How to Calculate & Plot an Estimated Position (EP)
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- čas přidán 12. 08. 2021
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#freesailingtutorials - Jak na to + styl
Such a useful video. The clear explanations of why things are done at different times for different objectives brought sense to something I'd been struggling to remember parrot fashion with very little success. Many thanks :-)
Another great informative video
Thanks for the great tutorial. I can't work out why we apply leeway at the beginning of an estimated position but at the end of the course to steer.?
Hi. We apply leeway at the beginning with an EP because we already know the heading we were or are steering. We will therefore measure or estimate our leeway based on our point of sail. Leeway will be greater with a vessel that is hard on the wind and lesser if the wind is abaft the beam.
Also, if you do an EP calc, try applying the leeway at the end. You will have a problem. Because you get a point (an EP). How do you apply a bearing angle to a fixed point?
With a CTS, we add leeway at the end because the leeway will be dependent on our angle to the wind. We don't know our desired heading (and therefore our angle to the wind - known as the TWA True Wind Angle or AWA Apparent Wind Angle) until we have calculated tidal effect. Once we know our desired boat heading to get to our destination, we apply leeway to obtain a final Course to Steer. We can apply a bearing adjustment to a line vector.
It is quite possible that if your desired heading before applying leeway already has you hard on the wind (beating or close hauled) then you can't make that destination on one tack. In that case, you may need to tack at some point, to reach it.
Hope this helps.
@@FreeSailingTutorials Thank you for your comprehensive answer, really appreciate it. It makes sense now after thinking through your points.
Aha! Just as I expected your response. That is confirming and I thank you greatly!!Aloha from Hawai'i.
@@FreeSailingTutorials
thanks for the video... what do you mean by "piloting"... as in "We'll probably be piloting anyway?"... Thanks !
Clumsy language, perhaps. I mean that we will probably be navigating largely by estimating our position by way of visual reference to fixed landmarks, bouyage, depths, etc.
In the Solent (used for this example) you can usually see channel markers, prominent landmarks and depths where shallows are present, at all times, making the need to plot an EP from this method less necessary in practice. It is used more on coastal or ocean passages when using paper charts.
Pilotage is navigating using visual signposts, such as buoys.
Where there are visual signposts calculating a Course to Steer, or Estimating a Position, is a waste of time. Plan the pilotage.
When you are clear of the channel (and of signposts) use the planned Course to Steer. If you can’t sail the planned CtS ask your helmsman for his/her best heading and speed to windward and immediately Estimate a Position for 1 hour; inspect the ground track for hazards
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It’s not free. Info on downloading this info is misleading. V.disappointing
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I have edited the pinned tweet accordingly. My time, as ever, in creating these tutorials is entirely free.
How I can do it, I don't have the boat speed and it's less than 1h?
Hi. You would have the boat speed because an estimated position is an historic calculation based on the boat speed over the last hour or part thereof.
If you are working on say 30 minutes, you just use a half hour of the tidal vector too. As long as the tidal vector and boat speed vector are over the same time period, the EP will be just as accurate.
In this video, I assume a boat speed - see 0:18
I’m not sure why you don’t have a boat speed; if the built-in log isn’t working get a towed log.
You should Estimate your Position for 1 hour as soon as your helmsman reports heading and speed.
Inspect the ground track for hazards.
An EP is predictive, not historic. It predicts your ground track for the next hour; this tells you whether you will meet hazards on that heading
@@johnstarkie9948 Hi John. Thanks for your comment.
Just to clarify the point you make, for others. An Estimated Position is historic when you are plotting from the log, but you can calculate an estimated EP if you so wish.
A Course to Steer is, as you know, is effectively predictive.
So if an EP is looking back historically and CTS is looking forward. Why would we ever need to do an EP if we always plotted a CTS?
Yes. Clearly, in this example, one can check position by using pilotage skills (looking at bouyage, etc) or even GPS, but a CTS before the fact and and EP is after the fact. In reality, it'd be more likely that you'd plot a CTS for say 4 hours and then make hourly log entries, then calculate am EP after say 3 hours, dependent on if you are in open water or coastal. Also, a CTS allows for tidal flows, possibly sometimes in opposite directions, so whilst a CTS over say a 6 hour periods and an EP after the same 6 hour period should both get you to about the same place the exact spot will vary (if only because of actual boat speed vs planned boat speed) and your position over ground through that 6 hour period will vary as tidal effect moves you both ways along the desired COG.
@@FreeSailingTutorials Ok so its a bit like double checking after the fact to see how your going, make sure your on track..
@@anthonyb8998 Yes. One is calculating a heading you should steer to get to a specific place with variables input. The other is a calculation based on what heading and boat speed and leeway you actually had. One plans for the future (CTS) and one calculates an end result (EP).
“Estimating a Position should always be predictive, very rarely historical.
You have set off on a heading because you cannot sail the planned Course to Steer; your helmsman has achieved best heading and speed to windward, and immediately reports heading and speed.
You have several questions:
Where will we be in an hour (20 minutes)(2 hours)?
What will our course over the ground be? (What hazards (rocks, restricted areas) will we meet on this heading?)
What time will we reach the EP?
What time should we tack to avoid the hazard?
You should Estimate your timed Position and ground track (and speed) within a few minutes of your helmsman establishing a heading and speed.
You can then calculate answers to your questions