I have an 18 foot Dux RIB (UD-550). It was left on the grass all winter, don't know if that affected this problem. It has six air chambers, and four valves. The aft valves just do the aft chambers. The middle port valve does both the port middle and forward chambers together. But the starboard middle valve only inflates the middle chamber, the forward chamber is completely limp. I can't find a leak. Could the internal part of the valve be damaged? When I bought it it was fully inflated. Any ideas?
Sounds like an odd arrangement but maybe a pressure relief valve is built in between the middle and forward chambers, built into the internal baffle? If so, maybe it is stuck? The inflation valves on mine can be a bit sticky if they haven't been topped up for a while.
__________________
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
