The Right Stuff
After correct design, the most important thing about any wet exhaust system is that it should be made up from the right materials and components. These days, anything marketed as suitable for exhaust applications is almost certainly adequate, but of course, quality varies. The cheapest materials deteriorate the quickest, and the expense and aggravation of replacement means they are hardly ever the best value for money, even in the medium term.
Exhaust systems and silencers can be made from a variety of materials, including stainless and galvanised steel, injection moulded polypropylene, rubber, and GRP All these materials have their pros and cons. Stainless steel may sound good, but it is poor acoustically (it radiates noise, rather than damping it down, and always seem to corrode rapidly around the welds). Galvanised steel is, surprisingly, better from the corrosion point of view but is still far from ideal acoustically and will produce nasty rust stains if the zinc coating gets chipped and steel is heavy. One advantage of stainless and galvanised components is that they can be made on a one-off basis fairly easily.
Rubber and Plastic
Polypropylene, rubber and plastic don't suffer from weld deterioration, are reasonable noise dampers, and are lighter than galvanised and stainless steel. However, polypropylene silencers are very vulnerable to overheating and can actually melt if the temperature exceeds 130° C - which can easily happen if the seawater inlet gets blocked. The same drawback applies to rubber, which tends to burn through rather than melt at high temperatures. It is also prohibitively expensive to produce non-standard moulded silencers because of the high tooling costs.
Initially, GRP may sound an unlikely material to use in an exhaust system. Ordinary GRP will burn better than wood, so the resins and material that go to make up a GRP silencer are highly specialised and extremely heat resistant. GRP exhaust components are constructed from sections of sophisticated filament-wound tube, in which several layers of glass and resin are carefully built up to produce the required thickness.
This type of advanced composite material has a number of advantages. For a start it is very light -an important factor on high-speed planing craft. It also absorbs sound very well, is particularly resistant to resonance (sound vibrations), and can cope with temperatures of up to 300°C for short periods if the cooling system fails. It is particularly resistant to the high transient pressures caused by backfiring. It is easy to make up special components by joining standard sections, although pure one-offs with non-standard diameters are naturally more expensive.
The need for special filament winding equipment coupled with the cost of the special materials means that GRP composites are generally more expensive than rubber or polythene components and usually about the same price as stainless.
The most common bodge -and one of the most dangerous - is the use of ordinary flexible hose in place of proper exhaust hose. Because rubber hose has excellent insulation properties, when everything is working properly it may well be possible to put your hand on the outside of the hose. But the temperature of the inner wall will be far hotter, and if the cooling system is blocked, substandard hose can catch fire in just a few seconds. Always use hose marked Marine Exhaust Hose, with the words 'Lloyds ,Approved' or 'DNV Approved' followed by a certificate number printed along it. In America, exhaust hose carries the marking SAE J2006. Basic rubber exhaust hose complying with the above standards is usually coloured black. While this hose can cope perfectly well with normal, everyday use, it needs to be inspected regularly and replaced at the first sign of deterioration or perishing. It can cope with limited overheating up to 110°C.
The next grade up is lined rubber hose, which has an increased overheat tolerance up to 170°C. You can expect to pay a bit more for lined hose compared with unlined.
Blue silicon hose is another rung up the quality ladder again, capable of withstanding a temperature of 170°C. This is more expensive than lined rubber hose, but gives a greater margin of safety in overheat situations.
The very best exhaust hose is red silicon. Capable of withstanding 250°C, it is usually found in military and safety-intensive operations requiring the very best protection against overheating available. Red silicon costs many times that of plain black rubber, and is not normally specified for leisure craft.
When it comes to joints, any hose is only as good as the clamps holding it in place. These should, of course, be of stainless steel and each joint should be secured with two clamps to ensure sufficient contact area between hose and pipe. Wormdrive clamps, as are commonly found in automotive applications have limited torque capacity and can fail to provide the necessary clamping force -particularly with large diameter hose. The tee-bolt variety must always be used in marine exhaust systems. Check that the nut is of the self-locking type. Tee-bolt clamps are capable of producing enormous pressures, and if not treated with respect, will cut through the hose and possibly crack the tube.
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