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Old 26-10-2009, 09:52   #1
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Tacking

Help me out here.....
What is the best tack angle in various condition ?
When close hauled, is it better to do many short small angle tacks or less long larger angle tacks ?
If VMG is the best indication of best tack angle, should I be looking for this on the GPS and disregarding tack angle and only tack when this starts to decrease ?

I have a 12m cat with fin keels
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Old 26-10-2009, 10:54   #2
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Unless you hear different I would keep it simple and stick to the angle that gives the best vmg.
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Old 26-10-2009, 11:03   #3
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Quote:
What is the best tack angle in various condition ?
The best tack angle is to trim the sails based on the apparent wind angle. In that sense there is always only one best tack angle the one going closest to the goal. If the crew are miserable then it's a bad tack angle too.

You VMG assuming a 45 degree angle made good would be actual velocity divided by the square root of 2. So you would divide the actual velocity over ground by the square root of two at 45 degrees. That assumes no leeway (and there would always be some) or current.

Sooner or later the leeway eats your lunch and the velocity made good turns poor. You can measure VMG on the GPS if you are heading to a way point. That can then compute in real time the actual progress toward the mark no matter what tack you are doing. Most GPS's will compute that and it's pretty accurate. If the delta between the starboard and port tacks is high and the leeway large you can end up with almost no VMG.

For maximum VMG you want to be sailing toward the mark as close as possible. The amount you are off is the VMG you are losing. It's a function of the Tangent of the tacking angle from the mark. When you are headed at the mark the VMG is the VOG. If you are headed anything less than 90 degree you might actually get there. Long or short tacks won't change much in terms of the math. There may be situations that prefer a shorter or longer tack based on currents, weather, or running aground. If winds shift over time then you may find equal tack angles need to be adjusted. Should the wind shift in your favor you would want to readjust the course.
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Old 26-10-2009, 11:41   #4
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Sail for the highest VMG and use fewer tacks. On any boat, but especially on cats, you lose a lot of speed on each tack, so more tacks will make your average speed lower.

Fair Winds,
Mike
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Old 26-10-2009, 12:40   #5
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Good question. I would back up all the others and say go with VMG to your destination.
FWIW one tack looses you around 4 boat lengths on a well sailed race boat - maybe 10+ boat lengths on a cruising cat - so it makes sense where you can to reduce the number.
Easiest rule is sail from layline to layline but this assumes you've the open seas to do this, and that wind is a constant direction.
Enjoy
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Old 26-10-2009, 12:42   #6
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Crak,
As computed by the nav system, VMG is usually the product of the speed (through water or over ground) with the cosine of the angle between the boat's axis and the true wind. This obviously neglects leeway and the possible influence of current.

Many nav systems give the instant value for VMG. This is risky to use for steering because if you luff, the cosine will increase immediately but the boat will take a few moments to slow down. Then, you could end with the sails stalled and the boat stopped.

In my experience, even without any current, leeway sometimes exceeds 20 degrees on choppy water, because each wave halts the boat and pushes it sideways. Then, you need to bear away slightly to increase the speed through water and the lift provided by the keels.

To check the result, you need to use your GPS and be careful to use measurements averaged over a few minutes, because your instant speed and course over ground keep fluctuating wildly.

As Yogao wrote, a cat doesn't tack quickly and needs a long time to regain full speed after tacking. Then, you have to judge carefully when a change in wind is worth tacking.

Alain
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Old 26-10-2009, 15:55   #7
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Go fo best VMG and avoid laylines.
Tack on headers.
In very stable conditions you can make fewer tacks - to get the crew more rest/sleep/comfort.
Cruising, why tack?

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Old 26-10-2009, 17:59   #8
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As a rule of thumb, unless there is a reason to go right or left - for example, favorable current or shore effect on the wind angle - I would sail within a 60 degree cone off the mark.

i.e draw the centerline straight downwind from the mark and then draw a line 30 degrees each side. Stay in the cone.

Of course the wind never comes straight from the mark and one tack or the other will be favored.

If VMG is reliable I would also sail VMG.
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Old 26-10-2009, 18:27   #9
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hmmmmm. someone is showing his stripes

[QUOTE=Ex-Calif;352533
i.e draw the centerline straight downwind from the mark and then draw a line [/QUOTE]

That's an old racing strategy to keep the young guns from running the laylines.

(But, in a steady breeze it was always so easy to beat the guy committed to staying in the cone.)
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Old 26-10-2009, 19:17   #10
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VMG sailing - if it were only that simple it would be very boring indeed. Loads more to consider;
is the more wind to the left or the right?
what will happen to the wind further up the course?
do I want to stay in the current because it will push me or do I want to short tack along the shore to get out of the current because it is against me?
do the waves favour one tack over the other?
Going to the laylines is OK, but it will leave me exposed to wind shifts
Is it better to tack on every header or will the act of tacking actually loose more time than I gain.
does my boat sail better on one tack than the other? (most do - especially cruising boats)

These and many, many more factors must be considered when trying to get to windward. On the boat I race on we have one guy who sits at the back of the boat with a computer and all he does all day is decide which way to go. (with varying degrees of success!!!)

But that is racing.

When I'm cruising I tend to sail to where I see the wind (if I need more wind), only tack on really big headers, or when I'm sure I'll make my windward objective on the opposite tack or when I'm about to run aground.

Short tacking a crusing boat, and especially a cruising cat is not going to be an efficient way of getting to windward - nor is it going to win you friends amongst the crew.
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Old 27-10-2009, 00:42   #11
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Quote:
Originally Posted by Bash View Post
That's an old racing strategy to keep the young guns from running the laylines.

(But, in a steady breeze it was always so easy to beat the guy committed to staying in the cone.)
When you are farthest left or farthest right, you then gotta ask yourself if you are the smartest guy on the course or the dumbest -

When in front priority 1 - cover. Priority 2 stay near the middle.

When not in front - Do something different. Anything different -


BTW - Ya'll are right. Racing ain't cruising and there is a high risk of spilling a beer on every tack. You have to ask if it's worth the risk to tack -
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Old 27-10-2009, 02:39   #12
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When racing around the 'cans' in Sydney Harbour we had a tacking compass. Basically it would just tell you when you have come off course a few degrees.... if you are being 'knocked' or 'lifted'.

If the weather mark is, for ease of explanation at 360 deg and you decide on the port tack and your course is 45 degrees. If the boat atually is doing 50 degrees you are being "Knocked" the wind instead of being 360 is actually 5 degrees.
If you are actually sailing at 40 degrees that would mean you are being 'lifted' the wind has come around to 355 deg.

So in close in racing if the tacking compass shows you are doing 50 deg you tack. If it shows you doing 40 you hang on!

These knocks and lifts may be every 5 minutes,or even more! An Olympic skipper of a 470 skiff may do 1 tack every 5 mins for wind change, a cruiser may do 1 per day.

In match racing it doesnt matter. As the lead boat you tack whenever the person behind you tacks.


In the ocean or with significant current then a whole other set of rules come into paly! Lee bowing the current etc.

So on a race boat the tactician just sits around on his banana chair saying to the skipper: *Fart* Time to tack, I reckon! *Belch* Where the bloody Beer!

The 'cone' dan is talking about may well be 30degrees in shore, but at sea might be 5 degrees. No use getting too far off the rhumb line as the conditions may change


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Old 27-10-2009, 04:28   #13
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Wow everything I read above is true but could confuse the reader who is new at this, when I taught at J-World, I made it easy for my students by saying "If you are not going where you want to go, then you are sailing the wrong way"! With that as a known and tacking and gybing angles drawn on the cockpit side deck they understood it quite clearly.... teach it simple and non convoluted and you will have better sailors. All the numbers in the world don't mean a thing if you don't know the basics.
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Old 27-10-2009, 04:28   #14
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You should be able to download the polars from FP, contact them for all the polars, both true and apparent. Most people sail to awa and aws since the intrument to output tws and twa are pricy and require a top end processor. (B&G and Ockam)

Here is the polar for your boat at 20 knots tws.



When sailing distance we almost always sail to vmc or ladders, that applies to upwind and downwind courses unless there is a reason to pull us off that philosphy. (ie wind on one side of the course or current)

Here is a primer from Ockam that will give you the basics.
Sailing in Shifty Wind

Or, you can just go reaching.
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Old 27-10-2009, 04:31   #15
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I always liked the Suunto tactical compasses. Green is good red is bad, lifts and headers at a glance! The nice thing about them is they kept your head out of the boat.

Quote:
Originally Posted by MarkJ View Post
When racing around the 'cans' in Sydney Harbour we had a tacking compass. Basically it would just tell you when you have come off course a few degrees.... if you are being 'knocked' or 'lifted'.

If the weather mark is, for ease of explanation at 360 deg and you decide on the port tack and your course is 45 degrees. If the boat atually is doing 50 degrees you are being "Knocked" the wind instead of being 360 is actually 5 degrees.
If you are actually sailing at 40 degrees that would mean you are being 'lifted' the wind has come around to 355 deg.

So in close in racing if the tacking compass shows you are doing 50 deg you tack. If it shows you doing 40 you hang on!

These knocks and lifts may be every 5 minutes,or even more! An Olympic skipper of a 470 skiff may do 1 tack every 5 mins for wind change, a cruiser may do 1 per day.

In match racing it doesnt matter. As the lead boat you tack whenever the person behind you tacks.


In the ocean or with significant current then a whole other set of rules come into paly! Lee bowing the current etc.

So on a race boat the tactician just sits around on his banana chair saying to the skipper: *Fart* Time to tack, I reckon! *Belch* Where the bloody Beer!

The 'cone' dan is talking about may well be 30degrees in shore, but at sea might be 5 degrees. No use getting too far off the rhumb line as the conditions may change


Mark
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