Get unlimited access to the best of MySeaTime for less than ₹100/week. Become a member

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 ?

Join now to access this article and much more.

Admin has placed this article behind a paywall, making it accessible only with a paid membership, which offers numerous perks:

Contribute to creating a platform for reading and writing without ads.
Enjoy all content here, including exclusive articles for members.
Gain access to all exclusive articles for members.

Comments (245)

Serj May 4, 2023

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

0

Login to Reply
Guru Bolar December 25, 2022

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

0

Login to Reply
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.

0

Login to Reply
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

0

Login to Reply
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)

0

Login to Reply
Deep steam cleaning in abu dhabi March 28, 2022

Thanks for posting this info. I just want to let you know that I just check out your site and I find it very interesting and informative. I can't wait to read lots of your posts. Thanks & Regards Deep steam cleaning in abu dhabi

0

Login to Reply
Apartment steam cleaning in abu dhabi March 26, 2022

Thanks for posting this info. I just want to let you know that I just check out your site and I find it very interesting and informative. I can't wait to read lots of your posts. Thanks & Regards Apartment steam cleaning in abu dhabi

0

Login to Reply
Steam cleaning service in abu dhabi March 25, 2022

This is really what I was searching for. I am feeling joyful to came here. I would like to thank you for sharing such info with us, I like to advise you that Keep sharing such ideas in the future as well. Thanks & Regards Steam cleaning service in abu dhabi

0

Login to Reply
manoj singh negi October 10, 2021

very good explanation in very simple manner

0

Login to Reply
Ibrahim alserafy October 3, 2021

Nice and detailed explanation,thank you very much.

0

Login to Reply
Viknesh August 14, 2021

Thank you capt it was of great help.

0

Login to Reply
Roman July 23, 2021

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

0

Login to Reply
Dai Software May 29, 2021

The blog is absolutely fantastic! Lot of great information which can be helpful about benefits of developing website. Keep updating the blogs. custom on-demand app development

0

Login to Reply
Rudra November 26, 2020

Sir good explanation...

0

Login to Reply
MAK November 18, 2020

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

0

Login to Reply
Rajesh Kumar November 5, 2020

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

0

Login to Reply
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.

0

Login to Reply
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.

0

Login to Reply
Pankaj Singh August 5, 2020

Very well explained sir !!!

0

Login to Reply
Lef June 2, 2020

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

0

Login to Reply

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.

More like this

In blog, Ship Navigation

A step by step guide to ordering and correcting charts on ECDIS

Many believe ECDIS has changed the way we navigate our ships. Rightly so, after all we see our ship all…

163

In blog, Ship Navigation

Here is All You Need to Know About Doppler Log

I am going to be brutally honest with you. To read about how Doppler log works is really a boring…

In blog, Ship Safety & Security

7 Medicines to Save Lives During Medical Emergencies on Ships

Have you faced any medical emergency on board during your career on board ? Medical emergencies at sea is the most…

In blog

Declaring Emergency: Important lessons for seafarers from air crash investigation

Have you been following the news about the disappearance of Malaysian Airline MH370? I am one among millions who agree…

In blog, Marine Conventions

5 Classification Society Terms You Need to Understand Now

Classification society !!! It is something that no ship can sail without. Drydocks surveys, statutory surveys, ship building, ship repairs,…

In blog, Ship Navigation

10 ECDIS questions SIRE inspectors ask and how to deal with it?

I cannot tell you how much do I enjoy writing on ECDIS. I truly feel that ECDIS has made the…

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

Join now to access this article and much more.

Comments (245)

About Capt Rajeev Jassal

Join our platform:

More like this

A step by step guide to ordering and correcting charts on ECDIS

Here is All You Need to Know About Doppler Log

7 Medicines to Save Lives During Medical Emergencies on Ships

Declaring Emergency: Important lessons for seafarers from air crash investigation

5 Classification Society Terms You Need to Understand Now

10 ECDIS questions SIRE inspectors ask and how to deal with it?

Login to Your Account