• High-output alternators need special attention given the inherent vibration in the installation. I have seen several fires started by output cables working loose. The cable attachment to the alternator should be designed to support the cable so that the cable’s weight is not hanging from the terminal. The nut attaching the cable to the alternator must be vibration-proof (i.e., Nylok or locking washer).  (Cruising Handbook, p. 178)
  • high-output alternator allows your engine to serve a dual purpose. It enables you to keep your batteries well charged without an expensive, separate, self powered generator set….To withstand extra heat and running time, these alternators need to be built with many extras, such as extra-heavy windings, bearings, heat sinks, terminals and terminal wiring, and diodes.  (All in the Same Boat, p. 158)
  • If you will be charging your engine running at 1,800 rpm, your alternator will put out according to that speed. Of course, you can vary the speed of the alternator with different-size pulleys, but with many configurations, this is true only to a certain extent.  (All in the Same Boat, p. 159)
  • You must also consider the loading the alternator places on the engine. On the one hand, you don’t want to use your diesel engine at anchor for long hours on a light load. It won’t be running at normal temperature, and excess carbon deposits will result….On the other hand, if the alternator takes too much engine power, it may interfere with engine performance while underway.  (All in the Same Boat, p. 159)
  • To minimize engine running time on a cruising boat, we want to be able to recharge house batteries as fast as possible whenever the main engine is running. In practice, there is a limit to how fast a battery can be recharged without suffering damage. Different types of batteries have different limits. Even when heavily discharged, wet-type discharge deep-cycle batteries cannot be charged at a rate that is much above 25% of their amp-hour capacity; cell takes up to 33% of rated capacity; and AGMs take up to 40% of rated capacity. Regardless of a charger’s output capability, as the batteries approach full charge, the maximum charge rates steadily taper down. What this means is that with a 600 amp-hour battery bank, there is little benefit in having an alternator output higher than 150 amps for wet cells (25% of battery capacity, 200 amps for gel cells (33%) and 240 amps (40%).  (Cruising Handbook, p. 163)
  • Alternators are typically rated “cold” (in 80 degree ambient temperature), but as soon as they warm up – which they do the minute they start producing power or in a hot engine room – the higher ambient air temperature reduces their output, sometimes by as much as 25%…The hot ratings are more representative of real-life output that the cold rating.  (Cruising Handbook, p. 163)
  • The manufacturer will have a curve that shows alternator output (amps) as a function of speed of rotation (rpm) and ambient temperature. Some alternators reach full-rated output at much lower speeds than others. The sooner the alternator reaches full output, the more desirable it is in a cruising application because it maximizes effectiveness on those occasions when the engine is idled at anchor solely for batter charging purposes.  (Cruising Handbook, p. 164)
  • The bottom line is that to minimize charging times, the boat needs an alternator with a hot-rated output in amps at typical charging rpms at least 25% (preferably 33%) of the amp-hour rating of all the batteries it is charging. Without this, it is extremely difficult to keep the batteries charged in a cruising environment.  (Cruising Handbook, p. 164)
  • …[alternators are] efficient, they are more dependable, they are lighter and they do not spark….they can endure higher rpm, which allows them to have a higher engine-speed ratio. Alternators typically develop full rated output between 5,000 and 6,000 rpm, but if you belt an alternator with a 2 1/2″ drive pulley to a 5″ crankshaft pulley, 1,000 engine rm will turn the alternator at just 2,000 rpm.  (This Old Boat, p. 286)
  • To increase alternator speed and raise output, you can run the engine faster or increase the pulley ratio. Either increasing the diameter of the crankshaft pulley or reducing the diameter of the alternator pulley raises the alternator speed.  (This Old Boat, p. 286)
  • To calculate alternator rpm, divide the diameter of the crankshaft pulley by the diameter of the alternator pulley and multiply the result by engine rpm. (This Old Boat, p. 286)
  • Most boat owners will find that their involvement with their alternators will be limited to adjusting belt tension and removing the unit to take it to a repair shop when it develops a problem. Good move. Alternators do not lend themselves to amateur repair efforts.  (This Old Boat, p. 286)
  • Sizing the alternator depends entirely on the battery capacity and how fast you want to charge. The maximum charge rate for deep-cycle batteries is 25%, so if you have 200 Ah of deep cycle capacity, the maximum charge rate is 50 amps. If the alternator is only charging the batteries, you need a 50 amp alternator…If you install a larger alternator to cut charging times, the reduction may fall short of your expectations. Suppose you double the capacity of your house bank to 400 Ah. Given the 25% capacity rule, you should be able to make good use of 100 amp alternator, but if you monitor the actual output, you will find that the big alternator will drive up the voltage sufficiently in just a few minutes to begin cutting back the output current.  (This Old Boat, p. 286)
  • discharging house batteries to 80% level will make better use of a high-capacity alternator.  (This Old Boat, p. 286)
  • In a perfect world you could convert 1 horsepower (hp) of mechanical energy into 746 watts of electrical energy….A closer approximation is achieved by cutting your expectations in half….Alternator voltage will be around 14 volts. Multiplying this by the rated current yields the power in watts – 700 watts for a 50 amp alternator. Dividing wattage by 373 (1/2 of 746) reveals that the alternator will require around 1.9 hp. The belt and pulleys consume an additional 1 hp, so a 50 amp alternator reduces the power to the prop by about 3 hp. A 150 amp alternator will require close to 7 hp under full load.  (This Old Boat, p. 286 – 7)
Alternator – Voltage Regulator
Fuel Generator
    1. The inverter should be as close as possible to its batteries (to minimize the lengths of the DC cables)
    2. It needs to be in a cool area with a good airflow over the unit (i.e., not in the engine compartment). Keeping it cool is necessary because an inverter’s overall performance is linked to its temperature; elevated temperatures lead to accelerated electronic-component failure.
    3. It must be kept dry and protected from spray or condensation because an inverter is a complex electronic device, incorporating different metals. If moisture is added, we have all the ingredients – water, DC electricity and dissmilar metals – for destructive galvanic and stray-current corrosion.
Manual Generator
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Water Generator
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