Inboard Hardware: Exhaust Systems

Dry Systems

Plate 55 — One method of insulating a dry exhaust line.

A common insulation method is the use of various types of lagging. One form of lagging uses asbestos rope or thick tape wrapped around the pipe and then wrapped with fiberglass tape or screen to prevent the asbestos from flaking off. An air space between the asbestos and the pipe can be used for additional insulative capacity. This can be accomplished by wrapping with heavy screen or expanded wire mesh prior to wrapping with asbestos (see Plate 55). Spacers can be used on the pipe between it and the mesh to create a "chimney" effect between the pipe and the insulation for the circulation of air as long as the ends are open. Another insulation method is similar to that used for steam pipes in shoreside systems using a lagging adhesive or compound. Lagging compound can be made by using a teaspoon of plaster-of-Paris to a pound of ground asbestos made into a heavy paste by mixing with water. This mixture is reinforced over the pipe with screen, expanded mesh, or wire. Another solution to the hot pipe situation is to use a pipe within a pipe to form an insulative flue. The outer pipe should allow at least 1" of space around the exhaust pipe at all points, and must be open at both ends. Where the pipe passes to the outside, however, there must be some means of keeping water from entering. In addition to insulation, protective guards may be necessary for the safety of crew members.

On vertical dry stacks, a funnel or "dummy stack" is often used to conceal the exhaust outlet. Regardless of this, rain and other water must not be allowed to enter the dry stack. The stack can be angled so that the exhaust exits in a more horizontal direction, and a flap over the opening is recommended. Nevertheless, a drain cock and condensation trap should be installed at the lowest portion of the exhaust system to get rid of any condensation which could freeze and cause damage during lay-ups. The outlet of the dry stack should also be fitted with a suitable spark arrestor.

A dry exhaust system should not be rigidly mounted to the engine, especially if the engine uses flexible mounts. A typical method is to join the exhaust to the engine by means of a flexible metal bellows section (see Plate 53). If these are used, they should be installed to be free from strain, and the stainless steel type is recommended for durability. Where a long horizontal run is necessary, a water jacketed pipe is recommended (see Plate 54). Cooling water can be directed to the pipe jacket and then to the muffler or other outlets where it can be ejected overboard.

Another type of dry exhaust system is often used on competition high speed racing boats. Basically, these boats used header-type exhaust pipes or manifolds which expel the exhaust nearly adjacent to the engine. Since these are often in open cockpit boats with only the driver in the boat, and are used only for regulated competition, their use is tolerated. This is one case where a hot exhaust system without any form of auxiliary cooling is used aboard a boat. Obviously such an installation offers no noise reduction whatsoever.