Ok, simple question ( I hope)- how much of an overlap to leave on a new dacron slot gasket? just touching, or 5mm, or more? I've got the bit about fixing it, keeping it taut, and securing it afterwards. Most of the fitting stuff I can find is about fitting mylar gaskets to wayfarers

Do a search on the forum - I'm fairly sure that there was something fairly detailed a couple of years ago but could be longer.

Hi, I fitted a new slot gasket to my old mk12 last Friday. 
Tips: measure the width from the outer edges of the brass beading and hence calculate the width of gasket required, that way you can cut the gasket tape before fixing it, if you trim it afterwards you will cut in to the paintwork. 
I left about a 5 mm overlap which seems to be ok, I wouldn't go anymore than that. 
To get the tapes tight screw the front down then stretch them really hard and put the rear screws in. I also burnt eliptical holes in the tapes with a soldering iron, where the screws go through, hence you can tweak the amount of overlap once the beading is on.
Be patient, so far it's been the trickiest job on the boat.
Good luck

Thanks,I searched the forum and not much came up suprisingly, there was a a few references to mylar, and the Mike McNamara video on wayfarers, and the use of neoprene pond liner as a gasket,but nothing definitive on overlap or merlin specific fitting.

I've been told subsquently that it's best to tack the slot gasket in place before the keel band goes on, and to 'close' the aft end of the gasket like the front, so its a snug fit all along the board and doesn't have any bits flapping at the back end. It makes complete sense, because otherwise there would be no tension in the sealing edge of the gasket. Some classes seem to leave the gasket open at the back.
Consensus seems to be to go for a couple of mm overlap, not more than 5, but probably a bit less, 2 or 3mm.
Thanks for the tip about pre cutting the gasket Alex, I could easily have ploughed into the job and forgotten that!

I have always cut an angle off the rear inner edge of each strip of the tape. This effectively leaves a triangle open right at the back of the slot to act as a "bailer" to suck the case dry. On my Contender (which has a completely open top to the centre board case) you can certainly hear, and sometimes see, the suction working. Since the tape material is very stiff there is still plenty of tension in the inner edges to keep the slot closed. It would be interesting to hear other people's experience on this matter.

From other responses it seems as though the practice of using the single sheet of mylar with a slit in it for the board to poke through has a parrallel in the dacron sailcloth ones, where the rear of the case doesn't have a drain vent cut into it. 
I don't know which is best or what pitfalls there are - hopefully not a Merlin with an on board jacuzzi for the crew!

oops - should read 'the gasket at the rear of the case doesn't have a drain vent cut into it'