Bad Alternator? Bad belt?

Had my 1990 sport Nautique out last night and on the drive back to the marina, the engine was squealing and wouldnt stop and eventually I heard a thump on the motor box and the alternator belt had broken off...

A little back story, I bought this boat a couple months ago with 920 hours on it (now has 932). It started to squeak a little bit during idle on about our 2nd or 3rd time out, and I assumed the belt was possibly bad because the squealing stopped once I put the boat in gear and we were under throttle. Thought I could live with the noise until I had a chance to change the belt. The squeal had progressively started to last longer and longer and I guess I held off on the repair too long.

After it broke last night, I'm curious if something might be also wrong with my alternator. After the belt broke off and I had it back at the dock, you could freely spin the alternator pulley. Any ideas? On the drive back, the engine temp remained at right about 150, which is where it's been ever since I've owned her. The volts dropped from about 13 down to about 9 or 10 though. Oil pressure was at 40 psi just in case that's applicable at all...

I'm not mechanic and hoping to learn some basic engine care from the old girl. I'm fairly handy with most things, which is why I'd like to learn more about engines and be able to do some basic maintenance that I'm unable to do now.

How easy/difficult is to put on a new belt? I've heard it's fairly simple, just curious if anyone has any instructions (pictures/videos even?).

Thanks in advance and sorry if I missed this somewhere in the forum already. I did a search but didnt see anything.

12 hours with a squealing belt. Well, good thing you're into boats and not planes.

suspect its just the belt, start there.

Yeah, there has to he some utube about old school fanbelt tensioning if one hasn't done that before.

Um, his circulation pump pulley should have 2 grooves. 1 for the RWP and 1 for the alternator. Both also go around the crankshaft. A missing alternator belt will not affect engine temperature.

Thanks HW. I was about to chime in. Two belts, one for each RwP and Alt.

Brad, welcome to the site. I totally agree with Larry and Gary (sounds like a standup comedian act?). I'm guessing you are just a belt away from a perfectly running engine.

Get both belts, the alternator belt and raw water pump belt. Both the alternator and raw water pump will have one attach point to their mounting arms that are fixed, the other can slide inward and outward when the bolt is loosened. Loosen, put the belt on, then use a large screwdriver or a pry bar to put tension on the belt while you tighten the bolts. Once all tightened up you want to only be able to push the center of the belt between the pulleys down 1/4" or so, in other words it needs to be pretty tight.

Once back running, and with a fully charged battery, you should see about 12 volts when not running, just over 14 volts when running. You won't see 14 volts until the battery is charged, maybe charge it while you are doing this repair so the alternator doesn't have to do all the charging. You shouldn't hear any squeal at any speed once this is done. Good luck and hope this helps!

Sounds like you're running cool, impeller is probably good but having a new spare on hand is even better.

I like the feedback on not changing out parts just to change them out. I dont have $200 laying around to throw at a new alternator. I'll do as suggested and get the new belt on and take her for a spin to see if the squeal is present or not. I'm inclined to think the alternator is OK as it has had good battery charge since I've owner her - always around 13 volts when under power - whereas the squeal has gradually gotten more progressive over the past handful of trips on the lake until it finally broke.

I'll test the new belt and see if that takes care of the squeal. I've also spent the majority of my day reading up on things to look for so I'll check some of the other items I've seen too. Mainly some other items about pulley alignment.


thanks again guys - love the feedback

Easy if you have a few tools with you. I carry a small tool box, has basic tools for small repairs like that. I carry both spare belts, impeller, spare drain plug, spare prop nut and key and cotter pin, spare kill switch lanyard. Maybe a few more things, would have to take a look, it kind of grows over the years. I like having the ability to do a 15 minute fix rather than being dead on the water and towed back to the ramp.

Someone correct me if wrong, but isn't the fan named Reid of race city marine on norman? If not, he would still be a good source of local help and reference.

Thanks HW. I was about to chime in. Two belts, one for each RwP and Alt.   
My 350 GM belt for the RW pump only went around the crank and RW pump.
My alternator belt turned the water pump and alternator driven by the crank.
Based on your comments the Ford is set up differently.
I have changed RW impellers several times on my first Nautique and once on my newer Nautique but can't remember the belt lay out on the 351W Ford.
If it was me and I did not know the history I would certainly change the impeller while in there right now. They are only good a couple years so do it now and take the next two years off.
Make sure the cause of your belt breaking is not a failing Water Pump since it is driven by the exact same belt. Both the alternator and the Water pump should spin freely with no grinding felt as they rotate by hand. Also check movement in all directions. The pulley should not be loose or have any movement other than the ability to rotate.

Seems like too much calculus.   Just replace the belt.
Might be wise to get both, then they don't work against each other, and be good for a another decade.

I buy gates belts for mine 1/4 the price of PCM belts haven't failed me yet.
The "teeth" are to help in cooling the belt.

The major belt builders of quality belts are Gates, Goodyear or Dayco. I would run any of the three.
Dayco does offer a belt with notches on TOP, the Top Cog belt. Those are for cooling.
The bottom cogs are for increased flex as pointed out.
Even Ford, GM and Chrysler do not build belts, they will contract with one of the big three to build them.

Seems like the primary function is still bending resistance though. The increased surface area gives other beneficial side effects

Undo the adjustment one. There may or not be play in the position of the alternator at that point. If it doesn't move, loosen the lower one too. Slide alternator in as far as it will go. Slip new belt on around the pulleys. As mentioned above, get a large screw driver or something similar to pry the alternator out to tighten the belt. Maybe to the point of 1/2 inch play in the belt, hold the alternator there, tighten bolts.