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Old 12-20-2004, 08:55 AM
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Question Selecting an Alternator

Are there any rules of thumb that will help in selecting a properly sized alternator?
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Old 12-20-2004, 09:07 AM
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I have heard (and am finally paying attention to) four rather simple rules of thumb regarding alternator sizing:

  1. The most basic rule of thumb is that alternator output should equal 25% of a boat's battery capacity in amp-hours. This rule would hold that for a 400 amp-hour battery bank, a 100 amp alternator would be a wise choice.
  2. There is fortunately a related rule of thumb for battery sizing which greatly simplifies the process of sizing batteries for sail boats. This rule of thumb is to install the largest battery bank that space (and budget) permits, and to then connect the batteries in such a way that allows you to isolate one battery while sailing or at anchorage, to be used for starting.
  3. There are a couple rules of thumb regarding belt sizing for alternators. A 3/8" belt is maxed out at approximately 100 amps, and a 1/2" belt at approximately 120 amps. There are 7/16" belts available in some brands, which tend to perform (as you would expect) somewhere between 3/8" and 1/2" belts. For alternators with capacities over 120 amps, you should consider double belts.
  4. Regarding power required to turn an alternator, an alternator draws at least 1 HP per 25 amps of actual output. This power requirement may not seem like much of a load, until you consider the problem most of us have in selecting a prop that allows for the delivery of a rather modest 20 HP to the prop shaft at maximum power available. In this scenario, if your alternator is producing 50 amps while you happen to be clawing your way around the proverbial lee shore, you'd be sacrificing 10% of your available power just to charge your batteries.
A couple personal notes regarding the above rules of thumb:
  1. Battery sizing: About 3 seasons ago, following the rule of thumb for battery sizing, Brenda and I installed three group 27 AGM batteries on our boat, for a total capacity of just under 300 amp-hours.
  2. Alternator sizing: While our 55 amp Mando didn't quite meet the rule of thumb for alternator sizing (theoretically we needed a 77 amp alternator with our new set of batteries), I left the Mando installed, if only to see how it would perform. Not surprisingly, it would more or less max out at 50 amps for a few minutes when we ran the batteries down at anchorage, but most of the time, the output remained between 10 and 25 amps (depending on battery condition). However, whenever we attempted to use our rather dated refrigeration unit, the Mando spent most of its time between 40 and 50 amps.



    While the Mando seemed to survive for two seasons, it ran quite hot most of the time, so we decided to install a bigger alternator. I selected a 120 amp alternator, produced by the same firm which has been supplying the 55 amp Mando. Partly for simplicity, and partly for economy, we decided to stay with a single output, internally regulated alternator. Since internal regulators are all designed to remain below safe charging levels as determined by the battery industry, the new 120 amp alternator produces approximately the same output as the Mando, except for two important considerations: it produces its output at a lower RPM, and it operates much cooler.
  3. Belt sizing: To this point, we haven't seen any output above 60 amps, so we're staying with the 7/16" belt which we used previously with the Mando.
  4. HP required: Since we're not seeing any increase in average alternator output, we're not noticing any increase in engine load. In the unlikely event that we let ourselves get into harm's way, needing more power from the engine, I would consider selecting only one battery (the one saved for starting) until coming through whatever challenge we've been facing.
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Old 03-31-2006, 10:53 AM
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bigger alternator effect on A4 engine

It's been suggested to me recently that putting a bigger alternator on an Atomic 4 is asking for trouble because the crank is only supported by two main bearings (front and back with nothing in the middle), and the additional side load on the crank will cause stresses on it which will likely cause it to fail in the long unsupported middle section.

I've just put a 70A Balmar (the 621) on to support the bigger house bank required for a new refrigeration installation this past offseason, and I know I can jumper the temperature control on the regulator to reduce the alternator output, but that would be a shame, given that a major reason for putting it on (aside from 3-stage charging) is in response to the rule of thumb that alternator capacity should be 25% of the bank's Ah rating.

I haven't been able to find a cutaway drawing of the A4 to check out the bearing story.

Can you comment?

Thanks.
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Old 03-31-2006, 01:28 PM
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Mark,

This question is revisited at least a couple times each year within the Atomic 4 fraternity. The short answer is that the Atomic 4 crank is indeed designed with no center main bearing. However, the engineers really did a good job in designing the crankshaft to withstand all of the normal stresses which they envisioned, up to the rated HP of the engine plus a generous safety margin.

As of the time Universal was sold in the early 1990's, their reported failure data showed that the number of crankshaft failures was less than the theoretical number of failures to be expected simply from QC defects of a crankshaft during manufacture. The failures that were reported were usually associated with a bubble in the casting from the casting process.

Our own experience tends to support the Universal assessment. We have only had the opportunity to investigate 3 crankshaft failures in over 15 years of servicing and rebuilding Atomic 4's, and each one had some preexisting defect which set up a premature cycle of fatigue.

Fatigue crack analysis is interesting because it indirectly demonstrates (after the fact of a failure) how much of the cross-section of a particular structure can be sacrificed prior to the ultimate failure, hence supporting the design claim of a particular margin of safety. In each of the cranks we analyzed, the fatigue zone of the crack (meaning the part of the crack that had existed for quite some time before the ultimate failure) was almost 50% of the total fracture surface. This means that a crankshaft can reasonably be expected to survive for a relatively long period of time even with up to half of its design strength sacrificed by the fatigue crack.

NOTE: The fact that a fatigue crack is developed over many years before an ultimate failure occurs means that "wet-magging" a crank during overhaul would normally have a good chance of revealing a fatigue crack before the crank would ever fail. More importantly, if a fatigue crack is not discovered during overhaul after many years of service, it would go a long ways towards confirming that no manufacturing flaw exists in that particular crank.

The naturally aspirated up-draft design of the carburetor on the Atomic 4 makes it virtually impossible to "over-torque" or "over-boost" the engine. The engine simply gets to an RPM on the power curve where it says: "I don't care what you do with the throttle; I can't give you any more power". This design concept is very friendly to internal stress-bearing members of an engine, the crankshaft in particular.

In summary, in the unlikely event that your crankshaft would ever fail, it is far more likely that it will do so from a pre-existing manufacturing flaw than from your Balmar alternator.

Don
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Old 08-26-2007, 10:17 AM
lhbradley lhbradley is offline
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I've been using a Balmar 912 (nominal 90 amp) for 6 years now with no problems, using a smart 3-stage regulator. The batteries (two 225 amp-hour golf cart batteries in series) take around 55 to 60 amps initially (I have to run the engine at around 1700 RMP to get this much into them), and after about 1/2 hour, things start to taper off.

The alternator is driven via the accessory drive, which is gear driven from the crank via an idler gear. I wouldn't be worried about that end of things. The place to worry is the bearings on the aux drive for the alternator pulley, if you put on too big an alternator on, and try to tighten the belt too much to prevent slippage.

Incidentally, re belt tightening - I bought the belt tensioning device from Indigo Electronics a few years ago, and it makes life MUCH simpler.
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Old 04-04-2010, 02:33 PM
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alternator selection

[as i am new to this forum and also to computers, this may not look too good but i do have over twenty years experience working on and using the Atomic 4 both earlyand late types including six trips to Florida in a Bermuda 30. After talking to Balmar and studying their literature, I have come to understand that the overriding consideration is alternator speed. The Atomic 4 suffers from a fundamental design defect in that the accessory drive sheave can only drive the alternator slightly over engine rpm, about 1.3:1 believe. The reason your alternators are running so hot is that they are running so far below design rpm. It is pointless to put a big alternator on that tiny sheave. I bought the PTO flywheel from Moyer Marine, then went to the McMaster book and ordered a double sheave for 1/2 inch belts and then fabricated a bracket to run the length of the (old type) head which carries the Balmar 712 up front. I ran this at 2 to 1 (backwards) all the way to Florida, cool enough to hold my hand on it. I am now moving up to 3 to 1 (9.75 inch double sheave). My wife likes the 1600 w toaster oven so I need big output from the inverter. Thats all for now. Thanks to Don Moyer and his organization for preserving this beautiful technology.[/B][/B]]
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Old 04-04-2010, 03:56 PM
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Any chance you could post a picture of your alternator bracket?

Qben Oliver
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Old 04-04-2010, 08:30 PM
hanleyclifford hanleyclifford is offline
 
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alternator bracket

sorry to say that i have not learned how to post pictures or do much of anything with this contraption. the bracket is simply a piece of 3/16 inch angle iron cut long enough to be held down by all 5 head studs in a row on the accessory side of the engine. the angle iron extends forward over the flywheel housing about 5 inches. two ears must then be fitted to enclose the two inch configuration of the alternator. this is very close engineering work as you can appreciate that the unit must run parallel to the axis of the crankshaft and must line up with the new sheave forward of the flywheel housing. i did this by first clamping the ears, then drilling and bolting, and finally electric welding the ears to the angle iron. the best way to drill the five hold down holes is to back drill on press using an actual early head. i bet the engineers at Moyer Marine could produce this item perfectly. Now in Nov 2010 have learned how to do pix. See "Engine" album for alternator bracket.

Last edited by hanleyclifford; 11-09-2010 at 04:12 PM. Reason: update
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Old 04-30-2010, 02:21 PM
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Smile photos of alternator bracket

Qben - Here are some pictures I couldn't send before. A computer wise friend helped me.

Last edited by hanleyclifford; 07-23-2010 at 11:14 PM.
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Old 05-03-2010, 08:35 PM
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Thumbs up

Nice!
Thank you.
Qben
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Old 05-03-2010, 08:46 PM
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Red face long way to go

Qben - As you can see I have a lot to learn about computers. Until a few weeks ago I was convinced I could get thru the rest of my life without having to acquire this skill. Wrong again. I am going to learn how to do pictures properly because I see the value of this. More to follow.
Old 07-10-2010, 11:26 PM
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This message has been deleted by hanleyclifford. Reason: resize photo
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Old 07-22-2010, 04:09 PM
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Balmar 712 (110amp) with 3:1 double sheave
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Old 08-01-2010, 12:20 AM
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Qben - Here are some pictures I couldn't send before. A computer wise friend helped me.
. This time I hope I got them resized correctly.
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Old 03-13-2011, 03:29 PM
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What About Wire Size?

I would think if you go from a 50 amp to a 110 Amp Alt them the wire size would need to increase. Is this correct?
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Old 06-18-2011, 03:17 AM
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bump..............
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Old 06-18-2011, 09:27 AM
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Talking

Not sure why you wanted to bump this thread. In the time that has passed since I first put that bracket on the engine I am more convinced than ever the alternator should be mounted this way. Quite apart from the charging advantage, this change frees up the accessory drive position for a water pump so the FWC system could have two mechanical pumps. I wonder if even the Catalina 30 crowd might be able to make room for that sheave on the flywheel (about 5" needed)?
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Old 06-18-2011, 10:40 PM
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Exclamation

Hanley, No way... even if we removed the flywheel housing, any pulley attached would rub the settee 'box'. The bottom of the flywheel housing is less than 1" from the dinette box, the top, maybe 4".

I'll take a picture the next time I am at the boat for you. But thanks for thinking of us!

FYI - when Catalina started putting in diesels instead of the Atomic 4, they had to cut into the seat cushion to accommodate the engine's height..it was already tight when they shoehorned the A4 in there!
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Old 11-18-2011, 03:32 PM
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New Alternator and Smart Regulators?

Hello, I'm rewiring my old 78 Ranger 33 this winter, adding a 200Ah house battery bank (probably gel cell), and replacing the stock alternator. Two questions:

Will the 55A alternator be sufficient or do I go with the 120A high-output alternator? After reading Don's old post in this thread, I'm leaning toward the bigger one.

These are internally regulated. Everything I've read says to use a multi-stage smart regulator to get full charge without damaging the batteries. My goal is to keep it simple, but the batteries are going to run $600-700. Do I invest another $300 in the smart regulator? And if I do that, do I need an alternator that is not internally regulated?

Thanks for the help!
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Old 11-18-2011, 04:32 PM
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Keep in mind, as Hanley has frequently pointed out, that you're not going to get 120A out of any alternator driven off of the accessory drive. You just can't spin them fast enough to get the rated output. It will run cooler, however, and probably last longer, so that's a reason in favor of a higher rated alternator.

Hanley had to mount a huge pulley on the front take-off and mount the alternator up there in order to get a 3-to-1 ratio to spin the alternator fast enough. And he found that it was lugging the A4 too much. You've essentially turned it into a 30HP generator, with little power left to push the boat! He has since backed off to a smaller 2-to-1 rig.

As for the multi-stage smart regulator, you really want to go that way. Not only will it help maximize your battery life, but it will minimize the amount of engine run time required to charge the batteries. The built-in internal regulators are essentially automotive regulators, and back-off to a trickle charge way too soon. Any decent alternator shop should be able to modify an alternator to bypass the regulator and bring out the feild terminals. Just be sure they know its for marine use so they install the proper seals to protect against igniting any gas fumes. The smart regulators will pound the amp-hours into the batteries at the fastest rate they will accept it, until they're about 80% charged. Only then will it back off to a slower charge rate for the last 20%, and then back off to a trickle/sustaining charge.

This minimized engine runtime becomes important when you're passage making, and have to run the engine so many hours each day to maintain your "power budget".
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Old 11-19-2011, 08:50 AM
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http://www.morganscloud.com/category/electrical/

Follow all the AGM discussions on there. You may end up thinking twice.


Quote:
Originally Posted by jerkeene@gmail.com View Post
Hello, I'm rewiring my old 78 Ranger 33 this winter, adding a 200Ah house battery bank (probably gel cell), and replacing the stock alternator. Two questions:

Will the 55A alternator be sufficient or do I go with the 120A high-output alternator? After reading Don's old post in this thread, I'm leaning toward the bigger one.

These are internally regulated. Everything I've read says to use a multi-stage smart regulator to get full charge without damaging the batteries. My goal is to keep it simple, but the batteries are going to run $600-700. Do I invest another $300 in the smart regulator? And if I do that, do I need an alternator that is not internally regulated?

Thanks for the help!

Last edited by joe_db; 11-19-2011 at 09:05 AM.
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Old 11-19-2011, 04:09 PM
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Quote:
Originally Posted by jerkeene@gmail.com View Post
Hello, I'm rewiring my old 78 Ranger 33 this winter, adding a 200Ah house battery bank (probably gel cell), and replacing the stock alternator. Two questions:

Will the 55A alternator be sufficient or do I go with the 120A high-output alternator? After reading Don's old post in this thread, I'm leaning toward the bigger one.

These are internally regulated. Everything I've read says to use a multi-stage smart regulator to get full charge without damaging the batteries. My goal is to keep it simple, but the batteries are going to run $600-700. Do I invest another $300 in the smart regulator? And if I do that, do I need an alternator that is not internally regulated?

Thanks for the help!
An important question...
Will you have SHORE POWER available on a "regular" basis?
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Old 11-19-2011, 06:07 PM
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Smile

I've had a Delco 90 amp with internal regulator on mine for 4 summers and am pretty pleased with it. I use the boat during summer almost every day and we spend most weekends aboard, either on a little cruise or in the surrounding area. Running two marine starter batteries and always have starting and light power.

Also have a 1000 watt inverter. The only thing about that is that I need the engine running to run the vacume cleaner. I think if I was cruising I'd have a larger battery bank, but for what I do it works fine. It's nice to have something that will throw a charge back into those batteries.
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Old 11-19-2011, 07:23 PM
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FYI : I am unhappy with my AGMs and will go back to gels next replacement.
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Old 11-20-2011, 12:15 AM
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Quote:
Originally Posted by roadnsky View Post
An important question...
Will you have SHORE POWER available on a "regular" basis?
No. We are setting the boat up for a year-long cruise, so shore power will be "irregular" at best. I'm planning to add two 80W solar panels to supplement charging with the engine.

I'm definitely concerned about robbing power from the prop - I think I'm over-propped already.
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Old 11-21-2011, 08:51 AM
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Quote:
Originally Posted by joe_db View Post
FYI : I am unhappy with my AGMs and will go back to gels next replacement.

Joe,

What's the source of your chart?
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