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

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.

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.

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 ?

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Comments (245)

Serj May 4, 2023

It seems to me that in condition B the sped over water will be 17 kn

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Guru Bolar December 25, 2022

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

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Mirza Monnaf December 22, 2022

In my first ship, One-day i asked my 2/0, why you put speed through water during SIRE inspection and what's the differences between them? He tried but i couldn’t understand. Today i get it clear.Thanks Sir for your well explanation with suitable example which has made the topic easily understandable.

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DIonysios Garmpis July 21, 2022

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

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Aviorish May 9, 2022

I don't agree with your statement "In normal wind conditions, this engine speed will be equal to the speed over water (or speed obtained from log)". I will never the same even though no wind at all. Pitch is not a foreseen distance travelled through the water when propeller turn one round, rather it is a direct proportion of the pitch angle multiplied by 2 pi and radius of any point along the blade. It is a mathematical 'conversion' from rotational distance to the liniear distance. When it operates on the water, it meets viscous resistence which consists of viscous pressure resistance and friction resistence. It is unfair to judge speed log as erratic in the absence of foul hull. Even a clean hull, the viscous resistance will always there whih makes the propeller less efficient. The faster the propeller rotates, the bigger the resistance which increase the gap between log speed and engine speed (by pitch & rpm)

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manoj singh negi October 10, 2021

very good explanation in very simple manner

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Ibrahim alserafy October 3, 2021

Nice and detailed explanation,thank you very much.

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Viknesh August 14, 2021

Thank you capt it was of great help.

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Roman July 23, 2021

Greetings from ukrainian seafarers! Great article and a great webpage in general!

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Dai Software May 29, 2021

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Rudra November 26, 2020

Sir good explanation...

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MAK November 18, 2020

Great enjoyed reading your blog . Please share your E-mail ID to get in touch.

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Rajesh Kumar November 5, 2020

hi sir, when to check from sow to sog any depth requirement or whenever we see coastal

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Shun October 19, 2020

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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PanagiotisT October 4, 2020

Captain Rajeev your work on the subject is the best that i found and with your permission i will use it for training purposes. Thank you.

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Pankaj Singh August 5, 2020

Very well explained sir !!!

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Lef June 2, 2020

Can someone please explain me the difference between course through water and course over ground ?

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About Capt Rajeev Jassal

Capt. Rajeev Jassal has sailed for over 24 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.