If you read any book on racing, you will likely see reference to high mode and low mode. Terry Hutchinson of the recent "American Magic" AC campaign sums it up nicely here:
But how do we apply this to radio sailing.
What are the two modes
High mode is where you are able to claw away from leeward boats with only a tiny reduction in boat speed. When in the groove, a boat will magically move its way to windward away from other boats but it does require practice. You use this mode when:
getting off a start line to maintain your lane
starting at the pin end of the line, you can squeeze the boats above you so they fall back and give you freedom to tack
squeezing up to keep on the lay line at a windward mark
getting away from a leeward mark when you have boats in front of you.
Low mode is where you put your bow down a couple of degrees to sail fast
You can use this mode when:
trying to get clean air off a start line if you have boats in front of you and tacking is not an option
Using the extra speed to sail fast toward a heading breeze
Using the extra speed to drive over boats off a start line where you have established a gap to leeward
coming out of a tack to accelerate up to speed.
On a dinghy or yacht you are able to tweak the rig to change modes.
For low mode you might increase the twist, slide the traveller down an inch or two, ease the jib slightly, ease the backstay to generate power. For high mode you will pull the traveller up to windward keeping the boom central but maintaining twist up the sail. Closing the leech of the mainsail or jib will make you feel as though you can point higher but it will kill boat speed and drive the boat sideways as you slow. Often you see people who pinch sail like this, They point high but end up sailing low.
How do you set modes on an IOM
There are only two controls, the rudder and the sheet. Assume you set up your A rig according to the bible of BG Sails and Design, your main boom will be 10mm off the centreline for your standard set up, with the main twisting so the top batten is parallel to the centreline of the boat.
To get into high mode you need the ability to bring the boom towards the centre but must maintain the twist in the sail. To determine how much to bring the boom in, test sail against another boat. I sail on a flukey pond with flat water and often sail with the boom centred when I hit puffs of breeze. Your mainsheet post must be set up so that the top is only a couple of mm below the boom when the boom is centred. This ensures that if you centre the sheet, there is no downward component of pull tightening the leech of the mainsail. The jib boom will also move a few mm towards the centre. Have a care when setting up the leech tension of the jib before the start. If the leech is set a fraction tight, when you go into high mode the slot between the main and jib will close and will kill your speed. If you sail with tell tales on the jib, you want to maintain flow on both sides of the sail. If the windward tell tale drops, you need to push the bow down to keep the flow and speed maintained.
Sailing in high mode requires lots of practice. The changes are subtle and your steering will need to be very precise or you will rapidly lose speed and therefore the height you were trying to gain in the first place.
For low mode, you push the bow down a degree or two and ease the sheet a few mm. Again you will have to test how much you ease the sail and push the bow down by sailing against another boat.
When using modes in fleet racing, your powers of observation will always be your best feedback to determine if your changes are effective.
One final point about high mode and low mode. When sailing in choppy water high mode is difficult to achieve as the waves are trying to slow the boat. One steering mistake and your boat could end up in irons. Far better to sail fast and free.
It is only worth using the modes when you need them. Your boat will have an optimal setup for maximum speed and height upwind. Far better to stick to that setting wherever possible.
"It is not the strongest or the most intelligent who will survive but those who can best manage change." ~Charles Darwin