Alternator Belt Issues

I’ve been suffering from belt issues for sometime, have discussed it with a few members off-list, and recently had success by rebuilding the alternator. Per request of a fellow BCC-er, I post this here for y’all (I’m pickin up the southern thang here in NC).

I don’t know who asked you to start a thread for you alternator story here on the forum, but I think it’s a good idea. I really enjoy you website, with its excellent pics, but the forum is also a great place to discuss this sort of thing. I’ve copied and pasted this from your website. How is the cruising going? ,

To answer a comment on your website, I think Ben was dealing with shot bearings on the alternator’s shaft, not a bent shaft. When the bearings go, the shaft can wobble and the pulley doesn’t stay properly aligned.

The question is what caused the bearings to go on a fairly new alternator?? There are lots of possibilities. The alternator’s pulley and the crankshaft pulley need to be aligned (by shimming the alternator). If the belt tension is not enough, the belt slips, especially if the alternator is working hard. A slipping belt generates lots of heat that is transmitted through the pulley to the shaft and its bearings. Too much belt tension is hard on bearings on both ends. Ample Power, the supplier of Ben’s alternator, strongly recommends using Gates Green Stripe belts. They are much more flexible that the solid Yanmar belts, and so grip the pulleys better. Of course, the higher the alternator’s output, the more heat is generated. Also, the alternator is attatched to a hot engine.

High output alternators like Ben’s sometimes use double, matched belts. If Ben’s smart regulator is the Ample Power SAR, he has the ability to set the alternator’s maximum output (and toggle it back to full output). With the Yanmar 3GM30’s modest HP, and his single pulley alternator, it might be a good idea to reduce the alternator’s maximum output to maybe 60A, especially if the batteries are fairly discharged. If the batteries’ resting voltage is low, say below 12V, the smart regulator will allow the alternator to go to maximum output (until the battery voltage gets to about 14.2, assuming the engine rpm’s are high enough.

I suspect there are many things that may either collectively or individually have contributed to the demise of Ben’s alternator. Perhaps not least of which is the fact that boat was laid up for some time maybe the bearings seized and then were freed but of course the damage was done.

I have pondered the high output alt scenario for some time, and the advantages are great, faster charging assuming of course all the other ducks are in row. eg the regulator and temp compensation and the staging of the charge into a batt bank that can take it.

The V3 is an interesting product and as mentioned allows the operator to dial it back releasing the power back to the engine where it might be needed in an emergency. I suspect that an oversize alt may suck up perhaps as much as 6 horses of power, I would be interested to hear from anyone who could quantify this from their own experience. A yannie mabe has 29 HP so its a sig loss.

As we know diesels like to be loaded when running so charging batteries with an oversize alt may be more beneficial to the engine than a smaller alt which may barely tax it at all. It also cuts the time needed to charge.

The other side of course is the extra load/friction on pulleys and bearings and then what happens to the belt which is the next limiting factor. Fit and surface area and then the material itself are all factors.

I guess also the final test is whether the batts chemistry can handle both the voltage and also the current.

Keep us posted Ben and let us know how the scenario unfolds, it will be interesting to see what the charging time of the batteries does. You see I have the same alt and it would be good to have some insight for when 126 is commisioned.

Hello Stewart,
The high output alternator charging an agm battery bank does indeed seem to be the way to go. I am going to try a slightly different tact… two 70 amp alternators running in parallel. The advertised advantages are that the two will have a lower cut in speed than a comparable high output single alternator and if one fails the spare is pre-installed. It’s going to be a while before I can report back with empirical data to support these manufacturer’s claims but you can read a bit more online at:


I think I screwed up on power plant selection though. I bought the 25 hp Beta instead of the 30. (See attached photo) If those little twin alternators skim 6 off the top when they kick in I may have to put the dinghy motor online to get out of the channel on a flood tide.

David. I think the twin alts is a great way to go, you might think about the sar V 3 which can dial the field current back and release engine power for when you need it. Apparently it works for twin alt situations. I looked at the Adverc stuff on the net, and some of the Euro design is very good, I wondered about the price though.

After a while your brain goes round in circles. The figure of 6 hp in power reduction was a vague calculation I made, it may be more or less depending on the individual situation. I look forward to someone chiming in with accurate info. There are a few variables to consider like how many alts, and their charging capacity, one of the nice thing about two is built in redundancy for back up. Of course the fact that the engine bearing is loaded bilaterally is better for it than just one alt to one side. I would have beta if had had good info on it, however I think I have a good system with Yanmar. Hows the boat doin?

Your alternator dust prompted me to check mine’s temperature as I have been plagued by the dust as well. My 80amp alternator 3/8 belt temp was okay (84). I am still at a loss for the source of the rubber dust as the belt was recently replaced. The previous belt lasted ~250 engine hours so I guess all is reasonably well. Your posts and website have been a good source of info and entertainmet. Great stuff! Happy New Year,


Hey Tom,

I still get some dust too, FAR FAR LESS then I did before, and the belt temps are fine. I looked online a bit and alot of Yanmar owners complain about dust. The dust I have now I can live with. The dust I made before was not healthy. 250 hrs seems short to me, I’ll let ya know how long mine lasts.

What RPM do people cruise at? Is there a chart somewhere someone has made of RPM vs speed for their boat? I find myself at 2600-2700 RPM most of the time. Normal?


Hi Ben , somewhere I read (probably Steve Dashew’s book) that the cogged Gold Label Gates belts were good ones to use with high output alternaters.

I have been using them to reduce the belt dust, but they haven’t reduced it, if any.

I think I have an Balmar, 105 amp alternater, so I too am looking for a solution to the dust problem .

When I removed the Volvo 2002 and replaced it with the Yanmar 3 GM 30 F , I had an over reving problem, using the thin two blade 16" prop that was factory installed on the Volvo.

To load the engine properly, I had to go to a 3 blade custom prop from Michigan Wheel props.

With this set up , I cruise at hull speed at 2450 rpm , but that big 3 blade causes heaps of drag when sailing. Couldn’t afford a Max feathering prop.

There is a rule of thumb about the prop tip clearence in a prop cut-out aperature,in the deadwood , that rule is that there needs to be a 20% tip clearence, which the 16" standard two blade didn’t have, so I had the prop aperature enlarged to accommodate this reccomendation.

I also installed a set of prop spurs and by the remnant threads of rope and line I see around the prop and shaft, from time to time , I can assume that those spurs have been working well.


I had a very small spot of rust on my alternator belt pulley. Each time it made contact with the belt, it generated a microscopic amount of dust. Over time, I had dust everywhere.

I sanded down the rust spot, polished the inside of the belt, and no more rust.

Not sure if this solves all rust problems, but it solve mine.

Mark Gearhart
s/v Godspeed

For what it is worth I questioned my local Yanmar dealer with the RPM question. Previously I would not let it get over 2700 RPM a few years ago. They said that cruising rpm should be over 3000 with a max of 3200 or 80% of max RPM. He said the engine would load up if used to long under 3000 RPM. I have not had a chance to test this since I’m in refit now but it makes sense, the engine needs to get to and stay at operating temperature. The link below is to the same question on the cruisers forum.

On another note I never noticed black dust from my 2GM. But just last year, after replacing multiple belts on my truck, the bearings in my alternator were found to be the culprit. I couldn’t tell they were bad by moving the shaft, but at high RPM’s even the smallest issue is going to be amplifed.


Yeah i’ve read about the rust/pitting on the pulleys on other forums, could be the source now, there is certainly some pitting on my crankshaft and freshwater pump pulleys. And well, some belts are better then others, like the green belts.

Interesting about the Yanmar RPMs. I know you can’t be delicate with them, and low RPMs will kill them. When I worked aboard the HMS Rose, the captain used to justify his seeming abuse of the twin 671 Detroit Diesels by quoting the manual: “Do Not Baby these Engines”.

Only thing I noticed is that to keep up with Teresa on her Nor’sea I have to run at about 500 more RPM then she does. My 2700 = her 2200rpm. I know I have ALOT more boat to push.

Happy New Years to all. I’m back to Miami tomorrow, hope the boat is still floating and the stuffing box didn’t leak too much!!

I allow that fighting belt dust is a constant battle. V-belts are after all consumables. Their dust is scarcely noticed in auto engine bays. But in a BCC engine room, the evidence of the crime remains to be seen.

FWIW my fighting strategy on Zygote includes:

  1. Clean pulleys
    As Mark of Goodspeed noted, pulley surfaces have to spotless.

I polish the v-belt pulleys once a year with 240 grit paper (if time weighs heavy on my hands, I then use 400 grit paper, but time is short and the law of diminishing returns is in operation).

  1. Correct pulley loads
    And when the v-belts are off the pulleys, I check play in pulley shafts and spin the alternator pulley etc to check that the bearings are good.

  2. New and quality belts
    I aver that it?s pointless buying new belts and storing them in a locker.

I buy new belts each year, fit them, and put the old belt (if it?s still good, as it usually is, in the spare parts locker and dispose of the older belt that was there). Regardless of brand and quality, the rubber compound in belts loses flexibility over time, at a rate partly determined by the ambient temperature.

I only buy quality belts. As Douglas of Calliste and Ben of Elizabeth noted, Gates produces belts of better quality than many of its competitors.

On Zygote, with its Yanmar 3GM30F, standard Hitachi alternator, and Technautics Global 12 refrigerator with engine-driven compressor, I use:

  • for the engine - alternator: Gates 9380 Green Stripe XL 13A0965

  • for the engine - raw water cooling pump: Gates Truflex 1190 (3L190)

  • for the engine - reefer compressor: Gates 7417 XL 11A1055

  1. Good belt tension
    The right belt tension is the minimum to stop belt slip. That minimum also means that alternator etc bearings are not stressed by excessive sideways forces.

Getting the right belt tension is not easy.

I use a pulley spreader to tension and hold tension in the belt. I can then use a Krikit gauge to measure the tension (Krikit gauges are a bit of a joke, but they?re better than the ?educated thumb? technique. Alternatives include the ?spring balance, straight edge, and rule? technique or an expensive belt vibration meter).

I use the pulley spreader to spread the pulleys until my Krikit gauge measures a belt tension of 27 kgf (60 lb.f). Then I torque the fixings.

New belts need to be retensioned after their first run, of course.

The spring balance, straight edge, and rule technique, a replacement for the educated thumb, is:

  1. place a straight-edge along the run of the belt;
  2. put the hook of the spring scale at the mid-point of the run of the belt and apply a sideways force of 111 N (11.5 kg.f, 25 lb.f) to deflect the belt;
  3. use the rule to measure the deflection from the straight edge to the deflected belt.

If you?re not familiar with pulley spreaders, see the attached Zpulleyspreader.jpg (about 10 KB ) to see the three spreaders Zygote has. You can make your own from galvanised steel turnbuckles and a welder.

Rudi Scholz at may still be in the business of making and selling (with shipping from Malaysia) pulley spreaders if you tell him what size you need (I use a #6 spreader for the engine-alternator belt, a #10 for the engine-reefer compressor belt, but I?ve never found a spreader that fit and do the job for the engine-raw water coolant pump belt). Rudi will warn you not simply to use a pulley spreader to get the maximum belt tension your arm can generate ? that?s a sure recipe for destroying your alternator bearings. You must use a measuring technique (Krikit gauge, or ?spring balance, straight edge, rule?) with a pulley spreader.

A #6 pulley spreader handles spreads of 115 ? 178 mm. A #10 does spreads of 150 - 225 mm (measure from inside of pulley to inside of pulley, using your target v-belt brand).

If you?re not familiar with a Krikit gauge, see the attached Zkrikit.jpg (about 100 KB, taken before I switched to Gates belts - the photo image is large in size, the better to see the effects of ageing on my skin) to see my right hand using Zygote?s Krikit and/or employ Mssrs Google?s big invention.




Ben: Hi!

On engine rpm for the Yanmar 3GM30F, I reckon a good engine speed for the longevity of the engine is around 2700 rpm (75% of maximum), but 2900 rpm (85% of maximum) can be sustained if fuel economy is not an issue.

Engine speeds below 2500 rpm are too low to prevent coking of the exhaust elbow and glazing of the cylinder walls. When motoring at low speeds (ie 2500 rpm or below - Zygote has motored around 2200 rpm for hours to get through the doldrums) for extended periods, run the engine at 3000 rpm for 5-10 minutes every five hours.

I understand that most sailingboat engines die of disuse, not from overuse.