How to choose the correct alternator output

[Jolly Roger]

How do I choose a new alternator for my boat? What are the parameters? I have a very large battery bank, but also many items like a 12-volt fridge and also a separate freezer, autopilot, lights, etc.
How do I figure out the correct alternator output?

[Pete7]

Do you know how much power you use both at anchor and when at sea per day?

Then, how do you intend to generate this power? Solar/Wind/Genny/Shore power or totally from the alternator(s).

Pete

[more]

Originally Posted by Jolly Roger

How do I choose a new alternator for my boat?

wrong question.
How do I choose a new alternator for my engine xxxx
you pick alternator for engine not boat

[desodave]

You mention a very large battery bank ... how big? You typically size alternators according to the size and demands of the bank (taking other charging sources in mind), and what the engine will physically accept. This article may help;
https://www.practical-sailor.com/sys...put-alternator

[Jolly Roger]

How do I choose the amperage of a new alternator for my boat? What are the parameters? I have a very large battery bank, but also many items like a 12-volt fridge and also a separate freezer, autopilot, lights, etc.
How do I figure out the correct alternator output power?

[Jolly Roger]

I really want to know how to calculate my needs for battery charging and choosing a new alternator. Some parameters are obvious, like the size of the battery bank, type of batteries, daily usage, etc. But calculating all this is also guesswork because not all discharge systems are working at the same time.
I need to buy a new alternator for my Perkins 4-236 85hp engine. It has an external regulator which I don’t know if is working or not, since the alternator failed. It was a very old Delco Remi type 7102, 70 amps I think, but of dubious history.
I have eight 6-volt Deka CC15 golf-cart batteries connected to give 4 x 12-volt banks. They don’t show the amp-hours on the labels. In addition, I have two 12-volt deep cycle batteries which can be isolated as engine start and windlass.
Charging is primarily from the engine alternator, but I also have a 6.5kw diesel generator, with its own smallish alternator and of course 120 volts, which I can run through my battery charger. I also have a recently restored KISS wind genny which runs very smoothly, but I’m not sure of the output, which varies according to the wind. I don’t have any solar – yet.
On the 12-volt discharge side, I have a refrigerator, separate freezer, autopilot, two Raritan electric heads with their waste treatment systems. Also 3 electric bilge pumps and water pressurizer. Then there are all the interior 12-volt lights, and external lights like 4 spreaders, foredeck lights, navigation lights, etc. I recently changed all these 12-volt bulbs to LCD, including the 25-watt navigation lights. This significantly reduced consumption. I don't have a watermaker yet, but when I buy one it will be run off the generator at 120 volts.
I know I have high consumption demands for a sailboat, and I really need a power station tagging along with a long lead...
The whole exercise has been brought about by the alternator failure, and our desire to go cruising in the summer which will entail marina free anchoring and energy production.
I know doing it right will cost money, but I don’t want to just buy the biggest alternator I can get, but the right system.
Thanks, DesoDave for the link, which was very enlightening, but a bit too complicated for my brain.
What’s the first thing to do—apart from calling in an alternator salesman?

[hjohnson]

Originally Posted by Jolly Roger

How do I choose the amperage of a new alternator for my boat? What are the parameters? I have a very large battery bank, but also many items like a 12-volt fridge and also a separate freezer, autopilot, lights, etc.
How do I figure out the correct alternator output power?

It's a combination of the size of your engine (how much drag you can put on the front of your engine), the allowed charging rate of your batteries, and your budget.

You didn't say whether you're just looking at a simple dumb alternator, or one with an advanced regulator (Balmar or Wakespeed). The thing to remember about dumb alternators is they will only run at their advertised output for a short while before they start backing off due to heat issues. Even smart regulated ones won't generally achieve their rated output for long.

[boatpoker]

What horsepower engine do you have. That is the main limiting factor.
I have 55hp engine and a 90amp alternator, Any more amps would cause problems
like premature belt wear and alternator armature shaft wear.
\
You could go to a serpentine belt system and use an even bigger alternator and eliminate
those wear issues but at great expense and the change over is not nearly as simple as you'd think.

[hjohnson]

Originally Posted by boatpoker

What horsepower engine do you have. That is the main limiting factor.
I have 55hp engine and a 90amp alternator, Any more amps would cause problems
like premature belt wear and alternator armature shaft wear.
\
You could go to a serpentine belt system and use an even bigger alternator and eliminate
those wear issues but at great expense and the change over is not nearly as simple as you'd think.

That's more about belt type than engine horsepower. A single Vee belt is limited to about 100A max, a Serpentine/poly-v can go significantly higher than that. Beta tends to ship all their engines with serpentines these days, meaning I can easily stick a 120A alternator on a 14HP engine.

You do need to manage things to make sure enough power makes it to the shaft, which is where things like advanced regulators come in handy.

[desodave]

I decided to upgrade my system a few years ago on our previous boat. I talked to the marine mechanic I was using. We discussed options, decided on one with a quote and he put it in for me.

[dlj]

Originally Posted by Jolly Roger

Thanks, DesoDave for the link, which was very enlightening, but a bit too complicated for my brain.
What’s the first thing to do—apart from calling in an alternator salesman?

I wouldn't contact a salesman. I'd contact someone that understands systems like Bob Cambell. He's outside Annapolis, so you may be a bit too far away for him. But you can ask. Home- Bob Campbell Marine Consulting

dj

[Dockhead]

Originally Posted by Jolly Roger

. .
What’s the first thing to do—apart from calling in an alternator salesman?

On an engine that size, you want a large frame school bus alternator of about 175 amps, possibly 200. It doesn't matter much which one. Leece Neville are popular.

What's tricky is the drive system. If you keep the normal alternator for the start battery, making this a second one (highly recommended), you'll need a bracket for the other side of the engine. In any case you will have to change the drive pulley. A serpentine belt drive is probably the most reliable, but dual v belts will work also.

A lot of boats with this engine (very many Oysters and Moodys from the 80s and 90s for example) were built with dual alternators, so you might be able to find the brackets already made up rather than having to fabricate.

You'll want a good mechanic helping you. Good luck.

My boat has the same size battery bank as yours, and was built from new with dual alternators. The second one is a Leece Neville 100 amps (x 24v). Engine is 100hp Yanmar. Drive is dual v belts. It works well, producing up to 2.5kW of power, enough to use a washer/dryer off the inverter while motoring (for example).

[GordMay]

The Deka “GC15 6V FLOODED” battery has a capacity of 230 Amp/Hours @ 6V [1380 Watt/Hours] @20 hr rate.
➥ https://www.nationwide-battery.com/pdf/GC15.pdf

If the Practical Sailor article, desodave linked [#4], is too technical, you might just take some experienced technical advice, on faith.
Dockhead gave you some good advice [#8].

[Jolly Roger]

Thanks for that info Gord’. My two 12-volt batteries are EverStart. Model 29DC 845 Marine Cranking Hours, 122 amp hrs@ 1amp
Unfortunately, I don’t know how to interpolate the different data of the two types of batteries into a common power output. I’m more of a mechanical man myself—as readers of Cruising World and SAIL magazines might attest.

Due to time constraints at the moment I’m going to order a new alternator the same as the old one. Powermaster 70 amp with a vee pully, so it will fit exactly where the old one came out. (I would have to modify a floor beam to fit any larger alternator, and I don’t have time just now). I’m also going to order a new regulator.

When I get the boat up to her new berth near New Bern, NC I will start to plan to fit a second alternator, as Dockhead suggests. I will have more time to shop around and consider brackets and belt drive(s).

Thanks for your advice everyone. I’ll let you know.
JR

[Baronkrak]

Originally Posted by Jolly Roger

….What’s the first thing to do—apart from calling in an alternator salesman?

You could also try to have the alternator repaired.
Feeding 175A into a 70A circuit may not be a good idea.