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Old 22 February 2008, 09:08   #1
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Compass navigation

I'm a bit confused. I read that a yacht that takes 18 hours to cross the channel following a set compass direction will be closer to its destination than a RIB that only takes 3 hours. Apparently the yacht does not get blown off course (wind or current) as much as a RIB.

Can somebody explain this phenomena to me please?
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Old 22 February 2008, 10:55   #2
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I'm not sure I buy that premise. It would be interesting to know what wind strength they're basing the idea on - it can't be too strong or the RIB wouldn't be able to do the trip in 3 hours. I think I'm right (but I'm not anywhere I can check this, so I'm relying on a very fallible and ageing memory that someone might well correct for me -- be gentle, please!) that a 10 degree deviation off course for an hour = one mile, so the yacht would be 18 miles off course at the end of it's journey by leeway alone. The RIB would have to be blown off course by 60 degrees to be as far off course over the same distance covered in 3 hours. And that doesn't account at all for the tidal effect. OK, over 3 hours the RIB is likely to be effected by a tide going in one direction only (or over a fairly slack period) and the yacht over 18 hours will be moved first one way then the other so the movement will cancel itself out a bit. Overall - it doesn't feel at all right to me, and I'd take some convincing
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Old 22 February 2008, 11:04   #3
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Variables!

Is the model based on the yacht sailing 'close hauled'? If so, a modern cruising yacht with a deep fin keel would expect 5-10 degrees leeway in a slight seaway and quite alot more in moderate/rough conditions If said yacht was running before the wind, the leeway would be minimal.

More questions than answers!
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Old 22 February 2008, 11:04   #4
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I suppose if you're going to take 18 hrs in the yacht then the tide will carry you almost as far east as west.... As for the wind unless it's dead astern then it's going to depend on the rib, the yacht , the strength of the wind, wheter the yacht is sailing or motioring......

In theory with the right wind & tide conditions then yes, it probably is true, but it is a bit of a daft comparison, because anyone doing a simple "follow X degrees heading" over that distance probably needs a lesson in naviation anyway.....

Even over short distances (both RIB and Dinghy) the wind & tide are big players if they are strong enough. I've steered a 5 degree upwind course in my SR4 when doing a 45degree to windward course before in something like a force 4-5 - and that was just on a Rothsay - Largs leg. (Clyde). I've also sailed my laser in a beating position to reach a mark that was on paper a broad reach. There was quite a tide flowing!
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Old 22 February 2008, 11:17   #5
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erm.....?

9D

On a point of pedantry!....... any yachty doing this length of journey would imediately realise that 18 hours of passage time would include about 3 tides of 6 hours each. If he/she were travelling north to south and tide was running east/west/east, (as in the English Chanel) then they would calculate the overall tidal effect east/west over that 18 hour period i.e. in this example the net result would be 6 hours east.

If in that 6 hours of east going tide the boat will be taken 10 miles off course to the east, then the navigator should give the helm a compass course to steer that takes this into account.

I have basically described whats called 'Course to Steer' (CTS) and following that compass course as accurately as possible, will be the quickest way of making that passage.
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Old 22 February 2008, 12:56   #6
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Originally Posted by Pablo View Post
I'm a bit confused. I read that a yacht that takes 18 hours to cross the channel following a set compass direction will be closer to its destination than a RIB that only takes 3 hours. Apparently the yacht does not get blown off course (wind or current) as much as a RIB.

Can somebody explain this phenomena to me please?
Completely depends on the wind direction and conditions. For a RIB and yacht going in the same direction and same speed with wind beam on a RIB would probably be blown more sideways simply because the yacht has a keel to prevent this. Wind just off the nose (45 degrees) the RIB will prob be less affected because of the windage of the yacht. This is assuming the yacht is motoring. If yacht is sailing and RIB is doing 25 knts, the effect of the wind on the RIB will be proportionally less because of the speed - I do not imagine a RIB "in the groove" will have much sideward motion due to wind. With a yacht beating, the CTS is much harder to calculate unless they are beating directly on course (and therfore a RIB has much higher chance of hitting a tgt)

Tide is compeltely different and again it's down to speed. Cross tide, the faster the boat, the less effect. Tide in front of you and you have a massive advantage in a RIB.. Tide just off the bow on a yacht and it'll get shifted a fair bit (I've been racing in the clyde with spinnaker up going backwards)

So - true CTS I would say a RIB has a much higher chance of hitting the target, mainly based on the speed it goes at.

As mentioned before, it all comes down to good navigation - knowing the effects of the tide and likely drift for your boat. GPS is of course very good at confirming an estimate on tidal effect and it's nice to see when they are spot on. A couple of years back I passage planned a 60 mile course in a yacht on the west cost to get us to a very tidal set of narrows at slack water - I was 10 mins out which is realy satisfying when you've done all the sums with tidal flow etc :-)
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Old 22 February 2008, 13:00   #7
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Forgot to add....

A modern yacht under bare poles with the rudder centred will pretty much sit beam onto the wind if "let go". My dad's yacht broke it's mooring a while back and ended up in a pretty accurate line between the mooring and the beach which was 90 degrees to the wind direction.

I know my RIB sits side on as well but haven't looked at it;s relative drift. I think I'll try that next time I'm out in any wind as it's an interesting question
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Old 22 February 2008, 18:19   #8
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I think I saw something in a magazine article to this effect recently. I will try to fish it out of my "archive" (i.e. stack in the corner!). Not sure if it was 18 hours or not, or even mentioned wind. You will gather I didn't take much in

My limited recollection/understanding was that the tides would cancel themselves out much more on a long trip than a short one - and I think the message they were trying to get across was that tidal streams running across your intended route DO matter to fast powerboats - i.e. tidal calculations are not just for yachties...
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Old 23 February 2008, 04:30   #9
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If it were 12 hour crossing the tides would be going in one direction for 6 hours and 6 hours in the opposite direction and therefore cancel each other out.
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Old 23 February 2008, 04:49   #10
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Quote:
Originally Posted by Pablo View Post
Apparently the yacht does not get blown off course (wind or current) as much as a RIB.

Can somebody explain this phenomena to me please?
Hypothetical rubbish, there are so many other variables that it just provokes 100 different opinions - if a RIB and a yacht crossed the channel, the likelihood of either reaching their target is down to the quality of the navigation - BOTH would be affected in different ways by the wind and the tide!
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Old 24 February 2008, 04:31   #11
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Hypothetical rubbish, there are so many other variables that it just provokes 100 different opinions - if a RIB and a yacht crossed the channel, the likelihood of either reaching their target is down to the quality of the navigation - BOTH would be affected in different ways by the wind and the tide!
Cookee - you are right that it is all hypothetical pondering - however I think the point you make is exactly the point that the article I read (I think it must be in a PBO I leant to someone as I can't find it) was trying to make --taking into account the effect of tide (and wind) is not purely the preserve of slow sailing yachtsmen.

Quote:
Originally Posted by BassBoy
On a point of pedantry!....... any yachty doing this length of journey would imediately realise that 18 hours of passage time would include about 3 tides of 6 hours each. If he/she were travelling north to south and tide was running east/west/east, (as in the English Chanel) then they would calculate the overall tidal effect east/west over that 18 hour period i.e. in this example the net result would be 6 hours east.
But that assumes you are leaving exactly when the tide turns. If you leave right in the middle of the tide then you get:

3 hours E
6 hours W
6 hours E
3 hours W

which will actually mean that in this hypothetical and theoretical situation the yacht can ignore tide when working out his course to steer (assuming he can make the whole trip in one straight line)
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Old 24 February 2008, 09:52   #12
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But that assumes you are leaving exactly when the tide turns. If you leave right in the middle of the tide then you get:

3 hours E
6 hours W
6 hours E
3 hours W

which will actually mean that in this hypothetical and theoretical situation the yacht can ignore tide when working out his course to steer (assuming he can make the whole trip in one straight line)
only if you assume the tide is constant across the Channel, which it is not. (i.e. tides of cherbourg peninsula considreably stronger than tides on S Coast England.
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Old 24 February 2008, 13:22   #13
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Tidal vectors

OK, so it's come down to the nitty gritty of course to steer! We'v gone from the hypothetic to reality.

Get out your tidal stream atlas, look at each hour of tide and your estimated progress across the channel each hour. As Doug says, this will vary every hour. Make a note of each hours tides direction and speed. Plot each hour of that tide on your paper chart as a 'tidal vector' (don't forget your 3 chevrons if you want to be proper about it). Lay your compass course from the end of your tidal vectors to your destination. You are now much closer to an accurate course to steer. Don't forget your leeway.... most people use 5-10 degrees dependant on your point of sail or........ the 'apparent' wind direction on your RHIB for the duration of the passage. Don't forget to get the best/most updodate weather info you can for the 18 hour passage, and don't just rely on shipping or inshore forecasts, look at the synoptic/ surface pressure predictions and make up your own mind!
Don't forget 'Deviation' either, but then I guess there aren't many leisure ribsters that have a swung compass and a deviation card!


Alternatively, get a good book/DVD/training course and get your head round it that way!
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Old 24 February 2008, 13:22   #14
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Variation

Forgot to mention magnetic variation! Don't forget that either!!!!!!!!!!!!!!!!!!!!
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Old 24 February 2008, 16:51   #15
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Quote:
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Lay your compass course from the end of your tidal vectors to your destination.
Almost, but not quite right. You should lay off your course to a point along your ground track equivalent to the distance you would travel during the time you have used to calculate your tidal vectors.

But hey, let's not split hairs. That degree of accuracy gives you a course to steer that you could only steer with an auto-pilot anyway

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Old 25 February 2008, 01:12   #16
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zzzzzzzzzzzzzzzzz

SS

Yup.

BB

Like you, I'm nodding off with this thread but hope it's been useful nontheless.
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Old 25 February 2008, 07:01   #17
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9D

On a point of pedantry!....... any yachty doing this length of journey
Errr
Pablo's original post just said "crossing the channel". If we're going to get pedantic, crossing the channel Plymouth to Roscoff is a slightly longer journey than Dover - Calais.

The original post also said (and I quote) "following a set compass direction".

My other post was basically pointing out that anyone who blindly follows a compass heading on a long (or short, depending on the weather) leg is not going to find themselves where they thought they would, unless they did a lot of maths first.

And having done the maths for one boat, it's going to be totally different for another - probably even if you compare two "random" 6m ribs!
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Old 25 February 2008, 09:12   #18
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I very much enjoyed reading all your messages, thank you.

I now have another question which will sort the seamen from the seaboys (or the inconscient from the safety aware). You've just come past the Needles on your way to Alderney and your GPS packs up, what do you do?

Do you :
A) Get your pocket GPS ou and put in your new way point
B) You've got everything on board to plan your route and you continue with your compas
C) Your GPS was only a backup and you where doing everything with a compass anyway
D) You turn around and go home quick before you lose sight of the coast

At my present state of experience and equipement I would probably choose option D but i'm interested in your replies.
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Old 25 February 2008, 11:27   #19
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....Get out your tidal stream atlas, look at each hour of tide and your estimated progress across the channel each hour. As Doug says, this will vary every hour. Make a note of each hours tides direction and speed. Plot each hour of that tide on your paper chart as a 'tidal vector' (don't forget your 3 chevrons if you want to be proper about it). Lay your compass course from the end of your tidal vectors to your destination. You are now much closer to an accurate course to steer. Don't forget your leeway.... most people use 5-10 degrees dependant on your point of sail or........ the 'apparent' wind direction on your RHIB for the duration of the passage. Don't forget to get the best/most updodate weather info you can for the 18 hour passage, and don't just rely on shipping or inshore forecasts, look at the synoptic/ surface pressure predictions and make up your own mind!
Don't forget 'Deviation' either, but then I guess there aren't many leisure ribsters that have a swung compass and a deviation card!


Alternatively, get a good book/DVD/training course and get your head round it that way!
Feck that. Isn't this why you have a rib?
Quote:
OK, so it's come down to the nitty gritty of course to steer! We'v gone from the hypothetic to reality.
That's better. Which way's the tide goin...west? That'll be a few miles off then. Which way's the wind blowin'...westerly F3? That'll be a few miles the other way. Just drive straight across. Given that it's only gonna take about 2minutes to correct a mile and I betcha a fiver you can't drive a straight line when the winds blowing, why bother with the hassle.
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Old 25 February 2008, 12:25   #20
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Seaboy!

Pablo

All things being equal, I would press on using 'dead reckoning' but might make the decision to put into Cherbourg as it's a safer approach (commercial shipping asside). A few miles out going to Alderney (Bray Harbour)can mean the difference between the Swinge or the Alderney race!

IF however the viz was very bad, I'd head back to Yarmouth!
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