Alternator Overheat: A Surprising Solution for Older Boats

Over the years, and with two different small diesels working in the tight quarters of the engine compartment of our 1978 Islander Bahama 30, we have consistently suffered from “overheated alternator syndrome.” Simply put, not enough airflow over the alternator results in ongoing repairs to this vital engine component. As the years have gone by, there seems to be more of an electrical demand on all boats and, without an onboard generator or a solar array, the alternator has to work harder to keep up with that demand. The generic 30-amp alternator on the old Volvo MD7A would soon stink up the compartment (and main cabin) with that hot metal odor. It was a simple system with no gauges, and somewhat limited knowledge on my part. I would have the alternator rebuilt every spring as a preventive measure, the technicians commenting on “overheating” damage.

NEW ENGINE AND BALMAR 614 SMART REGULATOR

I ordered a new Beta 20 with the optional 70-amp alternator. Even with all those amps available, the standard internal regulator would throttle back amperage long before the batteries were charged, a function of the typical automotive installation. Research indicated that an external regulator would solve this issue. I installed a Balmar 614 external smart regulator, designed to monitor and control battery charging in larger battery banks typical of the marine environment. I wanted higher amperage over a longer time period, something the Balmar promised. The unit included temperature sensors for the alternator and the battery bank. The external regulator was pre-programmed to throttle back output when the alternator frame reached 100 C (212 F). Higher output equals higher temperatures. This, I learned from the local technician, is the recommended maximum temperature to keep the alternator in good health.

SAME ALTERNATOR OVERHEAT ISSUE

During the first summer with the new system, I noticed the smart regular showed the alternator reaching 100 C quite quickly when there was a high demand from low batteries. This was typical after a few days at anchor. With the rapid rise in temperature, the regulator would cut back alternator output and the temperature would stabilize, just as the smart regulator was programmed to do. But, as a result, it would take too much motoring to charge up the batteries! There had to be a way of keeping the alternator cool and the amps coming.

Like many small, older sailboats, the Islander engine compartment is under the cockpit sole with access behind the companionway steps. When I installed the Beta, I fully refurbished that compartment to try to contain engine noise as best as possible. This included 1-in. foam sandwich glued and taped onto all applicable surfaces, as airtight as practical. The compartment had two external vents, a blower to extract dangerous fumes and an equal size air intake, both connected to vents on the stern. Both openings into the engine compartment were, by necessity of the location of the fuel tank, low at the aft end of the compartment. Turning the blower on moved a large amount of air at the rear of the engine compartment, but air movement at the front was negligible.

Temporary fix. Like most small marine diesels, the alternator on the Beta is high on the front of the engine. For my compartment, this was as far from the cool air intakes as it could be. Through experimentation I learned to remove the top step of the companionway stairs to open airflow through the compartment. This dramatically lowered the indicated alternator temperature and full charging would resume. This confirmed that the hot air at the front of the compartment was trapped, not circulating despite the internal fan of the alternator. But removing the step defeated the purpose of all that soundproofing. It also made traversing the companionway a bit of a hazard! There had to be an alternative that would increase airflow around the alternator without compromising the integrity of the compartment.

PERMANENT FIX: REDIRECTING AIR INTAKE TO ALTERNATOR AREA

Then, while watching a YouTube video (the font of all knowledge), a mechanic mentioned the vast volume of air consumed by a running diesel. A lightbulb flashed on! Could the air intake of the Beta be directed to the alternator area of the engine compartment? From a background in building high-performance cars, introducing hot air into an engine intake was against all my instincts. Cold air increases horsepower. But a typical sailboat diesel, even running at 80 percent of rated capacity, is well below its highest output. And I’m not pursuing every last available horse on a sailboat diesel anyway.

With that in mind, I used some inexpensive bilge hose of the same diameter as the engine air filter intake nozzle to build a snorkel to just above the alternator. The overheated air from the top forward end of the engine compartment near the alternator was now being sucked into the running diesel. The cooler, outside air entering through the rear vent at the of the compartment, flowed under the engine and past the alternator before being sucked into the snorkel.

RESULTS

Trials this past summer proved that, under heavy demand, alternator temperatures were now only reaching 85 to 95 C, not exceeding 100 C threshold programed into the regulator. Maximum alternator output continued for the programed time without overheating damage. Success! Although this practical solution may not fit all applications, if space is tight and airflow is an issue, it may be worth considering.