The latest tuning numbers for your SB20 by Rob Andrews

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It’s great to be back working on the SB20 and I will try not to call it an SB3! That is the boat that I spent a year sailing, modifying and racing with great input from Tony Castro the designer, sail makers and mast builders as well as lots of sailors and fitting manufacturers.

The development was a fascinating process, the prototype had a keel case where we could chock the keel fore and aft to change the balance, we cut down the mast and almost every time we changed a sail the kite got bigger! The aim was always to have a close tactical upwind boat that could sail in most conditions, and when you turned the upwind mark it also turned on the smiles, as she was easy to handle and drive fast downwind. Looking at the boats today, they are remarkably similar to the boat I last sailed and raced, albeit with many small significant changes that have refined the boat and made her suit the type of racing that people want in 2018.

As part of my induction back into the SB20 I decided to look again at the tuning numbers and see how this has evolved over the years since I was last involved. A quick trawl on the web and I was shocked to see that many of the old tuning guides pop up when you use Google searches. The best guide was found in a newsletter in April 2014 and with a few quick conversations with the current World Champion, I felt back up to speed. Below is the guide that I referred to above and the additions are a quick explanation as to the rigging terminology and also the recent change towards using more pre-bend, particularly when using new mainsails.

TERMINOLOGY

  • V1/D3 – Main shroud
  • V2/D4 – Cap shrouds or uppers, (top mast shrouds)
  • V1 – Lower shrouds
  • V2-2nd – set of lower

SETTING UP THE V2/D4

The V2/D4 are probably the best way to induce some additional pre-bend, particularly further up the rig which will slightly flatten the main towards the head and especially helps if the mainsail is brand new.

Historically the set-up was with the mast down concentrating on the relative tension between the V2/D4 and the V1/D3. This was checked by pushing the spreader down towards the heel of the mast and looking at the tension of the V1/D3 and V2/D4. Historically we expected the two V1/D3 and V2/D4 to both have similar tensions. Now by running slightly tighter V2/D4 you see that as you push down on the spreader the V1/D3 goes slacker before the V2/D4. By having more tension in the V2/D4 as you use the rig tension numbers for each wind speed below, the shorter V2/D4 generates more pre-bend in the upper section of the mast. This gives you smoother, greater pre-bend up the entire rig.

In future months I hope to post some videos of this, particularly as I get my head around all the rig set up changes that now make you fast in an SB20.

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, placing a screwdriver through the pinholes 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 ways to check this is by looking at them when standing in the boat with the rig set up and 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 so as not to 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 D2’s 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 the 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 takes a simplistic view in that the whole rig set-up is based on a set of compromises at different wind strengths.

Light airs (before needing backstay to de-power)

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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 D2’s 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 D2’s 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 V1’s to 31.

You should find you can carry this set-up into quite breezy conditions, as you 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 at 34.

Again, backstay should be on tight before you do anything else to de-power the mainsail. If you need to de-power further, you can release some mainsheet to twist off the top of the leech. The 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.

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