Looks like a 7-man Pro. What's on the back, 40 or 50hp?
Back to your question.
The Pro uses a rail system, where the inflatable PVC collar can be removed or refitted. The tube-set has a rail that interlocks with a recess in the fibreglass hull, and you can slide on.
With the collar now on, it's secured at the bow with a strap to the towing eye and at the transom on both sides. Critically, the tube doesn't just have to be inflated so it's hard, but to a specific pressure range. These Pro tubes don't have over-pressure release valves, and it sounds like it's lost significant pressure given ambient air temperature and the fact the lake is cold allowing water to enter between the rail system and the hull.
Critically - if you stuffed it (bow into a wave) you could theoretically rip the tube-set off.
You need to check tube pressures regularly, especially if you get variations in weather. Zodiac sell pressure gauges, so invest in one quickly. For the 7-Man Pro, the boat has 3 independent sections. You don't inflate to pressure individually, but gradually top -up, or you'll stress the internal baffles. I always check the pressure at rest on the trailer, launch, then check again. There-after, you just know.
I couldn't honestly tell you what the tubes are meant to be on the Pro 7-man (and I used to own one), but on my old Zodiac GT it was anywhere from 240-280mb that's about 3.5psi.
The bung is inexpensive. You would need to measure the xisting one. Ribshop sell 24mm, 40mm or 44mm variations.
Zodiac Rib and inflatable boat parts and accessories
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