Electrical System

This article is generously supplied by Roy Myers, owner of Minx, Hull No: 030 with minor updates by Sam L. Morse Company.

Decisions on electrical requirements are often left till late in the construction process. These decisions need to be made at an early stage, as the electrical system will affect the entire yacht’s construction. This paper outlines some of the considerations for marine electrical design and poses some questions for the new boat buyer.

Commonly, owners think their electrical demands are modest. The most revealing design process is to draw up a likely circuit breaker panel for the proposed system.

Here is a typical “I don’t have many electrics” sort of system. This is entirely the 12 volt system, doesn’t include any shore power at all. Typical power consumption in amps is listed as well.

Masthead Strobe 0.7A
Anchor light 1.0A
Tricolor 2.0A
Mast Deck lights 3.5A
Mast steaming light 0.8A
Mast navigation lights 1.5A


Wash down pump 5.0A
Power anchor windlass – varies, but assume 300.0A
Compass light 0.1A
Searchlight 8.0A


Radar – LCD Display – Variable on make/model 3.0A
Radar LCD Color Display – Variable on make/model 3.2A
GPS with Color Chart plotter 1.5A to 4A
Depth Sounder 0.2A
Knotmeter (0 Amps if combined with depthsounder) 0.2A
Wind Speed/Direction 0.2A
Electronic Compass 0.3A
VHF (Transmit > 4.0 Amps dependant on model) 0.25A
Single Sideband Radio (Transmit > 10.0 Amps dependant on model) 0.25A
Stereo system 1.5 to 4.0 A
Laptop computer/ DVD (Chip is governing factor) 3 to 7A
Cell phone /small battery charger 0.5A max


Cabin lights and power outlets Approx 1A per bulb
Pressure water  
Forced air heat  
Propane sniffer/shutoff system  
120 V Inverter  

Electrical system design is a challenge. It’s difficult to devote adequate space for the number of batteries to power this system, yet it is not uncommon to hope for such. Batteries are heavy, fussy things requiring space in the best parts of the boat and kind attention if they are to last. No, we can’t mount them in the Lazarette. Or the bilges.

The standard battery installation is 2 G27 Gel batteries, with a total of 172 Amp Hours. If we underpower the above system, the owner will constantly be running the engine to keep up with his electrical demand. That sort of behavior is not good for engine, batteries, or owner. It comes down to this- how much are you willing to spend to have all the amenities of the above system. Our BCC does not lend itself well to installation of solar panels in effective locations. Many attempts have been made, shading being the biggest issue. The cabin top or dodger top looks good at first, but is of no value if a boom tent is rigged. Roger Olson of NEREUS created a winglike affair with two panels attached between the mast and cap shrouds, but says it gets too much shadow from the rigging. METAPHORA sports two square panels on the boomkin, but doesn’t use the Freehand windvane. One likely looking location on the outside of the lifelines near the boomgallows gets fouled by the mainsheet when the boom is broad off. Most of us want the solar installation to be seaworthy as well, when the demand on the electrical system is highest.

There are several methods by which we can calculate energy requirements. One method commonly used is to add up the loads and guesstimate what the usage will be on a 24 hour basis. If you don’t want to charge daily (really not acceptable- you are confined to how far you can wander from your boat by the need to charge batteries?), then a daily use figure must be multiplied by the number of days between requested charging intervals.

Another method of calculating energy needs is looking at battery charging parameters. How is the bank most efficiently used, both in discharge cycle and charge cycle. As in the first method, you come up with the simple conclusion: use the biggest battery you can possibly find space for.

Perhaps the calculation method of most value is that of user experience aboard. Experience shows that with one person living aboard full time, being moderately conscientious about shutting off extra lights, uses 15 to 20 Ah per 24 hour period. Refrigeration just about doubles that for the most efficient system we are aware of. That jumps us up to 35 Ah per day. The second person aboard doesn’t double the figure again, but a 50% increase is to be expected.

Lets say we are up to 55 Ah per day. The next question we must answer is: How often are you willing to put up with running the engine? Daily, a 150 Ah bank will theoretically do the job, but if you actually have refrigeration, it’s not true. Most of us want to go two days at least without running the motor, which puts the bank up to 300 Ah pretty easily. And, in all honesty, a 388 Ah bank of 4 G31 Gel batteries, plus a smaller starting battery, is about all the BCC has space for. Sure, more can be made to fit, but the sacrifices become too great.

Simplifying this complex issue is not easy, but here is the solution. We have made up three optimized systems, increasing in capacity and complexity, and matched them to the loads they are to carry. Here they are.

BCC – Cruising short term system


  • Cabin lights in all needed locations
  • Simple GPS/ handheld
  • VHF
  • Depth Sounder
  • Simple stereo, 2 speakers, no amplifier
  • Masthead Strobe
  • Anchor light
  • Tricolor
  • Mast Deck lights
  • Mast steaming light
  • Mast navigation lights
  • Compass light
  • Searchlight
  • Propane Sniffer

Charging/ management system

  • Stock Yanmar alternator
  • Two batteries, preferably wired as one bank, and a dedicated Starting battery
  • Isolator Eliminator (optional) if wired as the preferred system
  • Metering by analog gauges or Link 10 as optional equipment
  • Three stage shore power charger

Voyager system

If our very first example looked like what you really expect from your electrical system, it is time to get quite serious about your energy equation. For managing such a system, we have found that Ample Power/ Power Tap products are the best choice. The system features a microprocessor/ monitor, the EMON II, which, unlike the Link system, has digital control over the charge process. Pricing is not greatly higher for this sophisticated system, room to grow is included (at your peril) and it is solar/ wind/ shore power ready. High current loads such as anchor windlasses and refrigeration are most demanding on batteries. As noted above, 4 G31 Gel batteries and a smaller dedicated starting battery can be tucked into a BCC without too much sacrifice in space. This yields 388 Ah total capacity, 194 Ah usable capacity to the 50% level. The engine run time required to maintain this system at anchor will be about 4 hours every 4 days, depending on usage. Shut down the high loads, and you will often go for weeks without the need to charge the batteries. If refrigeration is desired in hot climate conditions, the addition of solar equipment will reduce engine run time and heat output substantially.

Open ocean voyaging places the highest loads on the electrical system. Someone is on watch at all hours, interior lights are typically on all night, instruments (GPS and Radar) are often used round the clock, and navigation lights must operate at night. The above requirements are in addition to the normal loads experienced at anchor such as refrigeration. Most voyagers accept a somewhat higher engine run frequency due to the fact that one is not generally permitted to leave the boat on a long passage.

Marine electrical systems have increased in complexity during the past 10 years. The discovery that typical automotive charging systems were not up to deep cycle demands has generated many products and solutions to the energy equation. The more complex the system, the more aware the user must be to operate it correctly. Our Voyager system using Ample Power components is state of the art, but requires a fair amount of owner understanding to fully utilize. New boat buyers should prepare themselves for this learning curve.

The TWO BANK SYSTEM had the following disadvantages:

  1. Use of the One, Two, Both switch lead to the following problems
    1. To start the engine, the switch was commonly set to Both, left there (incorrectly; subjecting the starter battery to the house charge regime) to charge both banks.
    2. The switch was forgotten after the engine was shut down, house loads then draining both batteries.
    3. The One, Two, Both, Off switch could be moved through Off while the engine/ alternator are running, producing a spike which usually burns up alternator diodes.
    4. All load currents flow through the switch, yet another opportunity for losses, resistance and heat.
    5. The switch produced confusion as to what setting to use to provide charging for both batteries on alternator or shore power. It also provided a route for a low battery (usually the house bank) to drain the starter battery.
  2. Since the two banks could be used interchangeably, a Battery Isolator diode was used so the two banks could be charged together, but not discharge each other. These diodes produce a voltage drop, like a load, reducing the efficiency of the charging system.
  3. The two bank system takes up space, which could be better used by combining the batteries into one bank. Capacity is directly related to size. Especially where large loads are involved, a small bank will loose capacity disproportionately faster than a large one made up of the exact same batteries.