Beam strength:

The greatest load a beam can carry without breaking or becoming permanently misshapen, within the elastic limit of the material. Steel at about 1/2 of its ultimate strength will stretch permanently because of its quality of ductility.

Beam stiffness:

A beams unyieldingness or resistance to bending affects its amount of deflection under load. Two beams could have equal strength, but if of different section shapes will have different stiffness.

Reducing the length of a beam by 1/2 will will double its strength and increase its stiffness 8 times, i.e. it would only deflect 1/8 as much under the same load.

Beam depth:

The stiffness of a rectangular section (such as a flat bar or rectangular tube) increases as the cube of its depth.

What do we get from this? Parts which are associated together for the purpose of resisting the same stress should be as far as possible similarly stiff. If two beams are placed near each other and are similarly strong, the more flexible one will give less resistance than the stiffer one. The stiffer one might break (or bend permanently) when the more flexible one might not be strained near its breaking point. Ultimately the premature failure of one will lead to the failure of the other.

Now to apply this to our steel, aluminum, FRP or wood vessel. Notice the relative efficiency of the two sets of frames, longitudinal and transverse, i.e. of frames versus side stringers, in stiffening the shell plating and supporting the sides against deforming forces. Evidently, the most efficient stiffener is the one which is strongest and stiffest, the former quality varying as the length, inversely, and the latter as the cube of the length, inversely. Now, the transverse frames are short compared with the side stringers, for they only extend from bilge to deck, whereas the stringers span the long distance from bulkhead to bulkhead, or if no bulkheads that of the ship. If, for instance, in a hold 30 feet long between bulkheads, and 7.5 feet deep, there were, say, only one frame and one side stringer, identical in scantlings, then, although both parts, when under pressure from without, would necessarily yield alike, the frame, being one-fourth the length of the stringer, would give 64 times more resistance, It is clear, therefore, that light side stringers, on account of their great length and consequent elasticity, cannot by themselves give much resistance to widespread straining forces. This, however, is not their function; their principal duty is to give local support through their binding effect on the transverse frames. If, for instance, only one frame of a series happened to be subjected to intense pressure, it would, in yielding, at once be backed up by the side stringer, which, being connected to and supported by the unstrained frame on either side, would, in this short length, be so stiff that rather than yield it would force these two frames to yield, and these in turn would similarly affect the adjacent ones, so that the resistance to the local pressure would be distributed and result in a minimum stress and straining of the side.

The form or design of the side stringers has, of course, an important effect on their strength and stiffness. The stiffness of a beam of rectangular section increases as the cube of its depth; so that when a stringer is formed of a wide plate it becomes a very stiff girder, having greater individual strength than a number of smaller ones composed of flat bars, and having, perhaps, a greater combined sectional area. When so stiff and strong it is capable, apart from its mere binding effect on the frames, of affording direct and useful support to the side, over large areas. On the other hand, as the principal purpose of the stringers is so to unite the frames as to necessitate mutual and simultaneous action, it is evident that a large number, placed more closely together, may, owing to the distribution of their stiffening effect, give a better general result. In some vessels built on the deep-frame system, a reduction is often made in the strength of the side stringers,and a suitable increase made in the strength of the more important frames, the latter being increased in depth.

The above considerations as to the comparative efficiency of stringers and frames apply also to the floors and keelsons. The relative importance of these two parts is sometimes misunderstood, i.e. if, as a result of grounding, the bottom should be set up from bilge to bilge (so as to bend the floors upwards, say, by an inch or so at the centre), the keel naturally assumes an upward curve, and as it is this deviation from the familiar straight line that alone catches the eye, it is commonly assumed that lack of longitudinal strength in the keelsons has been the weak point, or primary cause of the failure; and so, to strengthen the bottom and compensate for any possible deterioration in the structure due to the straining, it is common, when making the repairs, to reinforce the keelsons instead of the more efficient floors.

Some boatbuilders today prefer a hull framed with flat bar stock for ease of construction. Definitely most owners today prefer the same for ease of maintenance, esp. scraping and painting. The designer prefers frames and girders with good heavy face bars making each frame into a 'T' . Of course these are hard to get behind and prepare for painting. But, the advantages of a heavy face bar for utility of material regarding strength and stiffness are undisputable. The section formed is not a 'T' but actually an 'I' section, because some of the shell plating is included as part of the beam. Now, the most efficient girder is one whose neutral axis is at one half the height of the web. Therefore a heavy face bar (or flange) as wide as the frame is deep and even thicker than the web of the frame would be superb. But, the cry will be heard "it is to hard to paint!" The solution is to delete the flange or face bar and substitute an intermediate flat bar frame, thereby doubling the number of frames. This has the advantage of giving the longitudinal flat bars a shorter span (1/2). This is extremely useful as flat bar longitudinals are exceptionally poor shell stiffeners when compared with angles or T bars. I have stood under an aluminum deck and watched flat bar longitudinals "trip" as people walked across the deck with the deck plates popping like a beer can. (not my design of course, but I make it a habit to learn from other peoples mistakes). So reducing the frame spacing from say (in a 45ft. steel semi-displacement type trawler) 42" to 24" and deleting the face bars, and close spacing the (now short) longitudinals is a good way of satisfying the modern 'flat-bar only crowd'. I usually shorten the span of the transverse bottom frames by including a longitudinal girder which also forms part of the engine girders and shaft tunnel. In slightly larger craft I use another longitudinal bottom girder, part of which becomes part of any tank longitudinal baffles or sides.

One area which always suffers from being underbuilt is the area of the side shell above the chine in the foc'sle of power boats. This area suffers from near head on wave impact creating panting stresses. These stresses are generally only held at bay by the support offerred by a well built collision bulkhead. The collision bulkhead should have transverse stiffeners to prevent it collapsing. To stiffen the hull topsides from the collision bulkhead to the next bulkhead aft (which is usually the engine room bulkhead), I usually show on my drawings an intercostal girder, running longitudinally halfway between the chine and the deck: After the shell is plated over and welded to the longitudinal stiffeners, the intercostal girder may be added from the inside. Its ends should be welded to the collision bulkhead, frames and next major web or bulkhead aft. I usually assume its function may be carried on aft by the guard welded on the outside which runs to the stern. The reason for the longitudinal girder is that, forward, the section shape of the hull is such that the point of the chine may not be considered 'a beam' or point of support for the frames because the chine or knuckle dissapears forwards into the stem, the bottom plating and frames running into the topsides plating and frames with a gradually decreasing knuckle. This in turn increases the span of each transverse frame from being merely from keel to chine and from chine to deck to that of one frame extending from keel to deck with no chine to support it. Obviously such long frames should be made much deeper, but due to their interference with bunks etc., the longitudinal intercostal of the same depth as the frames will suffice.

The interior joinerwork may be made structural and in support of the hull. Have you often wondered exactly what is connecting your interior outfit to the hull? Unfortunately tradesmen are specialists and sometimes unaware of needs other than those that suit their own. A carpenter may assume a steel or aluminum hull is stiff enough and does not require him to make real structural connections between his work and their work. Unfortunately much may be lost. It is known that a vessel may suffer a lifetimes stresses in one storm. Any added effort to stiffen a hull should not be missed. A bunk or 'berth flat' well glassed into a hull is one very deep girder indeed. Or for metal vessels the clips welded on to take the joinerwork should be well fastened to the woodwork. This means that all of the following may be made structural and in essence form 'egg-crating' for the hull: bulkheads, berthflats, shelves, cabinet tops, joiner partitions and cabin soles.

     As the longitudinals are continuous throughout the vessel from end to end, they must inevitably pass through tank bulkheads and watertight bulkheads. If the longitudinals are angles or T bars they will require a 'collar plate' around them to cover up the large cutout hole. Flat bar longitudinals only require a slot cut into the transverse members and are easily welded oil-tight or water-tight. It is good practise to build in all tanks to be part of the structure (except stainless steel water tanks) although this is not always possible in small craft as it may be too awkward for the welders to get into the crampt spaces to build integral tanks they can feel compentent about not leaking.  This shows another advantage of the all flat bar framed type of vessel, the fact it is easier to weld around flat bar longitudinals as the frames are simply slotted and require no cutout. Also for transverse framing with cutouts for angles or T bar longitudinals, the frame depths must equal 2.5 times the cutout depth to be considered sufficiently strong. This necessitates deeper frames than many people would like to see. When flat bar longitudinals are used the cutout is not considered as a loss, as all that is required, is a slot for the flat bar in the frame or tank bulkhead, which will be welded all around both sides, eliminating any loss of strength in the frame.

Your comment or questions to the author, Mr. Trevor Bolt, are welcome at..... tbolt@direct.ca

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