Marine and Riverine Projects:
Reinforced Concrete Pontoon for Govt of South Australia.

The story in brief.

In 1909, South Australia's Chief Engineer asked Monash if he could supply reinforced concrete "pontoons" for river work, of the type being built in Europe. Monash admitted that he and his workforce had no experience in this type of construction; but he was keen to open up a new line of business and negotiated to build a prototype with others to follow if the first was successful. Design of the hull, and stability calculations, were carried out in the Melbourne office under Monash's direction. His Resident Engineer in Adelaide worried about the problems of building and launching the vessel. Their calculations for flotation and for slipway friction involved principles of high-school physics, so have been covered in some detail below, in the hope that they will be of educational value. The prototype hull was floated on 16 March 1910 and was afterwards worked for some time equipped with a Priestman steam crane as shown in the drawing below. However, no further orders were made and there is evidence that the crane was removed within a year, and the hull used as a simple barge.

The concept.

The idea of concrete ships has excited the imagination of many an engineer. Joseph Louis Lambot built concrete boats in France from 1848 onwards. Monash's first and only venture was inspired by A B Moncrieff, Chief Engineer of South Australia, who wrote in November 1908 that he needed pontoons for work on the Port Adelaide River. He knew they had been built of reinforced concrete in Europe, and thought this might be inexpensive and require less maintenance than wood or steel. He asked Monash whether he had a suitable design available. Moncrieff sent a sketch showing a length of 50 feet (15.2m) and a beam of 22 feet (6.71m), specified a maximum draught of 2'-6" (762mm), and suggested a shell thickness of two inches (51mm). The vessel would be equipped with a Priestman steam crane, with the necessary coal bunker and water tank.

Displacement and draught

After initial investigation, Monash set J A Laing to work on calculations. The displacement (and thus total weight) of a pontoon with Moncrieff's plan dimensions and draught would be 71 tons. Assuming it could be made with a 2" shell, the dead weight exclusive of crane, gear, tools and stores would be 44 tons. This would leave 27 tons for the rest. But Monash reasoned that the shell thickness could not be less than 3 inches, because such a vessel would be subject to rough handling. For the same plan dimensions, this would mean a dead weight of 66 tons. Allowing 24 tons for the crane etc, the draught would be increased to 3'-2". The only way to keep it at 2'-6" would be to increase the plan to 63 ft × 27 ft. The dead weight would then be 85 tons and the total weight, fully equipped, about 110 tons.

A special contract

Assuming three pontoons would be ordered, Monash informally quoted £400 each for the 50 ft × 22 ft version with 3'-2" draught, or £500 each for the 63 ft × 27 ft version with 2'-6" draught. Moncrieff preferred to keep to the original plan dimensions, and decided to live with the 3'-2" draught. In February 1909, JM submitted a formal proposal with reservations. He wrote: "Although a considerable amount of work has been successfully carried out in this field elsewhere, we have so far had no direct experience of pontoon construction; and therefore feel some hesitation in taking indefinite responsibility at the present stage. We are, however, most desirous of embarking upon this class of work, and consequently beg to submit the proposal in a form which, while fully safeguarding your Department, will not impose upon us a serious responsibility, either as regards the efficiency or the cost of the work. In the event of your ordering one pontoon only, we desire that in the event of the work not proving successful, our liability shall be limited to loss of payment only, and that we should be under no ulterior liability, as under the usual form of contract. In the event of your ordering three or more pontoons, we desire that, in the event of the first pontoon not proving successful, we may have the option of not proceeding further with the order. Under above conditions our price for a single pontoon 50 ft long 22 ft beam will be £460, and our price for each of three such pontoons will be £400."

Moncrieff then issued blueprints showing the agreed dimensions. The details, method of construction, and reinforcement were left to SARC. However, workmanship and materials were to be "of the best quality" and delivery was required as soon as possible.

Finalisation of design

Stress calculations were carried out by the Melbourne office in the middle of May 1909. The structure of the pontoon was treated as an inverted building floor, with the ribs of the pontoon acting like beams, and the shell like a floor slab; but subjected to upward pressure from the water. Monash decided at this stage to check what would happen to the pontoon when the crane picked up a load. His memorandum to Laing reads like an exam question - he had recently been acting as an external examiner for the Faculty of Engineering at the University of Melbourne.

"Mr Laing. Please work out following problem:-

ABCD is a pontoon 46' long × 22' wide, weighing, empty, 62½ tons. Assuming this weight to be uniformly distributed along its length, the displacement (in fresh water) will be 2'-2.6" (check this) i.e. EB = FC = 2'-2.6". Now, suppose that a concentrated load W is brought upon the pontoon, at point H, on centre line, 9' from one end. Find the values of EB and FC if W be taken respectively as 12, 18, & 24 tons. Rough solution required as soon as possible, subject to any urgent work in hand. JM 16/5/09."

The results were somewhat alarming, as the expected freeboard for the bare hull was less than 2'-6".

[load]	EB	FC
no crane load:	2'-2½"	2'-2½"
12 tons:	1'-9½"	3'-5½"
18 tons:	1'-8½"	4'-0"
24 tons:	1'-6"	4'-72/3"

However, the SA Engineer for Harbors, J B Labatt, assured JM that the pontoon would not sink, as the figures were "undoubtedly wrong". "The utmost concentrated load down the central pillar of the crane would not exceed 10 tons, while the utmost load hanging from the end of the jib would not exceed 2 tons."

Computations for the strength of the pontoon resumed at the end of July and continued into August. Laing calculated the strength required of the cross-rib under the crane, treating it as a beam of span 6'-8", carrying a load of 12 tons. After calculating the bending moment and shear force, he decided to provide small tertiary ribs directly below the crane. The volume of concrete in the pontoon would be 964 cubic feet. Without the crane, fuel, etc, it would displace 2'-6" of water, giving a freeboard also of 2'-6". It would be able to take more than 60 tons uniformly distributed load "without losing buoyancy". Monash sent these details and a copy of the calculations to his Resident Engineer in Adelaide, W W Harvey and asked him to finish off the design by sorting out details of the fender beams, bollards, and manhole covers.

Practicalities of building and launching.

Harvey had started to think about how the pontoon could best be constructed. His first idea had been to build temporary walls in the river to enclose a makeshift dry-dock, running parallel to the bank. The water would be pumped out, and the pontoon built on a portion of the river bed. Water would then be let back in to float the pontoon, and the walls removed. However, locals had told him the land surrounding the Port Adelaide River was so sandy that the dock would never be dry. They claimed that water in pits dug one mile from the river had been seen to fluctuate 4 feet with the tide. Foreman Suffren assured him this story was true. Harvey therefore suggested making a slipway and building the pontoon either inclined at the angle of the slip, or on a launching platform. The first method would be cheaper, but would induce bending of the hull during launching because one end would start to float while the other was still on the slipway, leaving the middle unsupported. As the pontoon was intended for use in quiet water, this effect had not been catered for in design. The second method, suggested by Monash, would ensure the pontoon floated off the platform evenly supported by water along its full length. However, powerful jacks would be needed to move the assembly, and abutments would be required for them to thrust against. This option was chosen. Monash approved Harvey's sketch for the platform and agreed it would be best to launch the barge sideways.

Optimum slope for slipway.

Then followed much discussion of the optimum slope for the slipway. A steep slope would minimise the force required to move the assembly, but there would be a risk that it might get out of control. A gentle slope would need greater force and a longer slipway, extending further out into the water. The vertical descent of the pontoon to obtain flotation would have to be about 4 ft, even at high tide. Laing had calculated that with a slope of 20 degrees, a force of 10 tons would be needed just to get the platform and pontoon moving. He based this on coefficients of friction he found in texts by Rankine (0.25 to 0.5 for wood moving on wood) and Molesworth (wet wood 0.68, dry wood 0.5, at repose). The platform would need to move 11 ft horizontally to achieve flotation. However, Monash was concerned that the 20 degree slope would be dangerous and suggested a flatter angle, implying a horizontal movement of about 30 ft.

RCMPC's Managing Director John Gibson promised to "have enquiries made from some practical men in Melbourne". In Adelaide, Foreman Pratt obtained advice from a Mr Fairweather of the South Australian Tug Co. that the usual grade of slipways for launching ships was between ½" and 1" per foot [2.4 to 4.8 degrees]. The Tug Company's own slip was reckoned to be steep at 7/8" to the foot. Fairweather suggested that SARC's slip could be set at 1" to 1¼" to the foot - or more if they chocked the ends to prevent the platform running away.

Harvey's summary of the problem was as follows:
[skip summary]
He used T as the symbol for tons.

University of Melbourne Archives
Reinforced Concrete & Monier Pipe Construction Co Collection

After wrestling with the problem for several days, Harvey told Monash he agreed that a 20 degree slope was too steep, recommending 4 degrees, but suggesting further recourse to "practical men". However, he continued to propose new schemes. One was to face the skids with iron and use 1¼" bars as rollers, though the slope would have to be very low for safety reasons, and the length of the slipway correspondingly long. Finally, he came up with the method that was to be adopted: to use timber skids, but have them separated by wedges, "as done in the ship building industry". The timbers would not be greased until the pontoon was ready to launch. The platform would have to be stiffened to avoid distortion of the pontoon as the wedges were gradually removed. Harvey ended: "There are so many details in connection with this whole matter which are new to me that I will be glad of your brief advice on the various schemes mentioned".

Internal stiffness and strength.

An incomplete drawing in the J Thomas Collection (JTC) suggests that at some stage in the design process, the Melbourne office had decided to simplify the system of internal stiffening walls suggested by Moncrieff's team by providing only two, longitudinal, wall-girders. Stiffness in the transverse direction would be achieved by adding ribs to both the deck and bottom plates. However, Harvey was worried that this would not provide sufficient resistance to racking effects. "Regarding the general design, there is one modification which I strongly recommend viz the introduction of three cross stiffening walls."

Harvey stated that the advantages of his scheme were:

• Increasing the lateral stiffness.
• Reducing bending stresses where the deck and floor systems met the walls.
• Preventing racking, should the "punt" strike a bank at point F and be hit by another "barge" at point E.
  
• Stiffening the main longitudinal girders against buckling.
• Permitting division of the hull into 12 or 13 watertight compartments.
• Transmitting the crane load more effectively to the main girders.
• Providing extra stability to the side walls.

Harvey pointed out that parts of these walls already existed in the Melbourne scheme to provide a water storage tank for the crane engine. The weight of the walls would increase the displacement by only one inch - less, if the advantage were taken of the stiffening walls to reduce the ribs in places. For a while, he was worried that the concrete might absorb up to one third of its volume in water, thus increasing its weight and the displacement of the vessel. However, after "Calculations and Experiments" he decided the absorption would be only about 8%.

Final arrangements

A reinforcement drawing initialled by Harvey was issued by SARC on 19 August 1909. The first requisition for materials followed the next day. On 23rd, Monash approved Harvey's choice of site for the slipway and suggestion for increasing the lateral stiffness of the pontoon. Launching should be by means of a platform with a horizontal top, a slipway with a grade of 5 to 7 degrees, and skids separated by wedges as suggested, the whole being chocked for safety. For launching, preventer ropes should be attached on the land side to limit movement, the skids greased, the wedges removed, and the assembly lowered away using the jacks to assist.

Harvey now negotiated with Labatt a number of modifications to simplify construction. The latter agreed that the ends of the barge, instead of being semi-circular in plan, could be formed from five straight sections. However, he insisted that the lower edge of the hull, where the sides met the bottom, should be rounded, arguing that this would make it easier to refloat the pontoon if it became grounded. The contractors naturally preferred a simple right-angle. It seems that Labatt tried to convince Harvey by showing him a copy of the Scientific American of 5 Sept 1908, containing European examples of concrete vessels. Monash tried to hunt up a copy in Melbourne, but his approach to Gibson and Mitchell was unsuccessful. He suggested as a compromise that the corner be made as a double bevel to simplify formwork.

By 6 September, Harvey was having doubts about the wisdom of the project. "I cannot help thinking that we are working on a wrong basis by trying to mould the pontoon. I fear that if this were the right method of construction for floating craft, that there would be very little field for the application of Reinf. Concrete owing to high price of timbering and complicated nature of the work. Also the heavy nature of the work when done."

Construction.

On 3 November, Harvey reported the slipway and launching platform complete. Work had started on the formwork, and on 11 December the 'floor' of the pontoon was concreted. Monash became concerned about the high cost as work proceeded, and learned that the Government Inspector was insisting on giving directions to SARC's foreman, Suffren, about details of construction. Monash declared this "unsupportable" - SARC had guaranteed to supply a barge and had agreed to get no payment until the thing was finished and afloat. The pontoon was a product, like in a shop - the Government should not be involved. By this time H G Jenkinson had taken over from Harvey as SARC's Resident Engineer. He assured JM there would be no more interference, and on 26 January 1910 reported that the concrete work had been finished and stripped, and looked excellent.

Launching.

The first attempt at launching took place towards the end of February 1910. It was only with great difficulty that the platform was induced to move, although two 10 ton jacks were employed. The pontoon was slid to the end of the skids; but the tide proved not high enough to lift the pontoon. A small local tug named the Wallace was hired to pull it off, but could move it only 6 inches (15cm). During a wait for higher tides, someone realised that the platform had skewed and become jammed. Jenkinson tried to hire a more powerful tug, the Wato, but its owners were unwilling to risk its running aground. On 16 March, someone at SARC took drastic action, and removed two end principals from the platform substructure. The remaining principals then "slowly collapsed sideways" and the pontoon floated off at peak tide. Jenkinson reported that the draught was about 2'-10" with no load. The sides were "sweating a little", but no more than was expected and would "doubtless take up soon". The pontoon's water tank (for the steam engine) would be filled and kept full until its walls became water-tight.

Outcome.

About a fortnight after the launch, the pontoon was put on a slip for inspection. The SA Government Inspector declared it satisfactory and it was formally delivered to the Way & Works Department. SARC was paid £473-10-0d. Although Labatt had said he might order six more pontoons after the first had been tried out, Jenkinson doubted that SARC could supply them for a price acceptable to the Department. A photograph of the vessel in the RCMPC records carries the inscription: "Port Bridge, Adelaide. Reinforced Concrete Pontoon at work in canal, Oct 1910, with steam crane and grab". However, a photograph published in the Adelaide Register in May 1911 shows only the bare hull, apparently being used as a barge.

Footnote.

In mid-1917, the Adelaide Register reported that Monash's rival in reinforced concrete, E G Stone, was promoting the idea of concrete ships of 4000 tons and would guarantee them seaworthy. He stated "We [Stone & Siddeley] built the biggest pontoon in the world. It was 180 ft long with a beam of 80 ft. It is now in use in the Sydney Harbour."