Speed Through GPS Versus Old Fashioned Paddle Log

[Dockhead]

Originally Posted by Stu Jackson

Nope. For example: What would happen if you are sailing well, trim is great, but into a building current? Your SOG would drop, right? But your STW would remain the same.

Exactly.

Of course you don't need any kind of instrument to trim your sails. How many of us did it for years or decades with not log, no wind instrument, nothing but telltales and a Windex?

And you could use SOG to trim for instantaneous effect; absolutely. It might even be better for that, since your SOG data will be more accurate than any STW data you can have on a small boat.

But as Stu says -- when the current is changing, you will not understand what is going on. You won't be able to distinguish change of current from sailing better or worse.

This is maybe not a big deal in areas with weak currents. If the currents are under a knot, then that could be within the margin of error of a dirty paddlewheel transducer, and the paddlewheel may seem like a waste of time.

But where the currents are strong -- like over here -- you really want STW for sailing. You want both STW and COG, which are each highly relevant to a different set of problems. It would be weird trying to sail here without STW. Say you're on a beam reach in a F4 and know you should be doing 8 knots. If not, you need to fiddle with trim. But say you only have COG, which is showing 8 knots -- what does it mean? Well at springs, and with a fair tide, it might mean you're only making 4 knots through the water, which means you will be stopped dead in relation to land when the tide changes. You can feel it, and you can see the water rushing against the buoys, and you can dig out your tidal atlas and look up what the current should be, and calculate what your real STW is (will require vector math if current is not parallel to your path), but wouldn't you much rather see it displayed?

[Sailor Doug]

The reason I ask if you still had a compass is in part just as important as having a knot log. If you are pulling the plug on not log why do you not throw away your compass?


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[Stu Jackson]

Why don't you guys who keep arguing start a new thread? "How GPS works?"

It's really tiresome, boring and useless. Find a new playpen.

Then we can get back to CTS and the original poster's question about navigation.

[Sailor Doug]

Good IDE Stu. Unfortunately defining the value of a knot log requires understanding how gps works. I guess I am also at fault that I can not let misinformation pass by.


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[chala]

The GPS can only be as good than the software that produces the data on the display. At time the speed can be wrong, the distance travelled can be wrong and the time can be wrong also GPS signal can easily be jammed or interfered with.

Originally Posted by hoppy

To understand how your boat is performing under sail you need to know your speed through water. Without it you don't have the full picture.

I agree.

Now for the one that do not mind diving under the boat, the following may help removing and preventing marine fouling when mooring. Somme Eva foam, two chocolate mousse containers, some rope, some Copper sulphate, a strainer (in this case a piece of anti slip mat). The picture also shows an old disk drive magnet to stick to the metal hull and later on removed from the pad. Unnecessary because the foam provides enough buoyancy to keep the device stuck to the hull. The second container is used as a lid and removed just before applying the foam pad over the paddle wheel. A lanyard connects the pad to the railing and prevents the pad from drifting away. A tug on the lanyard just before getting underway is all what is required to bring the pad to the deck. Three days of treatment will normally free existing fouling but may require a dive to clear the wheel.

[Stu Jackson]

Paddlewheel question: Which is the right direction for the paddles to face?

[hoppy]

Originally Posted by Sailor Doug

Good IDE Stu. Unfortunately defining the value of a knot log requires understanding how gps works. I guess I am also at fault that I can not let misinformation pass by.

Does it really matter? Lets just assume that for this thread that the GPS SOG is 100% accurate, then we can get back to SOG v STW.

If you want to sail your yacht, having STW is valuable to understanding the performance of your yacht. If you are happy for your yacht to sail you, then perhaps it does not matter so much.

[chala]

Originally Posted by Stu Jackson

Paddlewheel question: Which is the right direction for the paddles to face?

The relevant instruction manual will let you know.

[Stu Jackson]

Originally Posted by chala

The relevant instruction manual will let you know.

And if I had that, I wouldn't have asked. Haven't found it on the internet yet either. Thanks anyway.

PS - just found it on the internet. IT DOESN'T say, and I've read it twice.

Thanks again.

Anybody?

[Dockhead]

Originally Posted by Stu Jackson

Why don't you guys who keep arguing start a new thread? "How GPS works?"

It's really tiresome, boring and useless. Find a new playpen.

Then we can get back to CTS and the original poster's question about navigation.

An excellent suggestion. I have created this thread:

How Does GPS Work? - Cruisers & Sailing Forums

Why don't you guys take that completely different discussion over there?

I will move a few posts on topic over there.

[chala]

Originally Posted by Stu Jackson

And if I had that, I wouldn't have asked. Haven't found it on the internet yet either. Thanks anyway.

So what you got a Raimarine?

[Stu Jackson]

Originally Posted by chala

So what you got a Raimarine?

1986 Datamarine Corinthian S-100KL Knotmeter

And I found the complete manual online, the one I have somewhere in my multiple boat boxes. Same manual. It doesn't say.

[chala]

Hope this may help.

[Stu Jackson]

Thanks, just what I needed.

[Robin3]

Originally Posted by Stu Jackson

Paddlewheel question: Which is the right direction for the paddles to face?

Flat side of paddlewheel faces forwards. straight fore/aft might not always give max reading sometimes s slight rotation of the transducer will make a difference, depends on the waterflow into the transducer which might be diverted by something like a depth transducer nearby, or even the keel, especially if the paddlewheel transducer is mounted to one side of that once a position is decided consistant positioning after a clean/replace is of course essential. I have had to calibrate most logs over the years, sometimes, even usually a small adjustment, once or twice a bigger one ( bad transducer position too close to keel, I blame the original owner)

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 )