My boat has large thick smoked perspex transom flaps (very 1970's) that have worn hinges, what is the best stuff to replace them with?

Forget all about hinges, etc and go for one piece of thin plastic that goes all the way across both flap holes and in simply screwed in place along the line of the rudder pintles.  Depending on the position of the pintles, you may need to run the plastic underneath.

You then need a hole in the centre of each flap through which some elastic (4/5mm) is threaded and taken into the boat to a hook or similar on the hog.  The tension keeps the flaps closed while the elastic allows them to open with water in the boat and you can flip the elastic off the hook to let them open fully.

Make sure that the surface of the transom around the flap cutouts is smooth and free of any muck.

You can get the plastic sheet that you need from Pinnel and Bax (and I'm sure many others) - see link below. There is also a picture on this link which perhaps better explains than the words.  Also do not be put off by the size given on the link - in practice, you need to tell them the overall dimensions across your flaps (leaving some overlap) and they will send you a sheet that you can cut to size.

How flexible is this? can you bend it so the opposite ends touch? will they fold back 90 degrees to the transom like hinged flaps? 
I am asking as I have a sheet of thin perspex in the shed, but I thought that might be too brittle.

The transom flap sheet supplied by reputable chandlers (c. 1 to 1.5 mm thick) can be rolled to c. 6" diameter and spring back flat with out problem.  Whilst it doesn't look quite as good as the smoked perspex flaps did on Smoked Salmon, it is an awful lot lighter (weight out of the ends and all that).

According to 'Essex in a shed' they do have an added bonus of bending away from the transom when the wind reaches 'staying in the bar' strength, thus proving a handy reference for those who wish to avoid strenuous or boat breaking sailing conditions...

Redditch Pond Sailor,

I have a small stock of clear complete with protective film - as I replaced mine not just a week ago - please PM me if your interested

Carl

The stuff from P&B is fine - it bends all the way back and will seal up again.  Your thin perspex may not be right if it is stiff and cannot be cut with scissors.

I thought I would try the thin sheet option to replace my perspex flaps but was put off by all the 'modern designs' of plastic bathtubs at my sailing club having thin flaps that seemed pemanently alightly open. For a similar look to the originals I was recommended 3 mm polycarbonate sheet by a local trader who sold me an offcut for a fiver. It shapes easily and took a thread to attach the nylon hinges with no threat of cracking. The new flaps are half the weight of the old and cost about the same as the P&B item in raw materials.