This article originally appeared on NorthernMarine.com.
For anyone interested in passage making trawlers, the number of production or semi-production builders is small. Unlike most cruising motorboat builders, who create competent cruising boats in all shapes and sizes, the construction of bluewater trawlers involve more rugged design and construction, complex systems for safety and self-sufficiency, and a host of other details that ensure safe voyaging far off the beaten path. If global travel is in your future, there is a short list of motor vessels to go aboard.
While the trend today is for faster, semi-displacement powerboats with sufficient range and reserves to go the distance, the sweet spot of the bluewater fleet has always been a full displacement mini ship with a large, single diesel. Often with round bilges for the utmost seaworthiness, these expedition yachts truly capture one’s imagination. I have spent time aboard a variety of these small ships, and they are the real deal. I even stand up straighter when I take the helm in the pilothouse. Images of Jack Hawkins always come to mind, standing on the bridge of his Royal Navy corvette, HMS Compass Rose, plowing into the head seas of the North Atlantic, in search of the Nazi menace, heavy pea coat against the cold chill of the cruel sea.
One assumption one might have of these capable sea boats is that the boat must have a dry exhaust system. A system that does not require seawater to cool the engine’s exhaust but rather uses a heavily insulated exhaust pipe and a hull mounted keel cooler for the engine cooling circuit. These dry exhaust systems are found on many commercial boats, and the dry stack and keel cooler proving to be exceptionally reliable. Walk around any Maine fishing village and you will see the inevitable coffee can placed on top of the open exhaust stack as the lobster boats sit at the dock. It is the way it is done Down East.
As much as one finds the dry exhaust system to be standard practice on some production passagemakers, they are not without their issues. For one, they tend to emit soot, not a good thing with teak decks and shiny white fiberglass. After a passage, soot may make it difficult to keep a boat clean, and there is not much to prevent it. (Some try putting filters on the exhaust stack at startup, when a cloud of soot tends to belch out over the boat just aft of the stack.)
Another issue is the noise coming from the exhaust. While I happen to like the throaty sound of a diesel exhaust, many find it tiring to hear this background noise day in and day out.
A properly engineered system, especially the heavily insulated and reinforced stack that must travel up the interior, often straight up through a major living area, takes up valuable space. And to avoid any possibility of a fire, it requires plenty of room around the stack, sacrificing living space that would otherwise be used for accommodations. There is no way around this.
The keel cooler component is theoretically less maintenance than a wet exhaust because it uses fewer parts than a wet system’s associated seacocks, heat exchangers, and pumps. But it is not without maintenance, and it is a vulnerable attachment on the outside of the hull. It does not get painted with anti-fouling paint, so if it sits idle for any length of time, growth will no doubt occur and require a diver to keep it clean.
A Design for All Reasons
As I mentioned, the engineers at Northern Marine have backgrounds in commercial boats, and this is no more evident than how the exhaust system comes together on Northern Marine yachts. I spoke about this subject with Stuart Archer, general manager of Northern Marine. To say these folks approach exhaust systems differently is an understatement. Perhaps it is the gear head in me, but I get excited whenever I come upon a solution that solves issues common on other boats. In the case of a trawler’s exhaust system, I believe they nailed it.
To begin with, the boat is built with a sea chest, an opening in the hull (often with a grate of some kind on the hull) that brings in a large volume of water, far greater than what is drawn through a small-diameter seacock. It provides an intake reservoir from which a pump pulls water for its intended purpose. As opposed to a seacock in the hull, drawing water from this reservoir all but eliminates the possibility of sucking in a sandwich bag as the hull passes by. Ditto sea grass and other debris in the water.
The newest concept we have designed and built is seen in the above photo. The idea being you can shut off ALL the raw water into the boat via the butterfly valve, or individually, to clear an issue without hauling the boat. In addition, there is the inspection port that is above the waterline that can be removed in the water to clear anything if needed. Lastly, there is a vent on the top when re-launching the boat—Stuart Archer.
Exhaust gases coming out of the diesel engine are treated as if the boat has a dry exhaust system, that is, the exhaust manifold and cowl muffler are wrapped in dry insulation to cover the exterior surfaces to keep heat out of the engine room. Wrapping exhaust components keeps the heat in the exhaust system, which helps move the gases out of the engine room as quickly as possible. (You will see racing motorcycles with wrapped exhaust pipes specifically for that reason. By keeping the heat in, the wrapped pipes get the exhaust gases out of the engine faster, offering better performance.)
The corrosion-resistant cowl muffler disrupts the sound waves, turning them into heat with as little restriction in the flow of the exhaust gases as possible. Water is then injected into the wrapped steel pipe coming out of the muffler and routed along the overhead to then drop down toward the exhaust opening in the round chine of the hull.
There are two unique elements of the Northern Marine system at the termination of the exhaust system. Some boat builders create exhausts that exit above the water, while others route the exhaust underwater. The problem with underwater exhaust when the boat is not moving is that back pressure builds up as seawater tries to get into the exhaust pipe when the engine is at idle. While it is much quieter than spitting exhaust out into the water above the waterline, it is not ideal.
Northern Marine installs a bypass exhaust off the main exhaust pipe. This bypass exhaust exits the hull 6-12 inches above the waterline. At idle, cooling water falls into the water in the main exhaust pipe, but the back pressure created by the seawater drives the exhaust gases out the bypass exhaust pipe. It is a great way to allow idling engine gases to exit the hull without back pressure.
On this new build, notice the bypass exhaust pipe that allows gases to escape well above the waterline. Also note the large diameter of the opening for the sea chest reservoir that will soon be installed.
Details of the completed exhaust. Note the fitting that injects water into the exhaust just as it turns down to exit the hull, and the bypass exhaust for the idling engine.
The other design feature built into the Northern Marine hull is a special fairing around the outside of the underwater exhaust. Once the boat starts moving, the fairing creates a low-pressure zone that draws the exhaust water and gases directly out of the boat without restriction.
Both features minimize pressure issues, while providing the quietest, most reliable exhaust possible. With no soot, no smoke, no noise, and without robbing the accommodations of valuable interior space.
Northern Marine continues to build world-class trawler yachts for global exploring. There is nothing these boat cannot do safely and comfortably. A side-by-side comparison of features, such as exhaust systems, will prove enlightening to anyone in the market for such a yacht.
And while the commercial approach to boat building may seem like overkill, I see it as peace of mind.