Froggy

Ok, thanks, i was forgetting how badly blocked those vent grills at the rear were, we were quite shocked when we took the alternator off, it wasn't evident from the top and front when checking oil and water levels.

I'm guessing though that a blown diode might possibly indicate too large a load could have been imposed on the alternator at the time of failure by running too many high-demand appliances?

You are correct, belt had been slipping for a while on startup, it usually resolved after 5-10 minutes, i tended to have to readjust belt roughly once a month but hadn't got around to it that time. What can you see in the photos to indicate slippage, is it the way the backing has parted company in the second photo? Ok, well that's reassuring, i just wanted to cover all bases since the repair is costing about £160.

Ok, thanks. Profiles looked more or less identical although photos don't really show this, but old belt has been binned so can't double-check. New belt ran about 6 hours before alternator failure. Hopefully just coincidence, there were no obvious indications of belt slippage.

Latest update on this is that a diode was blown. Alternator is getting an extensive refurbishment in addition to the diode. Is it possible that fitting a belt that isn't an exact replacement could cause issues? The replacement belt was already on the boat but i did note the teeth were a little chunkier but just assumed it was due to wear on the older belt and/or different manufacturing tolerances. Both belts were as good as identical in terms of width and circumference. Taking a closer look at these close-ups now though there does seem to be a real difference in the profile of the teeth. The older belt is at the bottom in both photos.

Thanks for the suggestion but domestic alternator is now in for repair.

Tony, i am asking a lot of questions because i am a novice with electrics, but very inquisitive (and need to make a decision quickly since we are currently without power and have had to defer a fairly lengthy week's cruise until this is sorted). I appreciate all the advice you have given. No, we haven't done a power audit but our existing charging regime has kept us in good stead in the four years or so we've had the boat. The batteries still seem to hold their charge well about three years in and the belt has only been changed twice in over four years. The engine is probably only run on average for about 10 hours a week, apart from occasional longer cruises. We are normally very frugal with power consumption: the tv, fridge and vacuum are used very sparingly (the fridge is only used in very hot weather, mainly to cool drinks) and the microwave almost never; most of our lighting has been converted to led. I'm going to phone Barrus tomorrow to see if they can shed any light on the alternators that might have originally been fitted / recommended for this engine. Then I'll await the refurbisher's report and take it from there. I'll report back on how this pans out, and obviously the advice you and others have given will help guide my decisions.

The main predicament here is that we don't at this moment know the output of our dysfunctional alternator. If this can be ascertained we can try to source a similarly rated alternator, or have the existing one repaired. If we can't determine this the dilemma would be having to guess and ending up with an alternator with an inferior rating to the original. If you think a 50 amp alternator would efficiently drive a service battery bank i will bear that in mind.

It might be because the boat was originally fitted out as a six-berth hire boat. At the time it had four Trojan domestics and from memory (we still have the original manual) it still had a separate battery for the diesel heater, which was originally a Webasto.

The reason I'm questioning this is that the guy who diagnosed the alternator as being defective reckoned that 50 amps wouldn't be sufficient to charge a domestic bank. It would surely at least add to fuel consumption and engine wear on account of having to run the engine longer?

Yes, 4, with two alternators, afaik the lower alternator just charges the starter. When new the boat was fitted with four Trojan domestics, but the engine has been replaced with a different model in the meantime so it's possible that the original engine used two belts.

Please note I've corrected my post above and also replied to yours. The Eberspacher actually has its own battery but i think is charged from the same alternator that does the main leisure bank, i e. the alternator that has failed.

Yes. But i don't think it's on its own alternator. That's what I thought I'd been told when we bought the boat, it's probably faulty memory, the fact that when starting the engine after the alternator failed the voltage didn't change on our Eberspacher-specific volt guage suggests i was wrong. If we can't determine the amp rating of the faulty alternator and can't have it repaired do you think a 50 amp alternator would suffice for charging leisure batteries (currently about 240 ah from memory but ideally we would upgrade in the future to higher capacity). The downside of a low rated alternator would surely be longer charging time and hence greater fuel consumption and engine wear. I think that would be too much hassle since the starter and Eberspacher batteries are on the opposite side of the engine bay to the main leisure bank.

Yes, the alternator below only seems to charge the starter battery. I had been under the impression that it was charging the Eberspacher but i started the engine before removing the faulty one and no charge seemed to be going to the Eberspacher circuit.

Ok, thanks again for all your help. I'll post an update when we have it sorted. As things stand we don't even know the rating of the alternator we've removed so it might be a bit of a gamble. I can't find the exact model anywhere on the internet, I'm guessing it went out of production about 20 years ago. I think I'll phone Barrus tomorrow, they've been very helpful in the past on two occasions, i even got to talk to a guy who helped develop this particular engine model (1950).