Heel Angle vs. Leeway -- Calculation

[Dockhead]

Knowing how much leeway you're making is typically information you want to know in order to have an accurate calculation of true wind.

You need to know true wind for different purposes, including knowing the exact direction of true wind so you can accurately measure VMG to windward. You need that in order to sail upwind optimally -- maximizing VMG to windward.

I was hoping to be able to directly measure leeway using the new Airmar transducer, but that proved to be unobtanium, so now I will have to calculate it. It seems fairly straightforward to do based on some empirical measurements of leeway vs. heel angle vs. STW. To measure it empirically I guess there are a few possible methods:

1. With as little tide/current running as possible, and with accurately calibrated compass (I now have an extremely accurate satellite compass), compared HDG with COG on both tacks and take average.

2. Trail a cork on 30m of mouseline from the backstay, and measure the angle with a sextant. This is potentially more accurate. Described here: https://www.sailnet.com/threads/calc...-leeway.54631/

This will be a tedious process as the measurements will need to be taken in different conditions and different angles of heel, and with sails perfectly trimmed everytime. Maybe best way to gather enough data for this would be to record HDG, COG, STW over a lot of different upwind legs using OpenCPN or Expedition.

This, however: https://www.l-36.com/leeway.php Suggests that merely subtracting leeway from AWA will lead to huge errors as there is another factor -- upwash (updraft).

So how do we deal with this?

When I did my electronics refit 8 years ago, I was rubbing my hands at all the information I would get from the new B&G instruments, particularly the "SailSteer" screen on the Zeus plotters. It didn't take me long to realize that this screen is utterly useless with my instruments calibrated as badly as they are, and after a lot of futile effort, I gave up on calibrating them.

I participated in a long distance ocean race last year, and found my instruments so useless that I fell back on the windex for upwind work -- right back to my dinghy racing days. I want to break through this before the summer. I have replaced my compass with a super accurate satellite unit, and I replaced my speed log with a new UST800 Airmar ultrasonic one. The wind instrument is a CV7 which has a good reputation, but I have not been able to calibrate it or figure out upwash. That is my task for the next couple of months.

[Don C L]

Are you aiming for a heel angle/speed = x degrees of leeway kind of reference guide?
I suspect spending a few days doing 1 and 2 and making your own guide to extrapolate from will actually end up being the least tedious compared to trying to coordinate electronic data. Your estimates within measurements you make will probably be fairly accurate, no?

[Dockhead]

Originally Posted by Don C L

Are you aiming for a heel angle/speed = x degrees of leeway kind of reference guide?
I suspect spending a few days doing 1 and 2 and making your own guide to extrapolate from will actually end up being the least tedious compared to trying to coordinate electronic data. Your estimates within measurements you make will probably be fairly accurate, no?

Yes. There is a well-accepted formula for calculating leeway as a function of heel angle and stw. Heel angle is an excellent proxy for lateral force on the sailplan so this works well.

[jordanbettis]

For a long keel, leeway is a function of the lateral plane of the boat, so taking into account the reduction in lateral plane caused by heeling should give a ratio for the increase in leeway.

For a fin keel it's a little more complicated because the keel can stall out. But if you assume that the keel is not stalled you can calculate the same ratio by looking at the component of the force vector that's wasted by pointing up.

[chrisr]

i'm not quite sure why you want to know 'leeway' ? isn't what you really want 'COG' ?

you start by saying you need 'leeway' to find 'true wind'...but nearly any worthwhile instruments will give you true wind at the flick of a switch (i know mine will)

slightly more advanced instruments (as normal on any race boat) will also give you VMG as a direct read out

on my last boat (B&G intruments) we had 11 seperate lines of information displayed on deck. get the right instrument package and set it up right, and you will have all you need

and : yes, you must calibrate AWA / BS / AWS / compass (ie all your raw inputs) accurately and frequently on the GIGO principal. not hard...

but perhaps i'm missing something in your question but i just cannot imagine why you want to separate out leeway instead of going directly to VMG ?

incidentally when i was racing we rarely even looked at VMG...we could read out boat performance as a %age of the polars for the situation (polars were stored in instrument memory)

cheers,

[dougweibel]

I gather from your number 1 option that you are looking at leeway as COG - Heading when compensating for any cross current. As a cruiser I am always satisfied with just knowing my difference between COG and Heading.

You appear to have some specific reason for wanting to know your actual leeway relative to the water. I'm curious why? You have to deal with whatever current you have so what can you optimize by knowing leeway (vs COG - Heading)?

Not wanting to argue, just to learn.

[Dockhead]

Originally Posted by chrisr

i'm not quite sure why you want to know 'leeway' ? isn't what you really want 'COG' ?

you start by saying you need 'leeway' to find 'true wind'...but nearly any worthwhile instruments will give you true wind at the flick of a switch (i know mine will)

slightly more advanced instruments (as normal on any race boat) will also give you VMG as a direct read out

on my last boat (B&G intruments) we had 11 seperate lines of information displayed on deck. get the right instrument package and set it up right, and you will have all you need

and : yes, you must calibrate AWA / BS / AWS / compass (ie all your raw inputs) accurately and frequently on the GIGO principal. not hard...

but perhaps i'm missing something in your question but i just cannot imagine why you want to separate out leeway instead of going directly to VMG ?

incidentally when i was racing we rarely even looked at VMG...we could read out boat performance as a %age of the polars for the situation (polars were stored in instrument memory)

cheers,

COG is not a substitute for knowing leeway. COG includes current vectors.


You need leeway in order to have accurate true wind. You can't know anything about true wind if you don't know in what direction you are moving in relation to the water. You need accurate true wind in order to have accurate laylines, accurate VMG to windward, in order to create or use polars, etc etc etc.


Your instruments may claim to tell you all of this, but garbage in, garbage out. Systems like the H5000 compute leeway in order to make this compensation. They will also compensate your STW for heel etc. in order to improve STW, which is likewise crucial for accurate true wind.

[Dockhead]

Originally Posted by dougweibel

I gather from your number 1 option that you are looking at leeway as COG - Heading when compensating for any cross current. As a cruiser I am always satisfied with just knowing my difference between COG and Heading.

You appear to have some specific reason for wanting to know your actual leeway relative to the water. I'm curious why? You have to deal with whatever current you have so what can you optimize by knowing leeway (vs COG - Heading)?

Not wanting to argue, just to learn.

Cheers.

As explained in the previous post -- the object is to get accurate true wind.

COG is useless for this. True wind is wind in relation to the water surface, so you need accurate information about your speed and direction through the water. With leeway, your direction through the water is not identical to your heading.

Accurate true wind is crucial if you want to be able to formulate or use polars, find optimal VMG to windward, know when you are at risk of gybing, etc etc etc etc. It's a fundamental question, which is very hard to answer. The top racing teams spend a huge amount of time and effort getting accurate true wind.

[dougweibel]

Originally Posted by Dockhead

COG is not a substitute for knowing leeway. COG includes current vectors.


You need leeway in order to have accurate true wind. You can't know anything about true wind if you don't know in what direction you are moving in relation to the water. You need accurate true wind in order to have accurate laylines, accurate VMG to windward, in order to create or use polars, etc etc etc.


Your instruments may claim to tell you all of this, but garbage in, garbage out. Systems like the H5000 compute leeway in order to make this compensation. They will also compensate your STW for heel etc. in order to improve STW, which is likewise crucial for accurate true wind.

Sorry, I don't follow your logic in some areas. I agree COG includes current vectors. If I want to create polars then certainly you want to eliminate the effect of current. However if you want to know VMG then COG matters, not leeway. Your waypoint, after all, is fixed to the ground and does not float along with the current.

If I know my actual leeway but I don't know the current vector (accurately) then I still can't accurately calculate a lay line, can I?

[Dockhead]

Originally Posted by jordanbettis

For a long keel, leeway is a function of the lateral plane of the boat, so taking into account the reduction in lateral plane caused by heeling should give a ratio for the increase in leeway.

For a fin keel it's a little more complicated because the keel can stall out. But if you assume that the keel is not stalled you can calculate the same ratio by looking at the component of the force vector that's wasted by pointing up.

Well, leeway expressed as lateral displacement of the boat is a function of heel angle, since heel angle is a function of lateral force on the sail plan. Add STW and you can get leeway as the angle between your course through the water and your heading.



Yes, you are right, that if the keel stalls, all bets are off.

[dougweibel]

Originally Posted by Dockhead

True wind is wind in relation to the water surface, so you need accurate information about your speed and direction through the water.

I have never seen true wind defined in relation to the water. I have always considered it relative to the ground. But, the only boats I have ever raced were Lasers (no instruments).

I did crew on a boat that had B&G instruments and saw in the setup that it was calculating true wind as you define and was a bit confused by that. Now I know better, though for a cruiser like myself I'm not seeing it as a distinction worth mastering.

[Dockhead]

Originally Posted by dougweibel

Sorry, I don't follow your logic in some areas. I agree COG includes current vectors. If I want to create polars then certainly you want to eliminate the effect of current. However if you want to know VMG then COG matters, not leeway. Your waypoint, after all, is fixed to the ground and does not float along with the current.

If I know my actual leeway but I don't know the current vector (accurately) then I still can't accurately calculate a lay line, can I?

I guess these calculations can be done in different orders, but normally you would add set and drift after you've figured out the rest of it. Your polars don't know anything about points fixed relative to the ground -- they only know wind vs water. That's why you need true wind -- relative to the water.


So COG as data doesn't help you. Your layline will depend not only on set and drift, but on sailing performance, which needs true wind -- tacking angle as a function of optimal TWA on both tacks. Nor do you know set and drift without knowing accurately leeway -- COG mashes up set and drift with leeway so with COG alone you can't predict what the other tack will look like.

Besides that, VMG to windward is crucially important for upwind legs, and for this COG is likewise useless.

[Dockhead]

Originally Posted by dougweibel

I have never seen true wind defined in relation to the water. I have always considered it relative to the ground. But, the only boats I have ever raced were Lasers (no instruments).

I did crew on a boat that had B&G instruments and saw in the setup that it was calculating true wind as you define and was a bit confused by that. Now I know better, though for a cruiser like myself I'm not seeing it as a distinction worth mastering.

Won't matter so much to you if you are not sailing to polars. But ground wind (ground-referenced true wind) is useless for that. Also useless for sailing downwind and knowing when you are at risk of gybing. Also useless for figuring out VMG to windward. For all of that you need true wind (water-referenced).


Ground wind IS however useful for knowing what the wind will be like after the tide changes. So I have compass-referenced ground wind on my weather display at the nav table.

[Dave_S]

VMG is an interesting subject, I think because it is more complex and open to personal interpretation than it would first seem. Sorry, I can't help with heal angle to leeway. But, can it be an accurate calculation with so many variables and roundings ?

Just my thoughts so value it for what you paid 🤪

VMG upwind as a method of working out your best tactics for getting to your mark is only really of value if the TWA remains the same otherwise your objective moves and tactics change. As soon as TW moves or changes speed your optimal passage to mark changes. You can't predict "to mark" velocity accurately at any one point in time because you are on one tack or the other and at any point in the swing of that tack, only once you tack or better still, reach the mark do you know if your choices were good.

It surprises me how little ground you loose in a tack so tacking on shifts makes sense to me. As good or otherwise your instruments are calibrated you should be able to get a reasonable idea of the degree change in a wind shift and make the call on "to tack or not to tack". If you tack you will have solid information to base decisions on. In a long event like the one you mentioned a while back, a test tack or two when things change would provide accurate information to make decisions on. Even if it turns out that the tack was a loss the information gained may make up the difference.

I don't think there is a perfect or even great system and that's the beauty of it, it comes down to the choices people make.

[Dockhead]

Originally Posted by Dave_S

. . . VMG upwind as a method of working out your best tactics for getting to your mark is only really of value if the TWA remains the same otherwise your objective moves and tactics change. As soon as TW moves or changes speed your optimal passage to mark changes. You can't predict "to mark" velocity accurately at any one point in time because you are on one tack or the other and at any point in the swing of that tack, only once you tack or better still, reach the mark do you know if your choices were good.

It surprises me how little ground you loose in a tack so tacking on shifts makes sense to me. As good or otherwise your instruments are calibrated you should be able to get a reasonable idea of the degree change in a wind shift and make the call on "to tack or not to tack". If you tack you will have solid information to base decisions on. In a long event like the one you mentioned a while back, a test tack or two when things change would provide accurate information to make decisions on. Even if it turns out that the tack was a loss the information gained may make up the difference.. . .

Well, this is of course true, but it's just another layer of complexity added to what we were discussing before.

"Sail the lifted tack" is kind of Tactic No. 1, isn't it? The most fundamental? So naturally you will be playing the wind shifts.

However, that doesn't remove the need for accurate true wind. We presume that the wind will be changing, but we want to know where it is -- we can't even evaluate those wind shifts without this infomation.


Happend to run across this really excellent discussion of the importance of knowing leeway: https://www.l-36.com/leeway.php