When it comes to speed over water vs Speed over ground, many of us get confused. May be, it is a confusing topic. What do you feel ?

And the confusion is not only what is the difference but also where and when these are used ? Why we need to have input of speed over water in radar ? Why do we have doppler log on board ? And many more questions like these.

Speed over water and speed over ground

So today i want to put all these doubt and confusions to rest once for all.  But before i start, I need you to ask yourself this question. Do you know the answers of questions I asked above ?

Let’s test it. Can you tell what is the speed over ground and speed over water in these three conditions ?

Condition A: No wind, no current, absolute ideal conditions. The GPS speed of the ship is 15 Knots.

Speed over water

Condition B: All other conditions being same but now we have 2 knots current from ahead. What will be speed over water and over ground in this case ?

Speed over water situation B

Condition C: All other conditions being same but now we have 2 knots current from astern. What will be speed over water and over ground in this case ?

Speed over water situation C

Write down your answers. Tick tick….Tick tick.

Ok, I hope you have your answers. If your answers for speed over water for all the three conditions is same, most likely you have got it. But if you have different speed of water for all three conditions, you must read on.

Speed.. What it is ??

There is nothing absolute in this world. Everything is relative to something. Speed too is measured with respect to something adjacent. While travelling on a train you might feel increase in speed when another train passes on opposite direction. Or Sometime on railway station, we suddenly feel our train moving even when it is other train adjacent to ours which has just started moving.

So what is the speed of your train in this case. You may say zero but I may disagree. Depends on with respect to what we are talking about ?

Speed with respect to station will be zero, but with respect to other train will be equal to the speed of that train.

In the same way, the ship’s speed is either measured with respect to water or ground.

Speed Through water & Speed over ground

Speed over water is the distance traveled in one hour with respect to water. Similarly speed over ground is the distance traveled in one hour with respect to ground. Now what does that mean ?

Let us understand this and look for the answers of our three conditions. Imagine your ship is moving from an island and there is a floating boat near to the island. Sea conditions are perfect with no wind and current and your ship’s GPS is showing speed of 15 knots.

In one hour you would have covered 15 NMs with respect to island as well as floating boat. This is because the boat would have maintained its position as there is no wind/current.

Speed over water Condition A

So in Condition A, speed over ground and speed over water will be 15 knots.

Now as in Condition B, rest of the things being same, we have 2 knots current from ahead. In this case ship would only cover 13 NM in one hour because of adverse current. So when measured from island, vessel has only moved 13 NM. So the speed over ground will be 13 Knots.

speed over water condition B

Because of 2 Knots current, the boat would move 2 NM away from the ship. The distance covered by ship with respect to floating boat will be 15 knots. So the speed over water will still be 15 knots.

Now finally let’s assume that there is 2 Knots current from astern as in condition C. In this case ship would cover 17 NM in one hour because of favorable current. So when measured from island, vessel has moved 17 NM. So the speed over ground will be 17 Knots.

speed over water condition C

But because of 2 Knots current, the boat would move 2 NM in the direction of the ship. The distance covered by ship with respect to floating boat will still be 15 knots. So the speed over water will still be 15 knots.

What were your answers ??

Interpretation of Speed Through water

As you would have noticed, current has nothing to do with speed over water. Irrespective of how much current you have, the speed over water will remain same.

How can we use this information ? Let’s see.

Speed through water for collision avoidance

If you have been sailing on tankers you would already know how much emphasis vetting inspections put on this fact. They want us to use speed over water in radars for collision avoidance. Do you know why ?

To understand this, let me give you a situation. I know many of us are fed up with ROR situations during competency exams but this one would be interesting.

Your vessel is moving on a true course of 000 Deg. You have another vessel right ahead on a course of 180 Deg. There is strong easterly current and because of that your vessel is making good a course of 040 Deg. Other Vessel is making good a course of 140 Deg.

Your radar screen would look something like this.

Speed over water in RADAR ROR Situation

If there is risk of collision, what action you will take and under which rule ?

If you think you would take action as per crossing situation, you would be wrong. Collision avoidance rules and situations are based on how we see the ship and not on how they are moving.

For example, as per ROR, Head on situation is when you see a ship right ahead or nearly right ahead. That is when you can see both her Mast headlights in line or nearly in line and both of her sidelights.

Now Just visualise the situation I described. Will the defination of head on situation fit in this situation ? Yes it would.

But the problem is that if you follow just the radar, It will give you a false sign that the situation is a crossing situation. This is because the radar would be showing the course made good of both the ships.

This problem is tackled by using the speed over water in Radar. And that is the reason vetting companies require use of speed over water for collision avoidance.

When we use speed over water the same situation would look as a head on situation. That is because when we use speed over water, it does not consider the effect of current on the vessel.

ROR Comparison with both speeds

But in both the cases, CPA and TCPA would not change. That is because CPA and TCPA are calculated with range and bearing of the target between two or more intervals. Remember Radar plotting techniques ??

Is the GPS speed required in RADAR ?

There is no debate that GPS feed is required in Radars for position. But if we have to use speed over water in radar, do we require GPS speed then ?

Yes, we do. Infact i have come across some vetting inspector who ask to put the radar on standby. They then check what source of speed it is showing. If it shows GPS speed, they have an observation such as..

“Vessel did not use speed over water while approaching the present port as was evidenced from the radar screen in standby mode.”

In my view, this is really incorrect observation. Navigators need to use both speed over water as well as speed over ground judiciously. Speed over 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 situation than otherwise. For example, if the ship is drifting towards a danger, navigator will know it better if he has speed over ground in radar. This is because, with speed over ground radar will show a vector of course made good. (Vector need to be selected as true in this case).

Benefits of using speed over ground

As you can see in above, with speed over water navigator may get an impression that ship is moving clear of the danger. In actual it may be drifting towards the danger.

So Navigators should use both the speed to their benefit and as a tool to better navigation.

Log speed as Engine Speed

As we have seen, current has no effect on the log speed. For this reason, in normal wind force, log speed is very close to the engine speed.

Engine speed is calculated by Propeller distance divided by the time. Propeller distance is obtained by multiplying total revolution to a constant. This constant is proportional to the pitch of the propeller.

So

Engine speed = (total revs x constant) / Time

In normal wind conditions, this engine speed will be equal to the speed over water (or speed obtained from log).

If it is usually not same on you ship, there are few important conclusions that can be drawn.

1. Log is erratic

This can be a good indication of you log giving erratic readings. The conclusion however need to be verified with other means too. These include comparison with the GPS speed in calm weather with no current.

2. Foul Hull

There is another conclusion that can drawn from the difference in actual engine speed and log speed. There are chances of hull having considerable amount of marine growth. This will increase the resistance to the ship and ship’s speed over water will reduce to that with the clean hull.

This can give considerable difference in engine speed and speed over water. There is another way to confirm this finding. If the hull is fouled, ship may not be able to run on NCR rpm because of excessive torque. In run on NCR with foul hull, it is very likely that you will get torque limit alarm on the engine.

Conclusion

Speed over water and speed over ground are two distinct tool. One does not replace other. Navigator should make full use of these speeds where these best fit.

For example, navigator should use speed over water for collision avoidance. Speed over ground should be used for navigation.

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.

Share this:

Related Blogs

Follow us Today on
our social network

Search Blog

Capt Rajeev Jassal

About Capt Rajeev Jassal

Capt. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers. He holds MBA in shipping & Logistics degree from London. He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping.

150 Comments

Amit
Amit
Apr 2, 2016

Very good explanation , only a guy whose been there can highlight the concerns of a mariner in -charge.Simple explanation.....great effort!!!!!

Rajeev Jassal
Rajeev Jassal
Apr 22, 2016

Glad to hear that Amit... Keep me posted on any other topic you want me to write about..

nic
nic
Dec 23, 2016

so speed over water is not affected by current????

Rajeev Jassal
Rajeev Jassal
Dec 29, 2016

Yes, you are right NIC..

naveen
naveen
Jan 12, 2017

sir , i think i want to add one simple thing that during collision avoidance speed has nothing to do with roc, only thing changes is that when we select STW, radar takes heading feed instead of cog. its nothing to do with sog or stw.this is what i had learnt. thanks

Rajeev Jassal
Rajeev Jassal
Apr 30, 2017

That is the most simpler way of putting it but in reality it does not take the heading. How will Doppler and radar know the heading of the other vessels ?

Vivek Kothambath
Vivek Kothambath
Jun 15, 2017

Sir can u explain what is speed by bottom track? Is it log speed or from GPS

Mradul
Mradul
Apr 3, 2016

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 !!!!!...

Rajeev Jassal
Rajeev Jassal
Apr 22, 2016

Thanks for reading Mradul... And yes many more blogs will be coming your way..keep reading

Sourav Chhapola
Sourav Chhapola
Jul 24, 2016

Thank you sir, it was very informative and was in simple explanation

Sourav chhapola
Sourav chhapola
Jul 24, 2016

Sir, can u plz give some info what does a vetting ins.expect from 2nd officer on a paperless ship.what all ques he mighg ask. I am just promotted to 2nd and next port going to have vetting inp of shell.

Rajeev Jassal
Rajeev Jassal
Jul 24, 2016

You should how to do everything on ECDIS that you do on paper charts. For example marking of parallel indexing, Abort point, contingency anchorages etc. Apart from that how to correct ENCs and how to plot Nav warnings & Navtej warnings on ECDIS. There are couple of post that I have written on ECDIS, you can also go through that. If you have any specific question, don't hesitate to ask me. All the best Sourav..

king
king
Apr 4, 2016

v.good and thanks

Rajeev Jassal
Rajeev Jassal
Apr 22, 2016

Glad you liked it king

Taranand
Taranand
Apr 27, 2017

Sir i wanted to know, even if ship is stationary the log will show some speed and log gives speed through water,!

Rajeev Jassal
Rajeev Jassal
May 17, 2017

When the ship is moored, ship's log still can show some speed for example if the ship is moored in river with current. Log gives STW but modern doppler logs are also capable of showing SOG.

Karthick
Karthick
May 26, 2016

Really excellent sir. after being three years at sea,im well clear now. thank u soo much. great effor. keep posting more.

Rajeev Jassal
Rajeev Jassal
May 26, 2016

Happy that it helped you Karthick. Let me know if I can be of any help anytime.

AJAY
AJAY
May 30, 2016

Sir Brilliant explanation on sog and sow Please provide a blog on yaw control in autopilot

Rajeev Jassal
Rajeev Jassal
May 30, 2016

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

Gautam Dogra
Gautam Dogra
Jun 6, 2016

An infomative article in a very simple and lucid manner.Thankyou very much captain.Hope to get more fantastic surprises like this from you.Shiva bless you

Rajeev Jassal
Rajeev Jassal
Jun 6, 2016

Happy that you like it Gautam..

Rohan
Rohan
Jun 6, 2016

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

Rajeev Jassal
Rajeev Jassal
Jun 6, 2016

Happy to help Rohan.. Let me know if I can be of any other help anytime

Abinash
Abinash
Jun 6, 2016

Nice n simple explanation.....looking forward for use of Parallel indexing... And sir kindly reply in mail if possible

Rajeev Jassal
Rajeev Jassal
Jun 8, 2016

Glad you found it useful Abinash..Sorry but I didn't receive any email. You can contact me Through contact section in the top menu..

Druvil
Druvil
Jun 11, 2016

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.

Rajeev Jassal
Rajeev Jassal
Jun 11, 2016

Speed over water and speed through water are one and the same thing and I have heard both terms being used by many people. I am sorry to have you given an impression that GPS speed is speed though water but I am not sure why you got that impression as I referred GPS as speed over ground.

Aditya
Aditya
Nov 7, 2016

I think the three cases you illustrate give an impression that the GPS always measured 15kn, which happens to be speed over water in those examples.

Aditya
Aditya
Nov 7, 2016

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.

Rajeev Jassal
Rajeev Jassal
Nov 7, 2016

Thanks Aditya.. That may have caused the confusion for Druvil but I think most of the reader (including yourself) got it that GPS speed was mentioned just for the reference in the first example. I will think over it again how i can remove this confusion. Thanks once again for suggesting this..

Nic
Nic
Jul 9, 2017

I was in the same situation. I got confused by the way you introduced the speed in the 3 examples. It does seem that you are talking about a constant SOG of 15NMs and we have to figure out what would be the SOW for the three situation. If you have no clue about either it leads to confusion as it makes you think that it is the SOW that will change and not the SOG. Someone that knows a little bit about it will pass by it but someone that is not so well versed in Naval Nav will be left confused and might just stop reading. Here is a suggestion on how it would help alleviate the confusion: Condition A: No wind, no current, absolute ideal conditions. The GPS speed of the ship is 15 Knots and Log Speed indicate 15 Knots. Now Condition B: how would a current of 2 knots from ahead affect the SOG and the SOW? Then you ask the opposite type of current followed by : It might surprise you if you are new to Naval Nav but it is the SOG that will be affected. The SOW is the one that will stay constant. Here is why... then you carry on with your very well structured explanation of the both speeds and their difference. I know it is pretty much late ( Almost a year after you posted) but know that I just search this topic and your article was first on my google search. Thank you for the help cheers !

Henry
Henry
Jul 5, 2016

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?

Rajeev Jassal
Rajeev Jassal
Jul 5, 2016

No Henry.. if you are on one of the ship and when you observe other vessel, you will be able to see the vessel head on with both masthead lights in line.. Both the ships will be facing head on but will be moving in the line of their course made good. Hope that clarifies

Christin
Christin
Jul 9, 2016

Officers get easily confused when port state comes with such questions.. Hope you will post more of these stuff. And the figurative explanations are much more understandable than plain notes on these..

Rajeev Jassal
Rajeev Jassal
Jul 12, 2016

Happy that you liked it Christin.. I will be posting more of these in near future.. Thanks for reading.

mithilesh
mithilesh
Jul 9, 2016

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...

Rajeev Jassal
Rajeev Jassal
Jul 12, 2016

Glad that I could be of some help to you Mithilesh..

TAUFIQ S BHOMBAL
TAUFIQ S BHOMBAL
Jul 11, 2016

Dear sir,thank u for an easy n much useful explanation.Very well n clearly explained.Hats off to u.

Rajeev Jassal
Rajeev Jassal
Jul 12, 2016

Glad that you found it useful Taufiq..

jino job
jino job
Jul 14, 2016

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

Rajeev Jassal
Rajeev Jassal
Jul 14, 2016

Glad that it could clear some of your doubts Jino. Let me know if you have any question anytime..

Vivek kothambath
Vivek kothambath
Jul 14, 2016

Thank you sir for nice explanation

Rajeev Jassal
Rajeev Jassal
Jul 14, 2016

Glad you liked it Vivek...

Utkarsh
Utkarsh
Jul 14, 2016

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

Rajeev Jassal
Rajeev Jassal
Jul 14, 2016

Happy that you liked it Utkarsh..

Aditya shankar chakraborty
Aditya shankar chakraborty
Jul 14, 2016

Very practical explanation wd more relevent examples.....thank you sir.....its very helpful:):)

Rajeev Jassal
Rajeev Jassal
Jul 15, 2016

Glad you found it helpful Aditya..

Vincent D'Mello
Vincent D'Mello
Jul 14, 2016

Excellent explanation.

Rajeev Jassal
Rajeev Jassal
Jul 15, 2016

Thanks Vincent...

Clinton
Clinton
Jul 15, 2016

An excellent & simple explanation. Very well written & illustrated.

Rajeev Jassal
Rajeev Jassal
Jul 15, 2016

Glad you liked it Clinton..

Gurpreet
Gurpreet
Jul 15, 2016

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

Rajeev Jassal
Rajeev Jassal
Jul 15, 2016

Together we will make sure better future of Mariners Gurpreet..

cecilio rahon Jr
cecilio rahon Jr
Jul 15, 2016

Thanks for the post. May I share it?

Rajeev Jassal
Rajeev Jassal
Jul 15, 2016

In fact I will be obliged if you can share the link Cecilio..Thanks.

abin Robert
abin Robert
Jul 15, 2016

I smell a subramaniam in you, Great explanation. I never going to confuse this now onwards

Rajeev Jassal
Rajeev Jassal
Jul 16, 2016

Thanks Abin for your kind words.. Glad it could be of some help to you..

Levon
Levon
Jul 15, 2016

Thanks Capt.

Rajeev Jassal
Rajeev Jassal
Jul 16, 2016

Thanks for reading Levon..

Jitendra
Jitendra
Jul 23, 2016

sir very useful, great explanation and explaining abilty. you make coplicated topics to understand very easily.

Rajeev Jassal
Rajeev Jassal
Jul 24, 2016

Thank you Jitendra.. Happy that you found it useful..

Gilson
Gilson
Jul 24, 2016

Hi, thanks for the explanation, it was very helpful. I would like to know if this blog is still working, Could not find new tops on it. Thx!!

Rajeev Jassal
Rajeev Jassal
Jul 25, 2016

There are many blogs that I have posted after this Gilson. You can visit the list by clicking "Blog" on top of this page. Or you can go to www.myseatime.com/blog . If you still cannot view it, let me know. I will get it sorted out.

Abdul
Abdul
Aug 11, 2016

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.

Rajeev Jassal
Rajeev Jassal
Aug 21, 2016

The power and fuel consumption is not calculated by the the speed. Log speed is the speed made through water and observed speed we refer to the distance made between point A and B. Observed speed could be different from GPS speed because we might have done few alterations to avoid traffic and would not have proceeded directly from point A to point B. So observed speed is the effective speed made by the vessel.

cLivanz
cLivanz
Aug 17, 2016

i like your blog Mr. Rajeev indeed its helpful. Id like to ask you something, about T&P for Jeppesen C-map, it seems it doesn't show the visualization of T&P unless you have list of T&P from NTM.

Rajeev Jassal
Rajeev Jassal
Aug 21, 2016

Yes, Infact T&Ps through AIO only shows the affected area and T&P number which user need to view the information by going to the properties. UKHO is trying hard to come out with a better way to display T&Ps. Till that time we have to live with it. It would be a good practice to keep the print outs of the T&Ps affecting voyage ENCs and keep near the ECDIS for the watch keepers to refer quickly.

Vijeesh kumar
Vijeesh kumar
Aug 23, 2016

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

Rajeev Jassal
Rajeev Jassal
Sep 15, 2016

Underway is when ship is afloat but not at anchor or at berth and not using engine. Making way is when vessel is making way through water and using engines.

NICOLAS
NICOLAS
Sep 14, 2016

Hello, can clear a doubt for me ?, when we speak about SOG and we need instant SOG, for example in anchoring, which source is best, GPS or doppler speed log ? I think Doppler speed log is best for being instantaneous, and GPS needs to compare between 2 positions and maybe one of them is not so good to be consider for this purpose....Am I right ? thanks a lot

Rajeev Jassal
Rajeev Jassal
Sep 15, 2016

Nicolas.. Doppler does not give SOG. Doppler gives speed through water. Also GPS speed is good and accurate and we can use it during anchoring also.

NICOLAS
NICOLAS
Sep 15, 2016

Excuse me Sir but you can select in some doppler speed logs STW or SOG, obviously when sea bottom is available, next is an extract for a manufacter brochure : " The Doppler speed log measures a vessel's speed through the water or over the seabed ". I personally remembered this function onboard. Thanks anyway, very good article

Rajeev Jassal
Rajeev Jassal
Sep 16, 2016

I worked on doppler log which had the option of STW and SOG but SOG was taken from the GPS feed but I never came across Doppler log which could also measure SOG. Thanks for that feedback. But coming to your original question, we still can use GPS speed during anchoring.

NICOLAS
NICOLAS
Sep 16, 2016

good information sir , you might think the equipment is working by itself and there is cross check but it is being feeding by the other, better to follow the input signal to find where they go...thanks

sushant singh rawat
sushant singh rawat
Sep 20, 2016

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

Rajeev Jassal
Rajeev Jassal
Sep 22, 2016

Glad you liked it Shushant. Next post in line will discuss the difference between underway and making way.. Hope you will find that helpful.

Prabhath Mathew
Prabhath Mathew
Sep 20, 2016

Amazing explanation sir. Will ARPA calculate different CPA for SOG and STW ? Which will be the accurate CPA ?

Rajeev Jassal
Rajeev Jassal
Sep 22, 2016

ARPA will show same CPA and TCPA with both the speeds. Both will show exactly same CPA & TCPA

saba karim
saba karim
Sep 24, 2016

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

Rajeev Jassal
Rajeev Jassal
Sep 26, 2016

Glad you found it useful Saba..

Gulbag Singh
Gulbag Singh
Sep 25, 2016

Thanks capt for taking the pain to gain us

Rajeev Jassal
Rajeev Jassal
Sep 26, 2016

Happy to help Gulbag..

Capt.Dr. russtam Mohammed Ismail
Capt.Dr. russtam Mohammed Ismail
Sep 27, 2016

Very good job! thanks

Rajeev Jassal
Rajeev Jassal
Sep 29, 2016

Thank you so much for your kind words Sir..

Fritz Gerald Roco
Fritz Gerald Roco
Oct 7, 2016

Good Day Sir! I would like to ask some question regarding this matter. You made a real good point in using the speed over water to determine the situation of both vessel if it is crossing or head on situation which is right. Now that you have determine the situation and apply the maneuver should you maintain using SOW or should you use SOG?

Rajeev Jassal
Rajeev Jassal
Oct 9, 2016

Once you know the situation, all you want to know is CPA & TCPA. Both speeds will give same CPA/TCPA so it does not matter which speed you use for that.

Hector Prado
Hector Prado
Nov 7, 2016

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?

Rajeev Jassal
Rajeev Jassal
Nov 9, 2016

Hector, the doppler speed is not affected by the current. It does not matter how much the current is, doppler speed (STW) will be same because of its principle of operation.

Harmanpreet
Harmanpreet
Nov 9, 2016

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

Rajeev Jassal
Rajeev Jassal
Nov 9, 2016

Glad you found it useful Harmanpreet..

Jack
Jack
Nov 9, 2016

This is the best explanation of STW and SOG that all Masters, Officers and Cadets should read. Appreciate your effort and time on this writing.

Rajeev Jassal
Rajeev Jassal
Nov 9, 2016

Thank you for your kind words Jack..

Kadali
Kadali
Nov 13, 2016

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.

Rajeev Jassal
Rajeev Jassal
Nov 17, 2016

Thanks Kadali..

Psingh
Psingh
Nov 29, 2016

a doubt: Does anywhere in RoR or as per IMO is given to use STW for coll avoidance? also how to explain that RoR is based on aspect? like heading of the vsl? thanks in advance

Rajeev Jassal
Rajeev Jassal
Nov 29, 2016

No.. It is not mentioned in RoR that speed through water to be used for collision avoidance. But it is rightly implied so by oil majors and by the industry experts. This is the main reason why we have feed of speed log in RADARS. During SIRE inspections on tankers, it is checked by the inspector if the ships are using STW. ROR is based on aspect because all the definitions of situation like Head on and overtaking etc are as per the aspect of the vessels and not how the two vessel are moving.

Swapnil
Swapnil
Dec 4, 2016

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

Rajeev Jassal
Rajeev Jassal
Dec 4, 2016

Glad you found it useful Swapnil..

Andy
Andy
Dec 6, 2016

I found very useful of your blog. i still have i more questions, what meant by sea stabilized and ground stabilized in radar setting? Thank you so much.

Rajeev Jassal
Rajeev Jassal
Dec 6, 2016

Glad you found it useful Andy. Sea stabilised Radar is when we use log speed (speed through water) in the radar. Ground stabilised Radar is when we use GPS speed (speed over ground) in the radar.

Angie
Angie
Dec 8, 2016

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

Rajeev Jassal
Rajeev Jassal
Jan 29, 2017

In ideal condition..Yes but the difference will be if any speed reduction due to foul hull or due to high wind.

Rigel CHEUNG
Rigel CHEUNG
Jan 2, 2017

Somethings in doubt, my ship's doppler logs have a 'bottom lock' function when UKC is less, it means the doppler log will not provide STW to APRA, If APRA are setting to true motion radar display, what problem will come out ?

Rajeev Jassal
Rajeev Jassal
Mar 13, 2017

In this case as the ARPA will have SOG, even if the radar is set to true motion, the targets will be shown moving along their course made good. We still will not have the aspect of the vessel on radar on which ROR is based.

Ian Kirby
Ian Kirby
Jan 2, 2017

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

Rajeev Jassal
Rajeev Jassal
Jan 29, 2017

Glad you found it useful Ian..

NIVAS
NIVAS
Jan 29, 2017

GREAT EXPLANATION SIR, SINCERELY APPRECIATE YOUR EFFORT FOR FELLOW MARINERS!!

Rajeev Jassal
Rajeev Jassal
Jan 29, 2017

Thanks Nivas..

Rajesh Sharma
Rajesh Sharma
Feb 20, 2017

Very good explanation

Rajeev Jassal
Rajeev Jassal
Feb 25, 2017

Thanks Rajesh..

Vasyl Plotny
Vasyl Plotny
Feb 26, 2017

Very informative article, all laid out on shelves and extremely affordable explanation. Thank you!!!!

Rajeev Jassal
Rajeev Jassal
Feb 26, 2017

Thanks Vasyl...

Nirmal
Nirmal
Feb 28, 2017

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

Rajeev Jassal
Rajeev Jassal
Mar 13, 2017

Nice to hear that Nirmal...

rakesh
rakesh
Mar 8, 2017

u made this concept looked so simple....short and simple..now i can explain anybody abt this concept

Rajeev Jassal
Rajeev Jassal
Mar 13, 2017

Thank you Rakesh.. Let me know if you have any doubts on any other topic..

Nish Walker
Nish Walker
Mar 13, 2017

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

Rajeev Jassal
Rajeev Jassal
Mar 13, 2017

Few Doppler logs can measure the speed over ground by "Bottom tracking mode". As this is speed over ground, it will be affected by current..

Nish Walker
Nish Walker
Mar 13, 2017

Thank you sir well understood! have you wrote any article on Auto Pilot systems. I've doubts in the Alarms especially with the Tokyo Keiki Systems 1)pilot watch alarm, 2)Heading monitor, 3)Off course Alarm ?

Rajeev Jassal
Rajeev Jassal
Mar 13, 2017

I haven't written yet on Auto pilot but I will surely consider writing on that..

Nish Walker
Nish Walker
Mar 13, 2017

Understood sir, you're blog is really helpful!

Rajeev Jassal
Rajeev Jassal
Mar 13, 2017

Thanks Nish..

nitish kumar
nitish kumar
Mar 23, 2017

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

Rajeev Jassal
Rajeev Jassal
Apr 17, 2017

Doppler log wil still give the Speed through water..In some Doppler log, it can additionally measure speed over ground too..

Capt.Vivek Trivedi
Capt.Vivek Trivedi
Mar 31, 2017

Hi Rajeev went through your blog,Excellent the explanation was simple and clear. Further i wish to work on Safety culture ............need to discuss further. My email ID is smrviv@yahoo.co.in.

Rajeev Jassal
Rajeev Jassal
Apr 17, 2017

Thanks Capt Vivek.. Will get in touch..

Paulus Pical
Paulus Pical
Apr 15, 2017

Thanks

Paulus Pical
Paulus Pical
Apr 15, 2017

Many thanks Sir,Your explanation very clear and helpful.Good info for mariners.

Rajeev Jassal
Rajeev Jassal
Apr 17, 2017

Glad you found it Helpful Paulus..

Brajesh kumar
Brajesh kumar
Apr 17, 2017

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

Rajeev Jassal
Rajeev Jassal
May 15, 2017

Glad to hear that Brajesh..

Gokul Teja
Gokul Teja
Apr 29, 2017

Thank you Sir for the simple and effective explanation. I am going for my mates orals in a couple of weeks and am a bit confused regarding the stabilisation display on a RADAR. How to identify by viewing a PPI if the stabilisation mode is sea stabilized or ground stabilized with an aid of a picture for better understanding. Thanks in advance.

Angelos
Angelos
May 6, 2017

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?

Rajeev Jassal
Rajeev Jassal
May 15, 2017

Absolutely Angelos.. With both the speeds, there is no change in CPA, TCPA..

Angelos
Angelos
Jul 15, 2017

Hi again and thanks for clearing things out. That article was really a relief since it explained all that someone needs to know on that issue. It is always very important to satisfactory explain to people "why" they need to do something in order to convince them do it even while you look the other way and your article does exactly that. Thanks again.

Franc
Franc
May 12, 2017

I still dont understand one thing - how Dopller measures STW in shallow water. The signals should reach bottom so this is automatically SOG?

Rajeev Jassal
Rajeev Jassal
May 15, 2017

No Frank. The doppler measures the speed through water by doppler effect which measures the shift in rays from the direction it is transmitted. The doppler shift exists and is measured in shallow waters too..

Franc
Franc
May 16, 2017

Thx Rajeev for clarification

Rajeev Jassal
Rajeev Jassal
May 17, 2017

You're Welcome Franc..

kumaran
kumaran
May 17, 2017

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

Rajeev Jassal
Rajeev Jassal
May 17, 2017

We have to use both as both gives us information that may be helpful for safer navigation. As I pointed out, STW is to be used for collision avoidance as it allows the radar to show aspect of the vessel. SOG is useful because it gives the "course made good" of own vessel which can be used to know if the vessel is drifting towards a danger.

Musthafa
Musthafa
May 18, 2017

Beautiful Explanation with Diagram. Had never been so easy all these Days

Prashant
Prashant
May 24, 2017

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.

Stephen Shantha
Stephen Shantha
May 25, 2017

The best explanation I read on this subject. Thanks . Please share with us more topics. One thing I am interested is plotting a celestial observations on ENC

prince
prince
Jun 9, 2017

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

prince
prince
Jun 9, 2017

Thanks, Sir, I found the answer in your Squat topic. It's really helpful. Appreciate....

Birgir
Birgir
Jun 9, 2017

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

Ahmed
Ahmed
Jun 23, 2017

Great article. Please can you provide explanation (difference) between for and aft speed, transverse speed and athwartships speed.

Ajay
Ajay
Jun 24, 2017

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

Leave Comment