OK so somehow we buy a 20 year old cruzy sloop, and now its time to turbo it.
First make sure what is there is in good order.
Then made sure nothing is missing, backstay, vang, adjustable everything. If you can't control it, its unlikely to be fast.
Sails. BUDGET this in with the cost, so you can get them right away. Almost nobody sells a boat with great sails. Obvouisly need a spinnaker if you are serious about going downwind.
Folding prop
Clean bottom
Low stretch (dyneema) cored halyards and sheets.
At this point you have a chance.
Then I would look at double ending control lines (backstay, vang, foreguy) so they can work form both ends.
Double the purchase on loaded block (jib car adjusters, vang, backstay)
Now your getting tricky.. look for things that make crew work faster, easier.
Jackdaw, IMO, gave you a great answer "philosophically." I agree with his perspective on what's important. My only (minor) point of difference with him is that I'd put a higher priority on a clean, smooth bottom than the folding prop.
I organize the project by focusing on eliminating deficiencies in three key areas. The precise order in which you do the upgrades depends on your budget and the cost/benefit analysis.
Great foil shapes above and below the water (including good halyards)
Easily operated trim controls
Reduce parasitic drag above and below the waterline. Have a smooth bottom, uncluttered mast.
But before you do any "performance enhancements" Fix anything that's suspect in terms of preventing sudden failure, including standing and running rigging, thoroughly inspect all components on the mast for wear, corrosion, etc.
A) Get beautifully shaped foils above and below the water. Sails should have have excellent shape, rudder faired, keel faired
B) Replace/repair hard sail trim controls that are hard to operate. Replace stretchy halyards.
C) Reduce drag above and below the water. Clean, smooth bottom. Get rid of extra weight and gizmos aloft.
D) Reduce weight below and above. Heavy boats are slow. Clean out all the junk you don't need aboard. Sand off 20 years of old bottom paint. Avoid heavy hardware aloft - it's antiballast. It will improve stability and you'll be able to carry more sail area.
I emphatically agree with Don G that sliding genoa cars are a must have if you really want to trim actively.I can't emphasize enough making sure that you can easily use the trim controls. If it's hard to use the outhaul, cunningham, vang, genoa car, or backstay, you won't use them.
Here's an example of how I approach my newest boat. My new toy is a 1992 Corsair F24 Trimaran with no structural or safety deficiencies. She came with an okay mainsail (not great) and a baggy genoa, and old-fashioned asymm spinnaker. Low stretch mainsail halyard in worn condition. Jib halyard is new but stretchy.
I consider myself "a performance" sailor, but not a racer. I like to have the boat trimmed in perfect balance, which is always fast. I also single hand frequently, so I want certain controls accessible from the helm.
I've budgeted for all new sails (main, jib, screecher/code zero for light air close angles, cruising chute for light air deeper angles), a new bottom job, fairing the rudder, low-stretch halyards for main and jib. These items are "must-haves" for performance, in my opinion.
I'm removing/replacing all the old bottom paint and fairing the rudder. That's a must-do's for performance, in my opinion.
I'm no gorilla, so the traveller needs upgrading to higher purchase. I want to bew able to play the traveller with one hand (for gusts and driving in waves, not for rounding marks) and drive with the other hand. I want 8:1 on the outhaul (at least), accessible from the helm, or else I won't use it enough. Fine and coarse time on the mainsheet.
Since this boat is 14' wide trimaran I'll be adding barberhaulers and twings in strategic locations for fine trim control of the headsails and spinnakers when reaching.
I don't need a backstay adjuster on the Corsair. There's no backstay on the Corsair F-24, due to the fathead mainsail and huge roach, but the aft swept shrouds have high purchase tensioners to control forestay sag when needed. But I've had and used backstay adjusters on all my boats, masthead and fractional alike.
Next year she'll get a modern high aspect rudder. It's not as pressing a need for this year, and it's too expensive for this year's budget, but it will make a huge difference in the boat's performance at high speeds.
Judy B
1992 Corsair F-24 Trimaran
San Francisco Bay, CA
Sailmaker