Speed Through water or speed over ground, Which one to use ?

Very good explanation about SOG n STW. Very informative sir.... !!! This is the 2nd blog i hv read from u sir.. last one being on ECDIS.. thnx.. awaiting many more such blogs frm u !!!!!...

v.good and thanks

Glad you liked it AJAY.. Thanks for suggesting a topic, I will write on that near future.

Great explanation in an simplest way possible! Really appreciate your efforts

1: In all my life I never heard before of speed OVER water? did you mean speed THROUGH the water, Sir? 2: in the examples you made, you talk of GPS speed reading. Well, according to the majority of seafarers out there, the GPS will give you ALWAYS a SOG and not a STW, which is instead what the Log will give you..... Just those 2 things let me realize I lost 10 minutes of my life reading this "article". Regards.

Hello Captain, thank you for your lesson which i can't even get in my institute. I have a question by the way. For the collision example, if i observe visually when 2 ships are in sight, there will be a crossing situation right?

awesome blog sir....it washed away my lots of confusions....u have explained it in such a easy language..which made it so easy to understand...

Sir I have asked many people about the same qustion but I ddnt get a good ans.. after reading this my all doubts cleared. Thanks sir

This article is so good! Great great refreshment for memory! Thanks a ton cap!

Excellent explanation.

I am sure, all of us are heading for a better future of mariners. Thank u for your efforts.

Thanks Capt.

Thanks Sir for such a simple explanation.Sir,can you please tell me what is the difference between Log Speed and Observed Speed.How do we determine these two.Also which one of these are to be considered for calculating the fuel consumption or the Power.Thanks in advance.

Thanks a lot......u did a great job...dont know...how to thank you...please tell me diff btwn underway and making way....waiting eagerly for more interesting stuffs from your blog

Superb explanation, was never so easy to learn. kindly comment on underway and making way.Thanks

Awesomeee explanation sir..... Now my doubt abt sog and sow is totally cleared. Thnxs for sharing such wonderful information sir.

Very good job! thanks

Awesomeee explanation sir. But I still have a doubt about how an instant STW measured by doppler log is affected by a current from ahead. If the vessel's speed is 10kn and the current from ahead is 2kn, the speed (if no current) would be 12kn? And the SOG would be same 12kn?

Just to clarify, since you do not explicitly state GPS speeds in cases B and C, that probably gave the impression of GPS measuring speed over water. I think you may clarify that speed over water in all cases remains constant, and then ask what GPS reading will be. That will be clearer in my opinion. It will illustrate more clearly how speed over ground is affected by currents given constant speed over water.

Thank you very much sir, long time I had been seeking the suitable explanation on this important topic. Really very helpful.

Capt.Sab this is the 2nd blog I have read, the explanation with examples made interesting,really very practical approach in explaining keeping in mind all ranks. Very clear on Sog and sow.

Superb explanation sir. It's very easy to understand and gives thorough understanding of the subject.

Hi sir, so can I just safely say that STW is equal to engine speed based on the RPM given?

You're very thorough sir. Excellent interpretation. Very uselful

Very good explanation

Sir,i dont know you.But the way u explained the stuff really appreciable and helpfull.

hi sir, What about the Doppler Log in Bottom Tracking Mode, is it also unaffected by set and drift?

Sr it was very helpful but in shallow area from which log on ship gives us stw???,that time doppler will be giving us sog na sir

Thanks

Great job sir my concept is clear on this point. Thanks a lot

Hi Rajeev. If I get it correct, the reason why STW is preferred against SOG for collision avoidance is because it makes it more easy to define which RoR should apply to a situation and has to do nothing with the accuracy of both ways?

sir , when we navigating in coastal waters we have to use SOG OR STW

Sir great explanation by you,thanks i understood what is stw and sog,when we use sog its remain same head on situation but Only the geopraphical postion of collision will differ,my question is that why we are not using sog even in both situation remain head on. Thanks a lot sir.

Sir If you could help me on this...Which speed (SOG / STW) shall be used for squat calculation and why......

Hi Rajeev. A fine article you wrote. Do you know where I can see in the IMO regulations that radar shall have input from log? and how about echo sounder? is the echo sounder mandatory for ECDIS? Kind Regards, Birgir

Very useful information sir. So glad to learn from you.

Very useful and very clearly explained. Thanks

Sir, if i am at anchor and have a current of 3 knots ahead my STW and SOG will still be zero, is that correct?

Thank You for excellent explanation.

Hi Captain,

It would be incredibly helpful if someone could clarify why STW would be 5 knots facing an adverse current on the bow which also has speed of 5 knots? Yes, yes... STW remains the same regardless of current or other environmental forces as opposed to SOG which is affected by tides,wind,current etc. I clearly see the distinction. The theory of a vsl that is drifting downstream on the river with engine off resonates with me, i.e. in this case it would have SOG 5 knots and STW 0 knots given that speed of the current is 5 knots; conversely STW is 5 knots if it is propelling against the current, SOG would be 0. Right. Favorable and adverse weather conditions are applicable just to SOG, I get it. Now... I got it , until I discovered that EM speed LOG as well as Pitometer as well as Doppler ultrasonic LOG... probably there are some others (devices that measure exclusively STW), they all collectively rely upon the movement of the vessel over the sea surface e.g. EM Log on the speed of cunducting seawater flow, PIT log on the dynamic/static pressure caused by the facing water flow and Doppler on transmitted versus returned sound waves/frequences while ship is on the move. The question suggests itself, since STW is not equal to the Engine speed, how come when it is propelling against the current (I suppose it is not moving, and SOG supports this, it is 'zero'), if it is motionless and only thing that is moving is propeller... what kind of speed measuring equipment would show speed 5 Knots stw? Is there any connection between engine speed and STW in this case? The same applies to the vsl at high seas with weather conditions that slows ship down or speed it up... it is physically affected, consequently all the equipment mounted below the waterline should be affected and show affected speed. I realize that engine may produce the same revolutions irrespective of what wind blows or where current runs from... but... ( If STW not related to the rpm of the propeller, how does it stay unaffected by current etc and on top of that shows 5 knots while we are not moving against the stream ? )Please help me to unravel this. I appreciate your time, thank you for reading.

I just did not understand it, I felt it

Sir by which regulation we use speed through water for collision avoidance

Thanks for your sharing captain..this important for us who have fresh graduated from school....

SOW should not be used in RADAR at all as none of these instruments are showing any consistent reading . Collision avoidance in this case must be using the visual means as these rules applicable to vessels in sight of one another , and in the case of poor visbility then a complete set of rules will apply . Many times if you have used SOW then you will find the vectors are never steady as the EM Log or Doppler log has huge variations in s short span of time . In case of Doppler log it may even change to SOG when the depth is low . The aspect is only valuable in vessel sight of one another .The wind drift factor is not even considered in this assumption , the notion that only current will affect is not correct and in low depth areas current may affect differently (places like Singapore straights or coast of Japan etc ) . The SOW water to be only used for collision avoidance is something which requires to be questioned , but sadly in this industry we tend to have some holy things and SOW is one such thing . A common SOG is best for good situation awareness .

Even a civilian can understand, its simply great.

Sir I have one question regarding navtex warnings. Most of them are automatically plotted on Ecdis but few doesn't get plotted. Can u please help me out

Dear Capt.My query is regarding the speed showed up on doppler log furuno DS-80, while vessel is moored, the STW shows 10 knots!!! Is it a error ? Kindly advice on same.

Thanks Rajeev sir informative

thank you sir, the diagrams that you have given are with true vectors, what would be different scenarios if relative vectors would be used vis a vis sog and sow. and does the ODMCS input come from log speed or gps and what would be the difference in both cases and finally annex 12 kts regulation for discharge cretria is it log or gps speed, awaiting your answer

Than you very much for this post Very useful information, however if the GPS speed given at the start is 15 knots so is the SOG in any given situation. Then in the first example STW =17 knots and in 2nd example STW =13 kts? Thanks rgds

Thanks capt.

Respected Sir, Now I'm very clear about STW and SOG thank you sir for making it simple.

Current is some kind of resistance

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Good day sir. I have a question regarding this matter. If I use Relative motion display and echo trace, do I still have to use sow for collision avoidance?

Brief and well explained..Thank you so much for sharing this information..really helps a lot.

Why would a reader think that GPS speed is only for the first A example? When B specifically states "All other conditions being same but now we have 2 knots current from ahead." So if B has ALL the same as A which states "No wind, no current, absolute ideal conditions. The GPS speed of the ship is 15 Knots." that would mean GPS speed of 15 knots ... which is always over ground. Yet your answer for B gives the wrong amounts. That is confusing and wrong. B should be with GPS speed of 15 knots, SOW of 13 knots (15 STW - 2 knot current) and SOG of 15 knots (same as GPS speed as that IS what its telling you). IF you want the other examples not to include GPS speed, then it should not state conditions are all the same. OR in condition A it should put the GPS speed as a question. For example "What would be the SOW and SOG if GPS speed is 15 knots?" Phrased that way, that is a question to answer condition A, and no longer part of an ALL conditions of A.

Maybe in the end of the article, is the opposit :"Using speed over water, a navigator can be sure of his assessment of colreg situation. Using speed over ground can give an early signs if the ship is drifting towards a danger". If SOW is for Collision avoidance, and the SOG is for navigation, this end frase is saying just the opposit.

Kudos sir. Thank you for explaining so nicely. This was the best way how one can know about the difference between STW and SOG. Dil se thanx.

Explenation principally correct, but is completely oblivious to what the GPS measures. GPS measures your position relative to earth however and will therefore always provide speed over ground. However, if the GPS readings in this articel are changed to a measurement reading relative to the water, for example through a pitot-tube, then the math would be correct. /Aerospace engineer

Having spent my entire career on the Great Lakes where there generally are no SOG equipment installed due to our operation in very shallow harbors and rivers where the equipment would not survive I now can see what it might be used for. Generally, on the newer radars we are getting SOG was something that needed to be locked out since it might revert to some manual entry that had not been updated and the operator was not cognizant of. With AIS you can determine aspect as well as by the very reliable visual observation. However, I would trust GPS speed much more than anything derived from being in the water especially considering marine growth or other faults that would be unknown to the operator, thus the big push by some State Control authorities seems overdone. Thanks for the clear and simple explanation.

Nice blog sir.....

Only one question please because i am confused . Course through water i see in harpa and course over ground. Please explain with few words the differences

Very well explained sir !!!

Hi Sir, I really like all your posts. Somehow you have the ability to explain a difficult topic in a simple way. Whereas most lectures in maritime school (both physical and online) are making things more confusing (I think that's because they don't fully understand a particular topic). Please post more or even consider doing YouTube. Best regards from Singapore.

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Thank you capt it was of great help.

very good explanation in very simple manner

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When we are talking about RADAR use it is true. But modern vessels have ARPA systems. These are used in Nup mode RM(T) and regardless the speed input sensor selected the results are the same. They give the true motion of all vessels and your own vessel. So to my opinion majors first have to understand what is the system the vessel is provided. Two radars and one ARPA system? Or two ARPA systems? Usually its the second configuration. If a surveyor puts an observation like this and vessel has only ARPA systems it means he has no idea what is he looking at

Capt. Rajeev, Excellent explanation. Thumbs up to you.