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Old 22-03-2015, 13:12   #331
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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. . .
But to stir this debart ( or is it debacle) further, I check and adjust the log reading using speed obtained by the GPS in SLACK water, ie find a place with little or no tidal flow or current and do the comparison at slack water from the tide tables, preferably double checking for little or no flow around a nearby buoy or marker. It is even possible to do a quick and pretty accurate compass swing by motoring at a steady speed on various compass headings whilst comparing the ships magnetic compass heading with the COG readout from the GPS. Otherwise you need to find a measured mile or distance marks and do the log calibration as per manual, with 'runs' over the measured distance in both directions. Compass swinging is another subject but the simple GPS comparison worked well for me even when later compared to my 'proper' swings using a pelorus with a shadow pin and a celestial almanac progam on my programmable calculator ( I did still use time by the GPS clock however )
That's exactly what I do, both with the compass and with the log (but for the "proper" compass swing which I am not capable of doing myself).

Of course, slack tide doesn't mean the water isn't moving. Needs to be slack tide and dead calm. Then a few two-way runs on reciprocal courses to eliminate any water motion.

It's really worthwhile trying to get compass and heading sensor properly calibrated and agreeing with one another and with COG in slack water. One problem I have is that my boat crabs slightly under power -- probably because the propeller shaft is slighly offset so that it can be withdrawn past the rudder. I wonder how many boats do this, and people just don't realize it? It's only a degree or two and only at higher power settings, but enough to throw you off if you don't realize what's going on.
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Old 22-03-2015, 13:24   #332
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

In this case for True Wind, SOG would be more accurate than STW. One of my chart plotters will allow me to use SOG instead of STW only if he cannot find STW at boot up. It would be nice if it was possible to chose between SOG and STW.
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Old 22-03-2015, 13:30   #333
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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In this case for True Wind, SOG would be more accurate than STW. One of my chart plotters will allow me to use SOG instead of STW only if he cannot find STW at boot up. It would be nice if it was possible to chose between SOG and STW.
Except that if you calculate True Wind with SOG instead of STW, it's not True Wind -- it's Ground Wind.
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Old 22-03-2015, 13:34   #334
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

The only reason the debate over GPS speed came up is someone complained that GPS speed update is "too slow" because it has to wait to get two or more position fixes to calculate speed. This is incorrect. GPS does not need a position fix to compute velocity. In fact is uses velocity to improve the position estimate. So GPS speed pucks are very accurate and fast at indicating speed over ground. I have no idea why this simple concept is controversial.
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Old 22-03-2015, 14:07   #335
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Except that if you calculate True Wind with SOG instead of STW, it's not True Wind -- it's Ground Wind.
If SOG equals STW then no one will know which one is used for the calculation.
So what do you call that, True Wind or Ground Wind? The dial will not change name. What is important is to know how strong will be the wind in the followings legs just for the sake of having just the right amount of sail.
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Old 22-03-2015, 14:13   #336
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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If SOG equals STW then no one will know which one is used for the calculation.
So what do you call that, True Wind or Ground Wind? The dial will not change name. What is important is to know how strong will be the wind in the followings legs just for the sake of having just the right amount of sail.
If there is no current -- if the water is not moving in relation to the land -- then SOG will be equal to STW, and Ground Wind will be equal to True Wind.

If, however, the water is moving in relation to the land -- as it is wont to do -- then all bets are off.
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Old 22-03-2015, 14:29   #337
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

Quote:
Originally Posted by chala View Post
In this case for True Wind, SOG would be more accurate than STW. One of my chart plotters will allow me to use SOG instead of STW only if he cannot find STW at boot up. It would be nice if it was possible to chose between SOG and STW.
I have STW displayed on a separate wheel pedestal mounted GMS10 Garmin that also displays wind ( app/true/ closehauled and also depth data. I can choose whether to use SOG or STW input for the wind data in the GMS10 setup, but not sure without going to the boat to check if the selection made on the separate GMS10 instrument then is used on the plotter display as well ( full NMEA 2000 network) or if I can choose separately on the plotter display . I don't really mind generally which it uses to give 'true wind' as it is apparent wind that we must sail on, but yes for predicting wind on next tack or gybe or when we turn to the next waypoint, and I guess SOG/COG are probably more accurate for that computation than HDG and STW.
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Old 22-03-2015, 14:38   #338
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by Robin3 View Post
I have STW displayed on a separate wheel pedestal mounted GMS10 Garmin that also displays wind ( app/true/ closehauled and also depth data. I can choose whether to use SOG or STW input for the wind data in the GMS10 setup, but not sure without going to the boat to check if the selection made on the separate GMS10 instrument then is used on the plotter display as well ( full NMEA 2000 network) or if I can choose separately on the plotter display . I don't really mind generally which it uses to give 'true wind' as it is apparent wind that we must sail on, but yes for predicting wind on next tack or gybe or when we turn to the next waypoint, and I guess SOG/COG are probably more accurate for that computation than HDG and STW.
It will be more accurate, but it will not give you True Wind.

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It won't matter if there's not much current running, but if there is, it will mislead you seriously.
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Old 22-03-2015, 14:54   #339
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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It will be more accurate, but it will not give you True Wind.

Attachment 99171

Of Instruments and Real Angles | Sailing World

I won't matter if there's not much current running, but if there is, it will mislead you seriously.
I need an aspirin! Yes of course but for me a good enough approximation until we actually make the turn.
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Old 22-03-2015, 15:06   #340
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

I'm a bit confused about TWD and ground wind and what's the best for the display. I believe our Raymarine plotter uses the paddle wheel to calculate TWD and I don't think it's possible to change it. So you can see on the plotter I have TWD displayed and primarily use this to keep track of direction changes in the ships log over time. However I have noticed when we tack the AWA is usually quite different on each tack. Would this be more likely to be a calibration issue? If so what is the best way to calibrate for accurate data. The main problem is each time we tack we might have to adjust the sail to wind angle between 35and 40 degrees, which isn't a big deal but it would be nice to just hit auto tack and have it pretty close to right...Click image for larger version

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Old 22-03-2015, 15:22   #341
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by monte View Post
I'm a bit confused about TWD and ground wind and what's the best for the display. I believe our Raymarine plotter uses the paddle wheel to calculate TWD and I don't think it's possible to change it. So you can see on the plotter I have TWD displayed and primarily use this to keep track of direction changes in the ships log over time. However I have noticed when we tack the AWA is usually quite different on each tack. Would this be more likely to be a calibration issue? If so what is the best way to calibrate for accurate data. The main problem is each time we tack we might have to adjust the sail to wind angle between 35and 40 degrees, which isn't a big deal but it would be nice to just hit auto tack and have it pretty close to right...Attachment 99172
Your issue with different AWA on different tacks has nothing to do with Ground Wind or True Wind. That's one of two things: (a) rigging not tuned well; and/or (b) wind instrument not calibrated.

Best way to calibrate the wind instrument is to go out in a dead calm and motor at top speed. That will give you 000 AWA; zero out the instrument. Then check your calibration by motoring in a circle when there's some wind -- True Wind should hold steady in all directions.

True Wind is what you sail in -- it's the relationship between wind and the surface of the water. It's what you need if you want to know what the wind will be like when you tack, head off, etc.

Ground Wind is the relationship between the land and the wind. You don't sail in it so you don't want to confuse it with True Wind. It's useful for knowing what will happen to the wind when the tide changes. It's "true wind" as experienced by landlubbers and is equal to True Wind when you're at anchor or tied up.
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Old 22-03-2015, 15:54   #342
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

Wot dockhead said plus

The sea state may be different after you tack, new wave direction now making you bear off a little to maintain speed. Are the sails trimmed exactly the same on each tack?, mainsheet traveller in similar place? Don't pinch too close to the wind after a tack but bear off a little initially to build speed before finally hardening onto the wind. Tack angles viewed as a plotter track may not really tell it like it was, was there any current? did the wind bend a little locally off the nearby land?

There comes a time when the instruments have done their bit, the rest is all down to the nut holding the wheel.
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Old 22-03-2015, 17:27   #343
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Wot dockhead said plus

The sea state may be different after you tack, new wave direction now making you bear off a little to maintain speed. Are the sails trimmed exactly the same on each tack?, mainsheet traveller in similar place? Don't pinch too close to the wind after a tack but bear off a little initially to build speed before finally hardening onto the wind. Tack angles viewed as a plotter track may not really tell it like it was, was there any current? did the wind bend a little locally off the nearby land?

There comes a time when the instruments have done their bit, the rest is all down to the nut holding the wheel.

In the example above the sea state was pretty mild, with no obvious better tack, traveller was centred, main sheet the same, and 1 k current from dead upwind. No obvious land wind sheer. Yes I often bear away 5 degrees or so just before the tack to maintain speed and make sure we don't end up back winded after the tack, then come up 1 degree at a time till in the slot. Jen was actually handsteering most of that day but still the tack angles were 5 degrees different AWA. So is the adjustment to make the one that centres the wind vane? I think there's an adjustment somewhere that allows you to offset it either way by degrees or fractions of degrees. Sounds like I could try alter it 2.5 degrees one way and see if it helps...
Thanks for the feedback
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Old 22-03-2015, 20:14   #344
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

According to Mareton:

Ground wind is the speed and direction of the wind referenced to a compass rose, which is different than apparent and true wind which are referenced to the bow of the boat. The apparent and true wind are interesting to see how the wind is effecting or pushing the boat (i.e., when sailing or docking), the ground wind is important for understanding the weather (i.e., where is the low pressure). Since the WSO100 only measures apparent wind speed and direction, display products such as Maretron's DSM200, DSM250, and N2KView require addition information to compute and display ground wind (i.e., vessel speed, course, and heading).

According to this thread: Differences Between Ground, Apparent, and True Wind Direction
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Old 22-03-2015, 22:23   #345
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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In the example above the sea state was pretty mild, with no obvious better tack, traveller was centred, main sheet the same, and 1 k current from dead upwind. No obvious land wind sheer. Yes I often bear away 5 degrees or so just before the tack to maintain speed and make sure we don't end up back winded after the tack, then come up 1 degree at a time till in the slot. Jen was actually handsteering most of that day but still the tack angles were 5 degrees different AWA. So is the adjustment to make the one that centres the wind vane? I think there's an adjustment somewhere that allows you to offset it either way by degrees or fractions of degrees. Sounds like I could try alter it 2.5 degrees one way and see if it helps...
Thanks for the feedback
It does sound like the wind vane is not exactly centred electronically to the displayed angle so the AWA is different each side of centre. yes you could try and adjust by +/- half the total difference between sides and see if that equals things up. Initial centring by driving dead upwind is not as easy to judge from down on deck as it sounds, so that might well be the answer. However I think yours is a cat so maybe there are other factors a mere monhull jockey like me might not understand.
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