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Old 22-12-2008, 12:51   #1
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Big A$$ alternator on a little diesel

Hi all;

Jubilee has a 18hp Volvo 2002 engine. The original owner ran Engine driven refrigeration and watermaker off of the diesel. This involved a twin-belt PTO pulley up front, belts going up to the top of the engine where they ran to two pulleys on a drive shaft that went to the rear of the engine. There one belt went to the watermaker and one to the refrigeration. Both systems are no longer installed.

We removed the shaft, but the PTO pulley, and the mount for the shaft above it, are still in place. The shaft contraption must have weighed 40 lbs!

I am wondering out loud about mounting a large alternator onto the pad where the shaft was attached to the front of the engine. It seems it would be easy to line it up with the PTO pulley. There is a used 165a unit for sale on our marina BB. I was thinking I could design the installation with a rheostat to adjust the output. This way, if we needed the 18hp for propulsion, we would have it, and could nicely load the engine for charging when required, for little $$$.

So a few questions.
1. How much load would a 165A alternator put on a 18hp engine. The stock alternator is a 50a unit, which I would leave on dedicated to the starter battery.
2. I have a 12v book with the scematics for a dial down system. Will this allow me to dial things down to 0 output?
3. How much drag do you think the freewheeling double belt alternator will put on the engine, when not calling for amps?

Anything else you care to comment on, please do.

Thanks in advance,

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Old 22-12-2008, 15:28   #2
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If I have my numbers right.. less than 3 HP. but you shouldnt be putting out 165amps long... although with a rheostat... you could consider cooking on the alternator also.... :>)

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Old 23-12-2008, 03:20   #3
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Approximate Rule of Thumb:
25 Amps of Alternator Output = 1 H.P. of Engine Power
Hence, full 165A output = 6-1/2 H.P.
More typical 100A ouput = 4 H.P.

Upgrading alternator
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Old 23-12-2008, 08:19   #4

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One way of dealing with an alternator that consumes a large percentage of your main drive engines horsepower is to put a switch in the wire supplying the alternator's field current. That way you can shut down power generation when you need the drive power. Gives you the best of both worlds.

You should be using a "smart" regulator which means that traditional means of dialing down the alternator are not going to work.
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Old 23-12-2008, 08:28   #5
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The switch is a good idea, if you just started your engine and are in a situation that requires a lot of push, you may want the extra power. The good news is that other than that, it is a good setup to have your engine well loaded when charging.... it's better for your engine. For your original question though, I would get rid of allthat extra hardware on the engine and just put the big alternator in place of the small.
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Old 23-12-2008, 10:43   #6
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A 165 amp alternator will need a double belt drive. Single belt alternators top out at around 100a and even then a double belt is reccomended. Unless you have a very large battery bank, 165a is way overkill. Doubt if you'd ever use anywhere near 165a and most likely almost never anything over 100a and then only for a matter of minutes. We had a 200 amp battery bank with a 45 amp alternator. Used a manual controller and only got close to full alternator output with a severely depleted battery and then only for the first few minutes of charge. As the charge built up in the batteries, had to dial back the alternator output, real quick,to stay under 14.2v.

If you get the alternator very cheap, I'd consider an alternator that large for your boat. If you are paying anything near retail, look for a 60 amp alternator. It would be better suited to the engine's power and should be more than adequate even for quick charging. If you have a welder or other high drain electrical needs and a 600 amp battery bank to go with it, then it's another story.

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Old 23-12-2008, 14:44   #7
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Using layshaft set up makes good sense as it will reduce sideload on main crankshaft which if excessive can lead to breakage, also by all means go for large alternator, if coupled with smart reg it will soon ramp down amperage and will most probably be running lightly which will keep it cool and give it a long life, also amps are there if needed

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