Main Belt on engine stretches

[Tunagirl29]

Main belt on my 2005 Beneteau 343 keeps stretching
Have to tighten every 15 hours of running engine.
Or replacing belt, but it’s not getting chewed up at all.
Any advice please?

 

[FastOlson]

Replace it with a known-quality belt. Likely this one is defective.
Once you take it up after a little bit of initial stretch, it should not keep on stretching.
Have you reached the end of the slot on the arm yet?

 

[Ron20324]

is that a 3YM30?

 

[Chris Patterson]

Belts do not stretch. The pulleys are worn out, or not the right ones.
They get loose by riding too low in the pulley.

 

[dlochner]

Use a good quality belt, Gates are well respected.

Make sure the pivot bolt and the adjusting bolt are tight. It is more likely that the alternator is slipping causing a loose belt than the belt stretching.

Here's a piece on tightening the belt. V-Belt Tensioning the Easy Way and v belts | Search Results | Steve D'Antonio Marine Consulting.

 

[Tunagirl29]

Replace it with a known-quality belt. Likely this one is defective.
Once you take it up after a little bit of initial stretch, it should not keep on stretching.
Have you reached the end of the slot on the arm yet?

Yes, it is at the end of the slot after only about 15 hours of running engine
and the belts I bought at yanmar dealer in Annapolis.
Have loosen all three bolts before tightening up on .
This has been going on for months, since I bought the boat.

 

[Tunagirl29]

is that a 3YM30?

Yes , it is a 3ym30

 

[dlochner]

Yes, it is at the end of the slot after only about 15 hours of running engine
and the belts I bought at yanmar dealer in Annapolis.
Have loosen all three bolts before tightening up on .
This has been going on for months, since I bought the boat.

The belt may be too long. Take the belt to a NAPA store and have them match the belt with one a little shorter. If the prior owner changed the alternator or the pulleys then the stock Yanmar belt will be too long. NAPA sells Gates Belts.

Edit: Rather than guessing at how much shorter, take an old belt and cut it. Put the alternator in the middle of the adjusting range and then use to the old belt to measure the size.

 

[Tunagirl29]

Belts do not stretch. The pulleys are worn out, or not the right ones.
They get loose by riding too low in the pulley.

Thanks so much,
Will check that out.

 

[Tunagirl29]

The belt may be too long. Take the belt to a NAPA store and have them match the belt with one a little shorter. If the prior owner changed the alternator or the pulleys then the stock Yanmar belt will be too long. NAPA sells Gates Belts.

That sounds good but when I put a brand new belt on it’s perfect tightness

 

[Ron20324]

... when I put a brand new belt on it’s perfect tightness

That may be because the belt has not taken a "set" to the pulley's shapes. If you are at the end of the adjustment slot, get a shorter belt. My 3YM20 had the same problem. It calls for a 20 belt, but I went to a 19 and all is well. On a marina work order I specified I needed a 19. Force of habit, they put on another 20, of which my old one was almost new. Called them on it, and the did the 19 and all is well.