Here are some thoughts on the rig set up on an IOM. Before you say that I am overthinking this, my goal is to find a single rig setup that will always be my starting point prior to racing which will give me great all round performance and can be tweaked slightly for changing conditions. Pictures below.
Articles on setting up the rig on an IOM may talk about how the mast control is split into 3 sections. The bottom third is controlled by the deck chock, the middle third is controlled by the spreaders and the rig tension and the top third is controlled by backstay tension. That is the easy bit. The hard bit is to determine what your initial racing setup should be based on your understanding of the mast bend/mainsail relationship. The experts are able to set their boats up almost by feel. They have done it so many times before and know what works and what doesn't work and have good reliable base settings.
To learn about the impact of different adjustments, I like to experiment by laying the boat with the mast horizontal. Bear in mind I use BG sails which has luff curve built in so giving great flexibility in the way you shape the sail and these are fitted to a PG mast (13mm prebend) on a Britpop. My goal in this exercise is to understand how the mast bend works given different settings from one extreme to another. Once I have been through this exercise, I identify one mid range setting, mark everything up to this setting and this will be my start point for racing. If there is more or less wind, I will make mm adjustments to the backstay, and maybe shroud tension.
The first thing to do is set the mast up with relatively slack shrouds for light weather. The leeward shroud must not go soft when on the wind and then apply enough backstay so the mast is as straight as possible even through the deck. You should have a couple mm of rake on your spreaders. This is the starting point for light weather.
Now if you apply more backstay you begin to de-power the rig, the mast will bend evenly but more in in the central third. If you keep the shroud tension the same and apply more backstay, the main will end up flat at the middle/ bottom third and be too full at the top and may even break down in the middle, if you overdue the backstay tension. Our goal is to keep the power low down and blade out or flatten the top.
As the wind builds above an imaginary 5 knots or so, gradually stiffen the mast low down by applying a little more rig tension and add a little bit of reverse bend at the deck with the mast chock. Remember the rig tension will affect the middle third of the mast. As you stiffen the bottom and middle third of the mast, you encourage more bend at the top which will help blade out the sail effectively depowering the rig.
The rig tension is gradually increased through the wind range, 0-5, 5-10,10-15. At the top end of the A rig you need max rig tension to get as much bend as possible in the top third of the mast so you blade out the top of the sail. You should end up with slight reverse bend at the deck level, an even bend through the mast with slightly more induced at the top. If it all works out when sailing upwind, your main should fill evenly top to bottom.
The goal of this exercise is to develop an understanding of how the rig works not to . develop a myriad of settings for different wind ranges.
Once you have tried this indoors and understood how everything fits together, find a mid point setting (5 to 10 knots) and test it out on the water. If you are happy with what you see, mark everything up so you can repeat the setup when you next rig your boat. The usual rules apply for rig set up, set the kicker for downwind, use the backstay for upwind leach tension, make the jib slot parallel to the main as well as using the BG measurement for distance from leach line to jib leach and main and jib foot depth.
From this setup you will only need to apply mm adjustments to the backstay, and maybe a slight increase of decrease on shroud tension.
When I set up, I have marked positions for the shroud bottle screws, sheet hooks, sail foot depths, mast rake and leach twist distances from the topping lift, sheeting angle for the main and jib. With these as a starting point, I know I have a reliable setup and if I need to adjust anything by more then a few mm, something is wrong in my initial setup. Now I am set for the season.
I hope article helps you to understand your rig a little better.
Some pictures of the extreme setting to see their impact on the mainsail
1 The shrouds are as slack as they can be with a straight mast
The effect of too much backstay with little rig tensions. The sail has inverted at the numbers and the top of the mast is still relatively straight at the top third.
The same setting as above only I have Increased the shroud tension to the max. Note how the middle has straightened out.
Max rig tension with just too much backstay. Note the bend now runs evenly through the mast and through the top third of the mast. You can see how the top has flattened towards the luff of the sail . The leech down to the IOM sign looks distorted but this is a trick of the image. My goto setting will be this with 1-2 turns of the shroud bottle screws and slightly less backstay to get the mast straighter and the middle luff of the main back into shape.