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I am replacing the genset on my 52 sport fisher. It has a 20kw Onan now but I am thinking about installing two smaller units. A freind has a 50 Bertram with an 8kw and a 6kw that seems to work fine. I am thinking about maybe a 12kw and a 6kw. any thoughts?
What is the maximum load you pull on the set now?
If you do split units you need to consider how you're going to feed the electrical system. In most cases you cannot parallel the sets (the switchgear to do this is somewhat complex and I don't know of any "small" marine sets that have it) so you have to split loads in some fashion.
unfortunatly the genset has been removed so I would need to go thru and add everything up. If I could only run the basics like refer, lights and heat off the six would be nice. With the 20kw I had to run all the time for the basics.
I can easily run two zones of AC, lighting, refer, hot water heater and house loads on an 8kw with some to spare. However, if I try to use the stove in the galley with all of the above on.....
I think you'd be ok with a six with two zones of AC up. The third one may hose you; those things draw quite a bit of current.
Don't just guess at what size gennys you need.
You should really determine the load (power) each appliance uses. Then add up those you want to use all the time, and those that are only run occasionally. The size of your main and smaller genny will be determined by how you use your appliances. If you plan to load each genny at 60% to 80% of it's rated maximum, you should be fine. Maybe you should consider two of equal size so you have full redundancy for all loads you anticipate. The incremental cost for such an arrangement may not be much more unless your regular loads are very small in comparison to full load.
For example, if you regularly run one of your air conditioners, that's a significant load. And if you want to cook with electricity frequently that will also be a big load. On the other hand, you may rarely use either of these. Only by adding up your planned loads can you make a sensible decision.
Anybody you purchase two gennys from will be more than happy to size them for you.