Squat on ships

Years back when I first came across the term squat, frankly I failed to understand it. Well if you just want to know the mathematical formula and calculate the squat, it is no rocket science. But to answer questions like “Why do squat effect take place” may not be easy to understand.

Are you too in the same boat ?

Most of us know that Squat is the decrease in ship’s under keel clearance due to vessel’s movement in the shallow water. And it is not a theoretical thing, it is a real thing.

Incident of sinking of RO-RO vessel “Herald of free enterprise” was the result of squat.

But squat is not always bad. In 2010, passenger vessel “Oasis of the sea” used the squat to its advantage. It allowed squat to reduce its air draft. This helped the vessel to safely pass under a bridge, which otherwise was not possible.

Incidents like these show how important the knowledge of squat is. But there are many questions related to squat, answers to which are sometimes difficult to find.

In this post I will try to answer five of these questions related to squat that mariners usually ask or enquire about.

5 Questions about Squat effect

Question 1: Why and how does squat effect take place ?

The ships float in water because of one simple reason. There is no net force acting on the ship. Now it is not that there are no forces acting on the ship. But all these forces are equal and opposite in nature.

Two of these forces that act in opposite directions are gravitation force and buoyancy. Force of gravity like to sink the ship and force of buoyancy likes to make it float. Force of gravity continue to sink the vessel until force of buoyancy becomes equal to force of gravity.

How do Ship floats

Even when we add a weight (cargo) on a floating ship, the gravitation force increases. This will cause the ship to sink up to a point when force of buoyancy (that increases as per the Archimedes principle) becomes equal to the gravitation force.

If you wish to read more about Archimedes principle, you can do so by clicking here, here or here.

The point I am trying to make here is that any increase or decrease in a force on or around ship will affect the ship in a way that depends upon the direction of force.

Squat is the decrease in ship’s under keep clearance when it moves in shallow water because of low pressure created under the ship.

Now the question is why do we have low pressure under the ship when it is moving in shallow waters. The answer lies in Bernoulli’s theorem.

If you are OK with reading little bit of physics, you can read about the Bernoulli’s theorem by clicking here or here.

But if you are in no mood to deviate from the topic of squat then you just need to know following from Bernoulli’s theorem

As per Bernoulli’s theorem, in a flowing liquid if the flow velocity increases, the pressure in the region would decrease. Above conclusion is drawn from the Bernoulli’s law of conservation of mass in a flowing liquid. As per Bernoulli’s theorem, the mass of flowing liquid in per unit area will always be same.

Now have you tried to run fast and felt air resistance acting on your chest ? You feel some pressure on your chest. But do you feel similar pressure on your back ? I am sure your answer is No.

You feel this pressure on your chest because your chest is trying to replace the air as you move (or run) forward. The air so replaced by you fills the vacuum you created by leaving your earlier position.

In the same way, when a ship moves forward it pushes the water forward. The water all around must flow under and around the hull to replace the volume of water pushed by the bow.

In open sea there is no problem for the water to flow under the hull. But in shallow waters, this flow is restricted. This results in higher flow velocity of water passing under the hull. And there is decrease in pressure because of high velocity of water (as per Bernoulli’s theorem).

Squat effect on ships

Now as the pressure at the bottom of the ship decrease, ship need to react in some manner to compensate that. Remember we said, ship’s float because net force acting on the ship is zero. This drop in pressure is compensated by the sinkage of the vessel as the direction of this force (low pressure) is downwards.

But will this sinkage be bodily, by stern or by bow ?  We will discuss it later.

Question 2: What are the factors that affects Squat ?

Now that we know the reason behind squat effect, let’s see what factors affects squat.

Speed of the vessel

As we know the squat is caused by the low pressure that is developed under a ship in shallow water. With more and more speed of the vessel, the squat will increase. This is because with more speed, the vessel will push more water forward and more water is required to fill that void.

This will cause more drop in pressure under the hull and vessel need to sink more to compensate for that drop in pressure.

But we need to understand that the speed here is “speed through water” and not “speed over ground”. Why, you may ask ?

Effect of speed on Squat

Consider a ship moving at 6 Knots GPS speed with 6 knots current from astern. Is the ship pushing any water forward ? No it isn’t because the water is flowing with the ship. In fact in this case ship will not be using engine as the ship will be moving with the current. Will there be any squat in this case ? No there will not be because as the ship is not pushing any water forward, no water is required to pass under the hull of ship.

So the squat in this case will be zero because the ship’s speed through water is zero. This is even when the ship is having a speed over ground (GPS speed) of 6 knots.

So we can say that squat depends upon the speed through water.

This is also the reason that vessel can experience squat while alongside in a river with strong current. In this case vessel’s speed over ground is zero but speed through water is equal to the the river current.

Block coefficient of the vessel

I am sure you already know what block coefficient of the vessel is. But I will refresh this for those who might need it.

Block coefficient is the ratio of vessel’s underwater volume (Displacement) to the volume of a box that this could fit in.

So for a box shaper vessel, the block coefficient will be 1.

Block Coefficient of ship

But how does block coefficient of a vessel affect Squat ?

Again it all depends upon how much water a moving ship pushes forward. Let me ask a question. Which ship will push more water while moving. A box shaped vessel or a vessel like this in the picture below.

Ship squat vessel with less block coefficient

 

I am assuming that you have got it right. Yes, a box shaped vessel will push more water and hence will have more squat compared to the ship in the photo above provided all other conditions are same.

So more the block coefficient of the vessel, more will be the squat.

Blockage factor of the canal and narrow channel

Canals and narrow channels creates a different scenario. In a canal, Apart from having shallow water beneath, even the sideways water flow is restricted. This creates additional low pressure which affects the squat.

But how do we know if the blockage factor exists or not.

Blockage factor is a ratio of ship’s immersed cross section to the cross section of water within the canal.

We can calculate the blockage factor by this formula

Blockage factor = b x h / B x H

Blockage factor

Blockage factor of less than 0.100 represents open sea like conditions and hence no blockage factor.

Blockage factor of 0.265 represents narrow channel.

Question 3: How do we know if the squat will result in forward trim, aft trim or no trim ?

As we discussed, in shallow depths, the water tries to fill the void created by moving ship. For the fine-form ships like Queen Mary 2, the bow of the ship will not obstruct the water flow as much as the mid and aft section of the ship. This is because of the shape of the bow.

In this case the effective low pressure will be aft of the midship. This will cause the stern to sink more than the bow and will result in trimming aft because of squat.

With full-form ships like super-tankers, it is other way around. On these ships bow shape is what we call full-form. Because of which bow obstruct considerable amount of water flow. The resultant low pressure created by the obstruction is forward of the midship and these vessel squat will occur more at the bow. This will result in trimming forward because of squat on these vessels.

The tendency of the bow to obstruct the water flow is related to the block coefficient of the vessel. Block coefficient of the vessel also defines if the vessel will squat bodily, by stern or by bow.

By various calculations, shipping scholars have got a defining value (0.7) of block coefficient. If the block coefficient is 0.7, the vessel will squat bodily. If the block coefficient is less than 0.7, the vessel will squat by stern. Finally if the block coefficient is more than 0.7, the vessel will squat by bow.

Dr Barrass has done extensive research on the topic of squat. And as per him, above rule will only be applicable when the ship is at even keel in static position.

As per him, if the vessel is trimmed by stern in static position, the maximum squat will be towards stern. And if the vessel is trimmed by bow, the maximum squat will be towards bow.

So we can conclude as per below

Squat trimming effect

Question 4: How can we calculate Squat ?

This is most important question. How can we calculate squat ?

There are two ways to know how much squat you can expect. One with the help of a software and second by manual calculation.

Calculating Squat manually

There are number of formulas to calculate squat. But Dr Barrass’s formula is widely used for calculating squat. Dr Barrass’s formula has several version ranging from the complex formula to the simpler ones.

Have a look at the complex one.

Squat Formula Dr Barrass

This formula has a simpler version which takes into account blockage factor.

Squat with blockage factor

And more simpler formula and which is used by most of the navigators is the most simplified version of Dr. Barrass’s formula.

Squat CalculationsIf you notice, the simplified formula above is derived by applying the blockage factor of open sea (0.100) and that of a canal (0.265).

Calculating squat with a software

There are plenty of softwares available to calculate squat. If you are using a software onboard for calculating squat, make sure that it has been provided by your shore office. Random softwares can give wrong values and as such can lead to mis-calculation of squat.

One of the authentic and good software for calculating squat is UKC manager.

To calculate the squat on UKC manager software, open UKC manager and enter ship’s static data.

UKC Manager Static Data

Next enter ship’s dynamic data. In the dynamic data we just need to enter the values of draft at forward and aft perpendicular. Rest of the data is not required if you only need to have the value for squat.

UKC Manager Dynamic data

Next enter topographical data. If you are in doubt about sea type (open, restricted or canal), assume canal for being on safer side.

UKC Manager Topographical data

Now under the “UKC Calculation options” we can tell the software what we want to know ? Do we want to know the speed at which we can achieve required UKC ? Or do we want to know at what height of tide we can achieve the required UKC ? or do we want to know what should be our static draft to achieve the required UKC ?

UKC manager UKC options

After choosing the required option, we can save and then click on results.

It will give the required results in complete detail which navigators can use for navigation.

UKC Manager Results

 

Question 5: What are the signs that show vessel is experiencing squat

While we need to allow the squat while calculating the ship’s UKC in all stages of voyage, there are certain signs which can show that we have entered in the shallow waters. Knowledge of these signs can help the navigators to be more vigilant and keep an eye on the echo sounder.

Presence of these signs is also a good time to re-confirm the squat with your calculation. For example if we expect our UKC to be 5 meters at this position and actual UKC is 4 meters, it would be better that we reduce our UKC by 1 meter in other stages of the voyage. We can then re-calculate if we are complying with UKC policy of the company. If not we can calculate at what speed we can comply and proceed at that speed.

So what are these signs which show that vessel is in shallow water and is experiencing squat ? These signs are

  • Ship’s steering becomes sluggish. That is it becomes comparitively difficult to steer the ship
  • Engine rpm will decrease to compensate for the load on the engine.
  • The speed of the ship will decrease. I have experienced with 0.7 meters UKC, vessel moving at full ahead only making 6 knots GPS speed.
  • The ship may start to vibrate
  • Mud showing up around ship’s hull
  • Vessel’s rolling and pitching reduced
  • Turning diameter of the vessel increases (it can become as much as twice to that in open sea)

Conclusion

Squat is not a theoritical term. It is a real practical phenomenon experienced on ships moving in shallow waters. People have lost life because of ships that sank because of squat. Ship owners have lost millions of dollars because of grounding of ships.

It makes more and more important to have complete knowledge of squat and answer to these five questions can help in that.

Do you know any other question related to squat that has gone unanswered ?

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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.

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70 Comments

Vivek Kothambath
Vivek Kothambath
Jul 21, 2016

Thnx. Gud explanation

Rajeev Jassal
Rajeev Jassal
Jul 21, 2016

Glad you liked it Vivek..

Rakesh
Rakesh
Jul 22, 2016

Very clearly explained sir.....Thank you.

Rajeev Jassal
Rajeev Jassal
Jul 22, 2016

Glad you liked it Rakesh..

Capt. Vijay Kumar Sharma
Capt. Vijay Kumar Sharma
Jul 22, 2016

A very comprehansive explaination. Nothing left to doubt

Rajeev Jassal
Rajeev Jassal
Jul 22, 2016

Thank you Capt Vijay for your kind words..

Vinay Singh
Vinay Singh
Jul 22, 2016

Just one question.. Can there be better explanation than that ? And the answer would be NO.. Thanks for your great efforts.

Rajeev Jassal
Rajeev Jassal
Jul 22, 2016

Happy that you liked it Vinay...

Rajdeep Singh sabharwal
Rajdeep Singh sabharwal
Jul 22, 2016

Great explanation sir

Rajeev Jassal
Rajeev Jassal
Jul 22, 2016

Glad you liked it Rajdeep..

Capt Sanjay Prasad
Capt Sanjay Prasad
Jul 22, 2016

Thanks Capt Rajeev for one more marvel. Your dedication is commendable. Very clearly explained and nicely presented.

Rajeev Jassal
Rajeev Jassal
Jul 22, 2016

Thank you Capt Sanjay for your kind words. People like you reading this is the fuel required for the hard work..

Siddhant chaturvedi
Siddhant chaturvedi
Jul 23, 2016

Very well explained sir.... it almost cleared all of my ambiguity , about squat

Rajeev Jassal
Rajeev Jassal
Jul 23, 2016

I am glad that it could be of some help to you Siddhant..

Naval Aranke
Naval Aranke
Jul 24, 2016

Good work on the blog Rajeev!

Rajeev Jassal
Rajeev Jassal
Jul 24, 2016

This comment coming from you mean a lot to me Sir. Thanks a lot Sir.

Akshay
Akshay
Jul 24, 2016

Clearly understood.Thankyou

Rajeev Jassal
Rajeev Jassal
Jul 24, 2016

Glad you liked it Akshay..

Selambharasan
Selambharasan
Jul 24, 2016

It was clear cut explantion sir ji

Rajeev Jassal
Rajeev Jassal
Jul 24, 2016

Thank you for reading Selambharasan..

vamsi
vamsi
Jul 30, 2016

nice explanation cap

Rajeev Jassal
Rajeev Jassal
Sep 5, 2016

Thank you Vamsi..

Alejandro Henao
Alejandro Henao
Sep 3, 2016

Thank you so much for your clear an compleate explanation . I would like to add, acordinly with some authors the squat will be at the head or stern with a ship trim by stern acordinly with the amount of the static trim by stern....static trim by stern leads to squat by stern ...it is no a fix rule.... Thank you.

Rajeev Jassal
Rajeev Jassal
Sep 5, 2016

Glad you liked it Alejandro.. Yes you are right about the static trim..

Gulbag Singh
Gulbag Singh
Sep 27, 2016

Thank you so much for a clear cut explanation

Rajeev Jassal
Rajeev Jassal
Sep 29, 2016

Glad you liked it Gulbag..

Subodh Patil
Subodh Patil
Sep 29, 2016

Thank you very much for explaining this in a very simple yet precise way.Already forwarded it to my colleagues and cadets.

Rajeev Jassal
Rajeev Jassal
Sep 29, 2016

Happy that you found it useful Subodh. Thank you so much for forwarding it.

Cem
Cem
Oct 1, 2016

Dear Rajeev, this is a very good explanation of squat phenomenon. Thanks a lot. I will forward this to my friends.

Rajeev Jassal
Rajeev Jassal
Oct 2, 2016

Glad you liked it Cem.. Thanks for sharing it with your friends..

Tim DeLong
Tim DeLong
Oct 1, 2016

I noted squat while alongside a berth on a river. We had calculated the expected draft before coming alongside and took visual draft readings, applied the salinity and the numbers were off. Then we applied the squat calculations due to the current and UKC and it worked out perfectly. It was a first for me.

Rajeev Jassal
Rajeev Jassal
Oct 2, 2016

Yes Tim, you are right. Vessel will have squat while alongside a river berth because of river current.

Istvan MAGAI
Istvan MAGAI
Oct 1, 2016

Good explanation! The Inverse Ship can avoid the Squat Effect. Visit the website: http://www.magaimotor.magai.eu/inverse_ship.php for more information. There isn't any back flow water outside of the hull. Almost no waves before and after the ship. See video: http://www.magaimotor.magai.eu/videok.php

Capt Jagdeep Singh Rangi
Capt Jagdeep Singh Rangi
Oct 4, 2016

Great job....Keep the engines running full ahead since lot to achieve...Thanks and kind regards.

Rajeev Jassal
Rajeev Jassal
Oct 9, 2016

Thanks Capt Jagdeep..

Alexander
Alexander
Oct 12, 2016

Thank you for good job. Can you explain please how squat affect vessel’s stability? Is it because change of ships underwater volume, change of center of buoyancy, change to KM? Is the difference between KM figures of corresponding drafts is directly applicable for calculations? And other question, is the no any changes to drafts when vessel at deep sea?

Rajeev Jassal
Rajeev Jassal
Mar 1, 2017

Yes, Squat will have some effect on stability as underwater volumes and so centre of buoyancy changes a bit. But it is not as easy as having and comparing stability of two conditions. In this case as there is no weight distribution change centre of gravity go the ship remains same. At Deep sea, the change in underwater clearance is negligible if any.

Raja B
Raja B
Dec 18, 2016

Very clear explanation. Tomorrow is my Orals, and I'm confident about Squat now.

Rajeev Jassal
Rajeev Jassal
Mar 1, 2017

Glad to hear that Raja..

tran hong long
tran hong long
Feb 28, 2017

thanks sir !!!

Rajeev Jassal
Rajeev Jassal
Mar 1, 2017

Thanks Tran..

Vijay
Vijay
Mar 6, 2017

Thanks sir...!!

Rajeev Jassal
Rajeev Jassal
Apr 11, 2017

Thanks for reading Vijay..

Sava Mitradzhiev
Sava Mitradzhiev
Mar 8, 2017

Awesome explanation, although some of these softwares that we encounter on board as the one shown in the article have this ship dynamic data which need to be taken into account. (change of draft due to effect of wind, tide, sea, atmosphere pressure and so), i think we will all appreciate if you can write down and advise us how to calculate or where to find them exactly. From my experiance some colegues are putting some values there just not to leave it blank, but infact after this during vetting or PSC it is embarassing when they have to explain how did they came with these values. Thanks in advance.

Indrajeet kumar
Indrajeet kumar
Apr 8, 2017

Nice and very neatly clearifi sir..

Rajeev Jassal
Rajeev Jassal
Apr 11, 2017

Thanks Indrajeet..

Capt.A.Athavan
Capt.A.Athavan
Apr 9, 2017

Great efforts

Rajeev Jassal
Rajeev Jassal
Apr 11, 2017

Thanks capt Athavan..

Capt.A.Bhattacharyya
Capt.A.Bhattacharyya
Apr 9, 2017

Very well explained with examples. Thanks for the great effort...

Rajeev Jassal
Rajeev Jassal
Apr 11, 2017

Glad you liked it Capt Bhattacharyya..

Maninder
Maninder
Apr 9, 2017

Very East to understand..

Rajeev Jassal
Rajeev Jassal
Apr 11, 2017

Thanks Maninder..

kumar Shitanshu
kumar Shitanshu
Apr 11, 2017

Very Well explained sir .. thank u so much.. for this beautiful knowledge.

Rajeev Jassal
Rajeev Jassal
Apr 30, 2017

Glad you found it useful Shitanshu..

NARESH
NARESH
Apr 27, 2017

Excellent

Rajeev Jassal
Rajeev Jassal
Apr 30, 2017

Thanks Naresh..

Varun talwar
Varun talwar
May 8, 2017

Very well explained..it helped me to clear my doubts ..good work sir

Rajeev Jassal
Rajeev Jassal
May 17, 2017

Glad it helped you Varun..

kumaran
kumaran
May 17, 2017

This is also the reason that vessel can experience squat while alongside in a river with strong current. In this case vessel’s speed over ground is zero but speed through water is equal to the the river current. PLS EXPLAIN THIS SIR

Rajeev Jassal
Rajeev Jassal
May 17, 2017

the ship will experience squat (and thus reduced UKC) in river ports with strong currents even when ship is moored. This is because the water is flowing under the ship's hull which will cause the reduced pressure.

Sai chandra Kishore
Sai chandra Kishore
Jun 10, 2017

Thanks very much sir .. Indian needs people like you to produce better captains .. for industry .

Hassan Said
Hassan Said
Jun 14, 2017

Comprehensive and simple to understand.

Rituraj Chaudhary
Rituraj Chaudhary
Jun 27, 2017

excellent explanation sir...nowhere else did i find an explanation as clear and helpful...the other topics covered by you are equally good...Thank You Sir..hope to see more topics covered...

Snehasish Padhi
Snehasish Padhi
Jul 8, 2017

Respected Sir, I would like to highly appreciate you for your kind effort you put in here to make things so simplified.Sir our Maritime Industry seriously need your presence .While reading your blog it seems like an idiot can even understand topics you covers.Sir You are absolutely great.Sir kindly keep on helping God may Bless you Sir

Dr C B Barrass FRINA
Dr C B Barrass FRINA
Jul 10, 2017

Rajeev, I have now read through your 5 Questions and comments. I find some of my/your quoted formulae are now out of date. Do you wish conversation about this? My UK telephone number is OO44 1704 569 454. Best wishes. Bryan Barrass.

Dr C B Barrass
Dr C B Barrass
Jul 10, 2017

Rajeev, Some of these formulae are out of date. Do you wish to have conversation about them? Best wishes. Bryan Barrass.

Dr. C B Barrass. FRINA
Dr. C B Barrass. FRINA
Jul 26, 2017

My Telephone Number is 0044 1704 569 454.

IGOR POBRIC
IGOR POBRIC
Sep 25, 2017

Nice explanation Capt. Rajeev. Kind regards

Farhan
Farhan
Oct 1, 2017

Very well explained....Could not find any better and simple explaination

Deck Cadet
Deck Cadet
Nov 2, 2017

as 50 confined water had breath restrictions ; as 100 open sea without breath restrictions , so is it explain why 50 and 100?

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