The alts on the W27 are a saddle mount Mitsubishi. The Balmar 6 or 7 series as well as the Electromaax saddle mount alts will usually fit though may require a shim or two. It is a tight fit on those motors due to the manifold so the small case alts like the 6 or 7 series or the Electromaax units seem to fit a little better than an older Delco 10DN style etc..
The W-27 engines have a 3/8" belt which means with a perfect alignment you can squeak about 80A or possibly 90A but the higher you go the more belt dust.
Of course the size of your battery bank and its state of charge will determine if the alt can even put out 80 or 90A. A 110A Balmar will often work fine of a well aligned 3/8" belt because when hot it won't be pumping out 110A for very long.
I am not a big fan of "dual belts" systems and find them only marginally better than a single belt. They often stretch at differing rates and it has become quite hard to get a well matched pair these days.
For the price of a custom "dual belt" pulley set up you can have a true 10 cog serpentine belt that will handle as much as 200A on a single belt. Electromaax has serpentine kits but I don't know if they have one for the W27 yet. They do show a kit for the Westerbeke 27 but I am not sure if this is the W-27 or not. Call Rob or Darren at Electomaax and they could help.
The serpentine kits are worth every penny if you are trying to drive high loads. A dual belt system is a band-aid approach, in my's, GEL's or wets in excess of 400Ah.
You can always go with a larger alternator and de-tune a Balmar regulator in belt manager or small engine mode then upgrade the pulleys at a later date. I really prefer the Balmar MC-614 regulator and even when I buy Electromaax, Powermax or Leece-Neville alts I still use Balmar regulators when installing external regulation.
Thanks for your very detailed reply. Here's my thinking so far:
I strongly considered getting a Delco (cheap, easy to find parts anywhere), but the mounting options weren't great. A Leece-Neville is much cheaper than a Balmar as well, but getting up near $300 and more once you get the regulator bypass kit. I'm not sure about amping up the current alternator as the cooling would stay the same, but agree that spares are cheap, so not a bad idea.
A few things finally made the choice for me (or at least the current plan unless I stumble across a problem). One, I have AGM batteries and like them. They can really suck up the amps, so I worry about alternator overheating. An external regulator has the temperature connector to the alternator, and I think that's a good idea for AGM.
Two, I feel that the W27 is a tad underpowered, especially with a feathering prop, so I like the idea of a "small engine mode" switch for when I need some more "giddy-up."
Three, I like the idea of a somewhat scale-able system with the possibility of going up to 100ish amps without too much more work. Getting a bit bigger alternator and then de-rating it gives me that ability.
These items really pushed me in the direction of an external reg, and the MC-614 in particular, no matter what actual regulator I buy.
Next was the fact that you can get some really great Balmar packages. In fact, Sailor's Solutions has a kit with a 7 series 110 Amp and and MC-614 for about $750 including the battery and alternator temperatures. If you add up the bits, the alternator itself is only about $330. Not too bad.
Next was a call to Balmar to talk pulleys and dimensions. Sounds like I can use the 70-110 with the dual foot arrangement. I'll need to likely fabricate some bushings and spacers, but that will be easy and not too expensive. You can choose which pulley you want for no extra charge. I plan to get the double pulley and only rig it with one belt. I can then play with the de-rating and if (when?) I decide to add another belt, I then need to source the Westerbeke parts for the crankshaft and water pump. Likely to cost me a few hundred for those two, I'd guess because they are OEM. I hadn't considered your excellent concerns about belt tension, but I think I may still go with the double pulley on one belt and then I can either go to two belts or replace all 3 pulleys and go to a serpentine.
The bolt to bolt distance is slightly different on the Balmar, but the adjustment arm is on a single bolt, so I'll need to loosen that bolt to tension the belt. Not the end of the world, and I can always buy or fab a new arm later if need be.
Last piece of the puzzle whether the larger alternator will foul on any other bits of the engine. I need to get to the boat to check on that, but I believe that I'm not the first to do this on a W27, so it should fit. After checking, I'll know if I need to go down to a series 6 or if the 7 will work.
Eventually, I want to upgrade and go almost totally solar, so the alternator will become the backup to the solar, but this system should hold me for a few years.