Speed Through GPS Versus Old Fashioned Paddle Log

[rwidman]

Originally Posted by MarkSF

Around here we have these things called tides and currents.

I think they are everywhere, not just where you are.

[rwidman]

Originally Posted by Robin3

But as a (competent) navigator it should be of concern to know how far you have travelled through the water...........

So a person without a way to measure speed over water would not be a "competent) navigator" ?

[Reefmagnet]

Same arguments against having a knotmeter can apply to knowing wind speed. My wind instrument isn't hooked up since I installed new instruments over a year ago and I can make do, but it'd be a lot nicer to have the info available. If my knotmeter wasn't working I wouldn't loose sleep over it, but ditto in being nice to have.

[transmitterdan]

Originally Posted by Robin3

I expect airline pilots still like to know both airspeed and groundspeed?

It is a must for pilots to know accurate air speed. That's the parameter keeping them and their passengers alive. A few high profile accidents have been attributed to inaccurate air speed indicators resulting in loss of lift because either the human pilot or Otto tried to gain altitude with insufficient airspeed.

For a boat STW isn't a life or death thing in most cases.

[boatman61]

Paddlewheel logs... don't trust them.. newfangled rubbish..
give me a wine cork and a stop watch any day...

[transmitterdan]

Originally Posted by rwidman

So a person without a way to measure speed over water would not be a "competent) navigator" ?

Correct, because they cannot tell how far they have come or when (or whether) they will reach their destination on a long passage. Fortunately speed over water/ground is easy to get from GPS. If you only have speed through the water then you also need heading plus estimate of current (direction and speed) to compute speed and course over water/ground.

I would say that unless currents are very fast such that it affects the safe CTS then knowing speed through the water isn't critical with GPS available. But it can help in choosing the optimal CTS to reach a destination the quickest. Sailors with this skill don't have to keep "hunting" for the course that gives the highest VMG. They "know" they are on the optimal course. That makes them a more competent navigator but if someone can't do it I would not say that makes them incompetent.

Please don't let us start a "GPS is gonna' die tack.

[transmitterdan]

Originally Posted by boatman61

Paddlewheel logs... don't trust them.. newfangled rubbish..
give me a wine cork and a stop watch any day...

That is more accurate the the paddelwheel on our boat for sure.

[TheThunderbird]

My God!

Racers use log
Istantaneous, dynamically true
GPS is an average speed, accurate, not precise.

Good for cruising

[Reefmagnet]

Originally Posted by transmitterdan

That is more accurate the the paddelwheel on our boat for sure.

Buried amongst the settings of my IS20 instruments was a speed compensation value for the log. I had to set to 90% on my boat to get the paddlewheel reading accurate. The reason for inaccuracy at 100% is that water accelerates over the hull and therefore runs faster than the surrounding water. On my previous boat with separate log and depth transducers, the knotmeter refused to work in one hole due to, I assume, turbulence from the forward through hull.

[Robin3]

Originally Posted by rwidman

So a person without a way to measure speed over water would not be a "competent) navigator" ?

I didn't actually say 'speed' specifically I said a means of measuring distance gone through the water and in which direction. that distance measurer device is generally known as a 'log' and typically gives a speed reading also (not the written log kind) and together with the compass heading? and an allowance for any leeway (mostly a sailboat thing) and with a tidal current atlas to predict 'stream' it is possible to calculate a pretty good idea of where you are. Or you could just look at the plotter or the appropriate Ipad app. These magnetic compass devices are still used are they on modern vessels or have they vanished from 21st century vessels?


Is navigation a lost art now ?

[Grateful]

As a 'recovering racer' I cannot imagine sailing without a knot meter. Where's the fun in tweaking that topping lift, trimming the out haul, and moving that barber hauler a scosh if you can't see the results on the knot meter?

No matter that we're still two weeks from landfall and the next watch is going to read a book, I still enjoy actually sailing now and then.

[rwidman]

Originally Posted by transmitterdan

Correct, because they cannot tell how far they have come or when (or whether) they will reach their destination on a long passage. Fortunately speed over water/ground is easy to get from GPS. If you only have speed through the water then you also need heading plus estimate of current (direction and speed) to compute speed and course over water/ground.

I would say that unless currents are very fast such that it affects the safe CTS then knowing speed through the water isn't critical with GPS available. But it can help in choosing the optimal CTS to reach a destination the quickest. Sailors with this skill don't have to keep "hunting" for the course that gives the highest VMG. They "know" they are on the optimal course. That makes them a more competent navigator but if someone can't do it I would not say that makes them incompetent.

Please don't let us start a "GPS is gonna' die tack.

The post I was replying to stated that without speed over water one couldn't be a competent navigator. That is what I was questioning. If a person lost the speed over water indication, I don't think that would change his/her competency as a navigator.

Speed over ground (GPS) is the one that tells you when you should get to your destination. If I could only have one, that's the one I would choose. It's also the most reliable and maintenance free.

[lordgeoff]

Originally Posted by Jonathancpwalsh

Can anyone tell me why so many boats still use the "old fashioned" impeller log to record their speed? GPS obviously gives much more accurate speed over the ground which is, ultimately all you really want, isn't it? Or am I missing something? I'm not a racer - just cruising! Would appreciate your thoughts!

I watch my paddle log closely. When it get too far out of wack to the true GPS speed I know the hull probably needs cleaning along with the paddle wheel ....... Log is not much use if you have access to GPS speed.

[Robin3]

Originally Posted by rwidman

The post I was replying to stated that without speed over water one couldn't be a competent navigator. That is what I was questioning. If a person lost the speed over water indication, I don't think that would change his/her competency as a navigator.

Speed over ground (GPS) is the one that tells you when you should get to your destination. If I could only have one, that's the one I would choose. It's also the most reliable and maintenance free.

This is what my reply actually said

<<
But as a (competent) navigator it should be of concern to know how far you have travelled through the water and in what direction because that data is required to manually do a deduced reckoning of position. The tuning come tweaking aspects are not the only issue. Generally IMO it is sail people that want the boatspeed data and mobo folk that don't bother as they have plenty of (engine) grunt in reserve. >>

Cheat's easy way out is simply nudge throttles to get an agreeable SOG and tweak autopilot/wheel to match COG to required BTW. Or you could do some proper navigation planning using predicted/anticipated changing streams/currents and offset their effect over the entire planned voyage or that leg of a route. That way the pilot isn't forever heading into or towards the stream to compensate for it's possible adverse effect. Those of us in rag and stick mode and with limited speed effect devices to hand must necessarily use more guile and less diesel!

[lordgeoff]

Originally Posted by Reefmagnet

Impeller log gives you speed through water which is probably a better indicator of sailing performance than SOG in regions with wicked currents like where I sail. Using the speed through the water and SOG together also indicates if you're going with the flow or against it (and at what rate), which is quite useful actually.

If you assume the log is clear of all growth, the shaft is not binding, or paddles are not worn and it has been installed correctly. Just have to accept what the log says.