Start with the well, not the catalog
Wikipedia’s automotive-battery article: BCI group size is the physical envelope. The tray already in the boat is the spec. Height under a Yamaha jet hatch, length under a Scout lid, and the hold-down ledge on a flooded 27 are what we measure. A battery that “should fit a 31” and does not clamp will move. 33 CFR 183.420 cares about that movement.
Posts and lids
Top posts that hit a hatch, side posts that miss the cables, and dual-post Optima layouts that do not match the old flooded cables are battery size and connection problems. We photograph posts with the lid closed in mind, not only the footprint on the shop floor.
Do not upsize for luck
A larger group in a smaller well is not more battery; it is a battery that will not sit. A smaller group in a large tray needs a hold-down that still meets the one-inch idea. Upsizing Ah inside the same group is a chemistry and plate conversation, not a magic bigger box.
When the tray is the part that failed
Cracked plastic and missing J-bolts are why a correctly sized battery still walks. The tray is on the quote. Replacing only the cell is how we see you again after the first hard run across Tampa Bay.
Pinellas and jet wells
Tight compartments are this article’s geography. Send a tape photo. We will pick a group that clamps, then a chemistry that matches the charger.
Cables that will not reach a different post layout
A group size can fit and the cables still miss a relocated terminal. Dual-post and side-post cases are the usual surprise. We dry-fit before we clamp. If the old cables will not land, that is a cable job on the same visit, not a reason to force a case that does not match the well.
Sources
- Wikipedia — Automotive battery (CCA, CA/MCA, amp-hours, BCI group size)
- 33 CFR § 183.420 — Batteries (Cornell LII)
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.