On a related note, when installing a high output alternator, I also like to keep the OEM alternator if at all possible. The small alternator charges the start battery and the HO alternator charges the house bank. It works well, provides online redundancy, and the emergency parallel switch serves to bring either alternator up on both banks.
I have done a number of these installs and agree. However, they can get very, very expensive if you don't DIY the project. Even if you DIY it can get a lot more costly than simply carrying a spare alt or regulator that could be swapped out in about 15-20 minutes.
Many boats these days lack the physical room in engine bays to even add engine driven refrigeration let alone the fitment of two alternators, the custom brackets and the forward offsets for a single v-belt plus a dual pulley or serpentine kit.
Also the engines these days are getting dainty and finding solid engine block to mount a 150A to 200A + alt to is pretty scarce. This often means mounting to the engine stringer, which I won't do, or custom fabricating new engine mounts with enough meat for an alt bracket. For some engines there are secondary alternator mount kits but they can get very costly too and the alignment is not always a guaranteed thing even with a "kit". Care also needed to be taken in the design stages of engine servicing and access. I see too many custom brackets that leave an owner with little to no access to some critical areas of the engine such as t-stats... R&Ring an alt bracket to swap a t-stat is simply crazy but on one boat that is what had to be done because the install was poorly thought out and rather poorly executed too..
With the advances today in wind, solar and other means of charging, and the reliability of HO alts, when installed properly, most of my cruising customers are now simply opting to carry a spare of the HO alt and a regulator as it simply costs less than the installation of an entire second alt bracket, the custom pulleys and the additional labor involved to install it, paint it and properly fit & align it.
The last one I did ran more than twice the cost of a spare HO alt and a spare regulator and I passed through the custom bracket & pulleys at my costs.... The custom made bracket required many man hours of templates, mock ups, measurement, machining, fabrication and fitment and the custom machined pulleys made the proper alignment of both alts extremely tedious.
In a perfect world this is a great solution/option but we don't always live in a perfect world. Unfortunately most of the secondary alt installs I see are under designed and often grossly misaligned thus emptying the owners pockets for no real gains. Correcting these alignment issues is often tedious and time consuming requiring more machining and fabrication. Once done right they are great but they are not inexpensive to do well, right or correctly.
If one chooses to go this route please be PATIENT and take the time to do it right. On one engine they bolted the custom made bracket to the gear case (in an attempt to get alignment with the crank I assume) and literally cracked it.. It left the owners dead in the water. A spare alt would have been a far easier swap out and a lot less expensive than the huge expense of replacing the gear case. Of course that job could have been done right, and that never would have happened, but it was quite cobbled together as many are that I see....