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Old 29-05-2008, 05:20   #16
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Originally Posted by Pblais View Post
A 150 amp output from an engine requires almost 2.5 horespower of input to spin the alternator. That means 10% of the engine is driving the fan belt. The loads on large alternators are indeed quite high as noted. You can do the math:

Volts * Amps * 0.00134102209 = horsepower

The 70 amp would be 1.1 HP and the 40 amp would be 0.64 HP.
I think it is actually significantly more. Alternators are not very efficient, the field current consumes quite a bit. 4 to 5 hp would be more realistic. I am putting a large alternator on a small, 20Hp, motor and I think it is a good idea to install a field shutoff switch for times when full power is required.


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Old 29-05-2008, 05:28   #17
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I think it is actually significantly more.
Very true. 80% efficient is not uncommon for converting forms of energy. It also brings up the second issue and that is the 20% wasted energy ends up as HEAT! If we take the 150 amp alternator and say that it requires 2.5 HP it also genrates 1200 BTU's per hour and ads another .5 HP to the load making it really 3 HP along with the trouble associated with 1200 BTU's per hour. The heat itself then reduces the efficiency of the alternator even more.

You want all the alternator power you need but anything extra imposes higher costs with all the problems they come with too.

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Old 29-05-2008, 20:57   #18
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Paul and Gord have it right in my opinion. Given the extra data you provided...
I think that:
1. You should get a 100-110amp alternator and stash the 40 amp that comes with the engine as a spare.
2. 100 amps is not too much alternator for a 25hp beta and will suck off about 2hp. If you go to the 150 level you will be pulling down over 3hp and also increasing load to the point of needing double sheaves and increasing side loads on engine bearings. I would not go that high.
3. Remember that you have 25-35 ah's daily being delivered by the solar panel. This should be between 15-25% of your total amphour usage daily so you an afford to be a BIT undersized on the "perfect" alternator for your 400amp bank.
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Old 29-05-2008, 21:18   #19
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Very informative, as always.
I think I'm going to stick with 25hp, and get the highest powered alternator option for that engine - the 70amp.
In the end I guess I'll be burning more or less the same amount of diesel to cover my needs - I just might be leaving the engine running a little longer.

Thanks everyone.
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Old 29-05-2008, 21:25   #20
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Camaderie: didn't see your post before I replied.
The only option on the 25hp is 70amps. I figured that getting a bigger engine just so I can run a bigger alternator would be overkill.
So I figured I'd stick with that, and if I do find myself missing that extra 30% I guess I could look into wind gen, more/better solar, or an alternate alternator (if you will) that would work on the 25.
I don't know what the fridge draws on the boat, but the fridge space is quite small compared to most boat fridges I've seen. So I'm hoping it will be relatively low on power draw.
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Old 29-05-2008, 21:45   #21
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For what is is worth --- we're running the Balmar 100amp small case alternator on the Westerbeke 21 HP engine through a Max Charge Regulator. Mathematically and use-wise, it works well. We have the ability to (a) turn the alternator off and on with a switch, as well as (b) utilize the 50% small engine mode that Balmar talks about when we are actually motoring to get somewhere and want to lighten the load on the engine. To date (one year and a total of about 210 engine hours) the 100 amp alternator on the 21HP has not been an issue and has served our needs. Our "load" is similar to yours, sans watermaker.
When is the last time you tried something for the first time?
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Old 30-05-2008, 17:45   #22
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Posts: 915 problem. It appears as if I was not very clear in my advice. I understand that the engine you are buying comes with either a 40 or 70amp alternator. I am suggeting that you get it with the 40 amp as that will be cheaper. Then I am suggesting that you remove the 40amp and replace it with a Balmar or similar 100amp unit along with an external regulator. Your engine will RUN a bigger alternator than offered by the mfr. do not need to upgrade the engine itself in order to install your own hi amp alternator.
I read that you are planning to go with the 70amp. I think that is a bit small but as you can just run longer. Be sure you are getting external 3 stage regulation with that 70 amp alternator or you risk killing your AGMs.

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