"Knots"?

[Welti]

Reading some miscellaneous posts, and mentioned going such and such knots, I wondered: do people generally mean SOG knots or STW knots if not specified?

[thomm225]

It depends on what the subject was.....

[GordMay]

Originally Posted by thomm225

It depends on what the subject was.....

Indeed, it depends; which is one reason to be precise, in one's communication.
A knot log will display Speed Through Water [STW], whereas a GPS will display Speed Over Ground [SOG].
Speed through water is only required to be used for collision avoidance, and not necessarily for navigation.
By using speed over ground, a navigator is more aware of the actual physical situation, than otherwise.

[SailingHarmonie]

Originally Posted by GordMay

Indeed, it depends; which is one reason to be precise, in one's communication.
A knot log will display Speed Through Water [STW], whereas a GPS will display Speed Over Ground [SOG].
Speed through water is only required to be used for collision avoidance, and not necessarily for navigation.
By using speed over ground, a navigator is more aware of the actual physical situation, than otherwise.

No! No! and NO!!!

The idea that STW is a silly old fashioned measure that has no utility to a modern navigator is just wrong, but one I see often repeated by people I would think should know better.

For a careful navigator, it is a HUGE help. Let me give you just one example from yesterday while I was approaching Charleston Inlet in 8 knots of wind, close hauled at about 5 knots.

Taking from the south, my MFD presents me with laylines, showing me my course on the opposite tack. These of course are REALLY helpful in determining the best time to tack. They are current corrected They show an accurate COG after I tack.

In this case, there was a significant current on my nose, about 1 knot. When you are tacking into a 1 knot current, it has a HUGE impact on your laylines. With a STW measurement, my MFD could not do this calculation. In fact, there would be no way of knowing how much current was even present. Add to that in light winds like this, with a significant current, your instruments can not calculate a meaningfully accurate True Wind Angle. All of which is very useful information to the smart navigator.

Of course you CAN navigate around the world with only COG, just as you can navigate around the world with just STW. But you will be a better, more accurate, more sophisticated navigator if you have BOTH and know how to use them.

50 years ago any good navigator would have sold his soul to have a real time updated SOG/COG and STW/HDG vectors. He would have sold his mother's soul to have a real time display of the actual current affecting his boat.

Dismissing STW as a useful tool is another symptom of amateurs who have never learned the basics of navigation and treat their MFD as a video game.

[Knotical]

When people mention “going at xx knots” they generally mean GPS speed, which would correlate to SOG.

STW is usually provided by a paddle wheel thruhull, which a lot of cruisers don’t have installed or it is simply non-functional.

[billreilly]

Originally Posted by Knotical

When people mention “going at xx knots” they generally mean GPS speed, which would correlate to SOG.

STW is usually provided by a paddle wheel thruhull, which a lot of cruisers don’t have installed or it is simply non-functional.

Without a paddle wheel there is no way for your sailing instruments to calculate true wind, so not having a paddle wheel is a big mistake.


If anybody is sailing without a paddle wheel then I suggest they install one.

[Tanqueray]

Originally Posted by billreilly

Without a paddle wheel there is no way for your sailing instruments to calculate true wind, so not having a paddle wheel is a big mistake.


If anybody is sailing without a paddle wheel then I suggest they install one.

I sail on an inland ocean (Lake Michigan), so no tidal current to worry about.

I have a paddle wheel, but not hooked into my chart plotter. I do have GPS SOG and that is used to calculate True Wind.

Greg

[Don C L]

When you maximize your STW with knowledge of sail trim, you’ll be maximizing your SOG. And STW is what you can measure fairly easily without either a computer via gps or a chart and some math and ded reckoning skills. To say SOG is all there is seems a bit odd to me. I don’t check my SOG when trimming.

[Hasbeen]

I had a paddle wheel speedo & on a few occasions towed a walker log. I have crossed the Coral sea 6 times, & sailed well over 30,000 nautical miles around the pacific islands.


In tropical waters I don't think I ever got more than 4 hours without a bit of seaweed stopping the things working.


Life would have been so easy if sat nav had been available at affordable prices in the 60s & 70s.

[TrentePieds]

"Knots" are always, by definition, speed THROUGH THE WATER. To derive "Speed over the ground" (SOG) a compensation needs to be made. That compensation derives most easily from taking fixes at known time intervals.

Any device that will read a satellite for latitude and longitude ("coordinates") will do, even an I-phone. No need at all to spend money on a chart plotter. Because I loathe i-phones, I use an antique retrofit GPS from my car. I take it with me to sea, and I read the coordinates every fifteen minutes just for something to do since 99% of the time spent on passage is utterly boring. I plot the coordinates given by the GPS on my antique paper charts. Picking up the distance twixt fixes marked on the chart using navigator's dividers and then laying the dividers against the scale ON THE SIDE of the chart, i.e. the scale that runs north and south, gives me DOG ("Distance over the Ground) directly. and the TDS formula gives me the SOG.

Once you have the coordinates there is absolutely nothing to coastwise navigation that you cannot do using the simplest of mental arithmetic. Obviously, since fifteen minutes is 1/4 hour, if your estimated speed is 6 knots you will have run a nautical mile and a half in those 15 minutes. How d'ye know you're going 6 knots? You know that because when it is "in the groove", that's the top speed of a thirty-foot boat. Or a 27 footer. Or a 32-footer. So long as you observe COLREG #5 there is no need for greater accuracy than that :-)

Cheers

TrentePieds

[thinwater]

Originally Posted by TrentePieds

"Knots" are always, by definition, speed THROUGH THE WATER.....

TrentePieds

I thought this was the one interesting bit in this otherwise obvious thread.


Yup, the term knots derives from using a chip log, and that it sounds like a shortened form of nautical miles per hour is sort of coincidental.


However, in the practical sense, knots can obviously mean either SOG or STW, and several pages of pedantic debate make it clear why most people just use GPS SOG. Keeping the wheel working is a pain, keeping it calibrated is more pain, and you can work out tidal effects without it. For example, you probably know you speed through the water under certain conditions, sail or power, so the difference between that and you SOG is tide. Yes, there are angles and all that. But, remember from basic geometry, you only need 3 parts of a vector triangle to define the whole triangle. Most navigation is simpler if you are not working from a moving frame of reference (tide) ... other than the earth's rotation.

[HeywoodJ]

Originally Posted by TrentePieds

"Knots" are always, by definition, speed THROUGH THE WATER.

If, by definition, a knot is speed through the water, what does that mean for aircraft that measure their speed in knots?

[GordMay]

Originally Posted by HeywoodJ

If, by definition, a knot is speed through the water, what does that mean for aircraft that measure their speed in knots?

AIRSPEED.
To generate lift, an aircraft must move through the air, or air must move past the ‘plane, or both. This relative velocity is called the airspeed.
While ground speed is the airplane’s speed, relative to the surface of the Earth, [true] airspeed is its speed, relative to the air it is flying in.
Airspeed cannot be directly measured from a ground position, but must be computed from the ground speed and the wind speed. Airspeed is the vector difference, between the ground speed and the wind speed.
Airspeed = Ground Speed - Wind Speed
Conversely, ground speed is simply the sum of airspeed and wind speed.

[thinwater]

Originally Posted by HeywoodJ

If, by definition, a knot is speed through the water, what does that mean for aircraft that measure their speed in knots?

Speed through the air measured by a pitot tube.


Or a very long chip log.

[Orion Jim]

My paddle wheel knot meter is speed through the water. I obtain SOG from my MFD. I can easily see what effect the current is having on my progress…although this info is always available, as is the direction of the currant and the speed of my leeway, from my wind instrument. Modern networked instruments are light years ahead of what was available in the near past.