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Old 12-21-2011, 06:18 PM   #34
RickB
Scraping Paint
 
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
RE: Engine Room Exhaust Fans

Quote:
Marin wrote:
Right, but wouldn't if follow that if the main engine was also driving an AC generator sized to run AC appliances like hot water heaters, stoves, and whatnot, as these loads were applied the main engine would have to work that much harder to combine the load of the generator with the load of moving the boat?**If that reasoning is correct,*you'd either need a larger engine to maintain the same propulsion power while powering the loaded-up generator, or you'd need to accept less propulsion power with a smaller engine whenever a significant load was put on the generator.
*That is correct, you can't create energy, you can only convert it. The shaft generator can only convert horsepower that is not being used by the propeller and it does take fuel to do that.

Shaft generators are usually sized to produce X amount of power at 100 percent shaft rpm or power (if a CP wheel) and a more or less linearily decreasing amount of power down to about 50 percent of engine power. You can't just add a shaft generator to an existing powerplant that is already required to produce 100 percent power to move at X speed and expect the speed to remain the same. You can't add a shaft generator to an engine that burns X gallons per hour at Y knots and expect to burn the same fuel at the same speed and produce Z kW as well.

Where a shaft generator would work on a trawler yacht would be on one that is grossly overpowered but normally operates at low speed so that there is a large margin of "reserve power" available to convert to electricity.

Given that most trawlers don't cruise at constant rpm for extended periods the chances are that the shaft generator would frequently trip due to rapid rpm and load changes. To negate this problem, a shaft generator can be paralleled with an inverter or smaller generator to share the load. Either method means that load management is still as important as it is on shore power. If you only have a 30A shore power supply you know you won't be running the hot tub, the dryer, and the galley oven at the same time and having a shaft generator doesn't automatically eliminate that issue.

Now, if you have lots of space for batteries and still need a lot of AC power, it might make more sense to keep it simple and just install a very large alternator on the main (if it is big enough) and feed inverters through the batteries. You can parallel inverters to support large loads and use the batteries to provide "peaking" power when needed or to make up for short term main engine power reductions.

The bottom line in all this stuff is that you are trying to change the fuel in the tanks to heat and lights or cold or some other more useful form and the means to do that are as varied as the reasons to do it. It is all a compromise and no one method is better than another for all people all the time.
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