The luxury of onboard electronics and electrical devices cuts both ways. What it gives in terms of convenience, it takes in terms of a load on your battery bank that needs recharging. There is tons of good info available on the net and in books on sailboat electrical systems. I especially like Sailboat Electrics Simplified by Don Casey. The information is concise and clearly presented in a way that makes it easy to refer to when I’ve forgotten some important point. I won’t bore you by rehashing all that but cut right to what I did and why.
Summer Dance has a single group 24 deep cycle battery. With the addition of accessories like a music system, LED strip lights, and a tiller autopilot, a single battery charge was no longer enough for a two-day cruising weekend, let alone those occasional great three-day weekends. I needed to find a way to put some amp hours back into the battery while we were out on the water. While the boat sits in her slip, I plug in a small, onboard charger.
At first, I considered the most popular solution, a solar panel. But after seeing the cost and complexity of a quality system and how little charge we could expect, especially at the northern latitudes where we sail, the results we could expect looked disappointing and my focus turned to our outboard motor. The winds are usually light in the North Idaho lakes in summer and we sometimes spend a lot of time motor sailing, more than I’d like, actually.
Most outboard motors (even those without an electrical starter) can charge a battery just like your car’s motor can charge its battery. Large displacement outboard motors do this as a rule.
The major components needed are:
- Generator coil(s) that create electrical current (unregulated AC) from the motor’s rotation
- Regulator/rectifier to convert the coil output into regulated DC current suitable for charging the battery
The good news was that our outboard motor, an 8HP Yamaha 4-cycle (model F8MLHC), already had generator coils installed (on top of the flywheel) at the factory. The manual states that the output is 80 watts maximum, or 10 amps at 12 volts DC. All it needed was a regulator/rectifier.
If your outboard motor doesn’t have generator coils installed already and you would rather consider a solar panel solution, check out Solar panels for boats: an easy installation guide.
Get a charge out of your outboard
I started shopping at eBay (where else?) and bought a used regulator/rectifier (Yamaha part no. 68T-81960-00-00). I had it mounted (to factory supplied holes for this purpose on the side of the motor) in 5 minutes and connected to the engine as shown in this diagram. The last step was to connect it to the battery onboard.
My goal for the onboard wiring was to be able to easily connect and disconnect the motor for trailering with a heavy-duty waterproof connector. I wanted the process to be as simple as connecting the fuel line. But I didn’t want to mount a cable connector in the hull that required drilling another hole.
At the motor, I made a short 2-wire harness out of 12 AWG cable. One end connects to the rectifier/regulator output and to ground on the motor. It then exits the motor at the front grommet and terminates in the male half of a 2-pole Delphi Packard Weatherpack connector.
In the boat, I ran a 12 AWG duplex (2-wire) cable from the battery along the bottom of the port side of the cockpit sole to the aft vent fitting. I ran the wiring right inside the short vent hose and out the scoop with enough cable to reach the outboard motor where I crimped on the female half of the connector. I covered all exposed cables with flame-retardant woven loom. When disconnected, the boat cable folds neatly out of the way along with the gas line. I spliced a 10 amp fuse holder on the battery end of the positive wire and connected it to the battery’s positive terminal. The negative wire is connected to the negative bus bar nearby.
The proof is in the put-putting
At cruising throttle, the motor puts out 2-3 amps to the battery, more than enough to offset the autopilot, GPS, and music system that are typically on at the same time. At full throttle, the battery gets a 5 amp charge.
I’ve been using this setup for five years now and I haven’t even had a low charge condition yet. I’m glad that we don’t have a big clumsy solar panel hanging off the stern. We might need one someday for week-long cruises, but even then it will be nice to know that we have more than one charging option.
Would you like to be notified when I publish more posts like this? Enter your email address at the bottom of this page to subscribe to this blog and receive notifications of new posts by email. You will also receive occasional newsletters with exclusive info and deals only for subscribers and the password to the Downloads page. It’s free and you can unsubscribe at any time but almost nobody does!