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chrisleigh 08-02-2012 08:37 AM

Navigation Problem

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?


wingNwing 08-02-2012 08:54 AM

Re: Navigation Problem
If you've got a smartphone there are apps that will show you this, try "charts and tides" $15 I think at the app store

hellosailor 08-02-2012 01:13 PM

Re: Navigation Problem
"recording the tidal speed when racing on a log - say every 1 minute preferably using a smart phone "
Not so simple. If you want tidal current information (tides are vertical, currents are horizontal, tidal currents are the horizontal movements caused by tide changes) you will need speed through the water, as measured by a knotmeter on the hull, plus speed over ground, which is the only number a smartphone with gps can give you.
So you're going to need instruments on the boat, not just software, to generate that information. I'd be surprised if the final price was attractive.

AdamLein 08-02-2012 01:43 PM

Re: Navigation Problem
HS is right. If you have a paddlewheel speedometer already, then maybe you can go out some time when you're not racing and sail different courses in various tidal conditions, and put the data together yourself.

Note, however, that

velocity of vessel over ground =
velocity of vessel over water +
velocity of water over ground.

Each of these three quantities has a magnitude and a direction. Your GPS will tell you the magnitude and direction of the first quantity. Your paddlewheel will tell you the magnitude, but not the direction, of the second quantity; that direction is determined not just by your heading but also by your leeway.

Here's an idea: is the water shallow enough for anchoring? Find a windless day and anchor out. Then your paddlewheel will tell you the drift, and your boat's average heading as it swings to the current will tell you the set (or rather, its reciprocal).

Alternatively if it's too deep to anchor you can try motoring into the current so that your GPS tells you you aren't going anywhere.

Sounds like a fun experiment!

WDS123 08-02-2012 01:45 PM

Re: Navigation Problem
be careful at getting too precise - you'd be surprised how much tides can be influenced by rain.

Is this just for beercans ?

BarryL 08-02-2012 03:25 PM

Re: Navigation Problem
[QUOTE=WDS123;904507]be careful at getting too precise - you'd be surprised how much tides can be influenced by rain./QUOTE]

And WIND!! Normally the currents in the Long Island Sound are very predictable. But add a 10k wind from the easy or west and the current will be doubled or cut to about 0.


WDS123 08-02-2012 04:42 PM

Re: Navigation Problem
Better to spend the time and energy making sure the Beer is precisely the right temp :)

chrisleigh 08-02-2012 07:40 PM

Re: Navigation Problem

Some good replies.... No workable solution I can see.

Smartphone would give me SOG, magnetic direction of boat and comparing consecutive gps positions would give me direction of travel. Dividing the beat angles by 2 should get the wind direction. Surely this lot should give me a good set of data to Sussex current out

It is the tidal currents I am trying to find as opposed to tides.

Thanks in advance

AdamLein 08-02-2012 09:13 PM

Re: Navigation Problem
Wind direction doesn't help you.

Here's an idea:

Mark waypoint A on your GPS. Sail on a constant heading for a fixed length t of time. Mark waypoint B. Let C be the place you would have gotten to if there was no current or leeway. The vector from C to B will give you the combined effect of current and leeway.

For example, suppose you sail due east for one minute at six knots. Had there been no current, you would have ended up 0.1 nmi due east of your starting position. But let's say you measure your new position (B) and it's 0.112 nmi east and 0.012 nmi north of your station position (A).

Let's assume there's no wind and you're motoring. That means the current pushed you an extra 0.012 nmi east and 0.012 nmi north in an interval of one minute. In an hour, it would have pushed you sixty times that, or 0.707 nmi east and 0.707 nmi north. By Pythagoras, it would have pushed you 1 nmi NE in on hour, yielding a set of NE and a drift of 1 knot.

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