So here in New York this past week we got a mother load of rain and i kept checking on my boat and it seems to be fine. First sunny day was yesterday so I pulled her in off the whips to go out and I found one of the bilge pumps stayed on and killed battery #1. I fired it off on battery #2 then switched over to batt #1 so it could put some charge in it. Went to go for a ride and the second I came up on plane I saw the volt gauge drop and it smoked the V-belt.

I of course could not find the correct belt but I was able to get a new shorter on one it, I went for a ride again and did the exact same switch to batt #1 and it again smoked the belt. I have a brand new 140amp alt in the boat does it but put a huge load on the belt since it was trying to charge a dead battery? Boat was out all season with NO issue and now it burns up 2 belts?

I took the batt home and put it on a slow charge and im going back with a new correct belt but I wonder what changed to cause this? All I can think of is the alt becomes that hard to turn under load from the dead batt?

Never seen something like that happen unless the Alt itself was the problem...maybe one of the bearings is going bad? Unless the battery is dead shorted...then you could also damage the Alt.

Alternator is brand new and put the battery on the charger, it took the charge no problem it wasn't 100% dead. I know AGM's don't like coming back from being stone dead. First belt I know was a little worn but that one it wrecked, the new belt it rolled. I need to confirm the alignment as well just it was fine all summer and yesterday I get a drained battery and wrecking alt belts. Weird for sure, I mean it is a brand new engine build so maybe I have a washer in a wrong spot throwing out alignment but it looked ok and was fine all season.

It is a JS alternator 170amp.

A big load on the alternator makes them much harder to turn for sure. Verify nothing is drawing power when it shouldn't (i.e. bilge pump/s, blowers, stereo, etc). A clamp-on ammeter would be helpful to see whats going on.

Make sure belt tension/alignment is good and belt does not bottom out on either pulley. Belt must ride on the sides of the "V" and not the inside edge.

I honestly think alignment is slightly out which is not helping the situation but the battery being so drained I think put such a load on the alt that when I went 3/4 throttle it just smoked the belt being its 170amp alt. How tight do you guys make these V-belts, Im used to serp belts but from what I remember a V-belt when tensioned should not be able to be twisted more than 90 degrees of its resting position anymore its too loose and any less its too tight. This is over the longest span in the belt.

This started happening to me after my rebuild. I have a 250 amp alternator and I had a little wiring oops moment and it was only charging one battery...long story short when I figured that out on the water and fixed it I couldn't keep an alt belt on it...would howl and squeal and do all kinds of weird sh!t until the batteries came back to life. Maybe the single vee belt just can't handle the high load you/we are putting on it?? I'm swapping everything over to a serpentine belt which I hope fixes it. Super embarrassing to have a belt squeal when you fire up a giant a$$ blower motor hahahaha. Mine didn't matter how tight or loose the belt was or how expensive the belt was, tried all brands from gates to napa and none would hold up. I know alignment was not my issue as I never changed anything alignmentwise from stock.....I think it's just the added load on the alt. I'm sure some will disagree and theirs works fine but that's just my experience.

Someone mentioned earlier in the thread of the belt riding on the tapered sides of the belt and not the bottom...that's some good info there! Might want to look around for a thicker pulley that would allow you to potentially run a thicker belt or add a second belt? Just trying to think of options.

I would not have thought an alt creates more load when it is under max load but my boat was fine for over a month, it get a dead battery and the belt issue shows. Stock alt is 65amp and my new one is 170amp so I'm sure it must generate more load than a 65amp would.

Tomorrow I get the new factory belt and a charged battery then go for a ride see what it does.

I figured an alt would generate more heat under load not torque to turn but I guess I was wrong, there is not much else that can explain what just happened to me and what your dealing with. I emailed the company I got it from but no word yet.

I'm no electrician but have always been told that an alternator only puts out what is called on...meaning if the batteries are calling for everything it can give then it's putting out in your case 170 amps/full load which makes it harder to turn to produce that power....when the batteries are charged and the only load is boat accessories be it lights, stereo, etc. it may only have to produce 30-40 amps to keep up with the load which is much easier to turn so it makes sense that a belt system designed to spin a 65 amp alternator is having trouble keeping up with the 170 amp load.....at least in my simple mind it makes sense hahahaha. If your batteries don't go dead you will probably never have another issue but I would keep an extra belt and jumper cables on the boat just in case...I have seen guys keep jumper cables on the boat to essentially "tie" the 2 batteries together in a straight shot to get rid of any voltage drop going through a switch or isolator or whatever until the dead battery gets some life back in it...might help distribute the load a little potentially instead of calling for all 170 amps like now cramming it into a dead battery? Again I'm no electrician hahahaha

A while back, I built a 100A gas powered battery charger from a used 100A car alternator and a brand new 5hp B&S eng. Also had a custom field controller for the alternator so I could control voltage and current separately. When charging my forklift battery, if I dialed-up 100A, but it would stall the 5HP eng. Had to back it down to ~80A to keep eng from being overloaded. At 170A, bet your pulling 8-10HP from the eng thru the alt belt.

When eng speed increases above idle, the alternator will produce more current and puts even more load on the belt. So at that critical moment when you have more load on the alt belt, the increased RPMs will cause any alignment issue to be more relevant. A double whammy. However, I doubt the alignment is all the sudden a problem when it wasn't before.

You can convert watts to hp.
Guessing 170amps is 7-10hp.
We are on the internet. Sure a quick formula exists.