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Old 02-08-2012, 05:39   #1
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Navigation - Help

Hi

I race in the same area which is influenced by tides and back eddies. Also different at springs and neaps. I want a method of recording the tidal speed when racing on a log - say every 1 minute preferably using a smart phone with compass and GPS - If not then a custom instrument.

I want take the data at the end of a sail and create an hourly tidal chart.

Any ideas? Any ideas of another forum which could help?

Thanks
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Old 02-08-2012, 05:53   #2
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Re: Navigation - Help

Interesting question. As a cruiser with fairly basic electronics I have no idea. There might be some expensive, integrated nav system that could generate this data but as I think about the calculations involved it sounds potentially very complex.

First question, how will you determine the speed and direction of the currents while you are underway? It would require knowing your exact speed and COG, easily obtained from GPS. But then you have to know your exact boat speed through the water and exact course through the water and the leeway of the boat through the water which will vary depending on the point of sail and wind speed.

Then once you have the speed and course OG you can generate a vector for that then subtract the vector for boat speed and course through the water to obtain the difference between the two which should be the effect of currents, unless there are other forces effecting the course I have neglected.
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Old 02-08-2012, 06:04   #3
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Re: Navigation - Help

An accurate log (electronic, or even towable) to give speed through the water, a hand-bearing compass to establish your leeway at various windstrengths at slack tide by taking 'back bearings', a piece of thin rigid plastic to lay over the chart, a protractor with a swinging arm, and a chinagraph pencil to draw vector triangles.

That way you'll know how it's done, and that it's done right, and you'll have the satisfaction of knowing you could do it all again.

I venture it would also be easier than researching to find and assemble the 'custom instruments' which you first have to assume even exist, and work the hours to pay for them.
And buy the new improved upgrades in perpetuity...

What's more (if it was me) I'd want to do the diagrams myself the first few times anyway, to keep it honest.
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Old 02-08-2012, 06:09   #4
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Re: Navigation - Help

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Originally Posted by skipmac View Post
Interesting question. As a cruiser with fairly basic electronics I have no idea. There might be some expensive, integrated nav system that could generate this data but as I think about the calculations involved it sounds potentially very complex.

First question, how will you determine the speed and direction of the currents while you are underway? It would require knowing your exact speed and COG, easily obtained from GPS. But then you have to know your exact boat speed through the water and exact course through the water and the leeway of the boat through the water which will vary depending on the point of sail and wind speed.

Then once you have the speed and course OG you can generate a vector for that then subtract the vector for boat speed and course through the water to obtain the difference between the two which should be the effect of currents, unless there are other forces effecting the course I have neglected.
I believe the latest nav systems (not mine!) will calculate this. They do it on the basis of boat speed and heading versus SOG and COG, which is not that complex. I guess this will not account for leeway but the data are probably usable anyway.

I bet one of the PC systems will log it for you, too.

Alernatively, almost any electronic nav system will give you COG, SOG, heading, and boat speed. You can log all of this every ten minutes (say) together with time and position, and make the calculations at home.

But this will tell you current set (ignoring leeway) at a given position and given time only. You would have to record a huge mass of data in order to make your own tide chart -- you would have to sail your route over and over and over again to hit every state of tide. I think it would be a fairly vast project. I can't imagine how it would be worth it compared to just using a commercial tidal atlas.
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Old 02-08-2012, 06:27   #5
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Re: Navigation - Help

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An accurate log (electronic, or even towable) to give speed through the water, a hand-bearing compass to establish your leeway at various windstrengths at slack tide by taking 'back bearings', a piece of thin rigid plastic to lay over the chart, a protractor with a swinging arm, and a chinagraph pencil to draw vector triangles.

That way you'll know how it's done, and that it's done right, and you'll have the satisfaction of knowing you could do it all again.
That would certainly work but you would be pretty busy using this procedure to generate a data point every 60 seconds.
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Old 02-08-2012, 06:28   #6
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Re: Navigation - Help

Greetings and welcome aboard the CF, chrisleigh.
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Old 02-08-2012, 07:15   #7
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Re: Navigation - Help

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But this will tell you current set (ignoring leeway) at a given position and given time only. You would have to record a huge mass of data in order to make your own tide chart -- you would have to sail your route over and over and over again to hit every state of tide. I think it would be a fairly vast project. I can't imagine how it would be worth it compared to just using a commercial tidal atlas.
It does sound like a huge amount of work to generate a complete set of current charts for every state of tide. But remember this guy is racing. You know, the sailors that spend a few grand extra for titanium and carbon fiber fittings to save a couple of lbs of weight in the boat. Knowing all the little eddies on the course could save them a few nanoseconds and put them in the gold.
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Old 02-08-2012, 10:22   #8
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Re: Navigation - Help

If the seabed topography changes a lot, as it does in places with a lot of currents, you'll never come up with an eddy chart for it...but some way of mapping the seafloor and measuring the actual current might have a future. How about your very own drone aircraft...never heard of a racing rule against RPV's for scouting sea and wind conditions. Or dropping JDAMS on competitors. Ooops, that's out of bounds. I think. Hmmmm.
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Old 02-08-2012, 15:07   #9
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Re: Navigation - Help

Ok, some more information
No carbon fiber on our boats. The design is 120 years old and all wood! 21 ft.

The racing is in a small area of about 10 sq miles so no tidal maps available. There is a rocky bay on one side so that is where the eddies are formed.

Yes a small amount of speed makes a massive amount of difference. Over 1 hour 0.1 knott of speed makes for 680 feet of distance.

It's not practical to take hand bearings.

I was kind of thinking that knowing the compass direction on the phone and the direction the boat is travelling from the gps plus estimating the leeway from the direction of the wind direction between tacks..... Got lots of data there to calculate tide. Maybe you have to have a manual log but doing it all through a smart phone would be great.

For cruising it would be good to immediately know you had entered a tidal flow against or tidal flow with you. I guess a couple of knots tide would make a lot of difference.

Thanks for feedback so far
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Old 02-08-2012, 15:18   #10
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Re: Navigation - Help

If it's rocky, then maybe the eddies will be steddie. Heheh. Ahem.

In that case, how about seeding the area with well placed anchored floats...obscure little ones, with enough rode to indicate the direction of current. If you had the time, you could make the rounds and check them in turn and get fairly accurate data on flow rate and direction. The folks with props will be suitably thrilled...I still think the drone might be a goer. Or, just lots of time on that patch of water, and detailed knowledge of what's down there. Got access to a good sonar?
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