Tim, I got this from Sandy, another 38 owner on the yahoo forum:
The battery box installed in the MK I & MK II is about 11 1/4" wide by about 28" long. It is set on top of a couple 2"x 4" rails attached to the sides of the bilge compartment so there is about 11-12" of space from the bottom of the box to the underside of the sole. The box was designed for 4 group 24 batteries (2 house 2 start). the 6 v Golf Cart (CG-2) batteries have the same footprint but are a bit taller and still fit in most 38 CCs. If you have your water heater mounted in the bilge just aft of the battery box extending the box back can be done once the WH is moved elsewhere. A new box can be made to add maybe one more group 24 (4 for the house and 1 for start) or maybe even a group 27 or 31.
These rails can be removed and lowered to enable taller batteries that are available in 6 v. You can get easy 100 Amp/Hr more using 4 golf cart batteries especially if you lower the box & use those 6 v batteries that are almost 12" tall. 4 of these can get you up close to 500 Amp/Hr for your house bank. Then you need to be able to add another battery for starting. My WH was in the way so I installed a Group 31 in the locker next to the NavSta for start and used 4 GC-2s under the sole for the house bank.
So MaineSail, as usual was right
The most important part is I can lower the rails to make extra depth for 6v's.
I'm going to re-do those rails (and epoxy coat them) and lower them enough to put 4 T-145's in (those are a tad taller) - they will give me 500+ AH in that bank.
I'm not stuck on Trojans, I use them for comparison.
IIRC it's a straight under the sole run from the locker beside the nav station to the engine, so that should do for the starter battery and a simple louver on the bottom will ventilate it.
Initially - due to budget - I'm going to settle for 2 Kyocera 240w's on the Bimini. Not optimal for clear shot to the sun, but it becomes what is possible. I'm hoping to average 150AH a day out of them using a MPPT controller.
That and a Honda 2000 should be enough - provided I carry enough gasoline in jerry cans.
I'm toying with the idea of a inverter/charger - but balking at the expense. I may just stick with the current Xantrex 20 amp. It's small but if I'm at a pier I'll have at least 24 hours to top off.
I'm not sure about a "tower" - but I think I will extend the davits upward via a arch arrangement that spreads to a "Y", and on the "Y" I will either add another panel, or put two up there. Those 240w panels are big - too many would look pretty bad and catch wind like a sail.
Radar, I'm going to go conventional - on the mast just above the spreader.
As it stands my Raymarine e7d chartplotter has built in GPS that works just fine even when it was on the Gemini under the dog house roof. Using the internal GPS saves me a NMEA 2000 port too. I may put a external antenna up on the aft rail just for a backup.