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Old 26-01-2020, 10:50   #1
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Speed and Direction if current

Using my standard sail boat instruments ( chart plotter, wind, speed and direction indicator) how can I measure the actual speed and direction of the current?
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Old 26-01-2020, 11:02   #2
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Re: Speed and Direction if current

You don't specify the source of "speed" in your list, but it probably is derived from a GPS, meaning it is speed over ground, not through water. That means that you don't have speed though waterto compare with speed over ground, allowing you to isolate the combined effect of current and wind.

There has been considerable discussion on this forum of ways of obtaining speed through water for just the reasons you imply. Speedometers work by impellers, paddle wheel, pitot tube, and log, all of which are prone to fouling and poor calibration, and electronic sensors, which are pricey. But, in the general case, you plot your GPS movement versus what the speed and heading predicts, and derive a leg that represents current and wind. On the other hand, with a bit of experience with your boat, you may be able to say "at this apparent wind and tack I sould be making 4 knots, and I'm making 3 over the ground, and I'm falling off more than expected, so..."
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Old 26-01-2020, 11:22   #3
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Re: Speed and Direction if current

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Using my standard sail boat instruments ( chart plotter, wind, speed and direction indicator) how can I measure the actual speed and direction of the current?
SOG ..speed over ground ..comes via GPS

STW...Speed thru the water comes via your log speedo paddle wheel sensor

How to determine accurate current velocity is a difficult

To many variableS... accurate paddle wheel calibration, angle of attack, surface current speed, GPS signal stability ...

Your best guess is the difference between SOG and STW measured over a long time period..perhaps five minutes
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Old 26-01-2020, 12:05   #4
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Re: Speed and Direction if current

Let’s just understand what you mean by “actual speed”. I guess you mean “how fast am I approaching that rock in the water”.

If by standard instruments you mean a typical log/speed recorder (normally paddle wheel, let’s say Raymarine) the simple answer is - you can’t. These measurements are taken simply by RPM of the paddle wheel against time. If you are in a current, the whole body in which you are sailing is moving so the paddle wheel is measuring your progress through that body of water and not relative to some fixed point on the planet.

It has no way of knowing that the body of water is moving at all, let alone at what speed or which direction.

If by standard instruments you include a chart plotter which calculates speed and direction off GPS, the plotter, if following a route, will automatically calculate XTE (cross track error). And then you have some simple comparisons like boat speed off the Raymarine instrument vs SOG on the plotter and heading on the plotter vs heading on your steering compass.

But analog accuracy now plays a part. Is your Raymarine speed/log accurately calibrated (probably not). Do you know what deviation exists in your steering compass?

The simple old pre-GPS navigation adages remain true. “You are never where you think you are” and “just because you’re pointing there doesn’t mean you’re going there”. A chart plotter takes out the guesswork - use it.
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Old 27-01-2020, 10:17   #5
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Re: Speed and Direction if current

You need to look up how to plot a vector tryangle. This should be in any basic marine nav book. The tryangle shows speed over the ground on one leg, speed through the water on another and current/tide on the third. You have speed and heading over the ground from GPS. You can approximate heading on the water speed leg buy a bearing on your wake course and can measure speed using a 'chip log'. IE you drop something that floats (attach a line to retrieve it) and time how long it takes to go from bow to sturn. A bit of math then calculates you speed through the water. You now have what you need to calculate the tide leg. Can be done on blank paper so you dont need a small scale chart. It's very 'old school' and with a short run for the chip log not super precise but should give a reasonable estimate and fun to do on a nice day.
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Old 27-01-2020, 10:20   #6
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Re: Speed and Direction if current

PS if you can find a used trailing log you will get a very accurate water speed and a bearing on the tow line gives the difference butween course through the water and heading iether with a hand compass or by horizontal sextant angle.
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Old 27-01-2020, 10:30   #7
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Re: Speed and Direction if current

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Using my standard sail boat instruments ( chart plotter, wind, speed and direction indicator) how can I measure the actual speed and direction of the current?
It is almost impossible to use instruments to calculate current speed accurately. In rescue work we found out by dropping a man overboard buoy marker in the water and noting the gps position and returning in 20 minutes and logging the position again and charting your findings you can calculate the speed and course very accurately. We could do three at the same time at different location and plot our findings on a chart and keep this chart for direction and speed only. You would be surprised at your findings especially around islands and deeper water compared to shallow water.
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Old 27-01-2020, 11:17   #8
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Re: Speed and Direction if current

Depends on your particular situation. Take something really simple: You are in the middle of the Straits of Juan deFuca. Lie dead in the water. i.e. zero speed through the water. Plot your position ex GPS. Make a cuppa and have it. Plot you position ex GPs. The distance twixt the plotted positions is the distance you've moved in the elapsed time. Now your standard TDS calculation give you the SOG, and because your STW has been zero, and therefore has no place in the calculation, the SOG equals the speed of the current. The bearing of a line drawn twixt the positions is the direction of the current.

Now, you can, as Roland correctly points out, get much more "scientific" about it than that. But if you start with my little exercise you'll find that other navigational problems become less daunting.

What about drift, you say? Well the short answer, in the context you posit, is that if you are dead in the water, drift due to windage is insignificant for this particular purpose.

What I'm suggesting works just fine in OPEN water, but obviously not in narrow waters such as e.g. Deception Pass in WA state. Or the Hole in the Wall in BC. But there your problem will never arise because you will be too busy just staying away from "hard bits"

A problem that probably has more relevance for most of us is one similar to crossing the Straits of Georgia from Point Atkinson to Gabriola Passage. Let's assume some (approximated) numbers: Distance to run: 20NM; Rumb line bears 230º; STW 5 knots, Current (per tables), slack as you depart Atkinson rising to 2 knots, 140º. What course would you steer to stay on your rumb line [more or less :-)]

As Roland implies, all you need to solve this problem is a pencil and a plain piece of paper. Best to be able to do it without reference to mod cons :-).

All the best.

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Old 27-01-2020, 11:21   #9
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Re: Speed and Direction if current

Come to a stop and wad up some tissues or towels and throw them over the side.
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Old 27-01-2020, 11:48   #10
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Re: Speed and Direction if current

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Using my standard sail boat instruments ( chart plotter, wind, speed and direction indicator) how can I measure the actual speed and direction of the current?
Don't overthink the question, enjoy the sailing experience.

Your chartplotter/laptop, with a good updated chart package, should give you the expected current speed and direction.

Noting the difference between your GPS and paddlewheel speeds (Yes, both should be adjusted for accuracy if accuracy is your goal; best done immediately after a haul out but how many of us really do that) can give you the net affected current speed (five knots on the bow is not the same as five knots abeam but it is still a five knot current).

Note that NOAA charts for your area may have an exact one mile area delineated for boat speed testing in your area...I have one in my area but haven't used it in years but notice that the land placards are getting overgrown with vegetation.

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Old 27-01-2020, 13:03   #11
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Re: Speed and Direction if current

Read about Vector Diagrams
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Old 27-01-2020, 14:33   #12
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Re: Speed and Direction if current

Read up on plotting Vector Triangle, most if not all basic navigation books will have information on this. Certainly Bowditch has all of the information.
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