New alternator needed

[Jolly Roger]

I need a new alternator for my old Perkins 4.236 engine. I believe the engine is original to the boat, circa 1978. I need it quite quickly for a proposed 500 mile passage. Where do I start?

[Uncle Bob]

Originally Posted by Jolly Roger

I need a new alternator for my old Perkins 4.236 engine. I believe the engine is original to the boat, circa 1978. I need it quite quickly for a proposed 500 mile passage. Where do I start?

A good local auto electrician, one that repairs and sells alternators.

[Jolly Roger]

Actually, I’m not even sure if the one that’s on the engine is the right one. There is no output on the positive terminal, and I don’t have time to have it repaired, and I’m happy to buy a reliable new one. But shouldn’t the output be related to my battery capacities, which is large with eight 6 volt golf carts and two 12 volt. All float.

[Sailmonkey]

If you rely on your alternator to charge your (roughly 750 ah) house bank, it should be sized appropriately.

So that begs the question, what are you expecting to spend?

You’ll likely want an alternator capable of supplying 75-80 amps for periods exceeding 4 hours. This points to a 120 amp alternator and an external regulator.

However if you rely on a generator for most charging (or solar, etc…) a stock bolt on alternator can work as well.

Here’s a link to stock type alternators.

https://www.parts4engines.com/perkin...or-12v-70-amp/


Keep in mind in the current supply chain mess you may be better served having your existing alt repaired.

[Bill O]

The 4.236 originally had a Delco Remy 10SI alternator. An upgrade would be a 12SI for better cooling of the alternator.

If you get either for a replacement, make sure it has the relay stud for the tach. output. (if it doesn't, then a reliable alternator repair shop should be able to install the relay stud).

Here is a link to a good article describing the difference between the 10 and 12SI alternators w/model numbers to go to the car store.

[Jolly Roger]

Thanks everyone. I’m going to take the alternator off the engine on Saturday, and see if I can have it tested and identified. I will also add up the total amp/hours of my batteries. I should add that Britannia also has a 6.5Kw generator, which can feed the battery charger. This also has an alternator.
No solar as yet.

[Jolly Roger]

I thank everyone for their input, but my situation is changing by the day, mainly because we are smack in the middle of moving house from Florida to New Bern, NC, and having a new roof put on our house starting today. The moving truck arrives next week, so you can imagine the state of our house. I wanted to get Britannia up there as well ASAP, but we’ve had northerlies for the past month, with big seas in the Gulf Stream. So what with the alternator, and other things which need fixing before we can leave, I have now finally run out of time and energy, and I must leave the boat in her safe berth until I can get things fixed, get a crew together, and a decent weather window. We might even try the Intracoastal, but she draws 6’6” and will take at least ten days, at about 550 miles.
So I’ll take the alternator off, and see if I can get one with a larger output, or have the old one repaired just to use it for the trip, then buy a bigger one, and carry this as a spare.
Actually, just to complicate things, I believe I have an external regulator wired in, but I’m no electrician, (hence this post), and if I can get her up to her new berth, I’ll get a marine electrician to look at the whole shebang.
I’ll keep everyone posted…

[Bill O]

If you can take a pic of the alternator (backside), someone here should be able to identify it.

Does it look like the 10SI in the link that I previously sent? If so, they are common and relatively cheap to purchase another one or easy for a shop to check/rebuild.
If you are mooring up the ICW, even a low amp output alternator should keep the bats up. If you do buy a new larger output alt., possibly stay under 90 amp otherwise you will want to change the belt to a serpentine.