I've had a few challenges with mine, but I simply can't be bothered swapping it out any more and often leave the blank in even when out sailing. It's one less thing to worry about where GPS speed is good enough.
To your question: IIRC there isn't just one flap, but two interconnecting flaps on opposite sides of the unit that close against each other - and I suspect mine is missing one, but I don't get all that much water in - certainly not full flow...
There are known issues around the size and lubrication of the O-rings on the body of the plugs, and if you try to force the plug in (or force it out more likely!) with an incorrectly sized or un-lubricated o-ring, you risk dislodging one (or more) flaps and losing them out the bottom of the boat. For this reason I'd find it really hard to believe they'd replace the unit no questions asked... but it'd be a bonus indeed if they were.
To your question: IIRC there isn't just one flap, but two interconnecting flaps on opposite sides of the unit that close against each other - and I suspect mine is missing one, but I don't get all that much water in - certainly not full flow...
There are known issues around the size and lubrication of the O-rings on the body of the plugs, and if you try to force the plug in (or force it out more likely!) with an incorrectly sized or un-lubricated o-ring, you risk dislodging one (or more) flaps and losing them out the bottom of the boat. For this reason I'd find it really hard to believe they'd replace the unit no questions asked... but it'd be a bonus indeed if they were.