NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: leeway
From: Clive Sutherland
Date: 2006 Jul 11, 17:54 -0500
George.
To confuse the issue further the I have found the expression 'Wind Drift'
( to mean leeway ) used on this side of the Atlantic also.The 'Nautical
Institute' here in the UK has argued that Leeway means a different thing to
Comercial shipping than it does for Yachtsmen, for example.
I was taught by the Royal Yachting Association that Leeway was the angle
between the boats Heading and the boats Course (through the water). and this
was applied to the navigation triangle as an offset angle to the Course
before applying the Tide vector. The plot then shows DR and EP.
The course through the water would change with time as the yacht tacks about
with wind shifts or whatever and the resulting DR position would be at the
end of a wriggly line. A single tide vector for the total time interval
would be attached to the DR to give the final EP. The trouble with this
method is; Where do you apply the Leeway? To be accurate, this needs to be
applied to each individual course, acording to the performance of the boat
on each leg.
If the leeway is the same on each course the it would be easy. as the DR
could be 'rotated' about the start position by the leeway angle assumed.
Commercial ship navigation sees this problem differently. Since these ships
stay on one course for days on end mostly out of tide streams, the EP can
be found by applying Wind Drift as 'distance' vector to the DR, in the same
manner as a current or tidestream vector.
' Horses for courses' you might think<G> But what do navy ships do?
I have no experience in big ship navigation.Perhaps they do it in a
different way altogether.
Clive.
----- Original Message -----
From: "George Huxtable" <george@huxtable.u-net.com>
To: <NavList@fer3.com>
Sent: Tuesday, July 11, 2006 2:25 PM
Subject: [NavList 657] Re: leeway
>
> Lu Abel sent a message on July 8 addressed to Doug, but quoteing what
> I had written to Doug, and Doug's response, so I suspect he was
> arguing with what I wrote, rather than Doug.
>
> I will interpolate some comments-
>
> | Any attempt to calculate and plot the position of a boat including
> | compensation for the effects of current or wind will have two
> components
> | (1) the boat's motion through the water, and (2) external effects
> | (current, wind) acting on the boat.
>
> Not such a simple division as Lu makes out. Wind, acting on the boat,
> affects the boat's motion through the water. For a sailing craft, it
> CAUSES the boat's motion through the water.
> |
> | #1 is covered by what has traditionally been referred to as the
> boat's
> | DR position. DR is calculated ONLY from speed, direction, and time.
>
> What I have questioned is simply WHY it should be done that way. If
> you know that your course through the water is different from your
> heading from the compass, why not enter your best guess as to what
> your true course really is, before drawing in your vector with respect
> to the water?
>
> | Is the result the position of your boat? Not necessarily. If you
> know
> | your boat is being acted on by current or has experienced leeway,
> it's not.
> |
> | Should a navigator correct the position of his/her boat by allowing
> for
> | leeway or current? Absolutely, if he/she thinks these might have
> had a
> | significant effect.
>
>
> We agree.
>
> | But, again by tradition, this is called an Estimated Position (EP)
> and
> | dotted line is drawn between DR and EP with the cause for the EP
> being
> | different from the DR shown by a label on the line. (Again,
> navigation
> | plots are usually meticulously annotated, both as a way of going
> back
> | and catching possible errors and to allow another navigator to read
> and
> | understand the plot).
>
> It's that "tradition" that I am questioning. For a sailing craft, it
> seems to me far more sensible to estimate, as precisely as possible,
> the path of the vessel through the water, to include any leeway, as
> courses and wind directions change. Then add in the vector for set and
> drift, for the motion of the water, independent of what the craft
> does. How does the "traditional" method that Lu advocates deal with
> the situation of a tack being made, so part of the day's work is on
> one tack and part on another? How does he allow for the effects of
> leeway then? Or is Lu's method intended to apply to powered vessels
> rather than sailing craft?
>
> | In addition to showing EP vs DR for a position at any point in time,
> you
> | can show a "live" estimate of Course over Ground on your
> navigational
> | plot. Simply extend two lines from your starting fix: one for a DR
> | course and one for your estimated COG. The former is labeled with
> | steered course (C) and knotmeter speed (S). The latter is labeled
> with
> | COG and SOG. And, again, there is usually some notation made on the
> | plot about how DR course and speed is being compensated to arrive at
> the
> | COG/SOG plot.
> |
> | As far as set, drift, and leeway are concerned: Leeway is sideways
> | motion of a boat due to the effect of wind on it. It's an angle by
> | which the boat's actual course deviates from the course steered and
> it
> | naturally depends on the strength and direction of the wind.
> Current
> | has a direction, called "set" and a velocity, called "drift."
>
> Yes, we've sorted that one out; It's one of those cases of two nations
> divided by a common language.
>
> | In this discussion I don't think anyone is arguing that the effects
> of
> | wind and current shouldn't be taken into account by a good
> navigator.
> | It's just that there seems to be a misunderstanding of the
> terminology.
> | But it's no different than why the latitude of the GP of a
> celestial
> | body is called its declination rather than its latitude.
> |
> | So: let's keep correcting for leeway and current. But it would
> | probably be best to use the notation that's been used by navigators
> for
> | centuries rather than redefining the terms.
> |
> | Lu Abel
>
> The difference between our attitudes is not whether one should or
> shouldn't correct for leeway and current. Nor is there any real
> difference about the terminology. It's simply that there are two
> vectors to be added together, and Lu thinks that "traditionally",
> leeway should be included with the motion of the water, whereas I
> think it should be included in determining the boat's motion through
> the water, because it depends so closely on course and wind direction.
>
> George.
>
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>
>
> >
>
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Clive Sutherland
Date: 2006 Jul 11, 17:54 -0500
George.
To confuse the issue further the I have found the expression 'Wind Drift'
( to mean leeway ) used on this side of the Atlantic also.The 'Nautical
Institute' here in the UK has argued that Leeway means a different thing to
Comercial shipping than it does for Yachtsmen, for example.
I was taught by the Royal Yachting Association that Leeway was the angle
between the boats Heading and the boats Course (through the water). and this
was applied to the navigation triangle as an offset angle to the Course
before applying the Tide vector. The plot then shows DR and EP.
The course through the water would change with time as the yacht tacks about
with wind shifts or whatever and the resulting DR position would be at the
end of a wriggly line. A single tide vector for the total time interval
would be attached to the DR to give the final EP. The trouble with this
method is; Where do you apply the Leeway? To be accurate, this needs to be
applied to each individual course, acording to the performance of the boat
on each leg.
If the leeway is the same on each course the it would be easy. as the DR
could be 'rotated' about the start position by the leeway angle assumed.
Commercial ship navigation sees this problem differently. Since these ships
stay on one course for days on end mostly out of tide streams, the EP can
be found by applying Wind Drift as 'distance' vector to the DR, in the same
manner as a current or tidestream vector.
' Horses for courses' you might think<G> But what do navy ships do?
I have no experience in big ship navigation.Perhaps they do it in a
different way altogether.
Clive.
----- Original Message -----
From: "George Huxtable" <george@huxtable.u-net.com>
To: <NavList@fer3.com>
Sent: Tuesday, July 11, 2006 2:25 PM
Subject: [NavList 657] Re: leeway
>
> Lu Abel sent a message on July 8 addressed to Doug, but quoteing what
> I had written to Doug, and Doug's response, so I suspect he was
> arguing with what I wrote, rather than Doug.
>
> I will interpolate some comments-
>
> | Any attempt to calculate and plot the position of a boat including
> | compensation for the effects of current or wind will have two
> components
> | (1) the boat's motion through the water, and (2) external effects
> | (current, wind) acting on the boat.
>
> Not such a simple division as Lu makes out. Wind, acting on the boat,
> affects the boat's motion through the water. For a sailing craft, it
> CAUSES the boat's motion through the water.
> |
> | #1 is covered by what has traditionally been referred to as the
> boat's
> | DR position. DR is calculated ONLY from speed, direction, and time.
>
> What I have questioned is simply WHY it should be done that way. If
> you know that your course through the water is different from your
> heading from the compass, why not enter your best guess as to what
> your true course really is, before drawing in your vector with respect
> to the water?
>
> | Is the result the position of your boat? Not necessarily. If you
> know
> | your boat is being acted on by current or has experienced leeway,
> it's not.
> |
> | Should a navigator correct the position of his/her boat by allowing
> for
> | leeway or current? Absolutely, if he/she thinks these might have
> had a
> | significant effect.
>
>
> We agree.
>
> | But, again by tradition, this is called an Estimated Position (EP)
> and
> | dotted line is drawn between DR and EP with the cause for the EP
> being
> | different from the DR shown by a label on the line. (Again,
> navigation
> | plots are usually meticulously annotated, both as a way of going
> back
> | and catching possible errors and to allow another navigator to read
> and
> | understand the plot).
>
> It's that "tradition" that I am questioning. For a sailing craft, it
> seems to me far more sensible to estimate, as precisely as possible,
> the path of the vessel through the water, to include any leeway, as
> courses and wind directions change. Then add in the vector for set and
> drift, for the motion of the water, independent of what the craft
> does. How does the "traditional" method that Lu advocates deal with
> the situation of a tack being made, so part of the day's work is on
> one tack and part on another? How does he allow for the effects of
> leeway then? Or is Lu's method intended to apply to powered vessels
> rather than sailing craft?
>
> | In addition to showing EP vs DR for a position at any point in time,
> you
> | can show a "live" estimate of Course over Ground on your
> navigational
> | plot. Simply extend two lines from your starting fix: one for a DR
> | course and one for your estimated COG. The former is labeled with
> | steered course (C) and knotmeter speed (S). The latter is labeled
> with
> | COG and SOG. And, again, there is usually some notation made on the
> | plot about how DR course and speed is being compensated to arrive at
> the
> | COG/SOG plot.
> |
> | As far as set, drift, and leeway are concerned: Leeway is sideways
> | motion of a boat due to the effect of wind on it. It's an angle by
> | which the boat's actual course deviates from the course steered and
> it
> | naturally depends on the strength and direction of the wind.
> Current
> | has a direction, called "set" and a velocity, called "drift."
>
> Yes, we've sorted that one out; It's one of those cases of two nations
> divided by a common language.
>
> | In this discussion I don't think anyone is arguing that the effects
> of
> | wind and current shouldn't be taken into account by a good
> navigator.
> | It's just that there seems to be a misunderstanding of the
> terminology.
> | But it's no different than why the latitude of the GP of a
> celestial
> | body is called its declination rather than its latitude.
> |
> | So: let's keep correcting for leeway and current. But it would
> | probably be best to use the notation that's been used by navigators
> for
> | centuries rather than redefining the terms.
> |
> | Lu Abel
>
> The difference between our attitudes is not whether one should or
> shouldn't correct for leeway and current. Nor is there any real
> difference about the terminology. It's simply that there are two
> vectors to be added together, and Lu thinks that "traditionally",
> leeway should be included with the motion of the water, whereas I
> think it should be included in determining the boat's motion through
> the water, because it depends so closely on course and wind direction.
>
> George.
>
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>
>
> >
>
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---