I recently acquired No.3101 (Storm 10)- I am puzzled by the mast on it which seems to be 22ft 9ins long (hog stepped, Proctor), seems a strange length for a Merlin? The latest Measurement Certificate shows the mainsail luff as 5715mm. The mast inverts when rig tension is put on (it is stepped as far forward as possible in the foot, and the spreaders are as swept back/extended as possible, which is to say not very much!).  Possibly the mast is intended for another class?

Ray your luff equates to 18'9" in old money! which should give you 4'from gooseneck to hog, which doesent sound to much off the mark. The invertion is probably due to the forestay being to short many boats of this age had multihole rigging plates fitted at the head of the jib connecting to the forestay. Without it the forestay is to short which has the effect of inverting the mast under load. There are some good articles elsewhere on this site but as a base trim you need to set the mast up with about 100mm rake aft at the gooseneck with about 20/25mm prebend.At this state the spreaders are likely to be deflecting 10/15mm forewards which will vary due to there height positioning. If your boat didn't come with a spare rigging plate in it's bag of bits you can get an idea of whats required by using a piece of good line (spectra/kevlar) on the jib tack to give the effect of lengthening the forestay, showing the increase required, to try out the idea, Please dont be tempted to sail it like that! I'm sure others who may live closer & have similar age boats may be able to help further on the actual mast measurments all the best Barry. (x sailmaker)

Much appreciated Barry, hadn't occurred to me but thinking about it your explanation is spot-on.