"Knots"?

[ZITIBOAT]

Just to be clear. Don't know how to use it don't be out there.

[wingssail]

Originally Posted by Bill_Giles

All information for navigation is useful. The difference between SOG and STW indicates the rate of the tide positive or negative. It also indicates when the tide is turned. The tidal rates and set from Tidal Atlases are not always very accurate. So any additional info is always useful.

BTW my log is always about 1kt shy of what the SOG says.

Bill, STW is most valuable when it is accurate. Most sailing instruments have speed calibration settings which you can change. STW calibration should be adjusted using SOG on a day without any current, (and two or more runs, opposite directions).

Next, it is very good practice to pull out the speed transducer when the boat is not in use, otherwise marine growth will affect the paddle wheel (assuming you have that type) and your calibration will be wrong. Finally, marine growth on the boat's bottom affects the boundary layer so the speed will degrade over time as the slime builds up on the bottom.

Keeping an accurate STW is a bit of a pain, but it is worth it if you are interested in measuring your performance (or calculating the set and drift of the current)

[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.

[Bill_Giles]

‘A nm never varies. It is defined today as exactly 6076.12** feet which is roughly one minute of latitude (1/60th a degree of latitude). How many nm there are in one degree of longitude vary.’

It is interesting that on a large scale chart the size of one degree latitude varies from North to South when away from the Equator. For practical navigation (never mind the strict definition) one NM varies. This is why international air routes are measured in kilometres where there is no doubt.

[Statistical]

Originally Posted by Bill_Giles

‘A nm never varies. It is defined today as exactly 6076.12** feet which is roughly one minute of latitude (1/60th a degree of latitude). How many nm there are in one degree of longitude vary.’

It is interesting that on a large scale chart the size of one degree latitude varies from North to South when away from the Equator. For practical navigation (never mind the strict definition) one NM varies. This is why international air routes are measured in kilometres where there is no doubt.

Latitude does vary (very slightly). One minute of latitude at the poles is
1.005 nm and at the equator it is 0.995 nm. It is 0.998 nm at the tropics.

A "modern" nm though doesn't vary. 1 nm is exactly 1852 meters (or exactly 6076.12 ft or exactly 1.825 km). It has been that way for the entire world ex US & UK since 1929. The US converted in the 1950s and the UK in the 1970s. If you have a distance in kilometers then you have a fixed distance in nm as well. Outside of the niche of very old charts there is a single conversion ratio for nm to km everywhere in the world.

Today a 1,120 km flight is 604.7516 nm everywhere in the world under all conditions.

Entirely possible international flights do use km (although feet for altitude) just to avoid confusion. Regardless though today a nm does not vary. I take credit for the confusion my historical reference probably just muddied the water. I just thought it was cool that despite the limitations of the 17th century they were really close. At the tropics less than 0.2% error and at the extremes (equator and pole) still less than 0.5%.

[TrentePieds]

Bill Giles said: "The difference between SOG and STW indicates the rate of the tide positive or negative." (emphasis mine).

How does that work, Bill? My rhumb line is 195ºT, my heading is 205ºM, the tidal set is 115º at 2 knots, Variation is 23ºE and deviation negligible. My STW is 6.5 knots. So what is the tidal "rate", please?

As a side issue, the words "rate" and "set" are NOT synonyms :-)!

And as a matter of philosophy: Spurious accuracy is the bane of the sailorman.

TrentePieds

[sailorladd]

What are SOG knots??? What are STW knots??? OMG WTBLEEP TTYL LMAO...

[JordanH]

Originally Posted by sailorladd

What are SOG knots??? What are STW knots??? OMG WTBLEEP TTYL LMAO...

I can't tell if this is sarcasm or not so I'll take it at face value.


SOG is "Speed over ground" (Basically how fast you are traveling over the seabed)

STW is "Speed through water" (Basically how fast you are traveling in relation to the water on which your boat is floating.)


When someone asks you, "How was your trip? Did you make good time?" Sailors often respond with, "Oh it was a great sail! I was making 7 knots!" So the question is, was the sailor giving his speed in SOG or STW?


I think general consensus is that most people assume 7kts implies STW unless otherwise stated with SOG.


J.

edit: Here's a real world example. My Contessa 26 is supposed to max out at 6.3kts hullspeed. However, in a big storm of 50+kts of wind, we hit a max speed of 15.6kts... that was clearly SOG, as it was recorded by our GPS and was caused by us being thrown forward by massive waves. We were not moving through the water at that speed. I always make sure to say "15.6kts SOG" so that people know it wasn't some uber-fast boat I sail.

[JordanH]

Other pet peeves...

When motorboaters call it "Knots per hour". I wonder if that's the rate at which they can tie bowlines...

When nonsailors try to be nautical and write them as "Nauts".

When armchair geniuses on youtube post youtube videos claiming that Nautical miles and the corresponding Knots of speed were called that because of of the old log devices with knots tied in the rope. They have that reversed. If that were the case, every captain would have different distances because every log would have different intervals between knots in the rope. <facepalm> The knots were tied in the rope at SPECIFIC intervals to match with the corresponding times to measure nautical miles.

[sailorladd]

Woke up in an unusually grumpy mood... Just fed up with how the English language seems to be changing at a breakneck speed of knots...everything is abbreviated now, my first assumption of that post was he was talking about rope knots...and not knot knots...

[Adelie]

Originally Posted by sailorladd

What are SOG knots??? What are STW knots??? OMG WTBLEEP TTYL LMAO...

SOG - Speed Over Ground
STW - Speed Thru Water
COF - Course Over Ground (direction the boat is moving)
Heading - Direction the boat is pointing.

If you are sailing, even if there is no current there can be a difference between SOG & STW
And COG & Heading due to leeway.

[Welti]

Good information! For my boat, I do not have a paddle wheel. I would like to at least have an estimate (ahead of time even) for currents where I am sailing. I know there are tide tables, but where can I find a current prediction/map? I mean, I have looked in Navionics and one or two other apps that I have and don't see it. Perhaps it is a Premium feature?

[Calif.Ted]

In practical terms it's usually about 1.5 knots downcoast, more during a winter storm from NW, however the California bight sometimes called the Catalina eddie creates a whirlpool most noticeable below Palo Verdes Point. From points downcoast the current is reversed and upcoast as you approach Catalina.
https://en.wikipedia.org/wiki/California_Current

[argonauta1]

Well Folks, I prefer to have all the info so I'll take paddle wheel distance recorded as well as actual GPS distance.

An example: on a 2909 NM passage Isabella Island to Fatu Hiva in the Marquesas, we anchored in Baie des Vierges 22 days and 10 hours out of Isla Isabella. Our paddle wheel log recorded 2726 NM for the passage.

One crosses 10 lines of latitude southbound and inevitably the course is never a straight line but is subject to the dictates of variation in wind direction to achieve best boat speed. So we covered certainly more than 2909 NM to reach destination. Maybe an extra 200 NM?? Subtract 2726 from 2909 and you get 183 NM. Throw in the fudge factor of a couple of hundred extra NMs and that is the benefit from the Equatorial Current. Nice to have that info.

Incidentally, our best 24 hour GPS distance was 167.5 NM. Worst 24 hour GPS distance was 117 NM.

[lestersails]

Originally Posted by sailorladd

Woke up in an unusually grumpy mood... Just fed up with how the English language seems to be changing at a breakneck speed of knots...everything is abbreviated now, my first assumption of that post was he was talking about rope knots...and not knot knots...

As goes the cute coffee mug; "Sometimes I wake up grumpy. Usually I just let him sleep."