Estimated Position
A lot of 'traditional' navigation has been largely superseded with the advent of GNSS and chart plotters, But electronics can always fail and the 'old-fashioned' techniques are fun(!) so it is worth knowing how to navigate just from the basics. OK. So given our start position and our water track and distance (from Compass and log), how do we establish an estimated position?
Well, let's start by devising our true water track. In this example, we have been steering 270 degrees on the Compass for the last hour. We need to correct this for Deviation (from the Compass Correction Card) and Variation (From the Compass Rose on the Chart:
We can then plot the vector of one hour's worth of boat speed on this track from our start position (I've noted my log reading at the start - 12.2nm) The cross at 1550 would be our position in the absence of any tide. I call it my 'water position'... other names are available!
We do have tide in most real-life situations, so how do we factor that in? Well, first, let us go to the tide tables for the day. I am using an example in the fictitious 'RYA land' used for RYA courses where all the tidal information in referenced to Port Victoria. Today is Sunday 23rd June so let us go to that page of the Victoria Tide Tables:
We can see that the nearest HW to our sailing time is 0850 UT (we are in the Southern Peninsula of RYA Land which runs Southern Peninsula Daylight Saving Time (SPDST) of UT +2hrs - so 0850 UT = 1050 SPDST) We therefore have a HW of 0850UT and a tidal range of 5.3m-0.9m = 4.4m. Both Tidal Atlases and Tidal diamonds refer to 'tidal hours' - i.e. the HW page/line is valid for thirty minutes either side of HW. Next then, we need to calculate which tidal hour(s) to use for our calculation. We can see below that, for our calculation starting at 1450 for one hour, we need to use 30mins of HW+4 and 30 mins of HW + 5:
As we can see on the chart, there is a tidal diamond, Tidal Diamond K, conveniently located in our vicinity. To get tidal data then we merely go to the tidal diamond data on the chart:
So we can see the tidal set and rate for both spring and neap ranges. We're close to springs, so we could 'guesstimate' the set... but to be completely accurate we will use the computation of rates graph, below:
So we can see the tidal set and rate for both spring and neap ranges. We're close to springs, so we could 'guesstimate' the set... but to be completely accurate we will use the computation of rates graph, below (don't worry about this if you aren't familiar with the COR... this is Yacht Master stuff and I may cover it in another 'top tip'). Once we have interpolated our actual rates we use half an hour's worth to establish our two tide vectors:
Plotting each vector in turn from our water position:
Et Voila! Our Estimated Position (EP):
As the lines we have drawn are vectors, we can derive our Course over Ground and Speed over Ground as follows:
Our Estimated SOG is therefore 9.1kts.