Limiting the variables

So in the last post I wrote down all the variables that are set when you build the rig

Prebend, how much and how far down the mast (two variables really)

Height of hounds

Height of jib luff attachment to mast

Spreader height

Spreader length

Position of jib attachment on jib boom

Piece of sail material under the bottom of the gooseneck

Other variables

Mast rake

Backstay tension

Mast bend

Shroud tension

Main twist

Mast ram position

Kicker tension

Main boom angle

Main foot depth

Cunningham tension

Main luff wire tension

Jib luff tension

Jib leech twist

Jib boom angle

Foot depth

So how do we manage all of these things to get the perfect looking rig

First I try to fix as many of the variables as possible. Before I start the setup, I make sure the backstay crane points to the stern or the boat and the main halyard swivel is aligned in the same direction.

Variables that I consider fixed

Prebend, how much and how far down the mast.

On my current A rig I have 16mm prebend running from the top of the mast to 800mm down. I have to confess I got someone who was a regular rig builder to prebend the spar. He did a far better job than me. My B rig has 8mm prebend down to 400mm from the top

Height of hounds, jib luff attachment, spreader height

I started out life with an alternative plan from BG Sails and design and still use the mast measurements from that design, except I have a different kicker and gooseneck position and the mast step to top of boom band is 135mm

Spreader length

I have experimented with various lengths of spreader, some too short or too long and have settled at 50mm for the A rig and 45 for the B rig. My shroud base is approximatley 160mm

Position of jib attachment on jib boom

On the A rig I set this at 80mm back from where the jib wire meets the boom and will experiment moving it forward in light weather to present more jib to windward when running downwind.

Piece of sail material under the bottom of the gooseneck

I use one square of hatch cover material with backing paper at the bottom of the mast. This will tighten the leech of the main when running.

Mast rake

I set my mast rake so the booms are parallel to the deck. The boat is balanced with this set up. The mast rake is set using the forestay and backstay. Some designs will have a measurement for the mast head to the stern or other similar measurement.

Shroud tension

I use a maximum of 18 on the John Gill tension meter. I will use less tension in light weather. When you think the shrouds are sorted sheet the sails in and lay the boat on the ground and look down the mast. If it is not straight you will need to loosen one shroud and tighten the other until the mast is straight. Remember to check the tension after you do this.

Main foot depth

I set this at 15mm from the side of the boom to the sail for the A and B rig. I may flatten the sails slightly in stronger winds.

Main boom angle

I set 10mm for A rig, 15 for B rig and 20 for C rig and then use mixing for fine tuning which allows me to bring the main boom nearer the centre line.

Cunningham tension

I apply enough tension to remove any creases, not so much to affect the shape of the sail

Mast ram position

A rig - I apply enough ram to create a slight reverse bend at deck level. For the B and C rig I look for a smooth curve through the mast.

Main luff wire tension

I apply enough tension so the sail does not scallop, i.e. blow away from the mast between sail ties.

Jib boom angle

I use just over three fingers between the mast and jib boom. I make sure there is enough slot room so that when I sheet in using the mixing I do not choke the slot.

Jib Foot depth

This is the distance between the side of the boom and the sail and should be somewhere between 15 and 25mm. The lighter the wind the deeper the sail.

So I have fixed 16 of the 21 variables and use those setting every time I go sailing. I may tweak on or two of them for different conditions but they are my goto settings.

Now we come to the tricky bit, which requires some experience and a careful eye

Jib luff tension

Jib luff tension which is driven by shroud tension and back stay assuming I have set the mast ram and spreaders as suggested above. I start by fixing the jib luff bowsie at a setting which will ensure when back stay is applied, I have the right mast rake to get the booms parallel to the deck.

Mast bend

If the weather is light I apply enough back stay to bend the mast to take out some of the luff curve built into the sail to achieve and cord curve with the maximum depth 40% aft in the middle of the sail. With looser shrouds you need less pressure on the backstay which allows everything to move freely in light winds. If you overtighten the shrouds, the middle of the mast will be too stiff and may invert. As the wind increase I increase shroud tension and backstay

Remember to think about the mast in thirds.  The bend in the top third is controlled by the back stay.  The middle section is controlled by rig tension and spreader angle and the bottom is controlled by the mast ram

Kicker tension

The kicker tension will control the amount of twist in the mainsail which I cover below 

The above are relatively easy to master. The real art is in setting the twist for the main and jib.

Jib leech twist, Main twist

I normally set the jib twist first and then set the main twist to balance the boat.  Set the jib twist (distance between the leech line and the sail) according to the sailmakers advice).  Then I try and set the main twist to match the jib twist on the shore using the kicker.  I look from behind below and above to check all looks good. After this I put the boat on the water and watch how it sails and adjust main twist to balance the boat. If I see the jib leech open excesively in gusts, I will tighten the jib luff which will firm up the jib leech.

And that is all there is to it. At the very least after reading all of this you will understand what each component of the rig does.

The pictures below are from 8/01/2025 at Burfield in 3 to 8mph of breeze. The sails are BG Sails and Design which look perfect set on a PG mast. The hull design is Proteus with a Craig Smith fin and bulb and John Gill rudder.