Thursday, 2 March 2017

R. J. Mitchell’s Wooden Hulls – structure and finish.

One of the fortuitous circumstances in R. J. Mitchell’s career was joining a firm which had adopted a method of flying-boat hull construction of considerable potential. G. A. Cozens relates how the Pemberton Billing firm that he joined began 'in a part of Mr.Kemp’s boatyard' and how the workforce were largely 'Kemp boatyard men.' Thus it would be no surprise that Pemberton Billing, in his stated concern to produce  'boats that would fly', basically adopted current boat-building techniques.  As Cozens related:
In 1914 the firm built the small flying boat PB I which was a credit to the workforce, and indeed it was judged to be the best example of aircraft construction at the 1914 Olympia Aero Show. The PB I hull was of round construction, built by small boat methods with closely spaced wooden ribs of half inch square section like girl’s hoops, joined by longitudinal stringers and covered by two layers of mahogany or cedarwood planking, laid so that the outside layer was sloping the opposite way to the inner layer, this was known as ‘opposed diagonal planking’. There was a layer of doped fabric between the layers and the whole fastened by brass screws or copper nails and in some cases the nails were turned over and clinched or riveted over a small washer.
With the outbreak of World War I, an Admiralty design team was sent down to Woolston to develop flying-boat construction and the basic structure with which Supermarine afterwards continued was established. F. Cowlin, the Technical Supervisor at the Royal Naval Air Station has recorded how he went down to the then Pemberton-Billing firm and ‘learned a great deal about hull design from Linton Hope'. As Penrose wrote: ‘the Admiralty had found floatplanes too dependent on smooth water; they were interested in the far heavier flying-boat hull which in the Linton Hope approach consisted of a double skin of mahogany planking with fabric in between, with rock elm strips forming almost circular ribs, longitudinally stiffened by closely spaced stringers’. 
The standard large flying-boat of World War I was the Felixstowe, but it had structural and hydrodynamic deficiencies and the Air Ministry were concerned to see if the Linton-Hope type of hull could be adopted on aircraft of the Felixstowe size. However, these types could not be contracted out immediately and it was to Supermarine and Mitchell’s good fortune was that the building of two prototype machines - known as A.D. Boats - had meantime been contracted out to the then Pemberton Billing firm. Although Hope did not entirely get his own way, as he indicated in a Flight article, the less than perfect machine embodied the alternative and much more promising approach to flying-boat hull design:
these boats were very difficult to get off the water . . . and with later experience it was obvious that the  main step was too far aft and the rear step much too far forward. . .  In spite of these faults in design, the A.D. boats showed the great strength of the flexible construction, and some bending and crushing tests carried out by the R.A.E. works at Farnborough show what they were able to resist.
The essential rightness of the Linton Hope approach was its flexibility, whereby a flying-boat could absorb the shocks of sea landings and take-offs when there could be no recourse to the normal springing of an aircraft undercarriage. Mitchell, as a young Chief Designer with a background in locomotive engineering and from the land-locked Potteries, was thus fortunate to inherit considerable sophisticated marine know-how and also a much more advantageous approach than that of the slab-sided Felixstowe flying boats. 
     He seized upon the basic principle of mounting necessarily rigid flying surfaces and engine mountings upon the Linton Hope flexible and robust hull.  [A small example of attention to the interface between rigidity and flexibility is mentioned in a Flight article about the Sea King:  ‘[the pilot’s] controls are mounted on the triangular tubular frame so well known in all Supermarine boats, and whose function it is to allow the circular hull to flex and "give" in a seaway, without interfering with the smooth working of the controls.’]
    As mentioned earlier, the Linton Hope hull was double planked although the positioning of the outer layer at about 90 degrees to the inner, diagonal, one was usually superceded by the former being laid longitudinally. What is less certain is whether the final finish was wood, varnished, or a doped-on fabric, also varnished. The fine reconstruction of the Southampton hull in the RAF Museum at Hendon [well worth a visit] shows the culmination of the Linton Hope approach which a Southampton apprentice of the time described as having the final layer of planking sanded down and varnished until it had a finish "akin to the best kept dining room table".  There is no final layer of fabric. And Cozens wrote of Supermarine aircraft having an "air of sturdy solidarity . . . due to the beautiful diagonal mahogany or Red Cedar planking of the hulls, covered by four coats of Copal varnish".
    However, Supermarine give a variant description of the construction with reference to their Sea King II: ‘the hull is of circular construction with built-on steps. . . the top side being of single-skin planking, covered with fabric treated with a tropical doping scheme’. Also a Flight description of the machine says that ‘the boat hull is of the typical Supermarine type, boat-built and through fastened, with copper or brass fixings throughout. The mahogany single-skin planking is riveted to rock elm timbers and frames, and covered externally with fabric suitably treated with pigmented dope.'
    Specific centre of gravity considerations or design requirements might very well have led to Mitchell requiring variations in planking and finishing in other of the firm’s aircraft at about this time – for example a Supermarine patent allows for planking to be omitted where external steps are to be added:
If it is desired to reduce the weight of the hull to the greatest possible extent, the skin-planking on the hull proper may be omitted where side wings or other projections cover that portion of the hull. Where this planking is omitted it is preferred to use a fabric covering for the hull proper so that it is maintained watertight, even although the wing or other projection may be perforated. The close spacing of the bent timbers and stringers provide sufficient support for the fabric to be a satisfactory watertight skin in cases of emergency.

Thus, when we consider Mitchell’s first complete designs, the Commercial Amphibian, Sea Eagle, Seagull II/III, Seal II, and Scarab/Sheldrake, which can be regarded as coming from a common stable, some or all may also have canvas exteriors – indeed, Flight describes the Seal II as ‘boat-built of planking over a light skeleton of timbers and stringers, and covered in fabric on the outside." The machines in this group are, however, all about 11 feet longer than the previously mentioned machines and would probably require the stiffening of double planking – with or without fabric doped on. There is an intriguing report in Flight of the visit of HRH the Prince of Wales to Supermarine where it is said that ‘the building of the Seagull flying boat hulls was greatly admired by His Royal Highness’: perhaps the company saw an advantage in producing hulls that were, like the Southampton, fine examples of the boat-builder’s art, bearing in mind their British, Australian and Spanish naval customers. But it could be that the prince merely saw well finished planking awaiting a protective layer of fabric to be doped on.

Incidentally, it is worth considering that wherever there was double planking, with the usual layer of canvas in between, fabric might also have been applied externally to protect the woodwork from splitting in the sun as well as to prevent the soaking up of water – one thinks particularly of the ‘tropicalised’ Seagull IIIs for Australia (see painting above). And the use of "pigmented dope", as quoted earlier, might account for the commonly held view that Supermarine hulls were a mahogany colour – either because of a varnished wood finish or because the doped-on fabric would allow the colour of the timber to show through – or, the ‘pigmented dope’ chosen might be mahogany in hue and none too opaque, given that Supermarine might very well have been strongly attached to reminders of their boating heritage.

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For reference sources, see my Blogpost: “Source Material and References" – an extended bibliography is included in my R.J.Mitchell at Supermarine; Schneider Trophy to Spitfire which also provides material for wider reading, grouped according to specific areas of interest. 

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