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Dick Barnes of Anchorage, AK asks:

You seem to be hooked on the air conditioner units, an option that our sailing friends lack. Rather than running the generator full time to support your comfort, what would you think of a hydraulic-driven cruise generator that gets its energy from the main engine? Would the net fuel efficiency be better or worse? It seems that limiting the run time on the Northern Lights unit would be advantageous.

Dan Streech responds: You have asked a very good question which has a thicker plot than you know. For over 10 years, we have been dabbling with the use of hydraulically driven AC “cruising” generators. As you know, an AC generator must spin at a constant specified RPM (usually 1800 RPM for 60Hz and 1500 RPM for 50Hz) to maintain the correct frequency. To maintain a constant frequency when using the main engine to provide the hydraulic power, one must deal with:

Changing RPM of the main engine.
Varying loads on the AC generator.
The variance in hydraulic oil temperature and thus its viscosity. Internal hydraulic leakage past pistons and rotors which varies depending on load and wear of components and thus causing imprecision.

Theoretically, a variable displacement pump on the main engine driving a fixed displacement motor on the generator can be tweaked to hold a fairly constant RPM on the hydraulically driven generator. This rather simple arrangement has been used successfully on fishing boats (especially in the Pacific Northwest) for years to operate various kinds of machinery which are not too frequency sensitive.

When it comes to pleasure yachts, the standards are higher and the owner/operators are typically less patient with the process of manually tweaking a cruising generator to maintain frequency. Thus, the more sophisticated installations seen on pleasure boats use a “smart” control valve which meters the hydraulic flow based on feedback from generator to maintain exact frequencies. It can work pretty well, but we have found that such installations are still problematic and prone to wide swings in frequency when heavy loads (such as air conditioning start-up or water-maker start-up) are placed on the system. If the frequency swings more than 10Hz, troubles begin such as the Trace (SW2500) inverter/charger rejecting the power and video monitors flickering.

Your question was whether or not the efficiency would be better or worse using a hydraulically driven AC generator rather than a conventional stand alone generator. To answer that question, one has to make assumptions regarding the amount of power used and degree of loading of the stand alone generator. My personal feeling is that if one is just considering efficiency, the two approaches are probably about the same. The heat loss of running hydraulics is probably about the same as the parasitic power loss of running a stand alone generator.

Then why do we keep trying to find the holy grail of the perfect cruising generator? Frankly, not everyone at PAE does. Jim Leishman and I have argued and debated this subject for years. Jim is quite happy to simply start up a generator when he needs AC power while he is underway and let it run indefinitely. The fuel efficiency is about the same, and modern generators are ultra reliable, run for many thousands of hours and if you do live long enough to wear one out, they are relatively inexpensive. I on the other hand, think that it is silly to run a generator while underway and feel that it is sensible to somehow draw power from the main engine to make AC power.

I personally would have a hydraulic cruising generator on my boat, but I no longer have it in me to face the wrath of customers who demand that their cruising generators act exactly like their 20KW Northern Lights generator. Therefore, I no longer recommend them or even permit them on Nordhavns.

As a footnote to the above discussion, we (I and John Champion of American Bow Thruster) have developed a new system that is showing great promise. As an option on the Nordhavn 62, we offer a hydraulically driven large alternator. As we all know, an alternator (despite its name) puts out DC current and thus is not temperamental if the RPM varies. In this package, we hydraulically drive a 250 amp 24V (6KW) Electrodyne alternator. Put this together with two series stacked Trace 4024 inverters, and you have 8 KW of AC power which has perfectly stabilized frequency. 8 KW isn’t a lot of power for a 62 foot boat, but with some power management, one can run the water maker, cook or run several air conditioning units.

We have completed the first boat with this system and it is working great. Stay tuned.


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