Thanks to Mike Sheppard who cut open this second hand exhaust he bought and then sent it to us. Examining it revealed several details we were not aware of, it also explained some problems we had experienced on Derek's D2 after a bad fuel problem, causing us to revise our conclusions. We did not expect a big deposit of limescale in an exhaust so this was definitely interesting. First 3 pictures are of a new silencer as this one had already been cut open.
New and unused vehicle type exhaust silencer.
Not gas tight so not safe for use in enclosed spaces.
Old used silencer cut open
Fibre glass is packed around the tube to absorb sound.
White coating of limescale is obstructing many of the holes and probably originally covered most of them as in some areas the coating has flaked off.
Upper layer of fibre glass removed.
U shaped tube has holes over the entire length.
These holes allow some of the noise which is basically air pressure variations to pass into the outer casing area filled with glass fibre, reducing the noise level.
Not easy to see but the central part of the tube is pinched in at the bottom narrowing the tube. Before seeing this one cut open we thought the silencers must have baffles as objects dropped into the inlet would not pass through. There are no baffles, the tube is continuous.
Side view also shows the tube narrowing. The outer case is only spot welded to the exit tube, one of the reasons this type of silencer is not gas tight and should not be used in enclosed spaces.
Two washers with a 5p coin for size comparison. Ideally we would have used spherical objects but had none of a suitable size for checking the tube's minimum size. The left washer 15.8mm will not pass through the silencer. The right washer 14.8mm diameter will pass through.
The outer edge of the washer held in front of the inlet pipe shows how much the silencer size reduces at the pinch. Some air may pass through the holes in the side of the tube and bypass this restriction.
The white limescale layer is quite thick.
The marked area has been immersed in kettle limescale remover. Compare this with the earlier picture. Most of the scale has been removed and the holes are clear. There is still a very thin layer of whitish deposit remaining which is not limescale. It would brush off but in practice you could not reach the surface without cutting open the silencer.
The method of construction using spot welds and folded seams means this silencer is not gas tight. It is designed for vehicle use and is unsafe in most boat installations. The silencer tube narrows in the middle explaining why objects dropped in to check if it is blocked need to be a lot smaller than the pipes. We also realised why Derek's D2 smoked after refitting the silencer when he had the bad fuel problem. Unburnt fuel had collected in the fibre glass, then as the exhaust gasses heated up the silencer the fuel was hot enough to produce smoke but not hot enough to burn. We solved the smoke problem by cleaning the exhaust but we also serviced the D2 again which was not necessary. If we had not cleaned the silencer the unburnt fuel would have eventually burnt away and stopped smoking but the continued smoke would have been anti-social. The fibre glass also explained why it seemed to take for ever to shake out all the water after cleaning.
Mike bought this silencer second hand and fitted it on a narrow boat. While investigating an overheating problem he found the silencer made little difference to the noise so did not refit it. We can see why it made no difference, the holes inside were blocked preventing noise passing into the silencer body. In effect all he had was a tube with a small restriction in the center, useless as a silencer. There is a lot of limescale so this must have built up over a prolonged time before Mike bought it. We would not expect water in diesel fuel to contain lime but we are not experts on fuels. Derek thinks the silencer could have been laying in the bilges covered in water. The silencer construction means it is not airtight so water might seep in. Alternatively water may have entered via an exhaust hull fitting due to splashes probably on a sea going boat. Rainwater would be free of lime so is not the cause. Hot gasses from the exhaust would evaporate the water leaving the limescale deposits behind.
Is it a common problem? Probably not but with a sample of just one silencer cut open we cannot say for sure. A much more likely problem is the fibre glass choked with carbon as most silencers are never cleaned. Unless the main through pipe is blocked the major effect would be gradual loss of silencing function. Ultimately as in this case the silencer stops reducing noise.