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Old 19-01-2016, 13:53   #31
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Re: Tacking 101

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Originally Posted by thinwater View Post
If the OP is an engineer, this is something that would have made an easy test question in high school. By now, he should be able to estimate VMG in his head with no more difficulty than you estimate a 15% tip. This is simple arithmetic.

Is he funning us?
Engineers can do Trig functions in their heads? I didn't know that.
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Old 19-01-2016, 15:29   #32
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Re: Tacking 101

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Engineers can do Trig functions in their heads? I didn't know that.
Easy peasy.

All you've got to do is convert degrees to radians by dividing by 2 x pi and then calculate the sine of your heading using a Taylor series
sin x = x − x3/3! + x5/5! − x7/7! + ...,

Can't anyone do that in their heads?
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Old 20-01-2016, 02:18   #33
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Re: Tacking 101

Guys, my apoligies for weighing in later in this thread. That said, there are several key things as yet unmentioned, which have very big factors on which tack you want to choose, when, & why.

And yes, the tacking on the shifts, etc. is important, particularly when racing. Especially in waters with no hypothetical currents, or where you & your competitors are all sailing in basically the same weather system & waters. But... having raced hundreds of thousands of ocean miles, I can say that there's a lot more to it than that, when trying to get somewhere. Even when racing.
And such is where the info in my below thesis, was learned. Or rather, tuned, & put into practice.
As lots of quality studying on the topics herein, are a Big help. But can comprise a good bit of time, & expense (plus headaches). That, & victories too, for the astute. Yea!

Firstly, & this may sound a bit wild, but which tack is your boat faster on?
As yes, there are a surprisingly large number of boats which will point significanly higher, & be much faster, on one tack vs. the other.
And not just upwind, but reaching & off of the wind as well, with various sail combinations, & in varying wind & weather conditions.

Like my Dad's old Cal 29 for example. Upwind, she was almost a knot faster on one tack than on the other. Which, sadly is often due to manufacturers cheaping out, when building boats. Doing things like pouring the lead for keels, with the molds laid on their sides, vs. being vertical.
So that you wind up with much more ballast, & thus sail carrying ability (speed) on one tack vs. the other. Ditto on other; hull, keel, & rudder building methods; etc. etc.

So it pays to take your boat out in different types of conditions, with different sail combo's, & test out your boat's true speed, on both tacks, at the same AWA, & thus, come up with a TRUE set of Polars for her.
As a half a knot of difference, can mean big gains or losses, depending on whether you're on starboard or port. In terms of getting to Bora Bora, ot making it into that favorable weather system or current, on/in which, you'd like to hitch a ride.

Also, I know that current has been mentioned, in terms of staying in it or out of it, depending on whether it's a help or a hinderance; regaring where you're heading.
But another Huge factor with regards to it, is that there are plenty of circumstances in which you might want to stay on an unfavored tack for a day, in order to get into or out of a certain current.
As the gains made by finding "favorable waters" (or winds/weather) are major, when compared to losing 10nm, due to staying on an unfavorable tack for a day.
Doing so can add (or subtract) a knot or two (Easily) from your VMG. For a much bigger part of the picture (passage), in terms of reaching your destination.

So... when you're down below "Naviguessing", you'll want to pull out you;
- Polars
- Pilot Charts
- Ocean Sea Surface Temps (reported), so as to know the true positions of currents, & to some degree weather systems (& their intensities, vs. what they're reported to be).
- Satellite pics of the weather systems, including temps, & moisture levels. Plus the same goings on in the things which affect weather systems, big picture wise. Like land mass temps, within a few dozen, to a few thousand miles of the big weather systems, & where you're headed. Especially ones with significant topographical features... Deserts, Mountains, Major Valley Systems & Mountain Passes, etc.
- Typical area Wind Patterns for the grid you're in; & those that you'll be crossing, & or aiming for (plus those of the above topo features).
- Reported Winds for same. And their variance patterns; both strength, & angle alterations. Be they cyclical repeating, or other.
- Reported Current locations & Strengths for grids X, Y, & Z (above). And again, their movement patterns of note.
- Ditto on what the weather systems are doing now in the same grids.
- Also, the weather forecasts for the same locales. Preferably from more than one source. As different weather bureaus/MET have differeing levels of (monitoring) assests in area X, or Y.
That, & varying levels of expertise on making predictions for said areas.

Meaning that, for example, you wouldn't expect a Hawaiian forcaster to be an expert on weather goings on, in Antartica. Where as you'd better trust the guys in New Zealand, Tierra del Fuego, or even better, McMurdo Station, for same.

Then, when you have all (or most) of the above data & info in front of you, comes the Art & Science of distilling it all into an optimum (weather routing) track for your vessel. So that you can better choose which tack(s) are the best, big picture wise.
Basing it on your likely boat speed, for the AWA headings which your entertaining. And using that, plus all of the environmentals, deciding which route(s) - Plan A, B, & C, are the best for you to take, in order for you to maximize your best speed to get to your destination.

So, yeah, you're both looking 2, or 10 miles down your intended track, at conditions, & what tack to best be on. Plus also, anywhere from 50nm to several hundred nm ahead. In order to best pick your most desirous conditions (& to avoid the bad stuff).
Ergo, a GOOD understanding of how the weather works, & all that goes into forcasting such, is cructial.

Also, to assist you with this, there's a good bit of software out there. Primarily targeted for racers. But as usual, it's trickle down tech.
That, & you can hire the services of professional weather routers. And they can tailor how much info that they lay out for you, based on how much data you can give them on your boat, including your real world Polars. Plus, how much you want to spend on their services. And give you very good routing suggestions.

Some of their reports & suggestions are Incredibly accurate & detailed. Being able to tell you what will happen in your area withing the next few hours... & (further) on down the line. And they'll even tell you when best to tack or gybe, even though it may (temporarily) put you on the (hypothetically) unfavored wind angle. In order to get hooked up with the best weather system in order to speed you towards paradise.

And most folks that use them, have found the services (of good ones) to be well worth it, particularly for long passages. As it helps them to optimize their speed, as well as comfort, in addition to keeping them out of the nasty stuff (& reducing wear & tear on the boat + the crew).

So, yeah, there's a Lot more to choosing the best tack, than just banging them out, on the 2 degree shifts.

PS: It's a lot easier for the helmsman, or on watch person, to be able to see such wind changes, with properly calibrated instruments; on deck, & at the nav station. Ditto on having other accurate instruments, like water temp measurements, & a good barograph, etc.
And even just a good pair of digital compasses will let the helmsman spot a 2 degree lift or header, where as, seeing such (especially @ night) on a regular compass card isn't nearly so easy. Especially for neophytoe type crew members.
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Old 21-01-2016, 08:36   #34
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Re: Tacking 101

Like everybody said .. I would emphasise the need to put yourself in the right position for coming wx. No point in battling away at the favoured tack if it means you'll be beating for the next week. Maybe a big bare away and screaming north for a day will give you a nice reach into the finish. As Uncivilized says it's the big picture and knowing your boat, comfort tolerances and those if your crew that make the decision.

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Old 21-01-2016, 09:13   #35
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Re: Tacking 101

everybody is still misreading the question which is how to know when to tack to make best vmg to weather. we can go on all day with what if the current this and what about the waves that and geez who wants to make so many tacks chasing shifts when the sun is out and it's so comfortable. fact is, if the only question is how to get best vmg to weather it's time to get to work. wanna sit back and cruise? fine. sit back and grab a cold one but if the goal is to get to that windward destination before the beer runs out, TACK ON THE HEADERS for crying out loud. there is nothing else in your power that you can do as regards WHEN TO TACK TO MAKE BEST VMG TO A WINDWARD DESTINATION.

and anybody who has a boat that sails a full knot faster on one tack, not considering wind and waves, you've got a rigging problem. never have I seen a boat that I could not rig to obtain maximum matching boat speed on either tack. again, like tacking it's not much fun when you're cruising and like tacking it's time to get to work IF YOU WANT TO MAKE BEST VMG TO WEATHER!!!
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Old 21-01-2016, 09:26   #36
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Re: Tacking 101

Often we tack well in advance of being headed, based on forecasts and observations. There's not much point in ending up 5 miles to windward of your destination, although it's nice to free the sheets for the last few miles. Currents also play a big role in tacking decisions. But in general, yup tack when we are headed by more than 5-10 degrees for more than a few minutes.
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Old 21-01-2016, 09:27   #37
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Re: Tacking 101

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Like everybody said .. I would emphasise the need to put yourself in the right position for coming wx. No point in battling away at the favoured tack if it means you'll be beating for the next week. Maybe a big bare away and screaming north for a day will give you a nice reach into the finish. As Uncivilized says it's the big picture and knowing your boat, comfort tolerances and those if your crew that make the decision.

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sure, bear away to find the best weather system but how does that answer the question of when to tack in order to make best vmg to windward? you don't want the favored tack if it means beating for a week? you favor beating for two weeks instead? on a cruise from san diego to san Francisco I could have fallen off because it was splashy and wet and perhaps diamond head, where i'm now pointed, would look better than the golden gate but what about those crab cakes I was looking forward to at joe's crab shack on fisherman's wharf? I know, we'll bear off, set the chute and man will we fly then huh? don't really want to go there but hey, it's comfy, right?
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Old 21-01-2016, 09:31   #38
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Re: Tacking 101

If you are racing, your device (say a tablet) will have both your polars and real time wind and ground data to compare. This will tell you the best place to tack or gybe. Then you will visually match this digital tip with where the opponents are and make a final decision to tack or to stay on.

In real cruising terms, I simply make a test gybe or test tack and stay on the opposite side long enough to even out momentary peaks of speed and course. Now and then I have to come back to the old tack too.

Over time you will develop 'mental' speed target tables in your head. Not too accurate but may give you a hunch as to whether you are sailing her up to her potential. Sure thing properly developed paper or electronic target speed tables are twice as good, just not truly required in recreational sailing.

It all depends on how far you are going and how important getting there sooner is. I often sail the wrong tack if I believe conditions will be nicer on the wrong side of the track.

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Old 21-01-2016, 09:33   #39
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Re: Tacking 101

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Originally Posted by Jim Cate View Post
As I read it, the OP was interested in a cruising situation, not a short race with an established course and starting line. Cruising often involves fairly distant destinations or in his parlance, waypoints. When a destination is many hours or days away, lots of the rules of thumb used in racing are inapplicable, for the winds and currents to be encountered are unknown or at best only approximations.
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I agree wholeheartedly with Jim.

The answer of when to tack goes to what your goals are. If all you want to optimize is VMG, crank up the auxiliary and motorsail. There may be many other factors, particularly on long passages. Perhaps one tack is more comfortable than the other due to sea-state -- so maybe you'll choose the uncomfortable one during the day and the more comfortable one for cooking/meals or sleep. Perhaps there are particular winds, sea state or current that you want to hit or to dodge. Perhaps you'd prefer an extra set of hands for the tacking, so you want until there is a change of watch. With 5,000nm under your keel, you've likely been exposed to these many other factors.
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Old 21-01-2016, 09:56   #40
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Re: Tacking 101

I think this thread got a little off track. It seemed to me the OP was asking when he should tack assuming the wind is constant.

He mentioned that he had heard 5 degrees off the rhumb line to which another poster responded that could work but you lose boat speed every time you tack and suggested 20 degrees from the rhumb line might be better.

Someone else might respond, forget the rhumb line and tack when you get to the layline, which of course introduces the risk of missing the mark due to windshifts but that would violate the original assumption of steady wind direction, not to mention if the layline is longer than a mile it is very difficult to do even if you know exactly what your tacking angles are, which can be very difficult to ascertain with any certainty.

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Old 21-01-2016, 11:00   #41
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Re: Tacking 101

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sure, bear away to find the best weather system but how does that answer the question of when to tack in order to make best vmg to windward? you don't want the favored tack if it means beating for a week? you favor beating for two weeks instead? on a cruise from san diego to san Francisco I could have fallen off because it was splashy and wet and perhaps diamond head, where i'm now pointed, would look better than the golden gate but what about those crab cakes I was looking forward to at joe's crab shack on fisherman's wharf? I know, we'll bear off, set the chute and man will we fly then huh? don't really want to go there but hey, it's comfy, right?

Read what I wrote before coming back.

I said bare off to put yourself in a better position. It could be that you beat for a 1000 miles or take a flyer and do the whole thing in a couple of reaches. Probably quicker and more comfy.

I agree with your comments on how to work out what the favoured tack is but it's rarely so critical in ocean sailing, in fact it's usually blindingly obvious. Choosing a favoured tack that requires such fine measurements is tactics . The op was talking of having sailed 5k miles and perhaps is talking long distances.

Sure your coastal passage for your can cakes might require fine tuning but on an ocean passage the best and indeed often the fastest course upwind is rarely chosen by pointing the bow at the destination and seeing from which side the wind blows.

Think outside of your little box ... Just sometimes hey!


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Old 21-01-2016, 11:21   #42
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Re: Tacking 101

Luddites! What you do these days is crank up your weather routing software, like Sailgrib or QtVlm and tack when it tells you.
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Old 15-02-2016, 17:19   #43
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Re: Tacking 101

If you knew unfailingly when to tack, you'd be earning $1000 or more a day as a professional tactician on a $1M boat.

Most of the advice on this thread is good. Pick a tack toward where you think the wind will shift (if you think it will go left, start on starboard tack). When you get headed, decide if you think it will continue in the same direction (in which case, keep going) or oscillate back (in which case, tack). Of course, you might get lifted instead, which is why you're not making all that money as a tactician. In that case, swearing is good, I've found.

If you're more than a few minutes' sail from the waypoint, always tack before you get to a layline. The wind will likely change; if it lifts you on the new tack you'll likely fetch the waypoint, and if it heads you you're "inside the shift" -- that is, better off than if you'd stayed on the old tack and been lifted. On the other hand, the 5 degree cone you mentioned is crazy -- the correct value should be something like 5 degrees from the layline, making a cone of 45 degrees or so on each side of the wind direction!

The one thing you can't use is VMG to the next waypoint. As long as you can't fetch the waypoint, the only thing that matters is velocity made good toward the average wind. I can't easily explain here why that's true but it's a well known principle with an easy proof (using ladder imagery) that you can find in any book on sailboat racing, or indeed almost any book on sailing.
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Old 16-02-2016, 10:15   #44
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Re: Tacking 101

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Originally Posted by Akka View Post

The one thing you can't use is VMG to the next waypoint. As long as you can't fetch the waypoint, the only thing that matters is velocity made good toward the average wind. I can't easily explain here why that's true but it's a well known principle with an easy proof (using ladder imagery) that you can find in any book on sailboat racing, or indeed almost any book on sailing.

This not accurate, VMG to the mark combined with weather forecast is exactly how routing software works. When combined with your boat's polar number, VMG wind is know and factored in. It does use a ladder type approach, but essentially starts with current position, port or starboard choice, with best VMG waypoint, over a given time. That puts you in one of two place, it checks the weather for each location, calculates two paths from each, and does it again. Eventually eliminating some of the impractical solutions. It finds the path with the fewest time intervals, and presents that as the route.


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Old 16-02-2016, 10:54   #45
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Re: Tacking 101

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The one thing you can't use is VMG to the next waypoint. As long as you can't fetch the waypoint, the only thing that matters is velocity made good toward the average wind. I can't easily explain here why that's true but it's a well known principle with an easy proof (using ladder imagery) that you can find in any book on sailboat racing, or indeed almost any book on sailing.
That sounds good in theory but what is the practical application? My handheld Garmin does not compute VMG to the wind. Does yours?
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