Three and a half metres of water under the keel, at all times, for the whole passage. It reads like a static number and behaves like a moving one, which is why the same instrument that sets it also caps your speed.
The Rule and Where It Comes From
The Straits of Malacca and Singapore carry a share of world trade out of all proportion to their width and depth, and they are governed by a specific set of rules rather than by general good practice. The Rules for Vessels Navigating Through the Straits of Malacca and Singapore were adopted by the Maritime Safety Committee at its sixty-ninth session under resolution A.858(20), and are reproduced in SN/Circ.198 of 26 May 1998.
Two definitions in Part I do most of the work. A deep draught vessel is one with a draught of 15 metres or more. A very large crude carrier is a tanker of 150,000 deadweight tonnes and above. Those two categories carry obligations the rest of the traffic does not.
The clearance requirement itself sits in Annex 1, Part II, paragraph 1: an under-keel clearance of at least 3.5 metres, at all times during the entire passage. The phrase at all times is the operative part, and it is the part most often read as though it meant on departure.
Clearance calculated once, at the planning stage, satisfies nothing. The requirement is continuous, and the vessel’s actual clearance changes with how it is driven.
Why Speed Is in the Same Instrument
The reason a navigation rule about depth also contains a speed limit is hydrodynamic rather than administrative.
A hull moving through shallow water accelerates the flow beneath it, which lowers the pressure there and causes the vessel to sink bodily and to change trim. The effect grows with speed and grows sharply as the ratio of depth to draught falls. In a confined, shallow channel a vessel’s real clearance at speed is therefore materially less than the figure taken from its draught and the charted depth, and the deeper the vessel the more of its margin is consumed.
Which is why Rule 7 caps speed. Very large crude carriers and deep draught vessels are to proceed at a speed of not more than 12 knots over the ground, as far as it is safe and practicable, in the One Fathom Bank traffic separation scheme, in the deep-water routes in the Phillip Channel and the Singapore Strait, and in specified westbound lane segments. Port authority guidance applies that limit in the westbound lanes between south of St John’s Island and Raffles Lighthouse.
Read the two provisions together and they are one rule expressed twice. A 3.5 metre margin assessed statically and then spent on squat is not a 3.5 metre margin. The speed cap is the mechanism by which the clearance requirement remains true in practice.
Rule 3, lane discipline. Use the appropriate traffic lane, proceed in the general direction of traffic flow, and keep as steady a course as is consistent with safe navigation.
Rule 4, defects before entry. A vessel with defects affecting operational safety is to take appropriate measures to overcome them before entering the Straits. The obligation is to resolve them beforehand, not to manage them during the transit.
Rule 5, leaving the lane. In an emergency or breakdown within a lane, leave it to starboard as far as practicable and safe.
Rule 6, precautionary areas. Vessels approaching Raffles Lighthouse or a precautionary area are to proceed with caution and avoid impeding vessels constrained by their draught.
Rule 8, readiness. Keep a safe speed, proceed with caution, and be in maximum manoeuvring readiness. Annex 1, Part II also requires maximum manoeuvring readiness in precautionary areas.
Rules 9 and 10, reporting. Vessels fitted with VHF are to participate in the ship reporting system adopted by the Organization. Very large crude carriers, deep draught vessels and difficult or unwieldy tows are advised to broadcast navigational information eight hours before entering the traffic separation scheme. Damage to or malfunction of aids to navigation is to be reported to the nearest shore authority.
Rule 4 deserves separate attention because it closes a common line of argument. A defect affecting operational safety is to be overcome before entry. That makes a steering, propulsion or navigational equipment problem a reason not to begin the transit, rather than a condition to be declared and worked around once committed to a lane with no room to stop.
The Reporting System Is Mandatory and Broader Than People Assume
Rule 9’s reference to the ship reporting system adopted by the Organization is STRAITREP, the Mandatory Ship Reporting System in the Straits of Malacca and Singapore, adopted by resolution MSC.73(69) on 19 May 1998, promulgated as SN/Circ.201 and in force from 0000 UTC on 1 December 1998.
Its participation thresholds catch more traffic than the phrase deep draught suggests. Vessels of 300 gross tonnage or more must participate, as must vessels of 50 metres or more in length, and tugs or pushers whose combined tonnage is 300 or more or whose combined length is 50 metres or more. Vessels of any tonnage carrying hazardous cargo participate, as do all passenger vessels fitted with VHF regardless of size. Smaller vessels fitted with VHF are brought in where they use a traffic lane or separation zone in an emergency to avoid immediate danger.
The area is divided into nine sectors, each with an assigned VHF channel, with defined reporting points including the line between Tanjung Piai and Pulau Karimun Kecil, and reporting on joining the traffic separation scheme or on leaving a port or anchorage in the area. Vessels arriving from other directions report on reaching sectors 7, 8 or 9 by bearing and distance from one of seven named reference points.
A tug and barge combination of 300 tonnes combined, or 50 metres combined, is inside the mandatory reporting system. Plenty of regional operators believe it is not.
What a report has to contain is equally specific. The essential content is the ship’s name, call sign and IMO number where available, its position, any hazardous cargo and its class, and any breakdown, damage or deficiency affecting the ship’s structure, cargo, equipment or normal navigation, including pollution or dangerous goods lost overboard. Course and speed are reported on request from the traffic service.
That deficiency element is the one to brief masters on. The reporting obligation is not simply positional. A defect arising during the transit is reportable content, and the same defect, had it existed before entry, would have engaged Rule 4.
What This Means for a Passage Plan
Pulling it together, a transit of these Straits needs a handful of things settled before the vessel commits.
The first item is where most of the genuine risk sits, and it is a planning rather than a navigation problem. A vessel with modest clearance on paper, driven at a speed that suits a schedule, can be closer to the bottom than anyone on the bridge believes, and the grounding reports from confined waters repeatedly describe exactly that combination.
The last item is the one that affects regional coastal and offshore support operators most directly. The thresholds are combined figures for a tug and its tow, so a modest unit can be inside a mandatory system its operator has never engaged with. Our consultancy services cover passage and compliance planning of this kind, which is cheaper to get right in the plan than to explain afterwards.
Frequently Asked Questions
What is the under-keel clearance requirement in the Straits?
At least 3.5 metres at all times during the entire passage, under Annex 1, Part II, paragraph 1 of the Rules for Vessels Navigating Through the Straits of Malacca and Singapore, adopted by the Maritime Safety Committee at its sixty-ninth session under resolution A.858(20) and reproduced in SN/Circ.198 of 26 May 1998. The requirement is continuous, so it has to hold through the transit rather than only at the planning stage.
Why is there a 12 knot speed limit?
Because speed consumes clearance. A hull moving through shallow water sinks and changes trim, and the effect increases with speed and with reduced depth-to-draught ratio, so actual clearance at speed is less than the static figure. Rule 7 accordingly limits very large crude carriers and deep draught vessels to not more than 12 knots over the ground, as far as safe and practicable, in the One Fathom Bank traffic separation scheme, the deep-water routes in the Phillip Channel and Singapore Strait, and specified westbound lane segments.
Which vessels must report under STRAITREP?
Vessels of 300 gross tonnage or more; vessels of 50 metres or more in length; tugs or pushers of 300 or more combined gross tonnage or 50 metres or more combined length; vessels of any tonnage carrying hazardous cargo; all passenger vessels fitted with VHF regardless of size; and smaller VHF-fitted vessels using a traffic lane or separation zone in an emergency to avoid immediate danger. STRAITREP was adopted by resolution MSC.73(69) on 19 May 1998 and has been in force since 1 December 1998.
What has to be in a STRAITREP report?
Essential content is the ship’s name, call sign and IMO number where available, its position, any hazardous cargo and its class, and any breakdown, damage or deficiency affecting the ship’s structure, cargo, equipment or normal navigation, including pollution or dangerous goods lost overboard. Course and speed are given on request from the traffic service. The area is divided into nine sectors, each with an assigned VHF channel and defined reporting points.
Sources: Rules for Vessels Navigating Through the Straits of Malacca and Singapore, adopted by the Maritime Safety Committee at its sixty-ninth session under IMO resolution A.858(20) and reproduced in SN/Circ.198 of 26 May 1998: Part I definitions of a deep draught vessel as one with a draught of 15 metres or more and of a very large crude carrier as a tanker of 150,000 deadweight tonnes and above; Annex 1, Part II, paragraph 1 requiring an under-keel clearance of at least 3.5 metres at all times during the entire passage, and Part II also requiring maximum manoeuvring readiness in precautionary areas; Rule 3 on using the appropriate traffic lane, proceeding in the general direction of traffic flow and keeping as steady a course as is consistent with safe navigation; Rule 4 requiring a vessel with defects affecting operational safety to take appropriate measures to overcome them before entering the Straits; Rule 5 on leaving a lane to starboard in an emergency or breakdown as far as practicable and safe; Rule 6 requiring vessels approaching Raffles Lighthouse or precautionary areas to proceed with caution and avoid impeding vessels constrained by their draught; Rule 7 limiting very large crude carriers and deep draught vessels to not more than 12 knots over the ground, as far as it is safe and practicable, in the One Fathom Bank traffic separation scheme, the deep-water routes in the Phillip Channel and the Singapore Strait and specified westbound lane segments; Rule 8 requiring safe speed, caution and maximum manoeuvring readiness; Rule 9 requiring vessels fitted with VHF to participate in the ship reporting system adopted by the Organization and advising very large crude carriers, deep draught vessels and difficult or unwieldy tows to broadcast navigational information eight hours before entering the traffic separation scheme; and Rule 10 on reporting damage to or malfunction of aids to navigation to the nearest shore authority · Maritime and Port Authority of Singapore Port Marine Circular No. 20 of 2019, applying the 12 knot limit in the westbound lanes between south of St John’s Island and Raffles Lighthouse · Mandatory Ship Reporting System in the Straits of Malacca and Singapore, STRAITREP, adopted by IMO resolution MSC.73(69) on 19 May 1998, promulgated as SN/Circ.201 and in force from 0000 UTC on 1 December 1998, with participation required of vessels of 300 gross tonnage or more, vessels of 50 metres or more in length, tugs or pushers of 300 or more combined gross tonnage or 50 metres or more combined length, vessels of any tonnage carrying hazardous cargo, all passenger vessels fitted with VHF regardless of size, and smaller VHF-fitted vessels using a traffic lane or separation zone in an emergency to avoid immediate danger; nine sectors each with an assigned VHF channel, reporting points including the line between Tanjung Piai and Pulau Karimun Kecil and reporting on joining the traffic separation scheme or leaving a port or anchorage in the area, with vessels from other directions reporting on reaching sectors 7, 8 or 9 by bearing and distance from one of seven named reference points; and essential report content of ship name, call sign and IMO number where available, position, hazardous cargo and its class, and any breakdown, damage or deficiency affecting the ship’s structure, cargo, equipment or normal navigation including pollution or dangerous goods lost overboard, with course and speed reported on request
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