Heel Angle vs. Leeway -- Calculation

[wingssail]

Originally Posted by boatman61

And people complain about drugs in sport...
What happened to old fashioned skills like intuition and reading the sea and sky..

Intuition has very little place when deciding tactics or making navigation choices, but sometimes years of experience has been boiled down and internalized and you "just know" something even if you can't verbalize the logic of it.

Reading the sea and the sky have always been important and remain so, they haven't been lost, nor have the watching of telltails and "feeling" the boat. These are skills that the best helm's persons use to their advantage, so no, Boatman61, nothing has happened to them.

But what has grown in importance is the information which is now available where feeling can be substantiated or invalidated when you cast a wider view than what you can see or feel with the seat of your pants.

None of this is new, just better, easier, and more accurate. We always had compasses to watch to see shifting breezes. We could always watch our progress against a landmark or plot our track on a paper chart to see if we had tide against us. We could always listen to weather forecasts to know of an impending high or low which could influence our tactical decisions. But the electronic information now available has given us new tools and levels of knowledge and accuracy. To turn away from them because "the old seamanhip skills were good enough in the old days, so they're good enough for me now" kind of thinking means one will be doomed to be left behind, just as relying on hand written letters stamped and dropped in a post box means your communications will be hopelessly out of date by the time they arrive.

We need to keep our skills and cherish them, but keep up with the times and use what is availabe now.

[boatman61]

Originally Posted by Dockhead

Well, sailing like that is fun. But try to win a long-distance race on intuition!

Worked for Knox Johnson... and excellently for Mottesier till he decided to go round again.
Level the playing field..

[Joli]

Originally Posted by Dockhead

Sure, but I've known that since my dinghy racing days, and everyone else out there knows that too.


It's not nearly enough on a long distance ocean race.

The two points I referenced are more important in distance racing then dinghy racing, by a very large margin. If you don't understand ladder concepts your gonna have a rough time competing.

Read, medium to large scale VMC racing, this is a very important concept for distance racing.

http://www.ockam.com/2013/06/03/how-...n-shifty-wind/

[wingssail]

Originally Posted by Joli

The two points I referenced are more important in distance racing then dinghy racing, by a very large margin. If you don't understand ladder concepts your gonna have a rough time competing.

Sailing on the favored tack sounds simple enough but there is more to it than which tack is closest to the rumb line.

First is it essential to know what your VMG is, because one tack or the other may be effected more by current and or leeway induced by sea state. Good instruments can help you determine this.

Secondly knowing what the true wind direction is helps you determine what heading and VMG you'll be able to make on each tack, therefore determining which tack is favored, and with good instruments you can see small shifts which otherwise you have to notice by watching the compass as you steer to the tell tails. Tacking to determine you heading on the other tack may be lost time towards the mark.

Finally, Having access to good polar numbers for your boat, along with anticipated wind and current information down the course, will help with routing.

So I agree with the approach that good numbers coming off the instruments is very beneficial especially when you cannot see the mark or your competition, which is often the case in long distance races, and when "what feels good" can easily be misleading.

But I also agree that the best helm's persons and the best navigators/tacticians utilize everything at their disposal, and that means boat feel, and head out of the boat, not just instrument data.

[jmh2002]

Originally Posted by wingssail

...Reading the sea and the sky have always been important and remain so, they haven't been lost, nor have the watching of telltails and "feeling" the boat...

But what has grown in importance is the information which is now available where feeling can be substantiated or invalidated when you cast a wider view than what you can see or feel with the seat of your pants.

...the electronic information now available has given us new tools and levels of knowledge and accuracy. To turn away from them because "the old seamanhip skills were good enough in the old days, so they're good enough for me now" kind of thinking means one will be doomed to be left behind...

We need to keep our skills and cherish them, but keep up with the times and use what is availabe now.

Thanks Fred All very nicely put

[Pelagic]

Originally Posted by Dockhead

Well, most of the racing I ever did was entirely without instrumentation -- just windex and seat of the pants.



I don't need decent true wind data so that I can sail with my head in the instruments. I need it to help with routing and tactical decisions. I need decent (I know "perfect" is impossible for me considering limitations in how much time and money I am willing to spend on it) true wind to develop more or less decent polars so that my routing will be more or less right. Updated every 6 hours as new gribs come out. This also doesn't need to be perfect, but the closer to right it is, the bigger the advantage -- I guess there's hardly anything more important than routing, on a multi-day offshore race.

Hi DH
Just wondering if there is a computer progam that could be used to plug in your parameters and help to develop the data you need?

I remember a French company had developed amazing software that replaced the need for tank testing and we used them to supplement tests where we could not scale inertia.

[Dockhead]

Originally Posted by Pelagic

Hi DH
Just wondering if there is a computer progam that could be used to plug in your parameters and help to develop the data you need?

I remember a French company had developed amazing software that replaced the need for tank testing and we used them to supplement tests where we could not scale inertia.

There is a polar generator program. But it's Apple only.


But I think I can start with the Hylas 54 polars I have and adjust them based on testing.

[Alan Mighty]

Having read through the thread to date, I have to say that I think DH's reasons for wanting a handle on absolute VMG (to give a name to what I think DH is after) and hence leeway via heel angle are perfectly sound and reasonable.

I managed to read a copy of Pedrick & McCurdy 1981. The logic seems sound.

I've attached a pdf of Pedrick & McCurdy 1981, at least for a short time, for anyone interested. It's about 900 KB. I'll delete it after 24 hours. My justifications for attaching it are that (1) digital copies are available for free download on the Net; (2) the work to which DH pointed (earlier in this thread) mangled the cited name of one of the coauthors such that it using the misspelled coauthor's name did not lead to the work; and (3) the SNAME paper has no clear statement of copyright and may be in the public domain.

Question for DH: have you read Ross Garrett's book _The Symmetry of Sailing: The Physics of Sailing for Yachtsmen_?. Garrett hails from Aotearoa, so it's no shame if N Hemisphere types aren't familiar with him.

Garrett spent much of his adult life as a physicist, working in the US. He was a recreational sailor. In the 1980s he did a series of workshops/lectures to yacht clubs in NZ. By about 1987, he compiled his lecture material into the book mentioned and had it published in the UK by Adlard Coles. The book fell off the Adlard Coles catalogue after a few years, but it was picked up by a US (I think) publisher.

If you've a copy, I recommend reading pp. 76 - 82.

Garrett does his own derivation, from first principles, of a slight variation of the leeway angle = k * heel angle/waterspeed^2 formula.

Before deriving that formula, Garrett lists and discusses a few ways to directly measure leeway angle (i.e. Garrett (1) regards finding the leeway angle characteristic to a hull, sailplan etc to be valuable; (2) lists three or four direct ways to measure characteristic leeway angle; and then suggests that the 'indirect' way of calculating leeway angle the heel angle to be the simplest way).

Garrett's formula differs only by using sine of heel angle, instead of the angle itself. Of course that makes k of a different order of magnitude, but for the usual range of angles of heel when heading to weather, I suspect the differences are not of consequence.

Given that I've not seen digital copies of Garrett's work, I'll aim to find time to scribble a concise summary of most of those pages. Don't hold your breath.

[Don C L]

Excellent! But where does k come from?

[Alan Mighty]

Originally Posted by Don C L

Excellent! But where does k come from?

K is just a constant to give proportionality.

[Dockhead]

Originally Posted by Alan Mighty

K is just a constant to give proportionality.

Indeed. Different boats have different "K" numbers -- usually between 9 and 12.

What is cool is that you can figure out your "K" number with a single empirical measurement, and then you are able to predict leeway over a pretty wide variety of conditions. That's the point of the formula.

Better than a single empirical measurement, do a series of them. Then you'll get a feel for what your true "K" is.

How I wish I had been able to buy that DX900+ which measures leeway directly. No software or instruments I am aware of are able to use the N2K PGN for "nautical leeway", or the 0183 sentence, which the DX900+ outputs. But you can read it off on a tablet connected with Bluetooth, and easily gather enough data to get a very solid "K" figure, and THAT you can program into your system (either Expedition or H5000, probably some other systems).

Maybe next year.

The formula for leeway does not, of course, work under all conditions. By the time you are pinching and sliding off to leeward, because you stalled your keel, that formula is out the window. But under a wide range of conditions short of that, it's supposed to be pretty accurate.

[Paul L]

Using the equation to estimate leeway you have
K - an estimated constant
Stw - a corrected value that the correction is dependent on the heel angle
Heel angle - in flat water, steady wind you probably have a decent value. In gusty winds or any seastate it it would have a lot of error

Seems like there is a lot of cumulative error in this equation

[Dockhead]

Originally Posted by Paul L

Using the equation to estimate leeway you have
K - an estimated constant
Stw - a corrected value that the correction is dependent on the heel angle
Heel angle - in flat water, steady wind you probably have a decent value. In gusty winds or any seastate it it would have a lot of error

Seems like there is a lot of cumulative error in this equation

I don't think it's extremely accurate. There is even a version which takes the cube of the STW rather than square.


But I don't think there is any significant error in heel angle due to gusts or seastate. Heeling is a very reliable function of lateral vector of force on the sail plan. Effects of seastate and gusts can be easily averaged out.


STW is more problematic, of course, but is pretty good under a wide range of conditions if you keep the paddlewheel clean and if you have proper calibration including heel and nonlinearity. I will look forward to seeing if my new ultrasonic speed log is more accurate than my old CS4500, which wasn't bad when it was working properly.


I think if you get leeway within 10 percent, this is already very useful data.

[Paul L]

How can a rough seastate that is knocking the boat around not affect the heel angle readings? They are going to bounce around.
I wonder if you could estimate leeway to 10% knowing the boat after sailing it in non current conditions and watching cmg and hdg enough times,

[Paul L]

At 6kts and 10* heel, if the actual heel is 9* you get about a 10% error in the leeway. And that's without any error in K.