LN GC AR R IE R

O ILT AN K ER

40F T.

SW L

400F T.

TW O SHIP TY PES W ITH SA M E D R A FT

BOW CUSHION

Bow cushion is experienced strongly when following conditions are met: 1. Proximity to a bank. The bow wave will be more easily dispersed when

the bank is submerged. Bank suction suffers to a lesser extent from loss in strength when the bank is submerged.

2. The ship must be on a parallel course to the bank. To build up the bow wave on the side of the bank, the ship must run parallel or close to parallel to the bank. Bank suction has a much greater tolerance for the angle between ship and bank.

3. The ship must reach a certain speed to build up a bow wave. In this respect the ship’s speed is relative to her size, for we must take into account the ship’s beam and bottom clearance as factors contributing to the height of the bow wave.

4. The ship must have a large underwater area forward of the pivot point, which is the case when the ship is in loaded condition. Trim by the head increases the underwater area forward of the pivot point, and consequently, accentuates the effect of bow cushion. In combination with point 3 above, we must also consider that the longitudinal component of the increased bow wave in shallow water tends to push the pivot point back, which, in turn, tends to increase lateral area forward of the pivot point and provides the transverse component of the bow wave with added

leverage. When a sheer develops the pivot point tends to shift even farther back, adding again to the force and the leverage of the lateral resistance forward, intensifying the sheer.

Case Studies: 1. Collision between loaded freighter (proceeding from sea to Amsterdam)

and an approaching vessel in Amsterdam-North Sea Canal. Findings: The vessel was kept off the centre line of the canal. Required some starboard rudder all the time to be kept steady. Uncontrollable sheer towards the entrance of a branch canal on the port side. Conclusion: On arriving at the entrance of the branch canal, the raised water level on the port bow dropped off into the branch canal resulting in predominating pressure of the bow wave on the starboard bow.

2. Collision between deeply-loaded ore carrier proceeding inward & an approaching vessel in Ymuiden canal. Findings: The ore carrier had a trim by head of 3 ft. which was being corrected at the time of accident. The pilot left the centre line of the canal too early. Once on the starboard side of the canal, the ship took uncontrollable sheer across the canal and – not with standing both forward tugs pulling to starboard and full astern on the engine- ran into the approaching vessel. Conclusion: The ships with a large lateral underwater area forward of the pivot point must avoid getting caught in a situation where the bow cushion will start a sheer, for that sheer is hard to break! Given the right conditions bow cushion can be embarrassingly strong, the more so since it is backed up and followed by an increasing effect of suction.

3. Collision in the River Danube at Sulina between Satya Padam carrying iron ore and at even keel draft and Valerie. Findings: When the Valerie was sighted, in daylight and good visibility, coming down the river, the Romanian licensed pilots on two ships agreed to pass port to port. At this point Satya Padam reportedly lost steering control and started to sheer to port. The wheel was hard over to stbd and the engine was on full speed ahead. It took two minutes before the ship’s head started to come back to stbd. The master of the Satya Padam, who was paying full attention to the steering problem, noticed suddenly when the ship pulled out of the sheer that the Valerie, in turn, had taken sheer across the river and now was trying to pass in front of his ship. The sheer of the Valerie in fact was a course alteration, made at the request of the pilot of the Satya Padam on VHF. The pilot of the Satya Padam must

have strongly believed in steering gear failure, for he left hard stbd rudder on while he requested his colleague on the other ship to alter course to port. Three cables east of milestone 24, where the ships collided, is the outlet of the Maliuc streamlet, an inlet in the river bank of the Danube into which the bow cushion of the Satya Padam could drop off and run up. Conclusion: As the steering had posed no problems until the meeting with the Valerie, we can assume that the Satya Padam had proceeded in mid-channel up to then. It was when the Satya Padam had moved over to starboard side of the channel a few minutes before the collision that the difficulty in steering was experienced. It must be considered that, at the time of the sheer , the Satya Padam was down by the head, due to squat. With her speed close to 10 kts through the water (there was a restriction to 8 kts) and her deep draft forward, there must have been a significant bow wave. The drop in bow cushion into the Maliuc stramlet caused the helmsman to take off starboard rudder that was on to keep the ship to her starboard side of the channel and probably he did even have to apply port rudder. In any case the helmsman was obviously unprepared for the renewed bow cushion effect immediately after the ship passed the Maliuc streamlet. The large underwater lateral area forward of the pivot point of deep drafted vessels makes them susceptible to the effect of bow cushion, Moreover large overall underwater lateral area gives the deep draft vessels a relatively smaller rudder area ratio as compared with ships in light condition.

4. Collision in the Mississippi River between bulk carrier Southwind heavily laden proceeding upriver and Astros coming downriver.

5. Collision in the River Seine between Japanese tanker Fuyoh Maru in loaded condition proceeding upriver, and a Greek tanker Vitoria in ballast, not gasfree, coming downriver on 24th June 1987 halfway between Le Havre and Rouen. Six people including the Master and the pilot perished. (Ref. Capt. H.H. Hooyer in ‘The Nautical institute on pilotage and shiphandling’)

GENERAL POINTS ON INTERACTION

1. Prior to meeting the other vessel in narrow channel, each ship should remain in the centre of the channel for as long as possible. Failure to do

so could expose either ship to bank effect, leading to sheer across the path of the oncoming ship or grounding.

2. Speed should be low to reduce the interactive forces. There is, then, plenty of reserve power for the use of corrective ‘kicks ahead’.

3. If the ships pass from deep to shallow water at any time during the manoeuvring, the forces will increase drastically and extreme caution should be exercised.

4. The smaller of the two ships and tugs, are likely to be the most seriously affected. Large ships should be aware of this and adjust their speed accordingly.

5. Pre-emptive and bold corrective action may be required to prevent or break strong sheers.

6. The engines should be brought to dead slow ahead for the manoeuvre, particularly turbine or fixed pitch propeller ships, so that power is instantly available to control the ship with ‘kicks ahead’.

7. On completion of the manoeuvre each ship should regain the channel centre as quickly as possible to avoid any furtherance of the bank effect.

8. Pilots who are engaged in canal work all the time become very specialised in this area and their advice should always be sought when in doubt.

(Ref. Captain R.W.Rowe, author, ‘The Shiphandler’s guide’)

SQUAT The reduction of underkeel clearance when a vessel is making way through the water as compared to when she is not making way through the water is called as Squat. For full form vessels such as supertankers or OBO vessels (block co-eff. > 0.7), change of trim due to squat will be by head whereas in case of passenger liners or container vessels (block co-eff. < 0.7) change of trim due to squat will be by stern. This is assuming that they are on even keel when stationary (i.e. not making way through the water). Vessels trimmed by the stern when stationary (not making way through the water) will trim further by the stern due to squat. Vessels trimmed by the head when stationary (not making way through the water) will trim further by the head due to squat. This is consistent with Bernoulli’s streamline flow theory.

Casualties of excessive Squat

Herald of Free Enterprise RORO vessel 06/03/1987 QE II Passenger liner 07/08/1992 Sea Empress Supertanker 15/02/1996 Diamond Grace 2,60,000t dwt VLCC at

Tokyo Harbour 02/07/1997

Ship type Typical CB,fully

loaded B Ship type Typical CB,fully

loaded B

ULCC 0.850 General Cargo 0.700 Supertanker 0.825 Passenger liner 0.625 Oil tanker 0.800 Container ship 0.575 Bulk carrier 0.750 Coastal tug 0.500

SIGNS OF SHALLOW WATER EFFECT (One or more of the following)

1. Wave making increases, especially at the forward end of the ship. 2. Vessel becomes more sluggish to manoeuvre. A pilot’s quote, ‘almost

like being in porridge’. 3. Draft indicators on the bridge or echosounders will indicate changes in

end drafts. 4. Propeller RPM indicator will show a decrease. If the ship is in ‘open

water’ conditions, i.e. without breadth restrictions, this decrease may be upto 15% of the service RPM in deep water. If the ship is in a confined channel, this decrease in RPM can be up to 20% of the service RPM.

5. There will be drop in speed. If the ship is in open water conditions this decrease may be up to 30%. If the ship is in a confined channel such as a river or a canal then this decrease can be up to 60%.

6. The ship may start to vibrate suddenly. This is because of the entrained water effects causing the natural hull frequency to become resonant with another frequency associated with the vessel.

7. Any rolling, pitching and heaving motions will be reduced as the ship moves from deep water to shallow water conditions. This is because of the cushioning effects produced by the narrow layer of water under the bottom shell of the vessel.

8. The appearance of the mud could suddenly show in the water around the ship’s hull say in the event of passing over a raised shelf or a submerged wreck.

9. Turning circle diameter (TCD) increases. TCD in shallow water could increase 100%.

10. Stopping distances and stopping times increase, compared to when a vessel is in deep waters.

CB X S0.81 X V2.08 Maximum Squat = metres. 20 CB = Block co-eff. S = Blockage factor = Submerged cross section area of ship Submerged cross section area of channel = b x d B x D

where b & d : breadth & draft of ship and B & D : breadth & depth of the channel resp’ly

V = Vessel’s speed relative to the water, in knots. If the vessel is in open shallow water, B= Breadth of the channel is taken as B = {7.7 + 20 (1 – CB ) } X b, known as the width of influence. B

2

The width of influence ranges from 8.25b for supertankers, to about 9.5b for general cargo ships to about 11.75 ship breadths for container ships. The presence of another ship in a narrow channel may cause the squats to double in value as they pass/ cross the other vessel.

SHORT-CUT FORMULAE

1. Maximum Squat = CB x VB K2 metres for open water conditions only

100 where D/d = 1.1 to 1.4. 2. Maximum Squat = CB x VB K

2 metres for confined channels only 50 where S = 0.100 to 0.265.

CONSTANT RADIUS TURN

The object of ocean navigation is basically to find a ship’s position. In coastal and confined waters, another dimension is added-the margins for errors are smaller, and actual ship handling must be integrated with the pure navigational disciplines. In restricted waters, the traditional philosophy of position fixing at intervals in terms of a point-e.g. cross bearings, bearing and distance off a reference object, or latitude/ longitude- is no longer a prime objective. In order to maintain safe positioning at all times, procedures to keep a continuous check on the cross track error must be used. Accurate cross track monitoring is as important when turning as on straight courses. These requirements make it obvious that a transit in restricted waters must be well planned, since according to the nature of the environment , time will not permit navigation in traditional sense-i.e. fixing the position and setting a new course. 1. Rate of turn indicators (Rate Gyros) are installed mainly on large ferries

(car/ passenger) running on time on a schedule between two ports in confined waters (e.g. between Sweden and Finland), ships trading on inland waters/ rivers and canals, large tankers, container vessels, and in ships with the bridge in extremely forward position. Rate gyros are very useful for turn control as they can sense/ measure the ship’s angular velocity much more accurately than is possible by the human eye. Some rate gyros are equipped with separate gyroscopes and others receive their input from the gyrocompass. The sensitivity of the rate gyros can be as low as 0.50 per min.

2. A type of autopilot, designed for ships navigating on the rivers and canals of Europe is known as the river-pilot. The river-pilot is normally operated by a tiller type lever with two alternative functions. One function is as in ordinary tiller steering, but the second and most important function for precise turn control is the automatic mode when a tiller order will cause the river pilot to apply a rudder angle to build up quickly a rate of turn (ROT) to the value ordered by the tiller. This (ROT) will then be maintained automatically until the tiller steering is changed. In order to maintain a straight course, a turn rate of 00 / min is ordered. Should the ship deviate from the desired course due to some outside force, the course must be adjusted manually with trim setting or with a new ROT order. Turns controlled by a river-pilot will be more accurate than those done manually, even with the aid of rate gyro.

3. Radius steering unit : A self adaptive auto pilot which will both maintain a straight compass course and perform accurate pre-selected constant radius turns is the Kockums Steermaster 2000. The input is compass heading and ROT derived from the compass. As the rudder angle required to maintain a straight course or a constant rate of turn depends to a certain degree on the water depth and the ship’s speed, input is taken from the echo sounder and a doppler log. The speed input is also needed to produce a constant radius and with a dual-axis doppler log in bottom track, any drift caused by wind or current can be automatically corrected. The navigators controls are a joystick to select a new course and push buttons to select the turn radius. The radius can be changed at all times, including when a turn is in progress. A standard feature of the steermaster is the override which in the case of a malfunction of the steermaster or the helmsman will give the navigator immediate direct access to the steering gear. Many of the large ferries in the demanding traffic between Finland and Sweden and many other Scandinavian passenger and cargo ferries are equipped with a radius steering unit. A number of deep sea vessels also have the equipment but, with the exception of the ferries and the car carrier company Wallenius and OK tankers, the training for the masters and officers in the use of the equipment has been poor. Many use the steering unit only as a conventional autopilot to steer a straight course and switch over to manual in narrow waters. Experiences from large ferries and car carriers have shown that with an advanced form of radius steering unit, such as the Kockums Steermaster 2000, it is possible to keep the cross track error at the exit of a turn to less than 20 m, even in very adverse weather.

COMPARISON : CONSTANT RUDDER ANGLE

TURN CONSTANT RADIUS TURN

1. Larger drift angle with a corresponding loss of speed

Lesser drift angle & hence lesser loss of speed

2. A large rudder angle is needed to steady the vessel on new course

At the end of the turn, the new course can be steadied with lesser rudder angle

3. Uncertainty of ship’s position during the turn

Proper control of ship’s posn. during the turn

4. Higher fuel consumption due to zig zagging with excessive use of helm.

Lesser fuel consumption, with reserve rudder and engine power available

Constant radius turn technique is based on the following formula: Rate of turn (Degrees/ minute) = 57.3 x V 60 R where V= Ship’s speed over ground, in knots and, R= Radius of the turn in nautical miles. The distance of wheel over point from the point where the turn is to become effective is usually taken as one ship’s length but it is recommended to find it out by some practice turns on the type of ship one is serving. Following formula can be used to find the distance of wheel over line from the new course line and the same can be used to set the parallel indexing line or the line of turn for giving the wheel over order: Distance of wheel over line from the new course line = F sin θ + R (1 – cos θ) where, F = one ship’s length (usually), R = Radius of the turn, θ = Change of course in degrees between initial course and final course