A new site reader and entrant to the fascinating world of radio sailing, currently racing a borrowed Britpop, asked the question,
"How do I understand a repeatable leech measurement for the IOM mainsail in various wind conditions"
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A repeatable leech measurement
On the face of it, this sounds quite an easy question to answer, just go to your setup notes and set the leech up to a measured twist (from leech to the backstay). If you setup with exactly the same mast rake, shroud tension, mast RAM position etc then a measurement will work but that will only cover one specific conditions. So how do you develop a process that will cover all conditions.
Lets start at the beginning with an admission. I don't have a mainsail leech to backstay measurement for twist but I do have a process I go through to set the right twist. First I set the boat up against all the standard Britpop numbers and start with a straight mast. If you have a Britpop you will know what these are and other designs have their own numbers. If your setup is different from the standard and are having issues, I would strongly recommend reverting to the original setup numbers.
I start by setting set the twist of the jib against a fixed number (deepest point of leech to leach line). If the wind is quite strong or gusty you may notice the jib leech open excessively. If this happens try tightening the jib luff a few mm until the leech is stable. This has two effects, first it tightens the leech of the jib and stabilises it and second pulls the mast slightly forward. Both actions will help the boat drive powerfully upwind in stronger conditions. If the jib leech is allowed to open uncontrollably then you will suffer weather helm in gusts and have trouble tacking in the top wind range for the rig. When it is right, you just have to get the boat through the wind and the sails will do the rest for you, pushing the boat off the wind onto close hauled and accelerating fast. You can help in this process by easing the sheets a tad until you are up to speed. Once I am happy with the jib, I adjust the mainsail leech twist for the run and then check it on the wind to see if it matches the jib and make any final adjustment with the kicker and mast ram.
Holding the boat on shore puts static loads on the rig which tend to be higher than when the boat is on the water and free to move. So the most important part of the process is to put the boat on the water to see if it is balanced upwind and the leeches look parallel and right. When I am happy I have the setup right, usually after several minor tweaks of the jib luff and kicker, I will find another boat to sail along side to see how my boat is performing. If I am not pointing high enough I would consider adding more kicker or mast ram. My goal in the setup is to be able to sail in a high mode off the start line when surrounded by other boats but also be able to accelerate when cracked off slightly.
When we sail, conditions vary across the race course. It may be calm at one end and windy at the other in which case you have to choose an average condition to set the boat up for that. It won't be perfect for all conditions but you will have chosen the best all round set up.
If it is choppy, you want to build power into the rig. It maybe you want a wider slot between the jib and the main so the boat can accelerate as it stops and starts in the waves. If the waves are short and stopping the boat, a high mode with twist maybe the best option, as all sailing free and trying to foot does is drive the boat to leeward and the bow deep into the waves. However if the waves are round and the boat can fly then speed is your friend and a slightly flatter, freer setup will work
After a windy series of races, or every month, it is worth while checking your rig setup in the quiet of home. Shrouds can stretch unevenly and the mast might start to lean to one side. The easiest way to observe this shore side is to set the boat up for a run and check the mainsail leech tension on each gybe. If the leech tension is different you know the mast is out of column. I use a mirror at home and once the boat is rigged with the boom in the centre I point the mast at the mirror with the bow pointing to the floor and check it is dead straight and adjust shroud tension if it is out of alignment.
One last comment on the A rig mast. With the BG sails I use, they are suited to mast bend in the upper third. If you get it right, as the wind builds and over powers the boat, not only will the sail be flatter in the upper third and backwind less but the top of the sail will blade out and invert , meaning only the bottom two thirds is powering the boat.
I hope this explaination helps. I am sure there are better sailors than me who may do this differently but from my observations over my 3 years of radio sailing, this seems to be the best way to set the boat up.
Try experimenting and if you have any questions, you know where to find me.
One final comment. None of this matters a jot if you cannot seek out the all important wind shifts. In district meeting at Chipstead, I watched one Peter Stollery sail an Isotonic in light weather and he seemed to know where every shift was coming from. It was a though he was sailing a different class of boat because he was so far ahead of the fleet. How do you develop this skill. Time on the water. About 42 years should get you up to speed with Peter.