Ferry glides and long crossing

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MikeB
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Ferry glides and long crossing

Post by MikeB » Wed Feb 23, 2005 10:18 pm

Laurie Ford (renowned Antipodean paddler, renowned for forthright views) suggests here http://members.iinet.net.au/~lford1/fools.htm that using a ferry glide on a long crossing is foolish.

Discuss.

Also refer Ferrero's work on Sea Navigation / BCU handbook / Hutchinson et al.

Mike.

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Post by Steve B » Wed Feb 23, 2005 10:26 pm

He's absolutely correct, right angles to the current is the fastest/least effort. It won't always take you where you want to go, obviously.
Steve Balcombe

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Zoe Newsam
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OOOoooooh, controversial!

Post by Zoe Newsam » Wed Feb 23, 2005 10:38 pm

Well, I'll skip over the stuff he says about 'shouldn't have been out there in the first place'... grumble grumble grumble. Lots of Grumpy Old Man stuff there.

But his point about ferry glides is spot on. If you're doing a long open crossing that is subject to significant tidal streams, particularly to a small-ish island, you cannot possibly calculate an accurate enough bearing to be able to get the ferry glide right and land on target. You'll also wear yourself out and bcome frustrated at constantly paddling into the stream. If, however, you do half the crossing before slack water and half after, and always point at 90 degrees to the current, you will end up back on the same bearing as you started. Simple and reliable. It'll feel disconcerting because most of the time you'll feel off course, but it does work and the maths is simple!

The situation where this needs tweaking is when a beam wind comes into play- a whole different ball game!

Zoe

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Geoff Seddon
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Post by Geoff Seddon » Thu Feb 24, 2005 12:55 am

Apologies, just returned from the pub, or I wouldn't be posting at all, but something doesn't seem quite right here. Help me out Jim. Ferry gliding has got to be good, although I accept that using an equal push, first left then right, whilst paddling the same bearing does the cancelly thing and gets you there. But this isn't always an option. Surely the point is that there is quite a lot of stuff occuring which you have to be aware of. That's the art of navigation- getting where you want to go with the least effort. Maybe that's a ferryglide, maybe not. The old joke re a chappie asking for directions and being told "If I were going there I wouldn't start from here" springs to mind, but I'm not sure in what context. Never give an old bloke beer.

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Post by RichardCree » Thu Feb 24, 2005 8:14 am

Hi Zoe, your ideas are grea as long as the ingoing and outgoing streams are the same speed (which is not always the case)

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Douglas Wilcox
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Example in practice!

Post by Douglas Wilcox » Thu Feb 24, 2005 9:47 am

Hi Zoe here is a worked example: we wanted to paddle across the Sound of Jura to Tarbert on Jura. Our departure time was 1 hour after a spring high tide. In the SoJ most of the ebb (to the SW) takes place in the first 3 hours after HW and the spring rate is about 4 knots. The shortest crossing to Tarbert is 7.5km from Keilmore but that would have involved ferrying into the current. So we went from Carsaig Bay a 15 km crossing from upstream so our ferrying would be with the current.

After leaving Eilean nan Coinean, we headed on a bearing just south of west, well upstream of Tarbert. The photo shows a white house at the point at Tarbert on the right hand edge of the photo, Jennifer's boat gives some idea of our ferry angle well upstream of the house. I calculated a landfall about 1 km north east of Tarbert as I did not want to overshoot and end up having to paddle back against the tide.

As you can see, the calculations were just about perfect.

Interestingly I had set tarbert as a waypoint in my GPS and had the GOTO display on the unit during the crossing. This shows the direct bearing to the waypoint which we were obviously not following. However as the GOTO bearing remained constant, I knew that our ferry angle and paddling speed were correct. The GPS also shows the distance to the waypoint and the hours till arrival based on your velocity over the ground since starting the GOTO.

The rate of the ebb was far from constant across the SOJ. Sometimes the time till arrival got shorter very quickly sometimes it and the distance to the waypoint even increased, when we came across an eddy going against the general flow. Sometimes these eddies were visible, sometimes not but the GPS GOTO screen showed them straight away.

Note on the way back (with the flood this time), how the tide accelerated very greatly about 1.5 km off the SE shore carrying us off course.

My conclusion is that ferrying works very well indeed but I would prefer not to ferry into the current on a long crossing. Much better to plan the tides.

Although not essential, I find a GPS is a very useful addition to traditional navigation tools like watch, map and compass.

Sometimes you have to paddle into the current but I am so lazy I only like doing this for short crossings.

On another occasion, we missed the slack in the Grey Dogs between Jura and Lunga so we eddy hopped up the South shore until the water began to slope towards us. In mid channel the Dogs were running like a river with some nasty looking water downstream near the island so we ferried across about 45degrees into the current, but this was just for a short burst.

Douglas :o)
Last edited by Douglas Wilcox on Thu Feb 24, 2005 11:22 am, edited 1 time in total.

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ChrisS
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Post by ChrisS » Thu Feb 24, 2005 9:55 am

Laurie Ford makes a thought provoking point. He seems to have in mind a particular incident on a crossing which he knows well. But applied generally the statement "sea kayakers ferrygliding across currents are crazy" is perhaps expressing it a tad forcefully.

"To ferryglide for 4 hours across a current that is as fast as you can paddle is not on. Should you attempt this then you will end up sitting out in mid stream not going anywhere - apart from more or less paddling straight into the current to try and stay on the straight line between the two points you are crossing to and from. "

edit. ...is right. As Bertie will point out you need to be paddling faster than the current.
Last edited by ChrisS on Thu Feb 24, 2005 2:35 pm, edited 1 time in total.

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Zoe Newsam
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Post by Zoe Newsam » Thu Feb 24, 2005 1:13 pm

I think Laurie Ford was referring to a simple crossing between islands with a mono-directional flow (unlike the Sound of Jura which is much more complex), where the destination is not in sight at the start of the crossing, meaning the ferry angle or use of the tide has to be calculated. If the speeds of flood & ebb are different, obviously you calculate & allow for that.

Douglas says:
My conclusion is that ferrying works very well indeed but I would prefer not to ferry into the current on a long crossing. Much better to plan the tides.
Surely, point proven. Ferrying into the stream is fine for short-ish distances, but exhausting for longer ones. Better to work out what the tide will do & work with it rather than fight it. Over a several-hour crossing (not the one & a half to two it takes to cross the sound of Jura), the effect will average out. Obviously my example was extremely simplified, and the basic art of navigating & using a pilot book or chart has to be brought into play. But surely that's where experience comes in??

Phew. Rant over!
Zoe ;0)

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Jim
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Post by Jim » Thu Feb 24, 2005 1:24 pm

Obviously it depends on what you are crossing.

There is a strong argument that if you have to ferry your planning wasn't right, but again it can not ALWAYS be the case.

For a really long crossing you can use the tide cycle to go both ways. Where your target is large and has an eddy or eddies against the direction of the stream it may be easier to go at right angles (or a shallow ferry that will lose some ground) because once you hit the eddy you wil be going much faster. Equally if there is an eddy on the nearside that means you can go upstream first with little effort that will help.
Where your target is small I think I would want to be very certain that my transits or COG were good enough to allow me to hit it.
If Phil Winch plans the crossing, more than likely the tide stream goes the same way as you at the time you are crossing so you just ride it!

But the one factor you have all ignored is the wind. I have done a number of crossings where ferrying againt the wind was more important than ferrying against the current. I'm not good at getting all my gear inside or at trimming the boat evenly so my boat will generally weathercock one way or the other and over a long crossing the angle that means I don't have to fight it is always good!

So yes paddling 4 hours against a tide stream is silly if you can have an extended lunch and go straight accross either side of slack water, I bet more often than not we find other constraints mean it's never that simple (darkness, weather closing in, only slack for 15 minutes etc.)

JIM

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Zoe Newsam
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Wind!

Post by Zoe Newsam » Thu Feb 24, 2005 1:32 pm

As I said above, wind in the equation means a whole different ball game. But if you're at 90 deg to the flow and use the triangle of vectors method to factor in the wind you're much more likely to get it right than if you're also calculating a ferry angle...

BTW Douglas I've just realised that your Jura example is a perfect display of both methods. Although it looks like a ferry glide at first you were paddling at roughly 90 deg to the flow & being carried downstream to your destination. ;0)

Zoe

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ChrisS
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Post by ChrisS » Thu Feb 24, 2005 1:49 pm

I agree that ferrying against the flow feels tiring. But why? If you keep to your usual paddling pace it should just mean that you are paddling for a greater distance. That should not be particularly tiring. Perhaps we try, even subconsciously, to paddle harder to overcome the feeling of losing ground and that is what tires us.

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Post by Bertie.. » Thu Feb 24, 2005 1:59 pm

surely this guy's arguement is that if you attempt to ferryglide accross a current that is moving as fast as you can paddle in a straightline AND try to chose a course over ground (COG) that is 90 degrees to the current THEN any attempt to remain on the COG will end with you paddling directly into the current.

Isn't this basic maths? I.e. if you want to maintain your lateral position in a current flowing at the fastest speed you can paddle at then any off-set angle (i.e. a ferry glide) will result in you drifting down stream.

For the same reason, a ferry glide across a flow that's faster than you can paddle will result in a COG which runs diagonally downstream.

or have I missed the point of the discussion (dangers of speed reading in lunchtime..)

Bertie..

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ChrisS
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Post by ChrisS » Thu Feb 24, 2005 2:29 pm

Thanks Bertie. That has got to be right. Very glad I discovered this on dry land in a warm office!

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Post by Bertie.. » Thu Feb 24, 2005 2:35 pm

thanks, thought I'd missed the point and was waiting to be put right! Instead a pleasant surprise...

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Douglas Wilcox
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Effect of wind

Post by Douglas Wilcox » Thu Feb 24, 2005 3:01 pm

Zoe>wind in the equation means a whole different ball game

Yes I know, its quite difficult when sailing up wind across a channel with the tide carrying you down stream. Knowing when to tack is a bit of an artform as the wind is never from the same direction all across a channel.

Here is another kayaking example Zoe.

When we left Torsa for the headland to the North of Loch Melfort (short crossing I know) the tide was in full flood up the Sound of Seil (3hrs after LW) and a force 6 NE wind was coming down the sound, so the water conditions were a little rough. If you look at our intial GPS track you will see we had undercompensated for the effect of the wind. There is then a sharp change in the angle of our course as we realised this and "ferried" more into the wind. Then when we began to get into the windshadow of the shore you can see our track headed into the wind too much.

Its quite interesting looking back at a GPS track and remembering what navigational decisions you made at the time.

Douglas :o)

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Zoe Newsam
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Post by Zoe Newsam » Thu Feb 24, 2005 3:28 pm

Thanks Bertie, spot on, just far more concise than me!

And ChrisS, I reckon you're probably right, we do try to make headway into the stream which is what feels tiring.

Zoe

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Post by Bertie.. » Thu Feb 24, 2005 3:50 pm

zoenewsam wrote:And ChrisS, I reckon you're probably right, we do try to make headway into the stream which is what feels tiring.
In the past I've run little experiments within my coaching sessions with the effect of paddling into headwinds, tidal flows etc. It would seem, albeit empirically, that paddling into head winds, tidal flows, etc will result in the paddler putting more effort in, achieving a faster speed, only to tire themselves out. ChirsS, this would seem to support your comments.

Bertie..

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Post by sub5rider » Thu Feb 24, 2005 4:38 pm

Bertie.. wrote:..... albeit empirically, that paddling into head winds, tidal flows, etc will result in the paddler putting more effort in, achieving a faster speed, only to tire themselves out.....Bertie..
I support this hypothesis, even from my limited experience. Head-down, slug it out, want to get it over with, etc. Hasving a "fat" boat doesn't help either.

All in favour of properly planned ferry glides when the destination can be seen - even better if GPS is used to adjust angle to get correct cog.

Nigel (sans beers)

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ChrisS
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Post by ChrisS » Thu Feb 24, 2005 9:50 pm

If you do find yourself ferrying against the current and unable to keep to the planned COG it seems your optimum course remains your planned Course to Steer. You won't reach your objective but you will get closer to it than you will taking any other course. I think.

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Post by Gavin74 » Fri Feb 25, 2005 7:36 am

Sorry newbie here................what the hell is COG!! Sorry again.

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sub5rider
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Post by sub5rider » Fri Feb 25, 2005 7:59 am

Gavin74 wrote:..........what the hell is COG!!
Course over ground. "Course made good" is the same thing.
In theory it's the resultant straight-line track achieved between two points when all the wind & current variables have been factored in calculating a heading bearing.

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Post by Gavin74 » Fri Feb 25, 2005 8:01 am

Thanks Sub.......I'll re-read the post above and see if they make sense second time around.

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Post by Bertie.. » Fri Feb 25, 2005 8:21 am

sub5rider wrote:
Gavin74 wrote:..........what the hell is COG!!
Course over ground. "Course made good" is the same thing.
In theory it's the resultant straight-line track achieved between two points when all the wind & current variables have been factored in calculating a heading bearing.
Alternatively, in practice, it's the imaginary snails trail over the flat sea bottom.. which may or may not be a straight line, dependent upon how you got from A to B (e.g. if you have to correct on route as is the case trying to get back on a transit if yu've lost it)

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