Questions about Alternator Choices

[San_Diego_SeaRay]

I have a 1981 Mercruiser MCM 260 engine that I need to buy an alternator for. When I search for a replacement, seems as though I have some ?upgrade? options available to me. I don?t know what the output of the original alternator was, but here?s what I?m finding as plausible replacements (the original was a Motorola 3 wire):

• 3 wire 63 amp
• 3 wire 63 amp w. (4[SUP]th[/SUP]) tachometer wire
• 3 wire 105 amp
• 3 wire 140 amp w. tachometer wire

Here are some questions I have and hope someone can help with:

• Other than the cost, are there any drawbacks to getting the higher output alternator? Do they produce too much amperage at times? Or would you pretty much always get the higher output if cost wasn?t an issue?
• I was under the impression that the tachometer was usually hooked up to the coil, and that?s how the RPMs were derived. Is the tach wire at the alternator really an added improvement?

Thanks in advance -

JC

 

[bruceb58]

You don't need the tach wire.

What are your electrical loads on your boat?

 

[gm280]

Just remember that nothing is free. The more amperage output, the hard it will load the engine...

 

[bruceb58]

Just remember that nothing is free. The more amperage output, the hard it will load the engine...

Of course it will just load it for less time so the average load is identical.

 

[airshot]

Your regulator will control how much and often your alternator is needed so why buy more power than needed. I only have a 55 amp and it handles all my electric needs on my boat with two batteries.

 

[San_Diego_SeaRay]

You don't need the tach wire.

What are your electrical loads on your boat?

Don't know yet. This is my first boat ever, and I'm not sure what I'm going to be running. Will say though that the main purpose will be scuba diving and there will be about 6 ppl on the boat on average. And we're in San Diego so this boat is going to get a lot of use. Also, it has an A/C system on board that I'm not entirely sure will be reactivated. And probably a 1000 watt windlass system. Pulling out the stove and refrigerator though.

When you say it will put an add'l load on the engine, that would mean that more torque is required to turn it? And per bruceb58's comments, that load eases up; comes off and on? I guess I never really realized how an alternator works. I thought it was a continuous load. If it is "demand driven", it would seem like more amps would not be harmful (except that I'm sure some might argue that lower amps over a longer period of time might be better...).

EDIT: I just Googled "alternator clutch pulley" and that pretty much explains it

So can pretty much any tachometer be wired to run off the coil?

 

[bruceb58]

The load the alternator presents to the engine is proportional to the amount of current it is generating. There is no alternator clutch. When the battery is charged, there is very little load caused by the alternator.

Sounds like you may potentially have some high loads. I would just buy the higher current alternator. You will need to make sure the output wire of the alternator is sized up appropriately.

The reason a tach output is offered is for diesel engines.

 

[Silvertip]

You cannot have too much alternator power, just too little. The primary job of the alternator is to keep the battery topped off. If the electrical load (the total of all devices operating at the same time) is greater than the output of the alternator you will eventually run out of power and you are dead in the water. An alternator cannot "force" excess current into a device. Devices "draw power" and will draw only what they need. If this were not true, an over the road semi tractor with anywhere from four to six 12 volt batteries in parallel would pop every 12 volt bulb on the rig on a regular basis. Therefore a 150 amp alternator cannot overpower a single little bulb that illuminates a gauge on your console. You mentioned you have a 1000 watt windlass system. That system under maximum load would draw 79 amps at 12.6 volts. If that system were used frequently, that places a high demand on a 12 volt battery. A 63 amp alternator might not be the best choice for you. You really need to figure out the power draw on your vessel and then use about 75% of that to size the alternator. It is highly unlikely everything will be operated at the same time nor continuously hence there is no need for a 1:1 relationship. Fact is, installing a 100 or even a 150 amp alternator would be a non-factor and the significance of the amount of power required to run it is not worthy of consideration.

 

[San_Diego_SeaRay]

You will need to make sure the output wire of the alternator is sized up appropriately.

Good point.

Well thanks for all the replies. I'm gonna go ahead and buy the 105 amp one as it cost the same as the 63 amp one, which are both half the cost of the 140 amp one. If I end up needing more power I'll buy another 105 amp one for my other engine or, worst case scenario, bite the bullet and upgrade both to 140 amps. Thanks again.

 

[UncleWillie]

Let's not give too much concern to the 1000 watt windlass.
Like most everything else electrical, it is rated at its MAX Capacity.
The windlass might draw 1000 watts (>1hp+) intermittently, with the motor nearly stalled trying to pull an anchor out of the bottom.
Look at its motor. A continuous rated 1hp motor is larger than a gallon milk container.

To put it in perspective (ignoring Efficiency losses) a 1000 watt (1hp+) motor will raise a 300lb anchor at over 100 ft per minute.
And a typical, 100 Ah battery, will operate the winch at this power level for over an hour.
If you are recovering a 25 lb anchor with less than 300 ft of rode, the winch is a non-issue, as it is an intermittent device.