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Old 16-09-2010, 08:21   #46
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OK, I'll bite - how do you get boat speed with no log?
As long as you have a NMEA feed from the GPS (which is essential anyway) it only takes a few button pushes to change the preferences on the B&G to SOG.
Not all units are as good, which is pity as it would be an easy software option. Even if you have a log it would be useful feature for if/ when the log fouls on a long passage.
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Old 16-09-2010, 08:49   #47
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Boat Speed is not SOG.

The GPS and log are not mutually exclusive and neither is more important.

Boat speed optimizes boat handling.
GPS provides SOG
If you enter a waypoint you can get VMG

With Boat Speed and SOG you can calculate current

With current you can plan the approach to the mark to optimize VMG.

Singapore gets a large east/west current on the southern shore. A direct line up the channel to Raffles light may put you in the worst of the current.

A better (faster) plan may be hug the Indonesian Islands or Singapore shore and dog leg the last bit.

You won't know that without current - You can refer to the coastal tide charts and hope they are right or you can have the info on the boat.

Of course the best plan is to arrive on a favorable current, however we don't always get to choose.

I've had 5 knots on the log and 2 1/2 knots SOG beating up mid-channel. Other boats tacked "long" across the channel. We tacked short maximizing time in the shallower water and made a lot of time up. Of course it was racing and it mattered.
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Old 16-09-2010, 08:56   #48
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With Boat Speed and SOG you can calculate current
Is this a feature included in consumer GPS systems ? Granted, it is an easy calculation to do manually but I'm curious if it is offered as a button-push ? It could certainly be a useful feature.



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Old 16-09-2010, 10:04   #49
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I am not familiar with higher end systems but if you can link boat speed with SOG it's an easy calc.

Actually as I think of it you might be able to derive current with heading and track data from the gps. You would have to add wind data to extract set and I don't know how you would extract wind set from current set. It would look the same to the gps.

My head hurts. I don't think you can get it without boat speed.
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Old 16-09-2010, 18:44   #50
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Well, there are cruisers who sail and sailors who cruise. With the first group generally cruising is more about going to great places in an economical conveyance - otherwise we would all just fly there. So comfort and simplicity takes the forefront and performance is put second. With the second group, performance (joy of sailing) is first and getting to the next destination an exercise in sailing acumen.
- - Their boats reflect their styles with the "barges" definitely being in the first group and the sleek rigs in the second. The first group likes to stop and smell the roses while the second group likes the smell of the ocean washing over the rails.
Guess I should have mentioned in my initial post that I'm definitely part of that first group.

So, who wants to buy a new speed/temp transducer and thru-hull?
Sorry plug not included until my next haul out.

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Do be careful to not get paint into the bearing area, though!
Oops! I think I already did.
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Old 16-09-2010, 18:58   #51
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My head hurts. I don't think you can get it without boat speed.
You can,t.

It's a comparison between the over ground track and through water track.
The velocity and direction of the current is derived from the difference between the two tracks.

You need GPS, compass and boatspeed information to calculate.

It's not a particularly 'high end' option. Most (perhaps all?) boat instruments suites have the capability.
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Old 16-09-2010, 19:33   #52
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Is this a feature included in consumer GPS systems ? Granted, it is an easy calculation to do manually but I'm curious if it is offered as a button-push ? It could certainly be a useful feature.
My Raymarine E-series system will not only do it automatically, but will represent it graphically as an arrow on the plotter.
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Old 16-09-2010, 21:20   #53
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G'Day all,
Am I the only one who carefully puts a thin coat of anti-fouling on my paddle wheel? Takes a Q-tip, a few drops of leftover paint and a couple of minutes, lasts for a few months, and markedly reduces fouling.
Do be careful to not get paint into the bearing area, though!
Cheers, Jim and Ann s/v Insatiable II lying Cairns, Qld OZ
The little paddle wheel contains two magnets that when they pass the sensor in the body generate the speed pulse. Putting a "copper" based or other antifoulant on the little paddle wheel -may- alter the ability of the sensor to detect the magnets as they whirl by.
- - Here is a page to "ultrasonic" speed transducers that do not use paddlewheels -
http://www.airmartechnology.com/2009...ch-results.asp
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Old 16-09-2010, 21:48   #54
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The little paddle wheel contains two magnets that when they pass the sensor in the body generate the speed pulse. Putting a "copper" based or other antifoulant on the little paddle wheel -may- alter the ability of the sensor to detect the magnets as they whirl by.
- - Here is a page to "ultrasonic" speed transducers that do not use paddlewheels -
http://www.airmartechnology.com/2009...ch-results.asp
G'Day Osiris,

Well, I appreciate your concern but in fact there is no effect whatsoever on the calibration or function of the knotmeter. Been doing this for more years than I like to think of, several different types of knotmeters, and no problems other than putting too much paint on and mechanically fouling the blades.

Cheers,

Jim and Ann s/v Insatiable II lying Cairns, Qld, Oz
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Old 17-09-2010, 05:05   #55
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G'Day Osiris,
Well, I appreciate your concern but in fact there is no effect whatsoever on the calibration or function of the knotmeter. Been doing this for more years than I like to think of, several different types of knotmeters, and no problems other than putting too much paint on and mechanically fouling the blades.
Cheers,
Jim and Ann s/v Insatiable II lying Cairns, Qld, Oz
Good news, how much do you thin the AF paint and are you using copper based AF paint? Copper % content? I would like to try this. One of the reasons I subconsciously never install the sensor is I hate cleaning it of the sea life debris.
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Old 17-09-2010, 05:45   #56
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Some solvent based AF paints attack plastics. I never figured out which ones are unsafe, it cannot be too many because many people paint their depth transducers.
Anyone know which are OK and the paints to avoid for this application?
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Old 17-09-2010, 06:15   #57
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The vinyl paints have a horrid solvent, yes? I think they lift all other paints. But no cruiser uses that stuff I suppose. ????
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Old 17-09-2010, 07:39   #58
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The little paddle wheel contains two magnets that when they pass the sensor in the body generate the speed pulse. Putting a "copper" based or other antifoulant on the little paddle wheel -may- alter the ability of the sensor to detect the magnets as they whirl by.
- - Here is a page to "ultrasonic" speed transducers that do not use paddlewheels -
http://www.airmartechnology.com/2009...ch-results.asp
That may be true, but the manual that came with the Raymarine unit recommends painting.
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Old 17-09-2010, 07:41   #59
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Some solvent based AF paints attack plastics. I never figured out which ones are unsafe, it cannot be too many because many people paint their depth transducers.
Anyone know which are OK and the paints to avoid for this application?
They make water based paints specifically for transducers. Comes in a small battle, pretty cheap, and has a little brush right on the lid. Perfect size for a paddle wheel.
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Old 17-09-2010, 07:41   #60
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True wind can be derived from GPS. Some instruments do refuse to calculate this, but the better instruments will do it.
The true wind derived from the GPS is slightly different from the true wind derived from the log. True wind from the GPS shows the wind that would be measured on land or a buoy anchored next to the boat. In my opinion this is the most useful and correct “True wind “, others disagree, but my experience is that true wind derived from the GPS is not only more useful, but more stable and accurate, as minor miss calibrations of the log affect true wind derived from this source and degrade the accuracy annoyingly
Tack and gibe on most boats using the log to derive the true wind and you will find the true wind varies considerably. Do so in a boat using the GPS speed input and the value is very steady (assuming steady true wind).
You are absolutely correct, GPS readings would be the better calculation base rather than the speed/log.

But can you name a manufacturer's wind meter that give the option to use GPS readings (rather than speed/log) for calculating this when you push the button to change from apparent to true readings?
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