I feel like I should know this by now but I’m not certain I have the alternator belt tight enough. There is about 1/4 inch of deflection when I give it a “good push” and with effort I can twist it 90 degrees. What makes me think it could be too loose is that when the engine is running I get some “flapping” of the belt. The alternator is 70 amp, in case that is relevant. I tried to to attach a video but this thing doesn’t allow for that format (m4v).
Appreciate the help and advise.
Ed- pretty certain its 10mm deflection
Ed
I attach a socket/wrench to the alternator pulley nut and turn the alternator pulley. If it turns over the engine without the belt slipping I consider it tight enough.
Is the “flapping” normal or a sign of misalignment or slack?
Ed
You possibly have a rusty pulley. If the engine is not run for a period of time, often the pulley will develop surface rust. The portion of pulley covered by the belt is protected, so little or no rust will develop under the belt. As the pulley rotates, the tension forces the belt deep into the groove against the friction of the rough, rusty surface. As the pulley rotates, this friction holds the belt deep in the groove. The length of unsupported belt stretches until it can pull the belt out of the groove with a “snap”. The sudden release of the tension in the belt, causes the unsupported portion of the belt to then stretch outwards from its normal position. The difference between the smooth, “low-friction” portion of the pulley and the rusty, “rough” portion causes a rhythmic stretching of the belt, often leading to the “flapping” that you see.
To reduce this flapping and you can try to sand the rusty portion of the groove to smooth the surface. But don’t bother to try and paint it. The belt will quickly remove any paint in the groove.
What its really telling you is that you don’t use the boat enough. 
Thank you, Dwight. Yes, winter does reduce its use. Hope to rectify that.
Ed
I use a Gates Krikit belt gauge to check tension and a Balmar BBU Universal Belt Buddy Tensioner to tighten the belt and prevent slippage. Both highly recommended.
What tension do you look for on your Krikit?
I don’t remember off hand - I’ll look next time I am on the boat. I have a different engine because the PO repowered with a Yanmar 3jh3e so the belt size might be different anyway. I believe Gates (belt manufacturer) has a website with recommendations based on belt size and type.
Michael Belanich
s/v Dolcetto, F38 #65
Ed,
Here is my belt story with an aftermarket oversized alternator on my 3GMF.
Acquired the boat with a lot of play in the belt as you described, and a lot of belt dust.
Tightened the belt to where it felt more secure. Reduced the dust, until the belt let go at a bad moment. Had a spare and it is a sailboat. I had crew so no damage was done.
Heard about the Gates belts. I have friends who swear by them, and so does my boatyard. They have a conversion tool on their website that will get you an exact match to your engine. You want the green stripe model, I think they call it HD or something like that. It is a much more robust item than what Yanmar sells, and for less money (maybe 10 bucks ?).
So far so good!
Bob, I tried to find the conversion tool on the Gates site, can you send me the link?
I 1) tightened the alternator belt and 2) sanded the alternator and motor pulley. The slapping seems to have gone away. Can’t tell you which had the bigger impact but the combination worked. Thank you all for your advice.
Hi Ed,
You have to enter your data here, and it will give you the answers you need.
https://www.gates.com/ais
Click the button for “find parts by application” and then choose “power units and marine engines.”
Your Yanmar model should pop up at that point.
Here are my results, and again I chose the Heavy Duty (green stripe) model:
Yanmar 3GM30F Diesel
Application Product Type Part No Comment
Belt Drive System
Front End Accessory Drive Components
W.P. FHP Medium Horse-Power V-Belt 6719
Alt. High Capacity V-Belt (Heavy-Duty) 9380HD
Alt. High Capacity V-Belt (Standard) 9380
Cooling System
Coolant Hose
Heat Exch. To Exhaust 90º Molded Heater Hose 28466