The standard reconnaissance flying boat of the First World War, derived from American models, had been the Felixstowe, but its slab-sided hull had structural and hydrodynamic deficiencies. The Air Ministry were concerned to see if the Linton Hope type of hull could be used instead on aircraft of the Felixstowe size and so, in 1917, specifications had been issued for this purpose. Captain David Nicolson, in two articles for Flight, described these and strongly advocated the Linton Hope approach they embodied:
Being of circular cross section, with fair and easy lines, they offer much less air resistance, consequently with the same horsepower are driven at higher speeds; they are much stronger weight for weight than the F [Felixstowe] Type, more seaworthy, and generally show the impress of the trained naval architect’s hand.
However, such hulls could not be evaluated immediately, as the manufacturers of larger seaplanes were fully committed to the wartime production of standard service machines. Supermarine and Mitchell’s good fortune was that the building of two smaller prototypes employing Linton Hope methods had been contracted out to the Pemberton-Billing firm and, although Hope did not entirely get his own way, as he indicated in another Flight article, the less than perfect machines embodied the alternative and much more promising approach to flying boat hull design:
Detail of the restored Southampton hull, showing the close-spaced hoops, the lengthwise stringers and the outer skin of longitudinal thin mahogany strips, also, the rear main point for the wing attachment.
These [Pemberton-Billing] 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 AD Boats showed the great strength of the flexible construction, and some bending and crushing tests carried out by the RAE works at Farnborough Show what they were able to resist.
When Mitchell became chief designer and chief engineer, he fixed on this Linton Hope structure as the standard company approach to flying boat hulls, particularly because of its shock absorption when landing on choppy water – there being no possibility of any of the conventional springing available to land planes. As C.G. Grey said of these hulls: ‘They were almost basket-like in their flexibility, and so got through the water without that jarring shock which was common to most high speed motor boats.’
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.
This approach and its method of construction were also spoken of approvingly in a lecture to the Royal Aeronautical Society given by Captain Nicholson, who had been involved with the alternative flat-sided hulls of the Felixstowe flying boats:
Construction is such that the structure is capable of resilient distortion, so that when alighting it can spring, reducing the shock. The hull cross section is egg-shaped, very light, possesses great strength, and is built of longitudinal stringers with bent hoop timbers inside and light frames outside the stringers, skinned with double planking, through-fastened together. No web frames or cross-bracings are required, and the hull is a continuous structure with steps externally added.
Webb, during his apprenticeship in the hull-building section, also noted how the brass nails and screws that held the final planking in place had to be fixed precisely in line and how the hull was then finally sanded down by hand and varnished until it had a surface ‘akin to the best kept dining room table’. Cozens used a similar comparison: ‘All through the lifetime of the wooden flying boats the air of sturdy solidarity was due to the beautiful diagonal mahogany or red cedar planking of the hulls, covered by four coats of copal varnish, which gave it a look of a piece of well-polished furniture.’
As Webb joined Supermarine in 1926, it would appear that he was speaking of work on the later Southampton flying boats, and it is also possible that Cozen’s account is influenced by the memories of the later machines’ finish – readers today are now able to see the magnificently restored Southampton hull at the RAF Museum, Hendon, and confirm for themselves the carefully aligned brass fixings and the ‘luxury yacht’ planked finish. (The positioning of the outer layer can be seen to be laid longitudinally; the practice of placing a layer of varnished fabric between the two layers was continued with.)
Unfortunately, photographs of the earlier aircraft do not show details of planking and so it is not clear whether the final finish was always varnished wood, as described above, or a doped-on fabric, also varnished. There is also evidence that at least some of the earlier machines, which were smaller and lighter than the Southampton, were not entirely double-planked but built with a single skin of planking: Supermarine themselves gave a description of variant construction, with reference to their earlier Sea King II:
The hull is of circular construction with built-on steps, which can be replaced in case of damage. The steps are divided into watertight compartments, 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.
Further, the Sea Lion III is also described as having ‘mahogany single-skin planking … covered externally with fabric suitably treated with pigmented dope’. One is reminded of Cozens’ report that, when the Sea Lion (Mark II, as it then was) was first started up, the vibration at the tail was so great that the pilot refused to fly it until the rear fuselage had been stiffened up – by wrapping and gluing more canvas around it.
It should be noted that the Sea King and the Sea Lion shared a common ancestry, and probably the same fuselage, with the earlier N1B Baby – hence the similarity in the above extracts – and so it would seem that Mitchell had inherited an aircraft whose construction had been significantly lightened in order to achieve the very sprightly performance it undoubtedly had. 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 had 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 11ft longer than the previously mentioned machines and would probably require the stiffening of double planking, especially bearing in mind the need here to design more staid, robust designs.
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 w
ere, prior to the Southampton, fine examples of the boat-builder’s art, bearing in mind the naval backgrounds of their British, Australian and Spanish customers. But it could be that the prince merely saw well-finished planking awaiting a protective layer of fabric to be doped on.
It is also 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 Scarabs for Spain or the ‘tropicalised’ Seagull IIIs for Australia. The commonly held view that Supermarine hulls were a mahogany colour might be accounted for either because of a varnished wood finish or because the doped-on fabric would allow the colour of the timber to show through. And, of course, 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.
Thus, more than 100 years after Pemberton-Billing and then Supermarine had begun to lay down these early hulls, there is a lack of information as to the precise structure of the various aircraft and one must be careful not to generalise. But, with this proviso, it can be clearly appreciated what was the essence of the Linton Hope contribution to flying boat design which Mitchell was fortunate to inherit and whose early structural virtues he must have valued.
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From Nighthawk to Spitfire Page 22
