Mosquitos...

Project Mosquito Net: Mosquito Nets on the Mekong

A primitive form of waterborne anti-ballistic missile defense systems for protection of riverine convoys.

by Louis Lemos, Former Project Leader, Engineering Design Group, Navy Division, United States Defense Attache Office, Saigon, Vietnam

At the height of the Cambodian campaign in 1974, US. military supply convoys en route from Saigon to Phnom Penh via the Mekong River consisted mainly of flat deck cargo barges towed by tug boats, operating under contract to the Military Sealift Command Far East (MSCFE). The Khmer Rouge Forces however, seemingly lacking an appreciation of American beneficence, took a dim view of this traffic and set up bases at strategic points along the embankment from where they ambushed the unarmed convoys pushing steadily upstream at five knots, with volleys of Chinese-made b-40 rockets. In response, MSC requested assistance from Defense Attache Office (DAO) Navy Division, Saigon, to devise a means of protecting the ammunition-laden barges from this bothersome weapons threat. Within a few days a U.S. Navy Tiger Team flew into Saigon, proceeded to draw a crude sketch of what appeared to be a floating carport, presumably intended to protect the barges, and left town the next day after leaving the following directions:

1: Only material readily available-in-country may be used for this project. No material to be imported.

2: Anti-rocket protection system must not interfere with the loading or unloading Of barge cargo.

3: Anti-rocket protection must not detract from minimum convoy speed of 5 knots.

4: The "system" must be ready for the next convoy within 30 days.

Accordingly, the Engineering Design Group of the U S. DAD (NAVY) Logistics Department was given the assignment and a three-man project team was assembled consisting of a Project Leader, a U.S. Engineer and a Vietnamese Engineer. None of who had prior experience with rocket type weaponry. The initial directions were to evaluate the floating carport sketch for feasibility and if not found suitable to devise an alternate concept.

It soon became evident that the carport structure, supported by two very narrow floats, would most likely collapse on top of the cargo barge if struck by a rocket and create a drag on the tow. Apart from the fact that the corrugated metal roof might keep the rain off the cargo, the structure itself offered no protection at all against the rockets. After considering several options and rejecting them all mainly due to the constraint against importing material, and the limited time factor, the configuration of the anti-rocket "system" was limited to, and entirely contingent upon, assets available in country. Having just completed an assignment with the Vietnamese Army Watercraft Transportation Command, the Project Leader was able to "borrow" a number of flat deck 750 ton BC barges from ARVN W.T.C. iNith the promise that they would be fully restored to new condition before being returned. These barges were 120 feet in length, and of 35 feet beam with a draft of 11 feet, fully loaded. “Since there was no means of stopping the rockets, the best we could hope for was to -intercept them at a safe distance". This being the case, the concept was to use the BC barges as-“shields" for the cargo barge. Specifications were then drafted and drawings made and the project construction work was awarded to three Marine Contractors, with all the work to be performed at one location.

Theoretically, positioning two shield barges in-tandem, to the port and starboard sides of the cargo barge should provide a reasonably safe distance of 35 feet at each side, with the cargo barge secured in the middle. The next requirement was that of "interception". This was intended to be accomplished by the use of cyclone wire mesh fencing, erected, in panels of ten feet long, by nineteen feet in height, erected on the port and starboard outboard edges of each shield barge; welded in place, and suitably stiffened with diagonal bracing. Panel framing consisted of 2 inch by 2 inch by 1/4 inch angle iron, to which two panels of cyclone wire mesh were welded, one on each' side with the one inch square mesh staggered, thereby reducing the effect mesh to one half inch squares. However, before construction could begin, the feasibility of '"interception" had to be proven and this required (a) acquiring a number of B-4O rockets, and (b) firing them at a mock-up wire mesh panel to determine the effect. Since the Khmer Rouge was not likely to provide such weaponry willingly, a request was made to the South Vietnamese military commanders to whose custody the U.S. military had entrusted several captured B-4O rockets prior to their departure from Vietnam.

As a further precaution against the barges being hit below the waterline, alternate ballast tanks were filled with styrofoam to provide reserve buoyancy if needed. Additionally, the cargo of ammunition would be
surrounded by a six foot thick by six foot high wall of sandbags. As a final safeguard, a test run was made up the river with a set of four shield barges and one cargo barge carrying a dummy load of comparable weight. Upon completion of the trip up river, the U.S. test monitor called back to Saigon by radio to report that the 5-barge tow assembly had maintained the minimum required convoy speed of 5 knots successfully, on the basis of which, the order was passed to proceed with construction.

A fleet of eight barges was brought into a small riverside boatyard and a large Diesel-powered welding generator was positioned to serve the welding crew, cutting and assembling the angle iron frames while other workers unrolled and cut the cyclone mesh wire into panel size pieces. At another nearby -facility hand-operated anchoring winches were being fabricated, four for each barge. In a move calculated to cut material costs, one of the Vietnamese contractors instead of buying rolls of cyclone mesh wire, opted to buy hundreds of coils of wire from which the fencing mesh is assembled by machine but lacking the machine, he hired a couple of dozen workers to stretch out the wire coils and weave the mesh panels by hand. Fortunately the finished product proved to be a neat orderly mesh that passed inspection. In the interests of fairness, two
Philippine inspectors were brought over from Subic Bay Ship Repair Facility and kept a close eye on the work for production quality control. A week after the start of construction a Navy commander from MSCFE, Yokohama, arrived to determine how soon the first set of barges would be ready. He was advised that if the work pace was hurried, twelve to fifteen days might be possible, but he was insistent upon a convoy departure date within ten days that the contractors could not meet.

In addition to running the gauntlet of B-4O rockets in the river, the ammunition barges were sitting ducks for sneak mortar attacks while the, cargo was being discharged by manual labor at the T -headed jetty at Phnom Penh. To help expedite the unloading it was decided to provide a fleet of battery-powered fork-lift trucks and in due course the shipment arrived in Saigon and a further decision was made to load all sixteen fork-lift trucks on the same barge along with the cargo of ammunition, rather than splitting the fork-lift trucks into two shipments. The barge chosen for this trip was the largest cargo barge in all of Vietnam at that time and rather than wait for the Shield Barges to be ready, the convoy departed on schedule. Khmer Rouge intelligence was apparent1y alerted and at a narrow stretch of the Mekong River the B-40's scored direct hits, exploding the cargo of ammunition and sinking the barge with what was left of the fork-lift trucks. This not only resulted in a serious loss, of vital supplies but also resulted in blocking the channel for further river traffic in both directions.

Eventually, the wreckage was further demolished enough to clear the channel for safe passage but details of this hazardous operation were not disclosed. By the time the first set of Shield barges was completed, orders came through to double the number of Shield Barges from the originally authorized level of eight barges, and in due course the modified vessels were all turned over to Military Sealift: Command Vietnam and hurriedly placed into service. As can be expected. Riverside dwellers were amazed to see what appeared to be floating tennis courts being towed upstream. A further safeguard was provided by stationing a Repair Barge at the mouth of the river loaded with rolls of cyclone wire mesh and angle iron plus a diesel-powered welding generator.

Needless to say, the welding crew was kept quite busy patching up the gaping holes in the wire mesh panels and the twisted angle iron frames but this was a small price to pay for delivering the cargo safely. In fact, even though Khmer Rouge is not a Cambodian cosmetic, the rebels must have been quite red in the face seeing their B-4O rockets hitting the Shield Barges, exploding and fading into the river, leaving great holes in the wire mesh as the convoys continued on their way. The Mosquito Net Project proved to be so effective that the Khmer Rouge eventually decided to lay acoustic mines to be actuated by the noise of the tugboats propeller. The response was to drag a large anchor chain strung between two boats to free the mines from their anchors while a third boat following behind detonated the mines by gunfire.

In all of South-East Asia the only anchor chain to be found was in a junkyard in the China Town of Saigon. However, with a cargo plane ready to fly the chain to Phnom Penh, the transaction was delayed for three days while the Chinese merchant consulted his horoscope to decide on what day the chain could be sold.