This is my first time posting here, but I've already learnt a lot from reading many of the other helpful posts and answers.
I've been having a problem with getting accurate STW and SOG. Here is the scenario:
Heading due north with no wind and no current. The maximum STW achieved at full throttle is 16 knots. As you would expect, STW and SOG are the same at this point.
I add 4 knots of current from 000 and after everything settles down again, STW is now 17 knots. SOG is shown as 13 knots, which is correct in relation to an STW of 17 knots, but the STW shouldn't have changed in the first place.
I then change the direction of the current so that it's coming from 180. I wait for everything to settle down, but now the STW has dropped to 15 knots with a SOG of 11 knots. This time, the boat has lost 1 knot through the water!
I've tried this with multiple boats at multiple speeds. The errors get bigger or smaller depending on the amount of current being used and the length of the hull, but there are always errors. The only time at which there is no error is if the boat isn't making way. Then, she drifts at the same speed and direction of the current.
Has anyone else had this problem, and is there a fix?
Thank you.
STW and SOG Errors
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Sailorjohn
- Posts: 105
- Joined: Sun Mar 07, 2021 2:13 am
Re: STW and SOG Errors
Welcome to the forum, Mtaylor.
I haven't done a lot of navigating in NG using set and drift, but I have noticed some anomalies as you've noted. Perhaps related to this is that after installing the latest update to NG (10/31/23), I noticed that vessels at anchor no longer respond to the wind as before, but only respond to the current. It appears the wind and current vectors may have been compromised. Alas, with Ilan gone, there doesn't seem to be a path to update NG.
I haven't done a lot of navigating in NG using set and drift, but I have noticed some anomalies as you've noted. Perhaps related to this is that after installing the latest update to NG (10/31/23), I noticed that vessels at anchor no longer respond to the wind as before, but only respond to the current. It appears the wind and current vectors may have been compromised. Alas, with Ilan gone, there doesn't seem to be a path to update NG.
Re: STW and SOG Errors
Hello Sailorjohn, and thanks for your response.
I thought that this might be the case. I've learnt a great deal from your responses to a number of other posts, so thank you for that too.
I thought that this might be the case. I've learnt a great deal from your responses to a number of other posts, so thank you for that too.
Re: STW and SOG Errors
Hi guys!
I was so intrigued by these problems that I couldn't resist seeing for myself, hoping to find a fix.
I did my own tests using the same procedure as mtaylor and with three different types of boats.
Here are my results:
No happy surprise.
We can clearly see the inconsistency of STW but the perfect coherence between STW and SOG.
My first thaught was that the STW error of about 1NM is irritating but negligible, isn't it?
If our purpose is to navigate it means that we want to know at what point we will arrive after having experienced the current on a given course (experienced current) or we want to arrive at a precise point, and therefore anticipate the current when calculating our route (predicted current).
In the first case we already know STW and SOG, we have just to accept the STW as it is because it is the one that our boat is experiencing .
In the second case we know the current we'll find ahead but not the STW to take in account as it will be slightly different from the actual one and we don't precisely how much different .
Let's check the discrepencies in STW when having side current at different angles.
we can see that are not that big, perpendicular current keep initial STW value and a precise SOG, for other angles the difference doesn't exceed 2kt (for a fast boat).
It doesn't seems to me that it would to much erroneous to take the initial STW value for define our route, even with experience we could adjust it more or less tenths of kt; further more, as soon as we are experiencing the current we got the STW value taken in account by the simulator and then we can correct our heading.
We can probably live with it, can't we? especially if we consider that these measurements are taken in hypothetical conditions (no wind, no waves).
But here's a last-minute thought: the difference between STW and initial speed tends to increase with boat speed, so what happens with a strong 12kt current (actually the strongest tidal current in Europe)?
Well what is shown here isn't the most shocking; 7kt difference between max speed and STW, so what? forget the STW, but further tests show that it's impossible to get a negative SOG! except if you turn off the engine... 
What final conclusion?
1/ no fix found
2/ for navigation we shall accept the STW as it is and consider the speedometer as imprecise (after all SOG is still credible)and avoid sailing in extreme currents low speed boats for which the SOG will be totally aberrant.
BTW what could also be confusing and irritating in some cases is that graphically the bow wave and the pitch (hydro parameters) are ajusted according to the STW values but the wake waves according to the SOG values!
But there is at least one good news!
more or less...
The good news is that boats still steer into the wind: of course it depends on the size of the boat (mass, surface area in the wind, wind strength...).
After some weird tests where the boats moved more or less and very slowly, I finally discovered the culprit: the damping command in the graphics settings sub-menu!
My test: 2 ships, a ferry the ‘MegaExpress4’ (174m long, 19,000T), an oil tanker the ‘Nicolaos’ (274m long, 84,800T) both heading at 113, no current, the box.cfg checked that they were taking the full height of the superstructures, wind18kt from 180, damping value=0 / the ships turned into the wind in 32 minutes!
But for sure any current effect will prevail over that of wind...In this last example a side current of only 1kt oriented west turned the ships to 260.
That's all...not that much in fact.
Cheers!
I was so intrigued by these problems that I couldn't resist seeing for myself, hoping to find a fix.
I did my own tests using the same procedure as mtaylor and with three different types of boats.
Here are my results:
- 01.JPG (40.03 KiB) Viewed 396 times
We can clearly see the inconsistency of STW but the perfect coherence between STW and SOG.
My first thaught was that the STW error of about 1NM is irritating but negligible, isn't it?
If our purpose is to navigate it means that we want to know at what point we will arrive after having experienced the current on a given course (experienced current) or we want to arrive at a precise point, and therefore anticipate the current when calculating our route (predicted current).
In the first case we already know STW and SOG, we have just to accept the STW as it is because it is the one that our boat is experiencing .
In the second case we know the current we'll find ahead but not the STW to take in account as it will be slightly different from the actual one and we don't precisely how much different .
Let's check the discrepencies in STW when having side current at different angles.
- 03.JPG (41.53 KiB) Viewed 396 times
It doesn't seems to me that it would to much erroneous to take the initial STW value for define our route, even with experience we could adjust it more or less tenths of kt; further more, as soon as we are experiencing the current we got the STW value taken in account by the simulator and then we can correct our heading.
We can probably live with it, can't we? especially if we consider that these measurements are taken in hypothetical conditions (no wind, no waves).
But here's a last-minute thought: the difference between STW and initial speed tends to increase with boat speed, so what happens with a strong 12kt current (actually the strongest tidal current in Europe)?
- 02.JPG (29.85 KiB) Viewed 396 times
What final conclusion?
1/ no fix found
2/ for navigation we shall accept the STW as it is and consider the speedometer as imprecise (after all SOG is still credible)and avoid sailing in extreme currents low speed boats for which the SOG will be totally aberrant.
BTW what could also be confusing and irritating in some cases is that graphically the bow wave and the pitch (hydro parameters) are ajusted according to the STW values but the wake waves according to the SOG values!
But there is at least one good news!
more or less...The good news is that boats still steer into the wind: of course it depends on the size of the boat (mass, surface area in the wind, wind strength...).
After some weird tests where the boats moved more or less and very slowly, I finally discovered the culprit: the damping command in the graphics settings sub-menu!
My test: 2 ships, a ferry the ‘MegaExpress4’ (174m long, 19,000T), an oil tanker the ‘Nicolaos’ (274m long, 84,800T) both heading at 113, no current, the box.cfg checked that they were taking the full height of the superstructures, wind18kt from 180, damping value=0 / the ships turned into the wind in 32 minutes!
But for sure any current effect will prevail over that of wind...In this last example a side current of only 1kt oriented west turned the ships to 260.
That's all...not that much in fact.
Cheers!
Re: STW and SOG Errors
Hello Rakom,
Thank you for taking the time to look into this in such detail. As you say, now that we know about the issue we can just learn to work around it. I just wanted to be sure that I hadn't missed anything which could solve it.
Thank you for taking the time to look into this in such detail. As you say, now that we know about the issue we can just learn to work around it. I just wanted to be sure that I hadn't missed anything which could solve it.