Tune for speed

In the first of a new series of tuning tips, we talk through setting up your SB20 rig.

Audi SB20 World Championships - Day 4

Initial rig set up
While the rig is down, you need to set up the top mast shrouds (cap shrouds). Make sure that they are the same length by detaching them, and placing a screwdriver through the pin holes and adjust so they are the same tension. Re-attach them to the spreaders and gently pull the spreader down – if the V1/D3 main shroud comes tight just before the V2/D4 cap shroud then this is a good starting point.

What to do with Caps
The principle purpose of the top mast shrouds is a safety factor to hold the rig in column downwind. So we are looking to ensure they are even side-to-side and not too tight. The way to check this is by looking at them when standing in the boat with the rig set up, but sails down, and slowly pull the backstay on. The tension in them should start to release fairly quickly and at the same time/evenly on each side.

Gooseneck lowers
We make sure these are loose enough to not interfere with anything, until we get to breezy conditions where we start to use vang to depower the main. We then have them set up so that they pull tight and stop the mast moving further forward when the lower part of the mainsail is at the right level of depth.

Mast tune
This leaves us using the main shrouds and D2s as our principle rig adjusters while out sailing. This is how we initially set up the mast from scratch.

Get the mast up in the air with forestay pinned in and each shroud loosely connected. Then, taking each shroud one at a time, completely unwind the bottle screws (taking the middle turnbuckle off from each of the screws top and bottom) and then take even turns on each pair of shrouds. Once they start to tension up, use the gauge to get to base setting (28/24). So long as each pair of shrouds are equal lengths (definitely worth checking before you start this process for the first time), with even turns taken on each side you should be able to look at the rig and confirm it is straight side to side and in column.

A basic simple to use guide for shroud tensions is as follows. You should check that the turns you add below correspond on your boat. Also check your rig gauge with someone else’s. Beware of worn out grooves on your gauge!

Light       28/24     +2/2 (+2 full turns on shrouds and 2 on D2s)
Medium  31/30     +4/1 (+4 full turns on shrouds and 1 on D2s)
Heavy     36/34

The thinking behind these shroud tensions is as follows. A simplistic view is that the whole rig set up is based on a set of compromises at different wind strengths.

Light airs (before needing backstay to depower):
Generally, SB20 mainsails are very full in the middle and top panels to suit the rig profile once backstay is pulled on. In lighter air before needing backstay, they need pre-bend to get an efficient shape. This is only possible by keeping a reasonable level of V1 tension, which causes a problem with getting any luff sag to drive power in the jib, as tension in the shrouds results in a tight forestay.

The minimum V1 tension our rig can handle without going too straight is 28 on a P2 loos gauge. We then adjust the D2s to achieve the desired profile on the mainsail for the right level of depth in the mid to lower section. All mainsails are slightly different, but 24 is a good starting point. More turns on the D2 give more mainsail depth and fewer turns flatten it off.

With this setting, you can still use some backstay as you start to get overpowered, but you’ll find that you don’t have much range in adjustment before you get starvation creases in the mainsail, due to over-bending the lower section of the mast.

Medium airs (frequently overpowered and needing backstay):
At this point you need to be able to use more backstay to further flatten the mainsail, which requires more D2 tension to stop over-bending the lower rig. We move the D2s to 30, and to allow for this amount of tension without over-straightening at lower backstay tension or getting an inverse bend in the rig, we tighten the V1s to 31.

You should find that you can carry this setup into quite breezy conditions, as can use a lot of backstay when you need to lose power to really flatten the mainsail off. In the lulls, easing the backstay powers everything back up nicely, so the primary controls become backstay and mainsheet, as you’ll find the leech profile changes massively at the different extremes of the backstay position and as the wind strength changes.

Strong wind (always overpowered):
When you are continually overpowered and never releasing backstay on the 31/30 setting, you are likely to benefit from de-powering further. We move the V1 tension to 36 to tighten the jib luff and the maximum D2 we can carry at this tension without over straightening the rig is 34.

Again, backstay should be on tight before you do anything else to depower the mainsail. If you need to depower further, you can release some mainsheet to twist off the top of the leech. Next step we use is to down-track the traveller so that the car is about on the centreline and then beyond that we use some vang to flatten the lower part of the mainsail.

Next month we’ll take a look at more sail control settings for different conditions…