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. As with most things in life, you get what you pay for. 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.

Heavy metal
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 like the bee's knees, but it is rather 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 -so long as the fabricator knows what he's doing!

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 degrees centigrade -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.

GRP
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 degrees 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.
cont....

Materials

Rubber Hose.

GRP Tube.