend the planing lines of the bottom.

3. A boat according to Claim 1 and 2 having a retractable centre plate.

4. A boat according to any one of the preceding claims, having a tapered rudder to permit area reduction as speed increases.

5. A boat according to Claim 4, having more than one rudder.

6. A boat according to any one of the preceding claims having the propeller shaft line aft of the step.

7. A boat according to Claim 6, having more than one shaft line.

8. A boat according to any one of the preceding claims having also sails for propulsion.

9. Afast motor boat having a conventionally shaped planing hull on the aft end of which is an extension providing a step at its forward end and curved upwardly from the step to reduce eddy formation at displacement speeds, said step being located at between 10% and 25% of the static waterline length of the extended hull from the stern.

10. A boat constructed and arranged substantially as described herein and shown in Figures 7 to 9 of the accompanying drawings.