I am trying to find out a little about the history of my Merlin 922. I have no idea what design it is. It was built by Jack Holt, I think in 1959, has wide rolled side decks, an aft deck and seems narrow even by the standards of the time.

I'm also trying to improve her rig/hull set up. How could I find the ideal rudder size? (the boat came without one, the present blade is a guess) and determine how high up the mast the spreaders should go? (they seem remarkably low, the mast has been cut down to take a more modern rig and the spreader bracket/hounds left unaltered).

The hounds would be lower (If original.) than today (No doubt you could raise them and cut new sheave boxes etc., the "I" being eaised in the early 70's to a Metric 16'?. A rule of thumb for spreaders if spreaders they be is 273rds of the wat from deck to Hounds if diamonds half way.

2/3rds!!!

922 'Capriol' is a Jack Holt designed and built Tequila and she was the second boat out of fourteen (I think) built to that design.
The 1962 year book shows her owner as Wing Cdr R.E. Havercroft, Vane Cottage, 435 Hook Road, Chessington, Surrey and Tamesis Club.
In 1963 she was owned by D.S. Scarfe, 41 Oaken Grove, Maidenhead, Berks and Cookham SC.
By 1964 she had moved again to D.W.C. Allen, 50 Ham St, Ham, Richmond, Surrey and Thames SC.
The 1967 year book shows the owner as N.G. Impey, 19 Sandy Lane, Petersham, Surrey at the same club.
In 1970 she had gone up the road to E.A. Shields of 31 Sandy Lane.Petersham & Thames SC.
From 1970 until 1994 the year books show no further changes of ownership, although there may well have been some. However the 1994 book shows the owner as Arethusa Venture Centre, Lower Upnor, Stood, Rochester, Kent.  
I am sorry but I can't help from thereon as the year books record no further changes.
Earlier this week I did similar research for Stuart Earl on another Tequila called Bluejacket number 963. You may wish to try and contact him through this forum and exchange information on the hulls,rudder and rig. 
Hope this helps.

My brother lives at Vane Cottage, 435 Hook Road now...how about that!

Before you ask...he doesnt have any boats...but he does have lots of pianos...

I believe D.W.C. Allen who owned 922 in 1964 is David Allen who's better known as Billy. By chance I had dinner with him and several other M/R oldies at Tamesis last evening.