Weight reduction in composite boats

Some time ago, I was asked by a free-lance writer working for one of those composite dedicated magazines an insight into “Weight reduction in composite boats”. What follows is the result of the job done.  It comes by reviewing the documentation I’ve been collecting over the years and from my direct experience. For instance, as structural analyst, I engineered the sheltered deck you can see in the picture below.

Of course, it can only be a starting point, moreover it is dated 2008. I’m pretty sure for most of you all of this could sound obvious, then I hope you will add and share your views.

The sheltered-deck

A bit of background

Why weight reduction in boats matters

Similarly to any transportation system, the lighter is the craft, the bigger is the payload with no extra consumption. Payload examples are from troops with their equipments in the defence applications to the galleys or fuel tanks in the high luxury motor craft sector. When payload is not the issue, fuel saving or smaller engines could be the options. Not to forget the reduced maintenance cost associated to the smaller propulsion systems.

In addition, the Physics of sailing says ;the lighter the hull, the heavier the bulb, the faster the boat in windward condition. Secondly, the lighter the mast and boom, the lower the centre of gravity of the boat, the faster the boat and, last but not least, the safer the navigation. 

History

The first experiments have been prior to World War 2 and first valuable results have achieved in the 1950s. Being able to combine the strength and low weight of glass fibres along with the watertight integrity of the polyester plastic matrix was the very first success. Monocoque construction capability followed, avoiding the risky welding processes always needed when adopting conventional materials.

Currently, GRP is still the dominant solution in terms of material in the boatbuilding industry but it is being challenged by other materials: lighter and more corrosion-resistant conventional materials as well as more advanced composite materials. Conventional manual wet lay-up of discontinuous glass fibres in open moulds is still the dominant manufacturing process. However, more and more stringent low-styrene emission regulation will force the change into safer, for the labor force, technologies.

Material trends for composite boats

Reinforcements

Glass fibre. S-glass provides better mechanical characteristics with respect to the original E-glass. Continuous fibre forms realised in unidirectional, woven or stitched fabrics allow the material to be placed in the most beneficial direction with respect to the randomly distributed chopped fibres. It is heavier than both carbon and aramid.

Aramid fibre. It is resilient and impact resistant unlike glass and carbon. Therefore, it can be adopted for the whole structure of rigid inflatable boats, canoes and small vessels likely to be grounded quite often in their life cycle. It also tends to be used for the outer skin of larger craft in order to provide damage tolerance to the hull. It is more expensive than glass but less so than carbon. In terms of weight, it is lighter than both glass and carbon.

Carbon fibre. It is the standard when you want to achieve the highest performance or, in particular, you want to go for innovative solutions such as free standing wing masts and canting keels or you wish to realize the hugest multihull. Therefore, it is mainly adopted in the sailing competitions. Though, given the state of the art mainly coming from the aerospace sector, you must still be ready to pay quite a large premium for that. A second major drawback is its inherent brittleness: basically, it gives you no warning before breaking. Not only could catastrophic failures waste big investments but also the reputation of lots of people: boat’s owners, designers, engineers…

Hybrid fabrics. These are very advanced textiles including two or even three of the above mentioned fibre types. Their usage should lead to faster design and lay-up processes.  One example is E-glass plus Aramid, leading to a fairly cheap fabric having good impact resistance. These dry layers perfectly fit the liquid resin infusion. Multilayered multi-oriented fabrics made of the same material have also been produced; the intent has been to provide balanced lay-up in “one shot”.

Matrices

Vinyl ester. It has higher mechanical properties, stability and resistance to salt water than polyester resin that leads to higher purchasing costs. The main issue is the rather poor adhesion to carbon fibres.

Epoxy resin. It has higher mechanical properties than vinyl ester. In terms of stability and resistance to salt water can, at least, be comparable to it. Surely, it is the most versatile solution given the capability of creating good adhesion to most type of fibres. Last but not least, styrene is not an issue. You can guess now which is the most expensive resin. R&D people has always striven to give the manufacturers the easiest to flow epoxy resins together with the lowest curing temperature, in order to ease the passage from open to closed-mould processes and open the door too low temperature curing stages.

Polypropylene. This thermoplastic polymer can be adopted as a matrix and long fibre reinforcement as well, it can be the alternative to glass fibre polyester resin boats.  In fact, waste material, scrap and the component itself are all fully recyclable, and this could make a difference for environmentally conscious customers.

Core materials. Sandwich construction is well established in boat building industry. In fact, there is no better way to save weight. There are plenty of different types having their strengths and weaknesses. The key is to put the right core in the right area of the craft. Polyvinyl choride, better known as PVC, is the most common foam, probably it is not the best solution where dynamic loads and impacts are the issue. Styreneacrylonitrile or SAN could be a good alternative foam for that. Polymethacrylimide or PMI is known for its good fire, smoke and toxicity (FST) characteristics, it is a reasonable choice near hot engines then. When lightness is the main concern, we should move to honeycombs. They are made out of aluminium or aramid sheets. Not to mention the former is more affected by salt water than the latter. The former to be used for interiors, the latter basically everywhere you wish, as long as you are ready to pay good money for it. Last but not least, there is the natural honeycombs end-grain balsa. Shear and compressive strength, which are the  typical mechanical properties requested by a material for core applications, are fairly comparable to PVCs. In addition, this type of wood has the same density as PVC.

Adhesives. Structural adhesives are now the standard when bonding composites to metal in primary structures or, generally speaking, composites to composites. In fact, you can forget bolting when joining hull and deck. As in every adhesive joint, you must pay a lot of attention

Manufacturing process trends for composite boats and components

What follows is a mere list of manufacturing processes and their most typical applications.

Spray-up: small dinghies.

Wet lay-up/hand lay-up: production boats.

Vacuum bagging: large, one-off cruising boats and core-bonding in production boats.

Resin Transfer Moulding or RTM: small complex components such as hatches.

Other infusion processes (VARTM, SPRINT, SCRIMP…): large panels or semi-production small yachts.

Autoclave: very high performance structural components such as masts and booms.

Low temperature curing processes: large racing and cruising yachts, rigid inflatable boats and rescue craft.

Conclusions

Hope you enjoyed this brief introduction to the very vast topic of composite boats. I firstly reviewed why having a light both is important and when composite became a viable solution. Then, an overview on the most common types of fibres,  resins and core materials leading to most advanced composite materials was given.  A glance at the manufacturing methods and their typical applications ended the discussion. I’ll try to keep this writing updated, as soon as I come across somethingworthwhile.