There seems to be a widely held opinion that transom flaps don't really work unless the boat was designed with them in mind and has a flat run aft.
Does the shape of an old Merlin with plenty of rocker and an aft deck make transom flaps pretty useless? Our elderly 'Gentleman's Merlin' has perspex hinged flaps held closed by elastic. The flaps don't leak and normally are well out of the water. All buoyancy is bags.
In extremis is the elastic supposed to be sufficiently weak to allow the weight of water in a swamped boat to push the flaps open or is one supposed to release it by hand?
When we tried to move weight to the back of the boat to shift said water the gunwhales went under rather negating the effect. If I install a central stern buoyancy bag under the deck the boat will float higher when upright (hopefully not too high when on its side) but the flaps will be inaccessible from within and I presume water flow to said flaps will be impaired. If I install longer tube shaped buoyancy bags to run under the deck on either side I suspect the boat will float too high when on its side.
Clearly the answer is not to lose concentration when planing on a dead run...but I don't need to be told that now...
In summary, do I prioritise buoyancy over efficient flaps? or what?
Thanks in advance,
Michael
Two pics from a while ago help to illustrate:

