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
You can’t look at this as gensets vs alternators, although that is a part of the equation. The basic issue is are you a generator dependent vessel, and one that cannot stray far from shore power? If the answer is yes, then the AC loads (start up and running) will be so high that the genset (and its backup, and maybe a smaller night genset) are going to be the answer. This is the way 99% of motor yachts are done, and many sailboats as well. It is easier for the boat builder because he is using off the shelf domestic appliances for all requirements.
Our approach is different. By carefully engineering the AC and DC loads, matching these with a large bank of traction batteries, massive alternators, and inverters, we are essentially genset free. A big part of this is using custom fridge and air conditioning gear which is much more efficient than the norm.
Bottom line is we can sit for five to seven days without charging the batteries. And we typically charge the batteries underway, so big alternators are required. If they are on the boat, lets keep them working.
When we are on passage the highest AC loads occur in the tropics, with air conditioning on. Between our alternators, which put out a combined 300 amps at 28 volts, and inverters, we can cover almost all AC loads, except for the drier during the clothes washing cycle. So, if all the air con and drier are on at the same time, then the genset is called into play. In temperate climates, without air con, the genset is never run during passages.
Now as to efficiency of the two systems. To make this work you need permanent magnet alternators with remote rectifier assemblies, and precisely engineered belt drives (hydraulics are to inefficient). Get it right, allow for belt, rectifier, and inverter losses, and you have one figure. If you compare this to a big genset running lightly loaded, the engine drive system will be at worst the same, and in many cases more efficient.
If you have very high AC loads, then the case goes in favor of the genset.
Of course, once you are generator dependent, this means you must have a second genset, as mentioned earlier.
I don’t think there is a totally right or wrong answer. But we like our approach because it allows us to sit for long periods without running a genset (our genset on Wind Horse has 550 hours on it in four years of cruising half time – engines are at 3943 as this is being written).