hlb
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Reged: 16/05/2001
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Theres just been a post about altitude.
Anyway Debs was saying a minute or two ago.
We must go twice as far as the chart says, going up and down all the time. So how much difference does it make. The wibbly wobbly way,has to be nearly twice as far as dead flat.
Is this why the GPS never agrees with the log, even more than the tidal situation. Or does it compensate for it in some way.
It's a gludy sort of question, sorry.
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rich
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Reged: 07/06/2001
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Loc: Jersey
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Think about it Hayden, the gps will give you the speed calc: between a & b ,doesn't matter how big the waves are 
PS I think thats wot I was tryin to say.
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hlb
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Well yep, I know all that. But debs theory is, cos we are going up and down all the time. The distance has to be twice as far. It's not a straight line. I understand what she's asking. Zoom up one wave, down the other doing 20 knots. Are you actually getting anyb where..... Speed over ground comes in to mind.
Still interesting to know. ow much does up and down come in to play.
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hlb
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It dont come in to play in YM ferinstanse. Up and down as debs says has to make a hughe diferense.
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hlb
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Well forget Debs. Lets say we are doing dead reaconing. speed over water, as opposed to spead over land. The lands not flat. Neither is the sea. So shoulld we knock off some of the log speed, cos most of it was just up and down.!
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sarabande
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Reged: 06/05/2005
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Loc: Thames valley, or up on the mo...
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never thought of it like that before, but I suppose that waves have an "uphill" bit above the average flat sea level, and also a "downhill" bit below the average flat level.
Second thoughts. If you get a piece of string a foot long and put lots of little waves in it, it doesn't measure 12 inches until you pull it straight again.
Maybe we should have a modification to the GPS which gives Distance Including Up and Down, as well.
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Enlightenment is motor-sailing
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Wiggo
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OK, I'm not up the maths for a sinusoidal wave, but if you assume triangular waves and...
you have wave crests 20m apart
you have waves 2m from trough to crest
you cross 10km of ground
Your distance through the water is 10198m
At 10m wavelength and 4m height, that goes up to 12806m
If I could do the maths, then a proper sinusoidal wave shape would give even longer distances through the water.
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BigNick
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Reged: 29/04/2003
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Waves are not really a sinusoidal, in practice, (he says boldly), so you are probably just as right, or wrong, as if you did the real sums.
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hlb
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So. When your doing your dazed kipper. Besides working out wind drift, sea drift. Should you not also work out, up and down drift as well, or all the sums will be out. Your not actually doing six knots, it's more like five or maybe four??
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Pete7
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Reged: 11/08/2004
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The maths sound a bit complicated especially at 20 knots up and down in big waves. Wouldn't the boats paddle wheel (speed or log) give you the actual distance travelled compared to the GPS?
Pete
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Moody 31
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Bod
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Reged: 10/11/2003
Posts: 11
Loc: South Coast
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Well its a slow day and I'm bored. Assuming a triangular wave I reckon:
distance actually travelled = DOG +(wavelength^2+waveheight^2)/(wavelngth)
So 20m between peaks and 2m height gives about an extra 0.5% of distance over ground, whereas a 3m height an extra 1.1% distance....
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Wiggo
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That was the point, I think.
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The nicest thing about not planning is that failure comes as a complete surprise, and is not preceded by a period of worry and doubt.
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hlb
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Reged: 16/05/2001
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So your saying, over say 100 miles, it only makes .5 or 1 mile difference?? dont seem much does it. would have thought much more than that. But what about swell we get loads down the SW, maybe going up and down 10ft.
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houghn
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Reged: 20/04/2004
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With a triangular wave, 20m between peaks, and 2m wave height, split the wave into two right angle triangles, hypotenuse is the root of the sum of other two sides squared, gives 10.2, so its 2% extra distance
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Wiggo
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OK, so that's 3m rather than 2, but the wavelength is huge, innit? Maybe 200m between wave crests, so still only 2%
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The nicest thing about not planning is that failure comes as a complete surprise, and is not preceded by a period of worry and doubt.
Nereid Charter
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Divemaster1
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Reged: 15/01/2002
Posts: 1494
Loc: Scotland
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You'll add more than 2% by course deviation I'd guess ..... as in up/down plus the zig/zag part ...
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Alf
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