Seagreen
Well-Known Member
When tacking along a coast, whats the ideal 'tack-to-distance gained' ratio? I know this depends on the 'wind to mean course' and the windward performance of the boat, but some ideas would be good. I'm not racing.
Coastal Navigation question.
- Thread starter Seagreen
- Start date
dt4134
Well-Known Member
I can answer from the racing point of view. I'd say cruising is the same except you occasionally might want to omit a tack or two if you didn't have enough wheetabix that morning.
Basically you measure the wind direction and tack when you get an oscillatory shift that heads you below the mean. If you're going to tack tack early or keep going and wait for the next one.
Watch out for progressive shifts, which you should be alert to from observation of your best course to windward and even what the forecast might have said.
Watch out for shifts close to land. It might be worthwhile going inshore, even with a header to get a lift coming out. Watch out too for the effects of topography on the wind.
Watch out for a sea breeze forming or dying that will help. Sometime they will extend less than a mile to sea, especially if it is not the height of summer and not on the south coast.
And then of course there's the tides. You might not want to short tack when cruising, but you still want to find less adverse tide and to benefit from back eddies.
Basically you measure the wind direction and tack when you get an oscillatory shift that heads you below the mean. If you're going to tack tack early or keep going and wait for the next one.
Watch out for progressive shifts, which you should be alert to from observation of your best course to windward and even what the forecast might have said.
Watch out for shifts close to land. It might be worthwhile going inshore, even with a header to get a lift coming out. Watch out too for the effects of topography on the wind.
Watch out for a sea breeze forming or dying that will help. Sometime they will extend less than a mile to sea, especially if it is not the height of summer and not on the south coast.
And then of course there's the tides. You might not want to short tack when cruising, but you still want to find less adverse tide and to benefit from back eddies.
dunedin
Well-Known Member
I plan the tacks to sail away from the black clouds and towards the blue sky - to keep dry.
Racer son tacks towards the clouds (or at least certain types of cloud) to get extra wind - even if he gets wet.
There are other strategies - but this is a good one for Scotland ;-)
Racer son tacks towards the clouds (or at least certain types of cloud) to get extra wind - even if he gets wet.
There are other strategies - but this is a good one for Scotland ;-)
sailorman
Well-Known Member
set-up a "Cone". 2 lines say 60deg apart radiating from the waypoint & "tack down the cone" tack each time your course intersects a line. each tack will get shorter as you travel down the cone
D
Deleted member 36384
Guest
The key as mentioned above is to use your ships head when close hauled. If you are getting lifted towards the upwind objective keep going, if getting headed tack.
On long coastal hops there are two navigation methods you can use. Tack along an imaginary corridor say 5 mile legs. Another method is to draw two lines out from your upwind objective in a triangle with the apex at your objective. Then just tack down the triangle. This is really only useful if you have position lines to tack down.
A GPS plotter is of course very good at visualising the VMG calculation. At a glance you can see how effective each tack is at making ground to your objective. If you don't have a plotter you can work this up on your chart.
Tide plays a big part in VMG the classic case being on your lee bow. Sailing close hauled with a strong tide on your lee bow could help your ground track towards the objective.
Hope this helps.
On long coastal hops there are two navigation methods you can use. Tack along an imaginary corridor say 5 mile legs. Another method is to draw two lines out from your upwind objective in a triangle with the apex at your objective. Then just tack down the triangle. This is really only useful if you have position lines to tack down.
A GPS plotter is of course very good at visualising the VMG calculation. At a glance you can see how effective each tack is at making ground to your objective. If you don't have a plotter you can work this up on your chart.
Tide plays a big part in VMG the classic case being on your lee bow. Sailing close hauled with a strong tide on your lee bow could help your ground track towards the objective.
Hope this helps.
D
Deleted member 36384
Guest
Make sure the position lines from your cone do not make you sail over any danger. So don't fix 60 degrees as a tacking boundary without checking on the chart.
sailorman
Well-Known Member
one imagined a modicum of common sense would be applied but were in these PC times
Seagreen
Well-Known Member
Just me to imagine there might be a majic formula. Eperience has already taught me there is no such thing, just thought I'd ask.
grumpy_o_g
Well-Known Member
That's an interesting question (for me at least). I didn't understand it at first until I saw the answers. I know the cone theory (and have used it in practice a couple of times) but is there an easy way to work out how far to offset the cone in a cross-tide situation, e.g. Cherbourg, or am I making it all too complicated and you just point it up tide a bit/a lot depending on the current?
john_morris_uk
Well-Known Member
Not so much an easy way as some suggested 'rules' (guidelines?)
1. Look what the tide is going to be doing when you get near your destination - and DON'T end up down wind and down tide. You'll regret it if you do.
2. Don't chase the cross track error and try to keep it close to zero. over more than one tide allow yourself to be swept one way and then back again - you sail less distance through the water.
This has drifted the thread from the coastal nav question, but you did ask...
lw395
Well-Known Member
Main thing imho is to have the big picture of how the wind will shift.
If it is going to swing much during the day/time you are beating, you want to be on the tack that gets headed first, so that after the shift, you tack and are upwind.
Unless it's going to change so much you will eventually reach in.
Sometimes you want to tack out into favourable tide or hide in the bays for less tide or a back-eddy.
When the wind is oscillating, it will often swing 10deg or so every 5-20 minutes, racing you would tack on this everytime. unless there is a 'big picture' reason to get out to one side. Cruising, you may let it average out.
When the wind is unpredictable, there is much to be said for simply being on the tack taking you nearer to where you want to go, ie best vmg.
And when the wind is going to die at the end of the day, you may as well have a nice sail while the wind lasts and motor in dead upwind at the end of it.
If it's not a race, do what you enjoy, whether that's settling the boat down on a long tack and relaxing, or working hard at making best progress.
Seagreen
Well-Known Member
I know that it is really just making the decision as it happens, as to wind, tide and hazards, etc., but what I was wondering:
If I was say sailing west from Start Point to the Lizard and I had a steady westerly wind heading me, and I think that my boat will sail 40deg. off the wind when beating, is there an optimal ratio between the width of the "corridor" one tacks across and the length of the individual tacks, so as to avoid unnecessarily long tacks for little ground gained.
There probably is, eg., a tack leg lasting one hour at 6kts will result in a corridor width of say 4nm (guess) with about 4nm gained to windward (wind direction and tide discounted) but was there an "official" optimal ratio for this?
If I was say sailing west from Start Point to the Lizard and I had a steady westerly wind heading me, and I think that my boat will sail 40deg. off the wind when beating, is there an optimal ratio between the width of the "corridor" one tacks across and the length of the individual tacks, so as to avoid unnecessarily long tacks for little ground gained.
There probably is, eg., a tack leg lasting one hour at 6kts will result in a corridor width of say 4nm (guess) with about 4nm gained to windward (wind direction and tide discounted) but was there an "official" optimal ratio for this?
LadyInBed
Well-Known Member
The formula for the slope of the straight line going through the points (x1, y1) and (x 2, y 2) is given by:
The subscripts refer to the two points.
(m=rise/run)
Well you asked for one
huzar30
Active Member
Sooner or later you're going to have to tack to make up for lost ground. With the wind on the nose, use the advice given by others: tack when your course is greater than 60 degrees to the direction you want to go. Otherwise, you can alternate long tack/short tack. On the long tack you've be gradually getting further from the line - what I often do is then tack to take you the same distance the other side of the line.
All this is ignoring tides/currents/windshifts. If there's a tide or current, try and make your tacks take you into places where the current will be less - bays and so on.
All this is ignoring tides/currents/windshifts. If there's a tide or current, try and make your tacks take you into places where the current will be less - bays and so on.
ff_nick
Well-Known Member
Start with Sailorman's advice and only modify it if there is an overriding reason to do so, if for example you are being persistently headed, sailing into adverse tide, into a wind shadow or into danger.
In other words, go down the middle of the course unless you have reason to do otherwise - this is what racers usually do too.
Agree that it is very helpful to have a means of calculating vmg, especially if you are unfamiliar with the boat or you've never bothered to check before. You'll quickly learn optimum close hauled angle.
You don't race? Give it try
Steep and fast learning curve.
In other words, go down the middle of the course unless you have reason to do otherwise - this is what racers usually do too.
Agree that it is very helpful to have a means of calculating vmg, especially if you are unfamiliar with the boat or you've never bothered to check before. You'll quickly learn optimum close hauled angle.
You don't race? Give it try
Steep and fast learning curve.dt4134
Well-Known Member
On a trip like that I would consider the tides carefully. Obviously nothing is simple, but let's assume the wind was expected to be pretty steady and pretty even, if I were beating against the tide I'd favour staying north of the rhumb line to get less adverse tide, if the tide were with me I'd stay further south to get more tide pushing down channel. And I'd plan to transition between one side and the other for when the tide changes.
However, on a trip of that length the wind is almost certainly going to change. Imagine if you expect it to back as a front starts to approach, you want to ensure you don't get trapped too far north. However even that will depend upon whether or not you expect the front to arrive before you reach your destination.
If it were light airs and a sunny summer day I'd be more tempted to follow the coast around for the sea breeze (however beware of the sea breeze disappearing after dusk and being left in a hole).
In short, there's lots of factors and you're trying to juggle them all and come up with a best guess. Racers don't call the navigator/tactician a naviguesser for nothing.
Cornishman
Well-Known Member
I guess Tim Bartlett is too modest to put in an appearance so let me recommend his book The RYA Navigation Manual here. Pages 137 to 140 will give you all you need to know on this subject in a clear and concise way.
oldsaltoz
Well-Known Member
.
All very good advice, however, I have won more races studying the wind shift, tide flow and VMG (Velocity Made Good).
I once sailed an extra 4 Nm with only 6 to the finish line located between two islands, 'position' took the short road and we finished 5 boat lengths ahead.
They sailed between tow islands to get to the line, we sailed outside the first and got less tide and more consistent wind. Hamilton Island race week.
All very good advice, however, I have won more races studying the wind shift, tide flow and VMG (Velocity Made Good).
I once sailed an extra 4 Nm with only 6 to the finish line located between two islands, 'position' took the short road and we finished 5 boat lengths ahead.
They sailed between tow islands to get to the line, we sailed outside the first and got less tide and more consistent wind. Hamilton Island race week.
Seagreen
Well-Known Member
I may actually have it on my shelf.
I don't think, from the nature of the replies, that there is any real formula, but then again, on second thoughts, how could there be?
Twister_Ken
Well-Known Member
One of my favourite quotations (from some Ancient Greek) is "The only constant is change". Except he said it in Greek I assume.
It applies to many things, including the wind. It won't be constant, therefore you need a general strategy for a long upwind leg (sail towards the expected new wind direction) accompanied by shorter term tactics so that you take the lifts, and tack on the knocks. Add in the tide (also not a constant) and you begin to see why identical boats can finish a trip hours apart, even if they started together.
Another quote, this time from my dad. "Use yer 'ead, son, it'll save yer legs." Never truer than when going upwind.
And as my 'owner' used to say. "Sail well offwind and you save minutes. Sail well upwind and you save hours."
It applies to many things, including the wind. It won't be constant, therefore you need a general strategy for a long upwind leg (sail towards the expected new wind direction) accompanied by shorter term tactics so that you take the lifts, and tack on the knocks. Add in the tide (also not a constant) and you begin to see why identical boats can finish a trip hours apart, even if they started together.
Another quote, this time from my dad. "Use yer 'ead, son, it'll save yer legs." Never truer than when going upwind.
And as my 'owner' used to say. "Sail well offwind and you save minutes. Sail well upwind and you save hours."
