The one-inch rule is written down
33 CFR 183.420 says each installed battery must not move more than one inch in any direction when a pulling force of 90 pounds, or twice the battery weight, whichever is less, is applied through the center of gravity — vertically, then fore and aft, then port and starboard, each for a minute. A battery sitting loose in a too-large tray, or a strap that is a bungee, does not meet that idea. We replace or repair the hold-down with the battery.
The tray has to match the case
A group 24 rattling in a 31 tray will move. A 31 jammed into a 24 well will not sit. Wikipedia’s group-size note is the envelope. The federal pull test is why the envelope has to be clamped, not merely adjacent to a wall of the box.
Plastic trays crack; aluminum corrodes
Florida heat makes cheap plastic brittle. Salt eats uncoated aluminum. We look at the tray as a part. A new battery in a broken tray is a callback when the case shifts onto a fuel line — and the same CFR section says a battery must not sit directly above or below a fuel tank, filter, or fuel-line fitting.
Jet compartments and consoles
Yamaha jet wells and Whaler consoles have little extra room. The hold-down that came with the hull is usually the right style. Aftermarket J-bolts that miss the ledge are how a pack walks. We use what clamps that case in that box.
On the visit
If the tray is broken, it is on the quote. St. Petersburg and Tampa boats that pound across the bay show this as often as they show dead cells. We come to you with the clamp in mind, not only the group number.
After a miss in a seaway
If a battery has already walked and scraped a hose or a fuel fitting, we inspect that fitting before we clamp a new case. 33 CFR 183.420’s fuel-spacing rule is why. A cracked tray that let the pack move is a safety part. We replace it. A pretty new battery in the same broken plastic is the same miss next weekend.
Sources
- 33 CFR § 183.420 — Batteries (Cornell LII)
- Wikipedia — Automotive battery (CCA, CA/MCA, amp-hours, BCI group size)
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.