A Fast Offshore Cruiser - 8. Materials and construction method.

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This chapter is in 3 areas, the selection, rules and construction. The construction section being split into hull and deck. This follows the progression that was taken in the production of the scantlings for the hull.

The selection of the material and method of constructing the hull cannot be treated as separate items as they are so inter-linked with each other they must be chosen as a package. This said the material is the starting place in this selection and the method selection, if any will follow.

Steel.

Steel is the ultimate material for a go anywhere cruising boat, it produces a very strong hull that is very robust and will probably survive pretty much anything. It is also relatively cheap and if any damage does occur it will be possible to repair anywhere due to worldwide use of steel technology. The reason that steel has not been chosen is its weight. It has strength to weight ratio of 51 (400/7.85) and 30 (235/7.85) which is very low compared to the other materials. Other problems include cosmetic maintenance, very careful finishing is needed to stop rust. There is also the need for insulation and the possible image of being a home built boat.

Aluminium.

Aluminium is an improvement on steel with strength to weight ratios of 98 (275/2.80) and 44 (125/2.80). There is also the improvement of cosmetic maintenance; it is possible to have a completely un-coated aluminium hull. The more complex welding that is needed for aluminium increases cost and restricts places that could repair damage. The need for insulation also increases the cost and complexity so aluminium has not been selected.

Single skin FRP.

Due to the nature of FRP were it can be made up in so many variations and directions, and the quality of the lay up then makes these numbers questionable, it can not really be compared by a strength to weight ratio. The reason that single skin FRP cannot be realistically be used for a small number or one off production is the necessity for a female mould, this being expensive and time consuming. It is on this point that single skin FRP is not being chosen.

Sandwich FRP.

This is an advancement on single skin construction and will produce a lighter and/or stiffer hull. The construction should be cheaper for one off because of the ability to use a male mould. Sandwich construction needs to be vacuum bagged to minimise and preferable post cured to enhance the bond between the core and the faces, the cause of delamination which is still a problem with this construction method.

Cold moulded wood.

This is a skilled construction method that can produce lightweight hulls. It is quite often used for pure racing boats and dinghies that can use the near monocock. It has not been selected here because of the extra complexity of construction and repair.

Carvel wood (speed strip).

Speed strip is the revolution in wooden boat building. It allows only relatively skilled workers to produce a large one off yacht quickly with minimal work. The finishing work is also minimised. The epoxy systems have improved the long-term usage of woods by sealing them from the environment preventing rot etc. The construction time is minimised by the speed at which the hull can be constructed.

Chosen method.

The construction method of speed strip has been chosen for the construction of the hull. It has been chosen on the grounds of ease of construction, cost of construction, weight and desirability. It will be combined with an outer layer of double diagonal veneers, giving increased resistance to damage and cross strip strength.

It is very useful to have a design accepted by a classification society. It gives customers comfort in the structure of the boat, allows it to be insured and should satisfy safety requirements.

The classification society rules that will be used to produce the scantlings will be the ‘American Bureau of Shipping’ (ABS) ‘1994 Guide for Building & Classing of Offshore Racing Yachts’ (5). This society and rule has been chosen due to fact that it is in touch with the design of this boat, in that it is a fast yacht. By working through the ABS rules and its requirements a set of scantlings can be produced.

Some designer’s say that the scantlings should well exceed the requirements of the classification rules, for example Steve Dashew in his book ‘Offshore Cruising Encyclopaedia II’ (3) states that the ABS rules should be doubled if not quadrupled. This seems a very pessimistic view seeing as the rules are based originally upon a similar pessimistic view, with the knowledge that the structure will be comfortably "up to the job". For this reason the scantlings of this design will all reach the requirements, but will not try to drastically improve on them.

Along with the ABS rules the author used ‘Cold-Moulded and Strip Planked Wood Boatbuilding’ (4) by Ian Nicholson for useful guidance in this form of construction.

The chosen construction method is a speed strip hull with two layers of veneer, in a double diagonal (45° , -45° ) orientation, on the outer surface.

Plating & frame spacing.

The available thicknesses, from ‘Joseph Thompson & Co. Ltd.’ (1), of approximately the required thickness, are 25 and 33mm. There are also sizes smaller and larger than this selection, but from reading about similar sized boats these seem to be the sizes generally used for this size boat for robustness etc. It is very important to know these dimensions, as these are the actual thickness that will be used, not what optimally produced by the equations.

The ‘speed strip’ construction method is a variation on the single-skin carvel method as used in the rules. For this it is recommended that the framing is transverse. Though this is not as resistant to buckling as longitudinally framing, by a factor of about 4, it does make sense. With the grain of the planking running along the length of the planks and the planks running fore and aft there will be great strength in the fore and aft direction. The weakness is in the transverse direction, were wood is generally 30 times weaker. So having extra strength fore and aft is not appropriate. There would also be the problem of long unsupported spans of planking in-between the stringers. The transverse frames will hold the planks together and provide strength perpendicular to the grain. The double diagonal outer veneer will also provide a large proportion of the transverse strength of the hull.

Any calculations that needs to include the hull plating or produce the hull plating scantlings will not include the effect of the outer veneers. This will mean that the strip construction will be strong enough on its own. The veneers will then increase the strength of the hull, and provide it with robustness to knocks and misuse.

The calculation of the plating thickness is an equation that relates the wood properties, panel span and a design head with a multiplying factor, which depends on the location of the panel on the boat. This equation can be rearranged to find the maximum frame spacing allowed for a set plate thickness.

See appendix A.1 for the equations and data for completion of table.

From the above table it can be seen that there are three possible solutions to the plating arrangement. 25mm plating throughout, 33mm plating throughout or 33mm bottom plating and 25mm topsides.

25mm plating throughout will produce the lightest hull shell weight but with a higher number of lighter frames, floors etc. 33mm plating throughout produces the heaviest hull shell but with fewer number of heavier frames, floors etc. With both these arrangements there will be a problem with the frame spacing being optimum for both the top and bottom plating. The frames will have to be spaced at the spacing for the bottom plating, because it is smaller. This will overbuild the topsides.

The third solution is to combine the two plating thickness, using the benefits of both. The frame spacing for the top and bottom plating, in the same position along the hull, are very close, if not the same. This would optimise the plating thickness for a given frame spacing. The one place that is not perfect is the plating supported by the frames that are in way of the keel, or where the multiplying factor = 1.8. Here the maximum frame spacing of the bottom plating is noticeable different from that of the topside maximum frame spacing.

The third solution will be used. The coincidence of the same frame spacing for the different plating thickness is an efficient use of the material and its weight.

The frames and bulkheads will have to incorporate the change in thickness of the plating. The difference in thickness is 8mm; this will have to be faired in so that there is no step, producing a discontinuity. A combination of epoxy filler and removing the top inner corner of the highest 33mm strip should fair this out.

Framing arrangement.

The arrangement of the frames and bulkheads is effected by the needs of the general arrangement. The needs of the GA can be seen in chapter 14.

The nature of the hull shape, quite deep, is advantages for the floors, allowing them to be deeper, therefore thinner and lighter. Also the wing water ballast tanks mean that the GA is cannot be pressed out to the full width of the hull thus allowing the frames to also become quite deep, reducing their weight.

The final arrangement comprises of two crash bulkheads forward, two bulkheads in way of the mast and shrouds close spaced frames between carrying the shroud bases. Another deep web frame separates the saloon and the galley/navigation area. Under the cockpit there are a further two water tight bulkheads. The transom is also a designed as a bulkhead.

Frames and floors.

The frames and floors will be made in plywood. This is possible because the frames can be deep, the narrowness of the GA because of the water ballast tanks.

The calculations can not include an effective breadth of plating, stated by the ABS rule requirements.

The frames between bulkheads C and D, in way of the mast, due to the GA (heads) need to be low profile, i.e. less than 10cm deep, between the wing water tanks and the local flooring. This is not possible with plywood only frames. To achieve the desired depth, or less, an aluminium flange will be bonded and bolted as the T to the reduced depth plywood frame. This system will be used on all the frames in the heads and only the forward two in the second cabin, were the intrusion into the lower berth needs to be minimised; the aft two will be normal plywood only frames. To ease the construction of the aluminium flange the frame will be straight. In the heads the frames will be boxed in to remove risk of injury to feet etc.

Bulkheads and bulkhead stiffeners.

The bulkheads will be constructed from plywood sheets. The ABS rule for the thickness of bulkheads is the same for that of the skin thickness. Plywood is classed as cold moulded construction so therefore the appropriate equation needs to be used.

The required ‘t’ of the bulkheads is dependent upon the stiffener spacing. The same spacing effects the dimensions of the stiffener therefore it is an iterative process to achieve the optimal numbers. A problem occurs with the internal shape, it varies so much there will be different number for each stiffener. Therefore a maximum spacing of 0.60m has been chosen and numbers produced for this value.

Wing tanks.

The head of water used to design these will be taken as the beam of the boat. This is the worst case scenario; the boat heeled with the mast parallel to the water, the system over filled so that the entire transfer pipe is filled with water.

Backbone and girders.

The backbone of the hull is solid wood. The minimal change in rocker mean that the wood does not have to be shaped that much, this reduces the time needed to form a laminated backbone.

The deck and super structure plating will be constructed from plywood. This material allows for easy, quick construction of the deck and superstructure. The flat nature of the deck lends it self well to plywood, there being minimal double curvature. The plating will sometimes be made from multiple layers of laminated plywood. This has the advantage of staggering the joints between the individual panels, producing a much stronger structure than just a single butt joint. This will be done in places were the required thickness is great enough too sensible allow multiple layers, without unnecessary extra construction.

All structural plywood will be marine grade (BS1088), and of mahogany.

The arrangement of the deck framing will be longitudinal. This is to reduce the panel size between the widely spaced transverse frames, allowing reduced plating thickness and therefore weight.

Plating.

The main deck plating will be constructed in the teak veneer method. This method is when the deck is largely made from plywood, single or multiple layers, and a thin, 5-6mm. The layer of teak finishes it off on top. It is said that this method may last up to 15-20years if not longer. The reasons for choosing this method is to still be able to have the luxury of a teak deck and all its benefits, looks, non-slip etc., whilst not having the extra weight of a teak only deck.

The ABS equations for deck thickness are the same as for the hull, with reduced pressure heads. This has the problem that the deflections produced by point loads on the plating have not been included. On the deck there will be points loads when a crewmember walks along the deck, even worse if two crewmembers happen to be standing very close. This deflection needs to be calculated and will be done by first principles. Only the panels that will be subject to these point loads have be analysed. The remaining panels, cabin side etc, will be produced by the ABS rule with a fudge factor.

The main deck, cabin top and cockpit will be limited to a deflection of y = span/200 and a FOS of 3. The pod top will be limited to y = span/100 and a FOS of 3 as it will only have a limited number of point loads on it compared with the main deck etc.

The boundary conditions will be taken as simple supported except for the cabin and pod tops that will be considered built in due to the number of frame that are supporting them.

Transverse and longitudinal frames.

The transverse frames are there to support the lighter longitudinal frames. The frames will be supported at the cabin/deck edge by treating the cabin side as a longitudinal beam.

Where the required section modulus of the beams supporting the cabin top is greater than that for the deck, the deck beams will be increased to the same section modulus.

The head used for the transverse frames will be the same as for the local deck structure. The longitudinal frames and girders will use the local head.

The transverse frames will be made from laminated Douglas fir, this has been chosen for its advantageous strength to weight ratio and that it will have greater aesthetic appeal over a plywood frame. Also in terms of aesthetics and headroom the depth of beams will be limited, also the cabin top beams shall be of the same size (that of the largest SM required) in an individual cabin to give a continuous appearance.

It was found that the longitudinal frames only needed to very small, because of the inclusion of an effective breadth of plating, therefore a uniform dimensioned longitudinal will be used though out.