Exhausting work – check and rebuild your wet system

Recently, I almost put to sea fully aware that my vessel’s exhaust system might be on its last gasp.

Fortunately, commonsense prevailed and I remained at anchor to unwrap the lagging and check it out, an act that caused the wet end of the dry exhaust pipe to fall apart in my hands. The lagging was the only thing holding it all together!

My crumbling exhaust system was fabricated from galvanised pipe, most of its dry section proving to be in excellent condition after fifteen years of service. Rust stains beneath the exhaust outlet had for some time been begging my attention, but never was there any sign of inboard leaking.

A lifespan of fifteen years is a fair innings for a plain steel wet exhaust system and, if compared with more exotic metals, it must be said that 304-grade stainless steel ‘tea-stains’ below its exhaust outlet and its superior relative 316-grade, the potential to crack without warning from work-hardening. Many stainless exhaust systems have been known to fail within ten years, let alone fifteen, unless chemical-standard 2205-grade stainless steel was used. This grade lives a much longer life but is very expensive and, being slightly magnetic, it might mess with the steering compass.

Regardless of the metals involved, the first section to fail in a wet exhaust system is usually where the dry exhaust becomes the wet exhaust at the end of the first gooseneck. A watchful eye on this area being important.

Beyond this junction all the wet pipe section can be industrial-quality hose for its flexibility and natural muffling qualities, or rigid PVC pipe and silicon joiners. When seeking joiners for this method check out auto and truck part dealers.

Wet exhaust systems can melt or burn if cooling water ceases to flow, demanding regular inspections be part of everyday seamanship by watching the temperature gauge as well as the volume of through-hull cooling water.

If the exhaust’s outlet spends much time underwater heeling to a breeze, audibility is muffled and visible water flow-checks are difficult, making the addition of an easily seen tell-tail above the exhaust outlet worth considering. Engines with separate oil coolers usually come with this extra outlet.

If the temperature remains normal despite diminished water flow, check the bilge into which an internally fractured wet exhaust system will run. If the vessel must keep powering under such circumstances a bilge pump capable of matching the
inflow is vital and fumes can usually be managed by ventilating the vessel whilst navigating from an open cockpit.

Having explored the possibilities of failure and how to cope, it is worth investigating how wet exhaust systems might be improved, especially those on yachts whose first gooseneck is constricted by a low deck-head, typically under the bridge or stern deck. The simple answer to this problem is to fit an easily accessed on-off valve into the wet pipe, but only if you are able match it with an unfailing commitment to turn it on before starting the engine and off when it stops.

One mistake here and the whole system can suffer appalling damage.

A safer more realistic answer is to fit a second gooseneck near the exhaust outlet, as illustrated, with a water-trap muffler placed between it and the first gooseneck. In this way the second gooseneck discourages back flooding whilst the water-lock constrains residual water after the engine is turned off. An exhaust thru-hull fitting with a one-way flapper valve is also a worthy ally for the way it discourages, but does not entirely prevent, back flooding when the engine is turned off.

Considering the challenges presented by wet exhaust systems, it needs saying that they can all be avoided by fitting a dry exhaust system. This minimises backpressure and eliminates the corrosive effect of saltwater throughout the whole system thereby improving engine performance and extending its life span. It is also much cheaper than wet exhaust, the only argument against it being its noxious fallout when expelled into the surrounding air.

Nevertheless, with careful planning it is possible to tolerate dry exhaust fallout, one method being to run its pipe through the mizzenmast. Another is to exhaust through the aft deck via a vertical pipe that clears all encumbrances, a cockpit Targa Bar promising a few creative opportunities here.

Motorboats and motorsailers typically exhaust through the cabin top with very few owners complaining of fall-out. It should also be pointed out that farm tractors need only short stacks to shoot their fumes well clear of the driver.

If dry exhaust is an option, a waterproof heat-shield fitted between pipe and deck is needed and it is well worth seeking advice from boat owners with dry exhaust, such as those with trawlers, motor cruisers and even small launches.

If seeking advice from marine engineers you will probably find that most see wet exhausts as being abominations best avoided whilst at the same time admitting that they are hard to avoid on a sailing boat.

All things considered, reaching the right decision on this subject can be very exhausting.

Alan Lucas

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JPK August 2023
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