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Old 03-06-2009, 14:09   #16
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Without measuring the speed through water, the navigation system cannot compute "over water" wind, it only gives "over ground" wind. But, if the current or tidal stream is significant, there is a strong difference between these winds, that you need to take into account for tacking or gybing. Then, IMHO, log speed is a valuable input for navigating.

In addition to this, I agree with Noelex and Wotname, some backup system is important for navigation. I am anxious to have my log calibrated again, after it has been reset in the repair shop. However, I keep in mind that log calibration isn't really accurate, and influenced by heeling and longitudinal trim. On a sailing boat, I expect accuracy to be about 10% of speed (much better on large ships).

About the update rate of GPS and log, it isn't relevant for trimming sails, because waves cause the actual speed to vary quickly, wether with respect to ground or to water. For this reason, a damping/averaging function is always provided in logs. Without it, you wouldn't be able to read the display, it would change too often. This damping doesn't always exist on GPS receivers (mine hasn't). However, I have seen one that would average speed over one minute, then display the same value during the next minute. That's really a slow update but it was on a 20,000-ton ship.

Averaging is also much necessary on wind measurement. When beating to windward, the masthead goes back and forth, so the apparent wind seen by the vane/anemometer keeps changing. To be able to do without averaging, you would need an accurate Inertial Measurement Unit providing pitch and roll rates to compensate for the lever arm between masthead and deck and compute the speed at the masthead.

In conclusion, log, GPS and vane/anemometer generally provide "filtered" values, for which the update rate (1Hz or 4Hz) isn't really significant.

Alain
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Old 03-06-2009, 14:37   #17
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Without measuring the speed through water, the navigation system cannot compute "over water" wind, it only gives "over ground" wind. But, if the current or tidal stream is significant, there is a strong difference between these winds, that you need to take into account for tacking or gybing. Then, IMHO, log speed is a valuable input for navigating.
"True wind" derived from the GPS amd padlewheel are going to be different, but which is more accurate.
Suppose 0 wind (Zip , not a breath) and a 1kn current. No motor.True wind derived from the GPS will show 0 true wind 1Kn SOG minus 1Kn apparant wind.
"True wind" measured from the paddlewheel will erroniously show 1KN of true wind. 0 speed and 1kn apparent wind.
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Old 03-06-2009, 15:11   #18
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noelex,
IMHO, "true wind" derived from GPS isn't more or less accurate than "true wind" derived from paddlewheel, it is just different. When using GPS, you obtain wind relative to ground. When using paddlewheel (speed log), you obtain wind relative to water.
In the case you describe, if you don't hoist sails, the boat will just drift away with the current, at 1kt with respect to ground, 0kt with respect to water.
*But*, if you hoist sails, you will be able to use this "inexistant" wind created by the current. IMHO, it is worth having an instrument to tell you that this "inexistant" wind is available for sailing.

For this reason, when there is some current, I avoid refering to "true" wind. I prefer to refer to "absolute" wind (what you feel when at anchor) and "surface" wind (what you feel when adrift without sails)

From the name of your boat and your cruising location, I guess that we might as well continue this discussion in French (I cannot write Breton).

Alain
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Old 03-06-2009, 15:31   #19
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Use this instead: BS XTL Fn 275-00-001 | B&G

ciao!
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Old 03-06-2009, 15:40   #20
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Thanks Alan
Yes the boat is french. but I am not. Its a bit of worry when the notes from the original owner are in french complete with warnings. Bable fish reassure me I have a Yacht with a "Fast skirt"!
In the Med the difference between " True wind" from the GPS and log is nonexistent, but I am still interested when I get into tidal conditions what I am loosing with "True wind " derived from the GPS.
In the above example my instruments would show true wind as 0 and apparent wind as 1Kn. If I want to sail I have 1kn of wind. The pressure in my sails is always going to be proportional to the apparent wind, but this apparent wind derived entirely from the current as I know the true wind is 0.
This is the correct information and just the sort of thing I want my wind instruments to tell me, but maybe I am missing something. It has been a long time since I raced a yacht and in days before SA was turned off, let alone SD fixes, GPS speed information was much too erratic to be of use.
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Old 03-06-2009, 15:52   #21
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Use this instead: BS XTL Fn 275-00-001 | B&G

ciao!
Nick.
Its also possible to fit an Aimar ultrasonic speed transducer that will fit an ordinary 50mm cut out. B&G offered one for a while , but apparantly its accuracy did not live up to their standards. I believe, however, under 10K it was OK.
You can now buy a transducer from Aimar and use it in most systems. no paddlewheel needed, but its not cheap.
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Old 03-06-2009, 22:01   #22
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5 Knots SOG against a 3 knot current= 8 knots thru the water?

That is "apparent" speed, correct?
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Old 04-06-2009, 00:30   #23
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Question about plugging in these paddle wheels - I have one too, lying in the bottom of my boat because, well, what sane man unscrews a hole below the water line?

Seriously, do you just unscrew the plug and quickly screw in the paddlewheel?
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Old 04-06-2009, 01:08   #24
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Wink Log & Blanking Plug Transfer

TOCTom,

Indeed, that's what you do. Some water will get in but the boat doesn't sink and when leaving it for a while, the blanking plug is better than the paddle wheel growing ocean weeds! It's a nuisance as I have two cable/thru-hull fittings in a relatively large compartment which we'd like to use for storage but have to protect, plus mopping up [fairly easily] the seawater which comes in during the transfer.

However, a caveat ! On my setup, there is a fore & aft alignment notch in the hull fitting which must be correct or the log/plug won't seal. You need to know if this is the case before you start. Of course, the compartment isn't lit and there's no third hand for the torch to see the little black-on-black arrow on both plug & log, so I've marked the arrow in white and put a marking pen arrow on the hull to show the alignment. This makes the switch easy with minimal water ingress.

In part, it's this transfer process which started the debate and prompted the original question.

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Old 04-06-2009, 01:08   #25
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5 Knots SOG against a 3 knot current= 8 knots thru the water?
Yup.

Imagine 0 knots SOG against a 3 knot current. The boat isn't going anywhere over the ground but the water is rushing by the boat at 3 knots. Actually the boat is moving "upstream" through the water at 3 knots.

4 kts of boat speed and SOG goes to 1 kt.
8 kts boat speed and SOG is 5 kts.
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Old 04-06-2009, 09:18   #26
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Racers need a knot meter, cruisers need a rubber squid and a good filet knife.... One of the requirements of Island Time...... ;>)
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Old 04-06-2009, 10:32   #27
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The other issue you might wish to consider is the log gives you a closer representation of the performance of the boat and its propulsion systems. I know at 1800 RPM, all things good, I should be logging 5.2 - 5.3 knots. If it is less than that, is it time to go clean the hulls? are my props clean? am I dragging something? did I get a nick in my prop (this is the most recent occurrence of my change in performance). It allows you to base line your performance a little bit better. With SOG, this is significantly influenced by tides, currents and winds. It is much more difficult to factor out these differences and make an informed decision to break out the diving mask and go have a look.

I also lets you gauge your fuel utilization a little better. Perhaps gives you justification for getting those new folding/feathering props!
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Old 04-06-2009, 12:53   #28
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noelex,
Be careful about current in the Med. Yes, there is almost no tide on the French Riviera (it's different near Tunisia or Sicily) but there are some permanent current patterns (up to 0.5kt) that the wind can increase up to 3kts. See your Sailing Directions.

In my job (hydrodynamics: manoeuvring, seakeeping, propellers and autopilots for large ships), we always refer to speed through water because this is what affects ship performance. We use speed over ground (GPS or inertial) only as a backup, then we work hard to compute and eliminate the current from the results. IMHO, SOG is only necessary for navigation (knowing where you are going), but you need to know STW for sailing.

As far as I know, ARPA radars also use STW to compute the relative motion of ships to provide CPA and TCPA because it is assumed that all ships in an area are submitted to the same current.

The trouble is, a speed log (Pitot, paddlewheel, electromagnetic) is expensive and needs calibrating (except for Doppler, the most expensive). GPS is cheap and fairly accurate, without having to put anything in the water.

Alain
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Old 04-06-2009, 13:06   #29
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My B&G Network Wind instrument can't calculate True Wind unless the boatspeed sensor is installed and working.

I'm all for having the speed log. When sailing around the islands, you can use the difference between boatspeed and SOG to figure out where the fair and foul currents are. They swirl around in patterns that depend on the tide, and you can add a half knot or even a knot to SOG by seeking out a fair current. Same for sailing offshore between the US East Coast and the Caribbean. The warm eddies spun of by the Gulf Stream can have a huge impact on SOG--typically in the 1-2 knot range, but as much as 3-4 knots. The difference between the paddle wheel and the GPS tells you where you are in relation to the center of the eddy. We've always planned our course to take advantage of the fair-current side of the eddies, or at least to avoid the foul-current side.
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Old 04-06-2009, 13:27   #30
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"....there are some permanent current patterns (up to 0.5kt)...." You're kidding right? Some of the local currents this month:
2009-06-07 2:07 PM PDT 8.48 knots Max Flood
2009-06-07 6:04 PM PDT -0.01 knots Slack, Ebb Begins
2009-06-07 9:15 PM PDT Sunset
2009-06-07 9:42 PM PDT -4.10 knots Max Ebb
2009-06-08 12:05 AM PDT 0.00 knots Slack, Flood Begins
2009-06-08 1:51 AM PDT 2.04 knots Max Flood
2009-06-08 3:57 AM PDT -0.00 knots Slack, Ebb Begins
2009-06-08 5:10 AM PDT Sunrise
2009-06-08 7:28 AM PDT -5.08 knots Max Ebb
2009-06-08 11:18 AM PDT 0.01 knots Slack, Flood Begins
2009-06-08 2:38 PM PDT 8.38 knots Max Flood

:>)
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