GB2465016A

GB2465016A - Tide ruler

Info

Publication number: GB2465016A
Application number: GB0820293A
Authority: GB
Inventors: Lawrence John Meakins
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-11-06
Filing date: 2008-11-06
Publication date: 2010-05-12
Also published as: GB0820293D0

Abstract

A tide ruler is disclosed for use with a tidal stream atlas. The ruler comprises, along its top and bottom edges, a series of hourly increments for a given estimated boat speed. The ruler can be overlayed onto the intended tidal stream atlas, and the width of the hour markers on the top and bottom edges of the tide ruler represent the distance travelled in an hour for the estimated boat speed. The passage of the boat may therefore be estimated on the tidal atlas for a given hour, and the rate and direction of a tide for that hour may be identified from the tidal atlas. The information may be used to calculate a true course to steer for the entire passage. The ruler may provide a means of identifying the rate and direction of tide for four different boat speeds (see figures 2 & 3). The ruler may also include, on a side edge, a means for verifying the device is of the correct scale to be used on a particular page of a tidal stream atlas (see figure 4).

Description

Tide Ruler This invention relates to a device that provides a means of identifying the rate and direction of tide for a given hour of a passage by a boat from a tidal stream atlas.

When a boat is crossing a channel such as the English Channel it is necessary to calculate a course to steer that takes into account the tidal streams it will encounter on the passage.

For example a boat that heads due south when there is a strong westerly tide for the duration of the passage will end up due west of the intended destination. A tidal atlas provides the rates and direction of tide based on a standard port and it is up to the navigator to try and calculate a course to steer based on this tidal information also taking into account that during the passage the tide is likely to change direction. It is not possible from a tidal stream atlas to know where the boat will be at a given stage of the passage in order to correctly identify the rate and direction at that position.

To overcome this, the present invention provides a means of identifying the rate and direction of tide for a given hour of a passage by a boat based on an estimated boat speed from a tidal stream atlas.

This invention allows the navigator based on a predicted speed to easily add up the rates of tide using a tidal stream atlas to calculate an overall balance and direction of tide that is used to determine a true course to steer. The course to steer could be that of the entire passage or the remaining part of a passage based on a current position.

The invention comes in the form of a ruler that has on one edge a mechanism to check that the ruler is intended for a particular tidal stream atlas page and on the other four sides different estimated boat speeds.

The invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows a page from Admiralty tidal stream atlas NP250 for the English Channel (4hrs after High Water Dover).

Figure 2 shows the front of a 4,5,6 and 7 knot tide ruler for use with Admiralty tidal stream atlas NP250 for the English Channel (4 and 5 knot boat speed side).

Figure 3 shows the back of a 4,5,6 and 7 knot tide ruler for use with Admiralty tidal stream atlas NP250 for the English Channel (6 and 7 knot boat speed side).

Figure 4 shows verifying the tide ruler is intended for use on a particular tidal stream atlas page by aligning latitude marks on both the tide ruler and atlas page.

Figure 5 shows using the 6 knot edge of the tide ruler on a page of the Admiralty tidal stream atlas NP250 for the English Channel (4hrs after High Water Dover).

Figure 6 shows using the 6 knot edge of the tide ruler on the next page of the Admiralty tidal stream atlas NP2SO for the English Channel (Shrs after High Water Dover).

Figure 7 shows using the 6 knot edge of the tide ruler on a later page of the Admiralty tidal stream atlas NP250 for the English Channel (3hrs before High Water Dover).

Figure 8 shows using a Bretton style plotter to determine a true course to steer.

In figure 1 a page from Admiralty tidal stream atlas NP250 for the English Channel is shown (4 hours after high water Dover). Such a tidal stream atlas is based on the high water time of a standard port, in this case Dover. Each page indicates the direction and rate of tide at a given hour before, on or after the high water time for the standard port. A tidal stream atlas will typically flow through one complete cycle of tide in the following order: 6 hours before high water hours before high water 4 hours before high water 3 hours before high water 2 hours before high water 1 hours before high water High Water 1 hours after high water 2 hours after high water 3 hours after high water 4 hours after high water hours after high water 6 hours after high water The cycle loops back round so the next page to look at after the 6 hours after high water page is the 6 hours before high water page. The arrows indicate the direction of tide and the numbers on the arrows give the rate of tide for both neap (the first and typically smaller number) and spring tides (the second and typically larger number). For example an arrow that points from right to left indicates a westerly direction of tide. If the numbers on the arrow were 15,27 it would indicate at neap tides the rate would be 1.5 knots and at spring tides 2.7 knots. The boldness of arrow also gives a visual indication of rate and where an arrow does not have any numbers it will typically be the same as a nearby arrow of the same boldness.

All a navigator needs to do is look up the high water time of the standard port in a set of tide tables and they can then determine the direction and rate of tide from the appropriate page in the tidal stream atlas. For example if high water Dover was 0500 and the navigator planned to be off the west tip of the Isle of Wight at 0900 the navigator would go to the page shown in figure 1 as it is 4 hours after high water Dover. They would then see at that position and time the tide is mainly in a westerly direction with a rate of 0.8 knots at neap tides and 1.7 at spring tides.

In figure 2 the front of the invention is shown. Along the top and bottom edges are the number of hours for a given estimated boat speed, starting at 1 and increasing sequentially. In the example tide ruler the top edge is for use with an estimated boat speed of 4 knots and the bottom edge 5 knots.

In figure 3 the back of the invention is shown with estimated boat speeds of 6 and 7 knots along the top and bottom edges respectively. The tide ruler can be produced with any combination of estimated boat speeds but will tend to be speeds typical of a relatively slow moving boat such as a yacht for which the invention is intended. The tide ruler will also indicate for which tidal stream atlas it is intended to be used with, in this case Admiralty tidal stream atlas NP250 -the English Channel.

The invention is to be used as an overlay onto the intended tidal stream atlas and the width of the hour markers on the top and bottom edges of the tide ruler represent distance travelled in an hour for the estimated boat speed. It can be seen that in figure 1 the width of an hour (and hence the distance travelled in an hour) along the top edge at 4 knots is less than the distance travelled (width of hour marker) in figure 2 along the bottom edge at an estimated speed of 7 knots.

The width of the hour markers and hence distance travelled in an hour is calculated from the latitude scale in the intended tidal stream atlas. in nautical terms 1 minute of latitude (represented as I') is equal to 1 nautical mile. For example in figure 1 the latitude scale is on the right hand side of the tidal stream atlas page. There are 30 minutes (30') above and below the 50° mark which added together make 60 minutes (60') or in nautical terms I degree (1°) or in distance terms 60 nautical miles. By measuring this (60'/l°) from this tidal stream atlas using a normal ruler gives a measurement of 84mm -note the tidal stream atlas page in figure 1 is much reduced so does not give a reading of 84mm but the real atlas does. So for this tidal stream atlas 84mm is 60 nautical miles, so to get distance for I nautical mile the following equation can be used: Measurement in mm for n nautical miles / n nautical miles = measurement in mm for I nautical mile In this example: 84mm / 60 = 1.4 mm for 1 nautical mile.

To calculate the distance travelled in an hour for a given boat speed the following formula can be used: Measurement in mm for 1 nautical mile X estimated boat speed in knots measurement in mm for distance travelled in 1 hour at estimated boat speed In this example for an estimated boat speed of 4 knots: 1.4mm X 4 = 5.6mm.

In this example for an estimated boat speed of 5 knots: 1.4mm X 5 = 7mm.

In this example for an estimated boat speed of 4 knots: 1.4mm X 6 = 8.4mm, etc. For the tide ruler shown in figures 2 and 3 the following simple spreadsheet can be used to determine the width of the hours for each of the boat speeds (only first 6 hours shown): Speed 1 hr in mm 1 hour 2 hours 3 hours 4 hours S hours 6 hours 4 5.6 5.6 11.2 16.8 22.4 28 33.6 7 7 14 21 28 35 42 6 8.4 8.4 16.8 25.2 33.6 42 50.4 7 9.8 9.8 19.6 29.4 39.2 49 58.8 The tide ruler is then created using these measurements so that the width of each hour on the tide ruler is equal to the distance travelled in an hour for the selected boat speed on the intended tidal stream atlas. Note the tide rulers included in the drawings are not exactly to scale.

Given the tide ruler is to be used for navigational purposes it is important that the navigator checks that the tide ruler is being used on the correct pages of a tidal stream atlas. Often a tidal stream atlas will have pages of differing latitude scales for each of the hours of tide. The tide ruler has a feature that allows the navigator to verify that the tide ruler is intended for use on that page. On the right hand edge of the tide ruler on both the front and back is a latitude scale that matches the latitude scale of the intended tidal stream atlas pages.

In Figure 4 the latitude scale of the tide ruler is shown being checked against the latitude scale of a tidal stream atlas page. They match exactly indicating this tide ruler is designed for use on this page. On the actual tide ruler the check latitude scal& text is in red.

The actual tide ruler is printed on white card with various options for boat speeds on the front and back. The card is then laminated in clear plastic for protection and ease of visibility. Whilst the examples given use an Admiralty tidal stream atlas and the English Channel this invention can be tailored for any tidal stream atlas and passage. Next follows a description of how the tide ruler is used to calculate a true course to steer.

In the following example a navigator is planning a crossing of the English Channel from off the west tip of the Isle of Wight to Cherbourg. They have estimated a boat speed of 6 knots based on the weather forecast and will be using the Admiralty tidal stream atlas NP250 -the English Channel described earlier. High water Dover is 0500 and it is a full spring tide.

In figure 5 the tide ruler intended for use with Admiralty tidal stream atlas NP250 -the English Channel is shown laid on top of that tidal stream atlas. The navigator has calculated they will be off the west tip of the Isle of Wight at 0900 which is 4 hours after high water Dover. The navigator has selected the appropriate atlas page (insert shows 4 hours after high water Dover) first verifying the page is appropriate for use with this tide ruler by checking the latitude scales match as previously described.

Referring again to figure 5, it is shown that the navigator has selected the 6 knot edge of the tide ruler which is the estimated boat speed and placed the start of the first hour next to where they intend to start their channel crossing. The tide is running in a westerly direction (the direction of the arrows) and the two figures opposite the first hour are 08 and 17. Given that it is a spring tide the second figure is taken and represents 1.7 knots of tide. This means in the first hour of the passage the boat will experience 1.7 knots of west going tide. For the next hour the navigator will need to turn the atlas page and place the tide ruler in the same position as the previous page, this is shown in figure 6.

In figure 6 the navigator has placed the tide ruler in the same position on the page that represents the next hour of tide (insert shows 5 hours after high water Dover). For the second hour of tide in the passage the information is taken from the arrow next to the second hour on the tide ruler. The arrow is still in a westerly direction but there are no figures on the arrow. As described previously the navigator would take the number from a nearby arrow of the same size, in this case the one just above and to the right. As it is a spring tide the second number of 18 is taken indicating for the second hour of the passage the boat will experience 1.8 knots of westerly tide.

The navigator would continue doing this for each hour of the passage. It can be seen that the passage will take approximately 10 hours since Cherbourg is opposite the 10 hour mark on the tide ruler. A quick sanity check can help double check this -the passage is nautical miles, the estimated boat speed is 6 knots per hour so the passage will take approximately 10 hours. During this time the navigator can expect the tide to turn to be in an easterly direction.

In figure 7 the tide is shown 7 hours into the journey (insert shows 3 hours before high water Dover). It can be seen the tide is now in an easterly direction and at the seven hour mark on the tide ruler is running at 4.2 knots.

Using the tide ruler in the manner described the navigator would complete a table for each of the hours of the passage noting the direction and rate of the tide. This can then be used to calculate an overall balance of tide. A table for the sample passage is shown below: Time Atlas Page Hr West Tide East Tide 0900 4 hours after HW Dover 1 1.7 ________________ 1000 5 hours after HW Dover 2 1.8 ________________ 1100 6 hours after HW Dover 3 _________________ 0.2 1200 6 hours before HW Dover 4 _________________ 0.8 1300 5 hours before HW Dover 5 ________________ 2.4 1400 4 hours before HW Dover 6 ________________ 4.1 1500 3 hours before HW Dover 7 ________________ 4.2 1600 2 hours before HW Dover 8 _________________ 3.3 1700 1 hours before HW Dover 9 _________________ 1.6 1800 11W Dover 10 1.1 _______________ Total ______________________ _____ 4.6 16.6 The navigator would simply subtract the smaller number from the larger to give a balance of tide in one direction. In this example: 16.6 knots east going tide -4.6 knots west going tide = 12 knots east going tide.

Using the invention the navigator has calculated for this passage they will need to take into account 12 knots of east going tide. 12 knots of tide in nautical terms is equivalent to 12 nautical miles.

In figure 8 the 12 nautical miles is shown plotted in an easterly direction from the starting point. A Bretton style plotter is then used to calculate a true course to steer, in this example it is roughly 1910 true. The navigator would then need to apply variation and deviation to get a compass course to steer, they would also need to take into account the boats leeway.

The navigator may chose to calculate a range of course to steers using the invention for varying boat speeds as weather forecasts can be inaccurate giving rise to a different boat speed than estimated. The invention can also be used to quickly re-calculate a course to steer based on current position if the boat speed or leeway is not what was estimated.

Claims

Claims 1. A means of identifying the rate and direction of tide for a given hour of a passage by a boat based on an estimated boat speed from a tidal stream atlas.
2. A means of identifying the rate and direction of tide from a tidal stream atlas according to claim 1, in which it is possible to calculate a true course to steer for a passage based on the directions and rates of tide for the entire passage.
3. A means of identifying the rate and direction of tide from a tidal stream atlas according to claim 1, in which it is possible to use four different estimated boat speeds per measuring device.
4. A means of identifying the rate and direction of tide from a tidal stream atlas according to claim 1, in which it is possible to verify that the measuring device is of the correct scale to be used on a particular page of a tidal stream atlas.