Tuesday, January 11, 2011

SHIPS SQUAT , BERNOULLI AND UNDER KEEL CLEARANCE -- CAPT AJIT VADAKAYIL


What follows is the text of an actual official email to my company.

Ships had been under loading due to squeamish company circulars on squat, written by landlubbers who do NOT understand this unique phenomenon.

Unless a navigator reads this he will NOT get a true handle of squat and UKC.  Posted in the interest of safety at sea --and NOT for charterers to load more cargo.


 #######

gentlemen,

what i present is a lateral study-it has nothing in common with 
what many powerhouses of nautical knowledge all over the 
world imagine what squat is. rather it is the essence of my
own squat studies backed by a strong belief in my own
lateral thinking ways----

--starting about 14 years back in the huge wave pool with 
ship models with delicate overhead suspenders and pulleys, 
at IIT chennai ( donated by holland ) -

--and is still continuing--.

--my last study was a 5 hour event at sitra channel on 
departure bahrain with a fully loaded siteam explorer 
with 4 mtrs water under the keel at a static 
depth-to-draught ratio  of 1.3.

--this study was more on engine load pattern behaviour , 
which i discussed with XXX- the use of piezo electric
bottom pressure transducers of draft indicators (which do 
not care for bow wave build up) to study squat , and the
effect of bernoulli’s theorem at bends and passing ship 
squat interaction

i will just breach the subject-and will not be surrendering
the rights, like what i did for the chemical stuff--

first of all there is nothing called as a -
a)open water condition
b)confined water condition
and a
c)pilotage water condition
--as highlighted in the excel sheet

rather it is a -
a)depth restricted condition
b)canal condition-say suez canal
and a
d)channel condition- now imagine suez canal is flooded by 
5 mtrs over its banks.

i will get back to squat later-first let me talk about ukc factors
which many have ignored and still not found out the hard 
way- DUH !

##########
the first is “barometric pressure”:

this can be best narrated by an incident when i worked on 
XXXX of XXXX new york-run by the yanks- my technical 
suptd  was XXXX who now works for XXX usa- he still 
thinks  vadakayil is ZE cats whiskers because he had 
predicted a fantastic  me compressor explosion one 
week in advance by a cynical  email, a fantastic boiler 
furnace explosion in the previous safety committee meet
minutes which was recd at office a day before the 
big bang, 

--and did what the combined might of usa could not
do for a whole week- tell why the sister ship got her bottom
holed by an age old rock plumb in the middle of boston 
channel spilling a large quantity of unleaded gasoline into the
harbour. i had a vhf conversation with the inbound master 
15 minutes before the incident, in which i had asked him 
what his inbound draft was . as soon as it happened i send 
message to XXX- “when the whole of american might
has given up-i will tell you what happened”--- my cockiness 
upset all my detractors at park avenue-- it was meant 
to be that way--. 

so the us coast guard and the
technical whiz kids brought out their underwater mini 
submarines with their robotic arms- scooped out the ships 
bottom paint from the rock-got it analysed , established
that the rock was always there over the centuries-the 
puzzle was still unsolved why did the ship hit the rock?

after 4 days i get a call from XXX---tell us what you think! 
 to cut a long story short -on that moment it was low water 
neaps-safe enough-but the aneroid barometer pressure 
needle was stuck at the max position which meant the 
pressure  was more than 1050-a record-which meant
the whole  ocean over thousands of square miles was 
depressed -more so in inland channels like boston-the 
uscg was quick to do the calculations after receiving this
 tip-- and declared  it within a week-the fall of channel 
depth was an unexpected 55 cms. in a hurricane of 
atmospheric pressure 925 mb--the surge is huge and just
the opposite--it sucks up like metal filings to a magnet--

the second is wind direction and speed- this is not a big 
secret- 
i learnt this from the argentinians. when the wind blows
the water into the river-they increase the loadable quality-
yes , there is surprisingly more water in the river-but 
what about the sangam where the river has its 
confluence with the ocean?-- 

the third is stability of the bottom-siltation. imagine a 
ship is anchored at bhavnagar where the high water 
spring tide is 11 knots- imagine a 11 knot current is 
passing under the keel of an anchored ship- what 
will happen? 
will there be scouring ?  will there be squating 
(aaha! -of an anchored ship? )-no easy answers 
gentlemen-ok-- let me make it more interesting--
take out the parameters from the squat excel sheet
you send me-dang!  my 2 metre freeboard ship 
is supposed to sink at anchorage ! help!!

the fourth is reduced depth over submarine pipelines -
i can see the XXX manual has covered this.

#############
lets get back to squat-

how many nautical powerhouses have published till now 
that the car carrier herald of free enterprise sank due
to an open bow door-due to squat-while pelting out of
harbour at 20 knots?

now you have V squared /100 and 2V squared/100-now 
what is this two? is it to be multiplied to open water 
condition blindly to get the confined water condition? -
no sir!-it is the sinkage factor due to squat induced
trimming -at 0.7 BC  it is bodily sinkage generally-at 
lesser or greater there is a trimming-hence the factor 2.

now to blast the myth of ready reckoner formulas-
imagine 2 ships are coming out of sitra port-both have
everything the same-when i say everything -it is all 
the factors which makeup the fantastic squat empirical 
formulas- “ blockage factor” / “ velocity factor” 
brainchild of geniuses who must not be questioned-
factors like block coefficient- breadth of ship-- 
width of channel-draft of ship---depth of channel- 
speed in knots --  now imagine one ship is 120 mtrs 
long and the other is 360 mtrs long-is the squat allowance 
of both to be the same, considering a fantastic draft 
increase due to trim will take place.

on the other hand herald of free enterprise already down by 
the head-at a very fine block coefficient of around 0.6 and
having an extremely fine form -will it squat by stern as is 
predicted by our empirical whiz kids ?- no sir-if it has a 
static trim by head -it will continue to have a squatting
trim by head too- and this has “F A” to do with the block 
coefficient-200 lives lost-  pin this on the  carpetbaggers.


what does this mean-simple-don’t ever give the ship a 
static trim-and hope the squat will even it out because 
of the block coefficient-people make block coefficient 
formulas -block coefficients change with the ships
draft--- on siteam explorer-the hydrostatics table 
gives the bc at various drafts.

on the other hand a vlcc left port with 50 cms trim by 
stern and squat pushed her bows down-this sir-is 
because trim is small compared to the length and the
bow experienced shallow water effect first.

now i shall be kind  and give a peek into the classified 
stuff-  squat has everything to do with bernoulli’s 
theorem-  Bernoulli himself stole his theorem from the 
indian vedic yogis ( Saraswati river banks ) of 11 
millenniums yore--who educted out their capillary 
sinus mucus by sending  driving water 
through the nasal passage-drinking water with nose 
and expelling out from mouth.

potential energy + pressure energy + kinetic energy  =  constant

if liquid does not change height and the potential energy term 
remains constant--:

and if the velocity of the liquid is increased then the pressure 
energy term must be reduced. if this happens beneath a ship
the reduction in pressure must result in sinkage and because 
the centre of buoyancy will move due to changes in distribution 
of' pressure the ship will also change trim.

--, in effect, that the total mechanical energy of the flowing
fluid, comprising the energy associated with fluid pressure,
the gravitational potential energy of elevation, and the kinetic 
energy of fluid motion, remains constant

a reduction in vertical pressure-force on the hull has to be 
compensated for by an increase in the buoyancy force 
(displacement volume), i.e. draught, in order to maintain 
vertical force balance. the ship will experience a mean
bodily sinkage and, in general, a slight trimming effect. 
squat is the algebraic sum of sinkage and trim-and
varies in proportion to the square of the speed.

a reduction in the pressure acting on the hull in the
vertical direction has to be compensated for by an 
increase in the buoyancy forces. thus, the ship has 
to increase its draught (displacement volume) to maintain
the state of vertical force equilibrium. as the ship is 
not symmetrical about its half-length and because of the 
viscous effects of water, which increases with depth , 
the changes in pressure are not identical for the
fore and aft parts of the hull. this causes the ship to trim 
forward or astern depending on the hull shape and
stationary trim. the effect of squat may increase 
significantly if the side clearance is restricted as 
well, as is the case in a confined channel. thus, the 
magnitude of squat depends on the ship (water) 
speed, hull shape, and the side and under- keel clearance.

using this theory the russians made their mighty heavy 
lift Antonov aircrafts --the air flowing over the upper 
curved surface of an aircraft wing moves faster than
the air beneath the wing, so that the pressure 
underneath is greater than that on the top of the wing, 
causing a lift of hundreds of tonnes.




when making worst case excel sheet thumb rule ready 
reckoner scenarios - be aware that a ship can load that 
much cargo less ,by faithfully following company 
instructions---on a ulcc with huge TPC that would be
 HUNDREDS OF THOUSANDS OF DOLLARS 
DOWN THE DRAIN---

-yes--

brgds
capt ajit vadakayil/

################## 

Bernoulli's law stated that, if an inviscid fluid flows along a pipe of varying cross section, then the pressure is relatively low at constrictions where the velocity is high and relatively high where the pipe opens out and the fluid stagnates.

now this is surely a paradox--- for to a not so bright layman -- a constriction should increase the local pressure rather than diminish it.  the paradox is solved when you term pressure changes along the pipe as cause and the velocity changes as effect, instead of the other way around.

here is a simple experiment .  hold two sheets of A4 size paper so that they hang vertically two centimetres apart and blow downward so that there is a current of air between them.  the sheets will be drawn together by the reduction in pressure associated with this current.  ships are drawn together for much the same reason if they are moving through the water in the same direction at the same speed with a small distance between them.

next time you are inside a lock gate tied up, and the water moves under the ship while pumping out--check out the squat. It hardly makes a difference if your ship move or the water beneath moves. 

In tidal ports when the bulk shippers tell your ship to move closer to land to make it more convenient for barges -- it also means that he wants you to have fast moving water under the ships bottom with limited UKC. You will be signing bills of ladings with imaginary cargo--and this quantity can be mind boggling on large ships.


CAPT AJIT VADAKAYIL
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