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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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