There's a one way rubber flap in there. They don't drain as well as a similar sized trunk, as there's a "X" in the middle supporting the flap. Mine is forever clogging up with dog hair so it leaks in almost as much as it drains.
They work, sort of. They won't drain a good sized wave though. Those look like they are centered on the transom so unless you have twin engines it will be hard to replace them with a trunk. I would just keep them and decide if they work for you.
Looks like you need 2 expanding bungs there i'd say, I have one similar on my transom, They insert from inside the boat and when you puch the expanding bung lever it causes them to lock in place through expantion, They can also be taken out whilst going ahead allowing a bit of green water to flow out, Personally I think I preffer the elephant trunk idea, But i'll make do with other for now.
Regards
They work (slowly) in theory, but first sign of any grit / weed / anything solid they will jam open.
Had them on a SIB. Phrases involviong chocolate teapots came to mind.
As said above, expandable bung in them & a trunk would be ideal (then if the boat is stored ashore or being towed, you can take the bungs out to get rid of the rainwater)
