Don'’t Be Under Alarmed: Part 1 - PassageMaker

Don'’t Be Under Alarmed: Part 1

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A few weeks ago a client of mine contacted me and asked about which, if any, alarms he needed to have installed aboard his boat while it was undergoing a refit. The which, if any comment caught my attention. Indeed, if there's one thing a skipper needs while he or she is cruising, be it under way or at anchor, it's information about the vessel, the propulsion system, generator, and the integrity of the hull. A vessel that includes complete, operational systems alarms is less likely to suffer catastrophic breakdowns, flooding, fires, and other seaborne calamities and is less expensive to operate in the long term.

Engines and Generators

One of the tests I carry out during a vessel inspection involves confirming that the engine’s own integral alarms work. Virtually every marine diesel engine and generator includes, or included when it was originally manufactured, an alarm to alert the user to a few key events. Typically, and at the very least, these include high coolant temperature and low oil pressure annunciators. The coolant temperature alarm threshold varies from engine to engine, however, it’s typically somewhere around 220°F. Oil pressure alarms also vary; their alarm set point may be as low as 8psi. Generally, oil pressure rarely has an in-between failure, either it’s within specifications or it’s zero. Both of these alarm scenarios call for immediate action on the part of the skipper, particularly the latter. While overheating is never desirable, depending upon the rapidity of the onset of the overheating event, it’s sometimes possible to motor a short distance, to leave a busy channel for instance, before shutting down. Loss of oil pressure, on the other hand, affords the operator no such option. If he or she ignores the alarm for more than a few seconds, the damage will have been done and motoring any distance, no matter how short, is likely to become impossible as a result of engine seizure. Pressurized lubricating oil forms a wedge of sorts between moving parts such as crankshaft journals, bearings, piston rings, and cylinder walls. Four-cycle internal combustion engines require this oil wedge virtually continuously in order to operate (an exception is at start-up), without it rapid wear and heat generation ensue. I’ve seen loaded diesel engines grind to a halt, literally, within 10 seconds of a low oil pressure alarm sounding.

The low oil pressure alarm systems on roughly half of the vessels I inspect are not operational. How do I make this determination? Easy, I turn the ignition switch or ignition circuit breaker to the “on” position (without starting the engine); if I hear no alarm then it’s likely it will not sound in the event of a genuine low or no oil pressure event. This test simply simulates a no oil pressure scenario by energizing the ignition system and, because the engine is not running and thus has no oil pressure, the alarm sounds, or should. Every time you start your engine make certain the alarm sounds briefly, if it doesn’t it’s possible it never will; troubleshoot and repair the problem immediately. This test, by the way, is far from definitive in that it makes no determination about the overheat portion of the alarm system. This alarm can and should be tested at least once if it’s never been done and every three years thereafter.

Generator high coolant temperature and low oil pressure alarms work in much the same way as those described above with one important distinction. When generators encounter either of these alarm scenarios they automatically shut down, preventing or minimizing damage. Propulsion engine alarm systems, on the other hand, will rarely, if ever, automatically initiate a shutdown, although some found on modern electronically controlled diesels will reduce rpm to minimize heat generation. The logic being that a propulsion engine shutting down without notice may be more detrimental than the damage caused by overheating or low oil pressure, when docking for instance, or crossing ahead of another vessel.

Another alarm found on nearly all generators but few propulsion engines is exhaust temperature. Typically, engines overheat acutely for one of two reasons: loss of raw water or loss of coolant. In the former case the engine will overheat, although it often takes a few minutes depending upon load. What will happen very rapidly, however, is overheating of the exhaust system, which is cooled with raw water on most inboard engines. The exhaust gasses of a diesel engine can be as hot as 1,000°F. At that temperature it takes but a few seconds for damage to occur to normally water-cooled rubber and fiberglass components. Thus, exhaust temperature alarms are not only highly desirable, they are required for ABYC engine installation compliance. Exhaust temperature alarms can be easily retrofitted to nearly any generator or propulsion engine that is not already so equipped. Make certain that alarm annunciators are located at both helm locations and ensure they are loud enough to be heard under all operating conditions.

A final note on exhaust alarms, they are available in several varieties, including those that are strapped to the outside of the exhaust hose, those that pierce the hose, and those that are fastened to a metallic riser or elbow. They all work, however, because of their mass and heat sink properties those that measure the temperature of a metallic riser are often slower to react to a temperature change than those strapped to the hose or those that actually reside in the exhaust stream by piercing the hose. The hose piercing variety has the drawback of living an especially harsh life as it’s bathed in a stream of hot sea water and diesel exhaust. As a result, they tend to fail prematurely as well as leak. Thus, my preference is for those that strap to the outside of the hose. The temperature sensors associated with at least one of these alarms, those manufactured by Borel Manufacturing, www.borelmfg.com, utilize a sensitive and quick-acting thermister and as a result the alarm nearly always sounds before any damage occurs to the engine or exhaust system.

The hose temperature should be tested using an infrared pyrometer while the vessel is and has been at 80 percent power for at least 30 minutes before installing the exhaust alarm. Choose the highest recorded temperature location for alarm sensor placement. In my experience, many vessels’ exhaust hoses already operate at the edge or in excess of their design limits (most conventional wet exhaust hose is designed to operate at no more than 200°F), and as such installing an exhaust temperature alarm will only confirm this scenario, i.e., the alarm will sound, while proving of little operational value.

Next month I’ll discuss other systems and vessel alarms.

Steve owns and operates Steve D’Antonio Marine Consulting, Inc. (www.stevedmarineconsulting.com), providing consulting services to boat buyers, owners, and the marine industry. He’s also PassageMaker’s technical editor.

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