Steve,
I had a question about the cruising alternators on the main engine. I have been reading Ken Williams’ blog and there was a significant amount of traffic on this subject. Bottom lime was concluding that they drew more power, hence fuel, then simply running there normal 20kw generator. They went on to suggest that Nordhavn as a company was no longer installing them on their boats. I know you area a big fan and I was wondering if you could shed some light on the subject
thanks
Scott
Howdy Scott:
This is a complex subject and starts with the AC loads. If they are so high that you are forced into complex hydraulic drives for the engine driven alternator(s) there are going to be big efficiency penalties, and you may not be able to generate the required power in any event.
Our approach works well for several reasons:
- Our AC loads are typically under six Kw when under way.
- We have a pair of poly-V belt driven 4.5kW DC alternators.
- The battery bank is massive (1200 amp hours at 24V on the FPB 64).
- If AC loads exceed what the alternator/inverter package can comfortably handle, as during a washing and drying cycle with air conditioning going, the genset is run for a short period of time.
In our case you have to consider the power losses of the inverters, the AC/DC conversion at the alternator rectifier assembly, and poly-V belt drive losses. If our loads were continuous and high these conversion factors would make the genset slightly more efficient. But since the loads are only high on a periodic basis, we come out ahead on fuel.
There is another factor to consider. If you are genset dependent, then you need a second unit, so you are 100% assured of reliable power. In our case, we have a choice of engines/batteries/inverter or genset, so with a single genset we are OK.
And then we have the advantage of being able to sit for days at anchor without needing to charge the batteries or run the generator.
Posted by Steve Dashew (September 10, 2009)
September 10th, 2009 at 1:39 pm
Let me ask a stupid question. Why doesn’t anyone take an alternator/generator (120/240volt type) and power it with a drive shaft off the front of the main engine? Aside from the long space required for the end to end configuration wouldn’t that work?
September 10th, 2009 at 3:49 pm
AC generators are RPM sensitive, so they only work at a specific RPM. However, in the days before inverters and big alternators, propulsion engine driven AC generators were common.
January 17th, 2012 at 8:05 am
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