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Old 06-23-2015, 02:19 AM   #1
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Default Alternator

Bomar 120amp alternator has had the gong. Buying a new one in the Bahamas is expensive. I have a new 80amp Hitachi. Apart from slower charging, is there any reason why I can't install the 80amp job. Have 3 x 8D Deka batteries (1 starter, 2 house). Look forward to any advice offered.
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Old 06-23-2015, 04:40 AM   #2
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Auzzee, there should not be a major issue with this. If needed you could run a smaller pulley size on the unit, to increase the rpm and hence the current output.

Many people choose the heavier duty alternators but they only help when the unit is really pumping all that current. That would only happen when the batteries are really low and able to absorb all that current.

Once the charging voltage has ramped up the current absorbed will drop, typically into the comfort level range of the lower rated alternator. It is only during the generally short initial boost stage where the heavier unit will come into it's own.

If you are running the engine near idle at anchor, neither of the units will put out their rated current anyway, as they really need to be cranking to do so. On my set-up anything below 1000 engine rpm gives me hardly any current, at 1200 there's significant current already and only above 1800rpm are we getting close to rated.

Ivo / SV Linnupesa
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Old 06-25-2015, 10:11 PM   #3
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Thanks Linnupesa, I am going to put the 80amp unit in and have confidence it will do the job. Thanks for your advice. The boat will be back in the water early mext week and then we'll see just how a 6'6" draft restricts movements in the Bahamas.
Cheers.
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Old 06-28-2015, 02:38 AM   #4
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Auzzee, The following are websites that may be of some help in your current (pun intended) situation: www.balmar.net , www.pkys.com & www.pbase.com/mainecruising/boat_projects . Hope these may be of some help,
George
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Old 06-28-2015, 03:07 PM   #5
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Thanks George. All these add to my knowledge which is growing by the moment. I am adding the 80amp alternator and hope to have ongoing access to cold beer as a result.
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Old 06-29-2015, 06:07 AM   #6
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Auzzee, We have an electrical engineer in our group and he researches and recommends all of our electrical decisions. When our alternator gave up the ghost about 3 years ago, he recommended a Balmar Series 6 - 70 amp alternator as its replacement due to the Balmar's power production curve as it produces its full rated power at a very low RPM and would not require a modification to our alternator drive set up (single V belt). We ran that configuration for 3 years, using the alternator's internal voltage regulator, until just recently when the alternator's regulator gave up the ghost. Further research revealed that the integrated regulator is basically intended as a back up, get you home system using only a single stage, bulk charging schedule. Balmar and Peter Kennedy of www.pkys.com recommended the use of a Balmar ARS5 external regulator, which is a 3 stage (bulk charge-float charge-maintainer charge) regulator, incorporating a battery temperature sensor and an alternator temperature sensor to better regulate the charging schedule. We have just installed the ARS5 with sensors and will see how it goes. We have 4 high dollar AGM batteries aboard (2 @ house / 1 @ start / 1 @ windlass) and an Air Breeze wind generator for a 24/7 topping current in the trade wind BVI's. Pardon my rambling on, but hope our experience helps. George
PS - If memory serves me correctly, we only need 1200 engine RPM to produce 2400 alternator RPM which delivers full rated current. The boat gets used 10 months per year, with a 2 month lay up. I just recently purchased for $300, a 1 year old Balmar 912 - 75 to 100 amp alternator with an ARS5 external voltage regulator from Cruiser's Forum Classifieds for use on my personal boat in NC.
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Old 06-29-2015, 04:15 PM   #7
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Auzzee,

Good luck with your new install. Having just returned from an Abacos trip with Linnupesa drawing about 5'6" your extra foot of draft should not be a major problem IMHO. We did pussyfoot over a few shallow areas but usually deeper water was available. Bottoms were mainly sandy so navigation by sampling the geology ( like in most of the ICW ) is sometimes embarrassing but not life threatening.

Ivo
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