GB2124374A - Navigational device - Google Patents

Abstract

The invention provides an instrument for use in sailing boats. The instrument is used in association with either a wind vane or a compass to provide in a simple manner information regarding true wind direction and critical arc (that range of bearings centred on the wind direction toward which the vessel is unable to sail). In the wind vane mode (Fig. 11) for example, sliding index marks define the critical arc 129 and are set for a given boat by sailing closehauled on either tack. A mark 128 shows true wind direction at any time when sailing to windward. In an alternative (Figs. 18-21) the instrument is aligned with a compass instead of a wind vane. <IMAGE>

Description

SPECIFICATION Navigational device This invention relates to a navigational device for use in sailing yachts and sailing boats. The device may be fitted to, or manufactured as part of, a compass or a wind-direction indicator.

Referring to Fig. 1, a sail-driven boat 1 may be sailed in a straight line to its destination except when the destination lies within a "critical arc" 2, which is an arc of about 900 (varying from boat to boat) centred on true wind direction (TWD) 3.

When the destination lies within the critical are 2, the boat must tack.

It is an advantage for the helmsman to know the precise location of the critical arc, but on a moving boat (see Fig. 2) the helmsman is conscious of the apparent wind direction (AWD) 7 which is the resultant of the true wind and the "wind" created by the boat's forward motion 6.

Determining TWD and critical arc in these circumstances has not hitherto been practicable except by the use of a number of instruments linked to an onboard computer.

It is therefore an object of the invention to provide a device which will assist the helmsman in handling the vessel in relation to the true wind direction.

A further object of the invention is to provide a device which facilitates tactical decision making by the helmsman.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 illustrates the critical arc; Fig. 2 illustrates the relationship of TWD and AWD; Figs. 3 to 7 illustrate various tactical situations found in sailing; Fig. 8 illustrates schematically a basic principle of the invention applied to compasses; Fig. 9 similarly illustrates a basic form of the invention used in conjunction with a wind vane; Figs. 1 Oa and lOb show the use of the principles of Figs. 8 and 9, respectively, in greater detail; Fig. 11 is a perspective view, partly cut-away, of a detailed embodiment of the invention applied to a windvane unit mounted on an associated base unit; Fig. 1 2 is a cross-section of the windvane unit of Fig. 11; Fig. 13 is a plan view of the windvane unit;; Fig. 14 is a plan view of the base unit; Fig. 1 5 is an elevation of the base unit in position on a gunwhale; Figs. 16 and 17 illustrate an optional modification of the embodiment of Figs. 11-15; Fig. 1 8 is an exploded schematic perspective view of another detailed embodiment applied to a steering compass; Fig. 1 9 is a detailed perspective view of the body and interfitting rings of Fig. 18; Fig. 20 shows in plan view to a common scale the body and two of the rings of Fig. 19; and Fig. 21 shows in plan view to a larger scale another ring of Fig. 19.

Referring to Figs. 3 to 7, various situations faced by the helmsman will be discussed.

When tacking (Fig. 3), boat 1 is steered as close to the wind as possible, to progress directly against true wind direction 3 in a series of port and starboard tacks 4A, 5A, 4B, 5B, etc., rotating through an angle of (say) 86 degrees at the end of each tack, with all tacks of equal length and boat mean heading 25 thus identical to true wind direction 3.

Should true wind direction 3 change to 3A (Fig.

4), helmsman then has to make the tactical decision either to remain for a longer period on a present and favourable tack (example 5B), or to change tack from a present and now unfavourable tack (example 4B).

In Fig. 5, destination 26 lies to port of true wind direction 3 but still within the critical arc 2 so that boat 1 has the option of proceeding to 26 via route A (two tacks, broken line in drawing), route B (four tacks, solid line), or route C (eight tacks, dotted line).

In each case the ratio, 'length of starboard tack/length of port tack' is the same, or about 2:1 in this example, with boat proceeding in a series of "assymetric" tacks centred on the tack mean heading 27. Note that in this form of tacking the boat still rotates through the same tacking angle at the end of each tack; while the port and starboard legs vary in length in a ratio which varies according to the position of tack mean heading 27 relative to true wind direction 3. Note also that in each case (route A, route B, route C) the boat always remains within a maximum and pre-determined perpendicular distance from mean heading 27, as indicated by the imaginary and parallel 'turning point' lines 28 and 29 associated with route B, with similar lines implicit for routes A and C but omitted for clarity.

In use, helmsman selects whichever route is applicable to prevailing circumstances (for example, lack of open water) and selects suitable units of either time or distance to suit the ratio indicated by navigational calculation (or by a scale which is a feature of the invention), to thus plot an optimum course to windward. Such a plot is useful when lack of open water prevents long tacks as in route A, and an assymetric tacking course facilitates both returning to the mean heading 27 and estimating distance remaining to destination (the former by sailing for units of time or distance equivalent to half of either tack, the latter by taking cross-bearings etc. each time boat traverses the mean heading).

When 'runing' or 'sailing on a gybe' (Fig. 6), boat 1 is sailed in a series of shallow-angle, windastern tacks (or gybes), with true wind 3 alternately 20 degrees to port or starboard of the boat's stern as it steers from a port gybe 31 to a starboard gybe 30 or vice versa.

In Fig. 7, a boat at the start line 32 of an Olympic racing course tacks directly into the true wind 3 to complete the windward leg 33, then is turned through a 135 degree angle onto the first reach 34, then through a 90 degree angle onto the second reach 35, then finally through 45 degrees onto the dead run 36 to arrive at the finish line 37, all as shown in heavy line.

Alternatively the course may be laid out in a mirror image of this arrangement, as indicated by the dotted line.

Various types of specialised compasses are available to the yachtsman to assist in one or more of the above situations, but these all require rather a high degree of skill in use, and none has the ability to function as a simple "closehauled and running" indicator.

Referring to Fig. 8, a basic form of the invention is illustrated. A conventional 0--360 degree compass grid ring 9 is made to carry on its upper surface a second ring, wind plotter ring 10, which is overprinted with a critical arc sector 11 centred on a TWD (true wind direction) index mark 12. CA Sector 11 also has provision (described later) for adjusting this to suit the pointing characteristics of a given boat. In use, the appropriate port or starboard end of CA Sector 11 is aligned with bow to plot TWD as indicated in the Figure, and this display is subsequently maintained in correct orientation with the bow by rotating the combined 9/10 ring assembly to maintain grid-ring north in alignment with compass card north.

With the addition of a stationary angular degrees reference scale 15, true wind direction 3 (and one or both of the critical arc limits 4 and 5) may be read off as both a wind angle relative to bow and as a (set of) magnetic compass bearing(s) relative to the horizon, or visually sighted across the instrument itself.

From the basic wind geometry of Fig. 9 it also follows that, when wind is of constant velocity and direction, as boat changes from one tack to the other (i.e. from heading 4 to heading 5), the shorter wind arc 1 7 traversed by a windvane 1 9 during the same manoeuvre is also centred on true wind direction 3: therefore a similar 2-ring mechanism may be used in windvane-based instruments, with the follower ring 9 aligned with the apparent wind direction pointer and the plotter ring 10 simultaneously aligned with boat's bow to thus convert AWD into TWD/critical arc, as shown in Fig. 5b.

In all embodiments of the invention, the principal ring (ring 9 in Figs. 3 and 5b) is always terms the "FOLLOWER RING" (either 'compass follower' or 'wind follower'), and the second ring (ring 10 in Figs. 3 and 5b) is termed the "WIND PLOTTER RING". In some versions of the invention a third ring termed the "TACTICAL RING" may also be provided.

In Fig. 1 Oa, boat is steered as close to the wind as possible then compass-follower ring 9's 'north' aligned with compass-card-north 20 before rotating wind plotter ring 10 until port tack limit 5 is aligned with the bow index mark 21, with true wind direction 3 and critical arc 2 then in the position indicated by, respectively TWD index mark 12 and CA Sector 11.

Thereafter, irrespective of boat heading, this display may be maintained in correct orientation with both boat and horizon by rotating the combined ring assembly 9/10 until the north follower index 22 is again in alignment with compass card north 20.

In Fig. 1 Ob the 'wind follower' ring 9 is first aligned with the pointer 24A of wind direction dial indicator 24, then 'wind plotter' ring 10 rotated until port tack limit 5 of its critical arc sector 11 is in alignment with the bow index mark 21, with true wind direction 3 then indicated as before.

Thus TWD/CA is displayed in correct orientation during tacking and closehauled sailing, and on other points of sailing may be shown in approximately correct orientation by rotating the combined ring assembly 9/10 until the wind follower index 23 is again in alignment with pointer 24A.

A detailed embodiment of the invention applied to a windvane will now be described with reference to Figs. 11 to 1 6. This embodiment provides a windvane unit which may be removably mounted in a base unit; suitably a small boat may have a base unit permanently attached to each gunwale, the windvane unit being moved from one to the other as the helmsman moves.

in Figs. 11 and 12, a demountable WINDVANE UNIT consists of a body moulding 101 which is provided with a vertical hollow shaft 102 into which fits the rod end of a one-piece moulded assembly comprises of windvane blade 103/1 03A, rod 104 and 'apprent wind' disc 105, so that this assembly is normally free to rotate (by wind action) in the body moulding 101: with disc 105 substituting for the 'wind follower ring' described earlier.

Windvane blade 103 is counterbalanced by weight 1 03A so that its position remains unaffected when rod 104 is tilted from the vertical: and rod 104 is equipped with two convex rings 106 moulded integral with the rod and serving as simple journal bearings, with water free to drain down the annular space between rod 104 and hollow shaft 102: and washer 107 (again moulded integral with rod 104/disc 105) serves as a simple axial-thrust bearing. Rod 104 is retained in position in shaft 102 by packing washer 108 and circlip/circlip retaining groove 1 09/1 09A.

Windvane body 101 is also provided with a horizontal hollow shaft 110 in which loosely fits a springloaded brake-rod 111 and brakepad 112, with the outer end of this rod provided with a thumboperated brake on/off lever 113 which may be rotated some 90 degrees across the face of a cam 114 moulded integral with body 101. Action of the brake is that when lever 113 is at the 'high' end of the cam, brake is held off: and when at the 'low' end of the cam, brakepad 112 is pushed against vertical rod 104 by the action of the compressed spiral spring 11 5.

The 'apparent wind' disc 105 carries on its circumference the 'plotter' ring 11 6, and disc 105 and ring 11 6 are separated by a friction spring washer 11 7 so that when helmsman grips the raised fingergrip section 11 8 of ring 16, with brake 'on', ring 11 6 may then be rotated relative to disc 105, in a single-handed operation, to align with a stationary bow index mark 11 9. At other times, with brake normally disengaged, disc 105 and ring 116 may be regarded as a composite unit free to rotate relative to the body 101 by the action of the wind on windvane assembly 103/104, with spurious movement of 11 6 relative to 105 prevented by the friction spring washer 117.

The lower part of body moulding 101 consists of an integrally-moulded square-peg section 120 which engages with corresponding socket 121, with socket 121 moulded integral with the base unit moulding 122. Base unit 122 is provided with fixing holes 123 and is one of a matched pair which are permanently attached by screws to, respectively, the port and starboard deck edges of the boat, so that when on a given tack the appropriate windvane unit is exposed to the relatively undisturbed apparent wind arriving from outboard and can be conveniently read and singlehandedly manipulated by the helmsman.

Each base unit is designed so that socket 121 is aligned at some 75 degrees to the plane of the base unit/boat deck interface: thus when peg 120 is plugged into socket 121, road 104 will remain in the vertical position while boat is heeled at 1 5 degrees to the horizontal, under wind pressure (Fig. 15).

When the demountable WINDVANE UNIT and permanently-aligned BASE UNIT are joined together, the body 101 's horizontal tongue 124 fits into the corresponding slot 125 to ensure that windvane unit is parallel to boat keel-line and in correct orientation with bow. Body 101 and base unit 1 22 are held together by a springloaded ball catch 1 26 and corresponding socket 127, with the latter moulded into the base 122.

Plotter ring 116 is provided with a triangular TWD index mark 128, and this forms the centre of an orange-coloured critical arc sector 129 which extends some 45 degrees to either side and is faintly engraved with a 'phantom' angular degrees scale 1 30 graduated at 5-degree intervals: with this scale intended to facilitate positioning sliding masks 131 A and 131B to simultaneously adjust CA sector 1 29 to a given boat and conceal the unwanted portion of this coloured sector. Thus to cover a CA range of 70-90 degrees (or 34-45 degrees at scale limits) each mask will be of length such that it spans 30 degrees.

Sliding masks 131 A/131 B are made of thin sheet metal or plastics material and are designed to be sprung into and retained by the angled rebates moulded into the top surface of ring 11 6, with friction such as to allow positioning but prevent subsequent spurious movement: and ring 116mask 131 A/131 B cross-section are as in Fig.

20. Ring 11 6 is held loosely in position on disc 105 by a retaining ring 140 which is attached to the underside of the latter by small screws, impact adhesive or other means.

Each of the masks 131 is equipped with a closehauled' scale 132A/132B, with this scale spanning 12 degrees and graduated at 2 degree intervals, and with a prominent 'CA limit' mark in the centre of each scale.

Ring 11 6 also carries the legends "PORT TACK", "STARBOARD TACK", and "GYBE", as shown, plus the gybe limit marks 1 34A and 134B: with the latter positioned at, respectively, 20 degrees to either side of a point diametrically opposite the TWD index mark 128, to thus enclose a total arc of 40 degrees, and this arc bearing the legend "GYBE", and to make it easier for the user to distinguish between a port gybe and a starboard gybe, 1 34A is coloured green and 134B is coloured red. (Figures 11 and 13).

The upper surface of body moulding 101 carries two 0-180 degrees reference scales 1 35A and 1358, with graduations at 1 5 degree intervals. Only the outermost scale is used for a given tack, with scale 1 35A used for a windvane unit plugged into a starboard-only base unit moulding 122A, and scale 135B for the (identical) windvane unit plugged into the companion portonly base unit moulding 122B. The upper surface of body 101 also carries the 'boat-bow-and-stern' motif 36, to improve graphic display of information.

The upper surface of base unit 122 is of such dimensions that it can accommodate a broad, large-diameter and semi-circular 0-1 80 degree sighting scale 137. Scale 137 is prominently marked, with graduations at 1 5 degree intervals and additional numerals at 30 degree intervals, and serves as an open-sighting device whereby the user can view the "critical arc" against the surrounding horizon: and since each base unit is of generally semi-circular form in plan the scale 1 37A for a 'starboard' base unit is a mirrored version of the scale 1 378 provided on a 'port' base unit.

Base unit 122 is dished so that the projecting bottom edge 138 of the sidewall can be trimmed with a hand-tool to suit the chamber 139 of a typical boat's deck (Fig. 1 5). To facilitate correct installation the inboard sidewall of each base unit bears an engraved or moulded legent "PORT I/B" or "STBD I/B" as appropriate, and the hollow interior bears the legend "KEEL LINE" (Fig. 12).

In an optional modification illustrated in Figs.

16 and 17, the disc 106 and ring 116are provided with two additional scales which improve the accuracy of TWD/CA indication on points of sailing other than closehauled.

When direction and velocity of true wind, and velocity of boat through water (ignoring tidal effects) are known, the velocity and direction of the resultant apparent wind can be calculated trigonometrically (Fig. 2), with displacement of AWD relative to TWD varying with both the angle of the true wind relative to boat's bow, and with the ratio of true wind velocity relative to boat velocity.

From this it follows that (Fig. 1 6) with true wind direction 141 established during closehauled sailing (by alignment of ring 11 6's critical arc limit 133B with bow index 119), and with windvane disc's AWD index mark (or pointer) 105A now displaced by the boat's forward motion and indicating the prevailing apparent wind direction: then the displacement of AWD pointer 105A relative to TWD index 128 may be regarded as a measure of the prevailing 'true wind speed/boat speed' ratio.Thus, depending on whether boat moves slowly or more rapidly in response to a given true wind speed, or whether a given boat is responding to either a light wind or a strong wind, 105A may of its own volition align with any one of the positions 1 42A, 1428, or 142C, or in any intermediate position.

In the optionally-refined version of the windvane unit (Fig. 17), CA sector 129 is provided with an "AW/TW ratio" scale 142, with this scale having three segments labelled A, B and C respectively and also coloured blue, yellow and red respectively: and ring 11 6 is now separated from the 0--180 degree reference scale 135A/B by a "TWD correction" scale 143. TWD correction scale 143 is engraved with three sets of curves (red, yellow, blue), each curve connecting points on the inner and outer circumference (see e.g.

curve 143A) calculated to correct for the angle between TWD and AWD.

In use, helmsman first aligns CA limit marker 133B with bow index 119, to accurately plot true wind direction as before (Fig. 1 6): then examines the position of AWD pointer 105A relative to scale 142 and in response to the remainder legend "NOTE AWiXW RATIO" which is engraved on disc 105, adjacent to pointer 105A, mentally notes which of the scale 42 segments A, B, C is in line with this pointer: for example red segment 'C'.

With boat steered so that true wind 141 is then on the port beam (Fig. 17), TWD index 128 indicates that true wind is at 135 degrees on reference scale 135A, or in approximately correct position. If desired, helmsman now uses the "AW/TW ratio" noted earlier on scale 142 (i.e.

ratio 'C in this example) and follows the red 'correction curve' which has its inner end nearest to TWD index 128 on the innermost circumference of correction scale 142 to its other end on the outer circumference of 142, and there reads off corrected position for true wind direction against the reference scale 1 35A (i.e. 143 degrees in this example).

"AW/TW ratio" scale 1 42 is calibrated to cover a range of typical true wind speed/boat speed ratios, with the three segments corresponding to: A (blue)-slow boat (or strong wind) B (yellow)-med. fast boat (or medium wind) C (red)-fast boat (or light wind) The three sets of associated correction curves are also coloured blue, yellow and red, respectively, with the entire scale 143 calibrated in accordance with trigonometrical calculations similar to those described, and two scales 142 and 143 applicable to the conditions most commonly encountered by the majority of sailboats.

Note that initial plotting and subsequent correction of TWD are both done instantly, without calculation and without requiring values for wind or boat speed.

Referring now to Figs. 1 8 to 21, a detailed embodiment of the invention applied to a compass will be described. This embodiment is in the form of an accessory to be applied to an existing compass, but it will be understood that a similar arrangement could be manufactured integrally with a compass.

In Fig.18, a body moulding 201 is provided with two circular recesses; a large-diameter and shallow recess 202, and within this a smailerdiameter but deeper recess 203 into which fits the body of a conventional spherical or edgereading compass 204; and fixing holes 212.

A compass follower ring 205 fits into recess 202 (Fig. 19), and ring 205 is provided with a raised and roughened fingergrip section 206, plus the slots 207 and 208, with "tactical" ring 209 fitting into slots 207, and "wind plotter" ring 210 fitting into slot 208: this construction allows the three rings to be rotated relative to each other or rotated as one unit around the compass 204, and also ensures that all of the rings' upper surfaces, with scales and markings, lie in the same horizontal plane as the stationary 0-1 80-0 degree reference scale 211, so that general appearance and operation of the rings is similar to that of a rotary sliderule, with rings retained in their respective positions, and their spurious rotary movement prevented, by conventional means not described here.

In Fig. 19, the main ring, follower ring 205 is shown in exaggerated cross-section, and likewise rings 209 and 210, all largely 2-dimensional.

In ring 205, the slot 207 is provided with a rebate 21 3 which carries the 0--360 degree (reciprocal) grid scale 214, so that when the (transparent) tactical ring 209 is mounted on follower ring 205, scale 214 will be visible at the scale area 214/A, with the rebate 21 3 preventing scuffing of scale 214 by the underside of ring 209.

"Tactical" ring 209 is moulded from transparent plastics material, with integral fingertip section 215: and ring 210 also has a comparable fingertip section 216, with 215 and 216 proud of the plane surface but to a lesser extent than the 'main fingertip section 206.

Fingergrip sections 21 5 and 21 6 are of rebated cross-section as shown and also serve to retain and allow positioning of sliding masks/limit markers 217 and 218:219 and 22). Each of the CA limit masks 217/A and 21 8/A is provided with a protruding wire limit marker 217/B and 218/B respectively, while the tack limit sliding masks 219/A and 220/A are provided with two such limit markers apiece, mounted at the extremities of each mask so that they are separated by 30 degrees (example: 220/B and 220/C).

Each of the CA masks 21 7 and 218 is provided with a "closehauled" scale 217/C and 218/C respectively (Figs.19 and 20), with each scale extending 8-degrees outward from the CA limit marker (217/B, 218/B) and graduated at 2-degree intervals, and totai mask length such that it covers 30 degrees of arc, so allowing it to conceal the surplus coloured portion of CA sector 222. No closehauled scale is fitted to 21 9 or 220.

Aft-reading compass 204 is provided with an aft-reading or reciprocal compass 0--360 degree scale 204/A, aft-reading compass lubberline 233, plus a series of compass scale sighting marks 234/A, B, C etc., located between the 5-degree scale graduations and each immediately adjacent to its associated 'thirty degree cardinal point', with marks 234 intended to facilitate using the appropriate cardinal point as a 'compass north' point as it rotates past a given scale on ring 210.

Marks 234 are all coloured orange-yellow.

"Plotter" ring 210 carries a TWD index 221, CA sector 222, and a "TW bearing" index 223 which is located diametrically opposite to TWD index 221 and is used for (visually) projecting across to the compass to read off true wind direction as a magnetic-compass bearing on either compass scale 204/A or the in-alignment grid scale 214, or as the central point from which to 'count off' a similar compass bearing for CA limit 217/B or 218/B. CA sector 222 is provided with a "mean heading displacement" scale 224 which is graduated at 5-degree intervals. Finally, ring 210 also carries a gybe scale 225 as described for the above embodiment, plus the legends "GYBE", "PORT TACK" and "STBD TACK".

The transparent "tactical" ring 209 is provided with two sets of "alignment marks" 227, diametrically opposite each other and normally positioned so that they simultaneously align with the appropriate 'thirty degree cardinal point' of compass scale 204/A while remaining at approximately right angles to boat centreline: this latter alignment then causes the (printed) gybe marks 228/A and 228/B and the (adjustable, preset) tack marks 219/B and 220/B to be centred on the corresponding thirty-degree cardinal point nearest to boat stern, with gybe and tack marks then straddled over an imaginary reference line which is approximately the same as boat centreline, with tack and gybe scales read from aft: and causing the Olympic scales 229, 230, 231, 232 to then be at the bow end of the boat, with this disposition of ring 209 used for 'general' sailing.

For Olympic course sailing, ring 209 is rotated through 1 80 degrees to bring the Olympic scales to the aft end of the instrument, then sliding masks 219 and 220 moved further round the fingergrip-cum-rail 215 to straddle the same magnitude of tacking arc, and again read from aft but now using the Olympic tack markers 219/C and 220/C instead of the limit marks 219/B and 220/B used for 'general sailing' tacking. Thus with the (now re-positioned) Olympic tack marks 219/C and 220/C, Olympic gybe scale 232, and Olympic course heading markers 229, 230,231 all to rear and in view of helmsman, boat can complete the ract without crew having to further manipulate the instrument.

To facilitate positioning the sliding masks to the required 'own boat' tacking arc, and repositioning as required for 'general' or 'Olympic' sailing, ring 209 is provided with two 'phantom' scale setting marks 236/A and 236/B with these disposed at ninety degrees to the tactical ring alignment marks 227/A and 227/B, and the sliding masks are colour-coded with a blue dot at the 'Olympic' end of both 219/C and 220/C a red dot at the 'general sailing' end of 21 9/B, and a green dot at the 'general sailing' end of 220/B.

As described, tactical ring 209 is rotated some 15 degrees to align with compass cardinal points for any use and may be rotated through 180 degrees as demanded by general or Olympic sailing. For setting a straight-line grid-steering course, ring 209 is rotated some 130 degrees to bring the least cluttered section of the Olympic scales area to a point adjacent to the lubber line 233; with slot 207 angled and of such width and fingergrips 206 of such height that compass scale 204/A and grid scale 213 can be easily viewed and kept in coincidence with each other.

Olympic scales 229, 230, 231 and 232 are printed on the top of transparent ring 9 and are geometrically disposed so that the 'start' ends of "Olympic reach 1 (East)" scale 229/A, and "Olympic reach 1 (West)" scale 229/B relate to a 'thirty degree module': thus with compass scale 204/A's '270' cardinal point in centre of the combined gybe/tacking scale bounded by 219/B and 220/B (with the latter then in correct position at the rear of the instrument), the corresponding '30' and '1 50' cardinal points will be at the 'start' ends of the 'east reach 1' and 'west reach 1' scales 229/A, 229/B.

This 'thirty degree design module' also means that a helmsman attempting to keep track of a 'selected for use' cardinal point (say 270) just moving onto scale start point (as boat commences to steer onto reach 1), and with this selected cardinal point near the limit of vision, may use the "alignment mark" 227/A (or 227/B) to take note of the '330' cardinal point then in simultaneous alignment to help verify that the selected cardinal point, now 2x30 degrees removed from 227/A, is indeed '270'.

Geometry and layout of scales and markings is as shown in Fig. 20, with ring 209 Olympic scales shown on larger scale in Fig. 21.

In Fig. 21, tack markers 219/C and 220/C are omitted for clarity but for Olympic use would be positioned as indicated by dotted lines 'A' and 'B'.

A boat sailing a 'west mark' course would see selected cardinal point start on scale 229/B at point 'C' and move to 'D' on completion of reach 1, then through scale 230/B to 'E' on completion of reach 2, then back to 'F' as boat turns 45 degrees onto final leg, and boat on 'east mark' course would rotate G to H, H to I, then I to J.

The 'end of reach 1' points 'D' to 'H' are both 5 degrees wide so that they can perform a double function; with the inner edges (30 degrees apart) serving as reach scale end points, and their outer edges (40 degrees apart) serving as a gybe scale 232 for Olympic use.

This embodiment thus permits a compass to be used as a grid steering compass, a tacking compass, a tactical compass and an Olympic course compass, and in addition provides for indication of TWD and critical arc. The same principles can be applied for example to a flat card compass.

Claims (10)

Claims

1. A navigational instrument for use in saidpowered vessels, comprising a body, at least one ring rotatable in the body and having an index mark for alignment with the wind, and adjustable indicating means on the ring which can be set at an angle about said index mark equal to the critical arc of a given vessel.

2. An instrument according to claim 1, in which a wind vane is rotatably mounted in the body, an apparent wind direction disc is drivingly connected with the wind vane, and said ring is mounted on the apparent wind direction disc for manual rotation relative thereto.

3. An instrument according to claim 2, including manually operable brake means for selectively preventing rotation of the wind vane in the body.

4. An instrument according to claim 2 or claim 3, in which the body is shaped to be removably received in a base unit attached to a vessel.

5. An instrument according to any of claims 1 to 4 in which the body presents sets of curves between said ring and a surrounding angular reference scale, each set of curves being for a given ratio of apparent wind to true wind.

6. An instrument according to claim 1, in which the body is adapted to encircle a compass.

7. An instrument according to claim 6, in which a second ring is manually rotatable in the body and bears compass markings adjacent the compass, whereby the compass can be used as a grid steering compass.

8. An instrument according to claim 7, in which said second ring is rotatably seated directly on the body, said first-mentioned ring is rotatably seated in the second ring, and a third ring is also rotatably seated in the second ring, the third ring bearing indicia geometrically related to a predetermined course of sailing in a number of given directions.

9. An instrument according to claim 8, in which the third ring carries a pair of index marks whose angular position can be manually set on the ring as tacking marks.

10. A navigational instrument substantially as herein described with reference to Figs. 3, 5A and 18-21, or Figs. 4, 5B and 11-15, or Figs. 4. 5B and 11-15 as modified by Figs. 16 and 17, of the accompanying drawings.