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Author
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Topic: Leeway Direction
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netwrangler
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posted July 12, 2011 10:09 AM
Is the 'leeway' vector always perpendicular to the heading by definition?
Thus, in a situation where there is no current, and knot-meter and heading inputs are 100% accurate, then the 'SOG/COG' vector minus the 'boat-speed/heading' vector yields the 'leeway' vector perpendicular to the heading.
Conversely, if that vector is NOT perpendicular to the heading, then there is some current, and/or there are inaccuracies in the data received from the knot-meter and heading inputs.
All of this is leading to my recommending, for our boat, knot-meter calibration runs up and down a measured course just inside the LA/Long-Beach breakwater.
The calibration exercise will present a good opportunity to practice radar piloting, since we want to maintain a constant distance from the breakwater during the runs.
And all of that is leading up to measuring leeway with the #1 and #2 jibs -- trying to develop some guidelines for when to change head-sails.
From: SoCal
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David Burch
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posted July 12, 2011 11:24 AM
it is tricky to think of leeway as a "vector" on the beam, as that implies the vector is added to your speed and heading vector, which is not the case.
leeway shows up electronically as COG always a few degrees downwind of your compass heading when on the wind on either tack, but the SOG still equals your knotmeter speed.
ie if it were a vector on your beam it would change sog as well.
we have a couple articles on the subject in our coastal nav course:
see, for example,
http://www.starpath.com/resources2/Navigation_with_Leeway.pdf
also from our coastal nav course there is an article on knotmeter calibration in the presence of current and leeway.... it is in our textbook, section 12.10 Log and knotmeter calibrations but we do not have a separate copy of it.
From: Starpath, Seattle, WA
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David Burch
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posted July 12, 2011 11:36 AM
"And all of that is leading up to measuring leeway with the #1 and #2 jibs -- trying to develop some guidelines for when to change head-sails. "
I think step one here would be to see what the sail maker (or polars) says is right, and then check that there is no current or at least known current... ie just drift and analyze to see what is going on...
then keep records of raw boat speed and true wind speed (do you have calibrated true wind instruments?) and study the results. and save the GPS tracks, which you can maybe export to gpx files to view in land based echart programs and google earth.
but in the long run this is a more complex question.... ie need to consider how much helm, etc, but raw speed is usually the key factor as long as you know your heading and your cog and your true wind speed.
From: Starpath, Seattle, WA
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netwrangler
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posted July 12, 2011 02:24 PM
Instructor wrote:
"but the SOG still equals your knotmeter speed"
Hey, I'll bet you a lunch against an 'attaboy' that you will reconsider that statement.
Seems to me that SOG and COG create one vector.
Boat Speed through the water [as shown by the knotmeter] and heading create another vector.
Consider this example:
You are entering SF Bay under power and doing revs to make 5 knots.
The tide is at full tidal inflow of 5 knots.
Your boat speed, as measured by the knotmeter, is 5 knots.
But your SOG, as measured by the GPS is 10 knots.
No way are the knotmeter reading and the SOG the same in that situation.
[If I am wrong, I will take you and a guest to lunch at a Zagat rated $$$ restaurant of your choice the next time we are in the same city.
Yes, I do get to Seattle from time to time.
'Attaboys' are due immediately after validation and should be published in this forum.]
And...
Sorry to play the role of 'wise-donkey' here.
But I'm back to the basic question, which is:
"Isn't 'leeway', by definition, a vector that is perpendicular to the heading?"
I still think that is true.
From: SoCal
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netwrangler
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posted July 12, 2011 02:53 PM
As for checking polars, there are a couple of issues:
** The polars that I see published by Beneteau don't specify what sails were flying at different wind speeds or wind angles.
I'd love to have that information.
** For the boat I am on, polars are aspirational.
We need to establish the 'as built' [or 'as sailed'] numbers.
In my experience, the mind-set of many racing Skippers is that shortening sail is for wusses.
Also in my experience, Skippers who shorten sail at the appropriate time win races.
They also have fewer equipment failures.
I'm trying to take my Skipper from category "A" to category "B".
He sees things visually, so vector diagrams have impact.
Thus, the most effective way to present comparative performance data for the #1 and #2 head-sails in winds of 15-20 is with vector diagrams.
[I am open to other presentation suggestions.]
Of course, all of this is dancing around 'VMG-to-the-wind'.
That's clearly the figure of merit that counts.
A vector diagram that includes leeway is simply a 'drill-down' on the VMG-ttw readings.
Still, I think that if I can show significant reduction in leeway with the #2 jib at, say 17kts of true wind, I will have his attention.
From: SoCal
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netwrangler
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posted July 12, 2011 03:17 PM
Lucky for me, your Marine Weather textbook and the pdf article you referenced answer my question.
On page 11 the textbook states:
"Upwind and downwind are directions relative to a line perpendicular to the wind direction, whereas windward and leeward are directions relative to the centerline of the vessel."
I read that as perpendicular to the heading of the vessel.
Agreed?
Combine that with this from your 'Navigation with Leeway' article:
"The term "leeway" .... [u]sually ... means how much a vessel slips to leeward as it progresses along its course under normal operating conditions of the vessel."
Seems to me that defines 'leeway' as being perpendicular to the heading -- QED
From: SoCal
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David Burch
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posted July 12, 2011 09:22 PM
may sound strange, but windward and leeward are names for relative directions. it is a concept that is unrelated to leeway.
In fact, the sentence of ours that you quote may in the strictest sense be wrong and should say downwind... but this is too hard to work out for now!
also though not often stressed enough, the sense of leeway that you are asking about requires the important qualifier (given) that the boat is operating in normal conditions. as soon as you stop the boat in strong wind, you continue to slip to downwind, but now this is an all new concept of "leeway." and in this case the term is unambiguously downwind.
From: Starpath, Seattle, WA
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David Burch
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posted July 12, 2011 09:55 PM
PS i do not follow the example above about the sog and knotmeter speed in current.
the question i was addressing was leeway... without current. leeway adds some uncertainty to your CMG without GPS but even without GPS it never adds uncertainty to your SMG which will be identical to your knotmeter speed and if you have GPS it will be the same as your SOG (assuming no current)
it will be very difficult, if possible at all, to pull out and measure leeway in the presence of notable current.... doable, of course, but very difficult. It is just something you have to know about and fold into the navigation if you do not have GPS.... if you have GPS and a plotter, then frankly you do not care about any of this at all unless you are preparing yourself for electronics failure.
i will look for another article we have on progress to weather that might help.
From: Starpath, Seattle, WA
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netwrangler
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posted July 13, 2011 09:23 AM
Oh, Pshaw!
Claiming that leeway cannot be thought of as a vector isn't 'reductio ad absurdum'.
That's called 'begging the question'.
Here is a link to the definition of the word.
http://www.science-dictionary.com/definition/vector.html
I'm talking about the velocity by which a vessel slips to leeward as it progresses along its course under normal operating conditions of the vessel.
Specifying both velocity and direction are sufficient to define a vector.
You say that is not 'leeway'.
Then give me another word for it....
[to paraphrase the Diamonds and Rust lyric].
At this point I'll stipulate that the RayMarine manual for the ST60 Speed Instrument calls for calibrating the instrument by setting the speed equal to SOG from the GPS while in slack tide.
You'd think that would end it. I mean, the two values are calibrated to be the same.
But read on.
Here is a link to the polars that Beneteau publishes for the Shallow Draft First 47.7:
http://www.beneteauusa.com/community/includes/tng/pub/tNG_download4.php?boat_id=47&type_id=1&KT_download1=4f32012a4df13a35e4c9508a1e108ad1
You will note that the polar chart shows values for 'white sails' and for a spinnaker.
Lots of good stuff in the tables, but one value caught my eye: 'leewy'.
Leeway here is expressed as an angular variation from the heading.
[One more way to look at it ... ah, those French!]
And this brings up the key question,
"Since knotmeters are oriented fore-and-aft on a vessel, how well will they display STW when going through the water obliquely?"
A starting point might be to expect the knotmeter to show a value that was approximately GPS-SOG times the cosine of the leeway angle.
SOG * cos(La)
If, instead of a paddle-wheel, the vessel has a pressure sensitive transducer, the nominal starting point might be
SOG * cos(La)*cos(La)
since pressure usually varies with the square of the velocity.
All of this could be carefully measured and copious data could be gathered.
But I am done with it.
I figure the French have it figured right.
If I keep my eye on the difference between the heading and the COG, and have a feel for where the current is, I'll have a real-time and easy-to-obtain figure-of-merit
[Heel angle isn't bad, either.]
As for the lunch, looks like I owe you.
The extreme over-powered and poorly steered sailing that got me started on this doesn't qualify as "normal operating conditions."
While there is theoretically some difference in the speed readings at smaller (and more realistic) leeway angles, that difference will be lost in rounding error.
Therefore, I concede the bet.
I hope you like seafood.
From: SoCal
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netwrangler
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posted July 13, 2011 09:32 AM
I wrote:
"since pressure usually varies with the square of the velocity."
and of course its just the other way around.
"since pressure usually varies with the square-root of the velocity."
From: SoCal
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David Burch
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posted July 13, 2011 11:30 PM
Please start this wind question in another topic if still of interest. thanks.
From: Starpath, Seattle, WA
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