A “built-in” genset can be a valuable amenity on any boat, but especially on a cruising boat. The need for a generator, like the need for air conditioning, space heating or a watermaker, depends on the personal preferences and the anticipated needs of the boat owner. In the case of a generator, considerations include the need to operate a battery charger away from shore power, use of 120VAC aboard, anchoring vs. marina preference, availability and costs of mooring fields vs. transient docks in planned cruising destinations, geographical range of intended cruising waters, etc.
If a boat’s battery bank is sufficiently sized, and the boat is moving by propulsion engine each day, battery charging from the propulsion engine alternator may be sufficient to deliver and meet AC electric energy needs, via an inverter. Especially so if AC electrical needs aboard are modest. Gensets can power larger loads like reverse cycle heat pumps for air conditioning and space heating. They can power emergency de-watering pumps, battery chargers, AC refrigeration, range/ovens, watermakers, washer/dryers and entertainment devices. They can provide emergency dockside power when commercial utility power goes away. They enable long term anchoring and the use of mooring balls in New England and Florida Keys destination towns, and along the shores of the Canadian Canal Systems. We aboard Sanctuary used our genset for refrigeration, heat and TV for several days in the aftermath of Hurricane Sandy, prior to the restoration of commercial utility power.
There are two choices of generator type that can be used very effectively on boats and in RVs. The most common choice is the combination of an alternating current (AC) alternator driven by a gasoline or diesel motor. Speed-of-rotation must be held constant – preferably 1800 rpm – to produce stable 60 Hz AC output. Speed control is generally accomplished with a mechanical governor that responds to changes in electrical load by adjusting the throttle of the drive motor. Generators are readily available in sizes of 7.5/8.0 kilowatts (kW), suited to boats with 2, 120V, 30A shore power circuits, or 12 kW, suited to boats with a single 240V, 50A shore power circuit.
The second type of AC generator consists of an automotive style alternator (DC output) driven by a gasoline or diesel motor. “DC generators” have internal electronic inverters that convert the DC produced by the alternator to 120V and/or 120V/240V AC at 60 Hz, usually in pure sine wave (PSW) form. These machines use high output DC alternators such as those used in commercial trucks and emergency vehicles, and produce +12V or +24V DC output. The design of DC gensets is mechanically less complex than AC alternators. The motor’s speed-of-rotation is not important to AC output frequency. Batteries can be charged directly from the 12V (or 24V) alternator output.
Regardless of the choice of technology, when installing a generator on a boat, be sure to purchase equipment that complies with the ABYC A27, Alternating Current (AC) Generator Sets electrical standard. Use marine rated materials and installation techniques that comply with the ABYC E11, AC & DC Electrical Systems on Boats electrical standard. Use marine-certified electrical technicians to perform any contracted installation work. The life you save by doing so may be your own, your children or grandchildren, contractors working on your boat, and/or your family pet(s).
As in all things, choosing the electrical rating, in kW, of the unit depends greatly on what one plans to do with it. That analysis and decision is the initial task to complete. One must balance several factors:
- continuous (sustained-output) vs maximum (peak-output) load delivery capacity,
- the average AC electrical load, in Amps, on the boat
- the maximum AC electrical load, in Amps, on the boat
- the percentage load factor (LF) of the generator on the diesel drive engine,
- capacity options as available from various manufacturers,
- device technology; i.e., DC generator with inverter vs AC generator,
- warranty terms and conditions,
- budget: i.e., used vs. new unit capital cost, and finally
- installation fitup.
Start by looking at the electrical load that must be supported onboard. The most simplistic analysis is based on the boat having either:
- one or two existing 30A shore power service cords, or
- a single 50A shore power service cord.
Doing the math for 30A services, the capacity of a single 30A shore power service is up to (30A x 125VAC) = 3750 Watts. A boat with two 30A services could use up to 7500 watts (7.5kW) of capacity at one time. Thus, a genset capable of a continuous output of 7.5kW would be needed to fully power everything aboard to the same level as available with shore power. Since 7500 watts at any one time is rarely needed, a 5kW machine might be an acceptable compromise, but would limit flexibility in maximum case situations. It does not allow for any further future expansion of simultaneous electrical load or for changes in how the Admiral may want to alter electricity use patterns in future months/years.
The capacity of a single 50A shore power service is up to (50A x 240VAC) = 12000 Watts (12kW) to fully power onboard AC electrical equipment. Thus, a genset capable of a continuous output of 12kW is required to fully power everything aboard to the same level as available with shore power.
Aboard Sanctuary, we do occasionally want to have nearly the maximum amount of AC electric power. That happens when we’re running the heat pumps for either A/C or heat, and when we’re also charging our batteries and simultaneously heating hot water and running the microwave. It’s obvious that this does not happen often, but it can and does happen, usually when we’re in a hurry to depart the boat for other (sightseeing) pursuits. That use could be balanced over time, of course; that is, if the time is available.
Unusual AC electrical equipment, like a washer/dryer, high-pressure pump of a watermaker or an air compressor, would also influence the foregoing load-planning scenario. Motor driven appliances introduce two technical considerations. First, they require dramatically higher currents to start the motor turning than they require to keep the motor running. The generating equipment must be able to handle that transient, short-period (peak) load. Second is a technical issue called “Power Factor.” In general, the implication of power factor is that larger ampacity wiring and higher capacity generating equipment is necessary to support the nominal load, often in the range of 20% greater than nominal. If this type of equipment is to be run by an onboard generator, special consideration must be given to wiring and steady-state power requirements.
If one decides to install a traditional rotating-alternator AC genset, I recommend gensets that spin at 1800 rpm vs 3600 rpm to product 60 Hz output. These small engine generators spin at 1800 (four-pole) or 3600 (two-pole) revolutions per minute (rpm) depending on the number of field poles of the stator design. Four-pole alternators are preferred to two-pole machines. The 1800 rpm speed-of-rotation of a four-pole machine offers longer service life and is quieter in operation.
If installing a new genset on a boat that was not previously fit with a genset, I suggest at least evaluating DC gensets that use high-output DC alternators to create regulated DC output driving an electronic inverter to provide the AC. Hybrid DC-with-Inverter gensets may have advantages over native AC alternators, including:
- more efficient in battery charging applications,
- no speed control issues for maintaining 60 Hz AC output,
- better fuel efficiency,
- inverter can be run from battery bank when genset is not running, and
- easier parts replacement – especially major components – if/when maintenance is required (dependent on manufacturer; go with one that uses non-proprietary components).
With either native AC alternators or DC-inverter hybrids, I recommend diesel-powered drive engine units. *NEVER* gasoline! And, *NEVER* Honda EU2000 or big box household portable units on boats!
The major project cost categories involved in installing a generator are:
- The machine, including delivery charges,
- the physical installation of the unit on the boat, including the cost of a suitable mechanical platform and a crane or other means of lifting the machine into place,
- components, materials and parts (mechanical, plumbing, fuel and exhaust systems and electrical), and
If installing a net new genset, plan for and fund the following:
- the machine itself, ABYC-compliant,
- mechanical preparation of the install location on the boat,
- raw water engine cooling intake thruhulls, sea strainer, and intake raw water plumbing,
- waterlock muffler, exhaust plumbing and exhaust discharge thruhull,
- diesel fuel supply, filter(s), and excess fuel return line,
- house AC electrical installation, including:
- an ABYC-compliant Generator Transfer Switch,
- power feed wiring from the genset to the Generator Transfer Switch,
- power wiring from the Generator Transfer Switch to the existing AC service panel,
- relocation of incoming shore power lines from the existing service panel to the Generator Transfer Switch, and
- remote controls for starting, stopping, and monitoring genset operation,
- DC power wiring for the genset starter motor
- a genset start battery
- labor on above items, and of course
- sales taxes
Notes on the above:
- genset waterlock (muffler) must be sized to handle the exhaust and discharge cooling water,
- plumbing fittings must be bronze or non-metallic; never brass,
- all parts and custom-fabricated components must be ABYC-compliant,
- the selection of Generator Transfer Switch will depend on how you choose to wire the genset (as a 120V machine or a 240V machine; ours aboard Sanctuary is wired as a 240V machine. There are pros and cons about that choice).
- electrical and mechanical skills are required to do this task. It can be done as a DIY project of advanced complexity. With time availability and skills, significant labor dollars can be saved on the installation. Labor hours will be significant: 40~50 hours, minimum. I did all the installation work for our genset myself. It took a couple of weeks elapsed time; not full days, just tasks interspersed with other projects and tasks.
- the AC wiring, DC starter motor wiring, raw water plumbing and diesel fuel supply and return lines will have to be custom-fabricated.
- a fiberglass waterlock will cost around $150.
- the Generator Transfer Switch (I used a Blue Sea Systems #9093) was the single most expensive component part, at $350 in 2004.
- for AC power wiring, I used a length of 50A shore power cord, #6 AWG, BC5W2, triplex because of the convenience of handling that multiplex wiring package.
Finally, to minimize the number of current and future hull penetrations, consider a single large raw water inlet with a single, large sea strainer, feeding a raw water distribution manifold (“sea chest”).