Thanks for the input everyone, I thought I'd share my final decisions.

For bouy racing, all I need is ability charge my TackTick stuff, and emergency lighting. My LED nav lights draw about 250 mah total, so I'm just going to do 8 x AA batteries in a case in tupperware and replace them a couple times a year. This will plug in where the "offshore/cruising" battery does. The batteries will (eventually) be mounted in the bilge if I can make it work.

For offshore work, I've calculated that given a 24 hour period in "race mode", I will consume ~32.5 amps if I'm monitoring VHF continually, and 17.8 amps if I am not. We plan to monitor VHF on handhelds when the rules allow.

Given a 20% "discharge floor" for lead acid batteries, that means I need ~21.3 amps for a 24 hour race, so I plan to buy a simple 20 amp sealed lead acid battery (scooter battery sorta thing) that weights 14 lbs.

My calculations included assumptions for use of:

VHF at idle, VHF at receive (5% of time), VHF transmitting (5 min over 24 hours), TackTick trickle, Nav lights (50% run time), Interior LEDs (8% run time), Charging a smart phone 1.5 times every 24 hours, Charging a GoPro once every 24 hours, Running a Garmin handheld GPS continually

I feel there is some margin in these numbers, the real need is probably more like 12-15 amps.

For longer races, obviously we will need a bigger battery or a way to augment.

I'd be happy to share my calculations with anyone who is interested.

-dan