Effects of Current and Tidal Stream
Tidal current is a very helpful force as long as it is used with thorough understanding. We Must understand that tidal current force is a much stronger force as compared to wind force hence we must understand that exposure of tide at broad angle must be minimized. Ignoring tidal stream/current can lead to very disastrous results hence it must be given lot of forthought when planning and executing a maneuver. In case of something going wrong in strong tidal current, anchor is your best friend. Good thing is that tidal current is very easy to understand and practice for everyone concerned with shiphandling.
Few basic principles wrt using tidal current in ship handling:-
As far as possible tidal current must be from head while berthing or unberthing.
Berthing/unberthing in tidal current from stern should be avoided except in case of emergencies as it can lead to below two problems:-
Due to current from stern, stern lines while being passed ashore or heaved onboard will tend to float and drift in forward direction and may foul with rudder or propeller and you may loose ability to use engine/rudder. (Specially be careful with CPP vessel while casting off stern lines)
Regular astern movement would have to be used to check vessels tendency to move forward. (Astern power is usually less than forward power hence one will have to use higher engine rpm/power this may increase transverse thrust and vessels behavior will be erratic and difficult to control.)
Force on vessel due to tidal current will increase rapidly with decrease in UKC. This is very important factor and must be kept in mind. Vessel will take longer to reduce speed, vessel will take longer to increase speed and turning circle will be much larger. Hence sufficient space must be allowed for turning.
While using tugs to pull vessel sideways in very less UKC scenario, the effectiveness of tugs will be greatly hampered.
While executing turn in tidal current, we must try to build good ROT(rate of turn) and must finish the turn as quickly as possible. As vessel will experience strong set due to current until vessel is brought head on to current.
While planning the movement transit time and height of tides at shallow patches to be expected must be kept in mind to prevent risk of grounding.
To simplify understanding of tidal stream first it is to be imagined as movement of entire water mass in which ship is being maneuvered. Hence imagine if you are turning a vessel in its own position and it takes 6 minutes with 3 kt of tidal stream, so vessel would have moved 0.5 mile by the time turn is completed. So we must ensure that there is sufficient sea-room available to allow for such large drift downstream.
“It is very common to allow 2 to 3 ship lengths downstream drift while turning the vessels in strong tidal ports like Kandla, Even more space is allowed in Mississippi river and Dahej etc.”
Off course the presence of various features like jetty, bend, shallow patches etc will influence the flow pattern of tidal current. Hence local knowledge and experience of pilot becomes extremely important.
Understanding parallelogram of forces:-
Consider a vessel approaching a berth port side alongside with tide from forward keeping slight cant angle (q) towards berth, direction of flow in line with berth.
If we adjust vessel speed (V) almost equal to tide strength (V CosQ) so that vessel just maintains longitudinal position wrt fixed jetty (i.e. Speed over water is equal to current and SOG in for/aft direction is zero), so if we resolve parallelogram of forces, the lateral movement of vessel will be V SinQ.
We have tabulated the lateral drift for various strength of tide and angles of cant.
Here we don’t mean that you should calculate this every time you are maneuvering a vessel. Our intention is only to highlight that large angle of cant in strong tides can create very large lateral drift and once vessel is close to berth it would be very difficult to reduce sideways momentum if tugs are not made fast or tugs are not sufficiently powered.
Normally ports have criteria for lateral speed to be kept below 0.3 kts or 0.2 kts (15cm/sec to 10 cm/sec) based on berthing energy calculation while coming alongside on jetty i.e. touching fenders. However ideally one must try to completely stop the lateral movement about 2 to 5 meter before landing on fenders, then vessel should be pushed very gently to come alongside. We have highlighted the generally permissible speeds in yellow in above table.
Landing on fenders with continuous momentum or high lateral speed can put huge forces on berth/jetty structure as well as on ships hull. In addition to vessels momentum, there is even more force exerted due to 'additional mass'. Will discuss more about additional mass in further chapters.
“Hence an experienced ship handler will initially keep fairly large cant angle and will keep on reducing the cant angle as vessel closes to berth and finally vessel should be brought almost parallel to berth to reduce lateral speed.”
“ Same way you can effectively reduce the drift of vessel towards berth due to strong onshore wind by keeping your cant outward”
Minimum speeds to counter drift:-
In very narrow channels or breakwater entrances with current/tidal stream flowing across, sometime it may be necessary to keep a certain minimum speed to keep the drift/set of vessel within the channel. If someone keep speed too low by being over cautious, it can result in large drift and and vessels hitting the buoys or breakwater. I strongly recommend masters to discuss/ask Pilots about the same.
Ways to estimate/judge flow of tidal stream (Very important to practice) :-
Difference between ships log(Speed through water) and GPS(Ground) speed.
Look at buoys (Tilting of buoy, flow of water near buoy).
Look at flow of water around fixed objects like jetty pillars etc.
Look at overboard discharges of nearby vessels, once this water reaches the sea surface, you can easily make out the direction of flow.
Wake of tug: - This is very important method. When tugs are pushing at 90 deg to own vessel, take a look at propeller wake of tugs. After leaving the tugs vicinity the wake will flow in the direction of tidal stream.
Vessels at anchor in vicinity. Normally you will find small crafts, tugs, boats etc at anchor or secured to mooring buoys in tidal boat. This can give indication of tidal stream.
So even if you are new to a harbour you can still get good idea of tidal stream with above visual cues and predict how vessel will set. In narrow harbours, channels it is very important to try to understand the strength and direction of tidal stream and apply correction to your courses proactively rather then wait to check drift of vessel on ECDIS or RADAR and then take reactive action.