Rig variables

People talk about investing large amounts of money in acquiring the latest design, eg VISS. V12, Venti, Britpop etc, you only have to look at facebook to see the shiny new models on display. But how many invest the equivelent money or time in getting the engine of the boat just right. The latest hull with a poor rig will not deliver the expected results. The following two articles are designed to help build and set a winning rig.

This article was inspired by a comment from the PDT during a discussion on rig setup. It describes how each variable affects your rig. Only by understanding the impact of each variable will you master getting the rig set up right. Remember this is my view and may not always be 100% correct but will give you a head start..

In the next article I will describe my rig setup and how I fix the majority of the variables so tuning is relatively straight forward.

What you need to know

• The designers guide should be your goto starting point for tuning the rig..

• If you go it alone there are approximately 21 variables to play with in rig setup.

• Each variable has a role which I will try to explain and then offer some advice.

The Detail

If you are new to IOM racing or have been racing for a while but not yet unlocked the secret of good rig setup, my advice would be to first seek out the designers guide for your boat and find out how that works out before making any deviation from the guide. 

If you want to go your own way, first consider that there are 21 or so variables that are on an IOM rig to play with.  Each one can make or destroy a good set up. It is no wonder these boats are so hard to tune properly.

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 what do all these things do and how do they affect tuning.

Pre- fixed variables

Prebend, how much and how far down the mast

Prebend is the amount of bend you build into your mast.  The amount bend can be anywhere from 10mm to 25 or so mm and this will be evenly spread over the top 600 to 850mm from the top of the mast.  The aim of the bend is to provide force when the mast is straightened by the backstay which generates jib luff tension.

Height of hounds

The hound is where the shroud meets the mast (usually with a hook) and unless you have the latest Sailboat RC VISS, your hounds will be fixed.  Because the hounds are attached to the front of the mast, they provide a force (in conjunction with the spreaders of pulling the middle third of the mast back.  If you squeeze your shrouds and look at the upper end of the mast, you can see the reverse bend effect.  I have not played with hound height so am unable to comment on the effect of moving it up or down.

My guess is that if you move the hounds up you stiffen the mast higher up but decrease jib luff tension. On the VISS, there is a very clever fitting where the shrouds meet the mast which allow the shrouds to run all the way to a termination point above the upper band. The hound can be moved up and down by loosening a grub screw. Running the shrouds up the front of the mast should make the top of the mast much stiffer and allow for more jib luff tension as the backstay will not bend the top of the mast.

Height of jib luff attachment to mast

Generally, the jib luff is attached by a hook a few mmm down from the upper band on the mast.  The only time I have lowered the hook is to get the jib boom parallel to the deck.  Lower the jib luff attachment and the back of the jib boom gets lower.  You may affect the mast bend slightly, but it is negligible.

Spreader height

The spreaders do two things.  First function to support the mast sideways and keep it in column.  Second function is to restrict or encourage fore and aft bend by raking the spreaders back or forward by a few degrees. Raising or lowering the spreaders will affect the mast ben in the middl third.

Spreader length

Shortening the spreaders will allow the mast to bend sideways which has the effect of opening the slot between the jib and the main.  I experiment shortening the spreaders to 45mm, but I think the side bend was too extreme. You can also rake the spreaders fore and aft which increases or decreases mast bend.

Position of jib attachment on jib boom

This has a couple of affects.  The further forward you make the attachment point, the more sail area you will present to windward of the mainsail downwind.   The downside of this is that the further forward the attachment point, the weaker the tension in the leach of the jib.  i.e., if you hit a gust and the attachment point is too far forward, the leach will open and the boat will not accelerate properly.  I am experimenting with moving the attachment point forward in light weather and moving it back as the wind increases.

Piece of sail material under the bottom of the gooseneck

This is a vital piece of kit if you want to maintain mainsal leech tension when running. If your gooseneck is screwed flat to the mast you will be able to tighten the kicker and control the leech upwind, but when you bear away downwind the leech will become too slack. The bottom of the kicker needs to be moved away from the mast by a tiny amount, usally a small square of patch material. There are some kickers on the market that have a grub screw which provides the adjustment necessary to get the leech tension just right.

Other Variables

Mast rake

Probably the most critical setting of all.  The mast rake will determine if your boat has neutral helm, weather helm (too much rake), or lee helm, too little rake.

Backstay tension

The backstay will affect the upper third of the mast helping provide jib luff tension and controlling the fullness of the mainsail in that area.  As a rule the mast should be straightish in light weather and for max power, and the as the wind increases a few mm backstay added to flatten the sail in the upper third.

Mast bend

There are several things that affect mast bend.  Split the mast into 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

Shroud tension

See above.  As a rule, light tension in light weather increasing to support the mast as the wind builds

Mast ram position

See mast bend

Main foot depth

This is the depth from the side of the main boom to the foot of the sail.  It is usual to have this around 15mm although it will vary depending on the sailmaker.  As the wind increases this depth can be reduced to flatten the sail

Kicker tension

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

Main boom angle

Usually there will be a gap of 10mm or so from the mainsheet post to the side of the A rig main boom.  This distance is increased for the B and C rigs. Using mixing, one can sheet inside these measurement for a high mode.

Cunningham tension

The main purpose is to remove creases in the luff of the main.  If tensioned excessively it will open the leach of the mainsail a bit.  Always remember to release this after sailing otherwise you will damage your sail

Main luff wire tension

If you have a luff wire, set enough tension so the sail does not scallop, i.e. blow away from the mast between sail ties.

Jib luff tension

This is set using the backstay and shroud tension.  One can use light tension in light weather and increase as the breeze picks up.

Jib boom angle

This will vary depending on your rig.  If you do not have a set up guide, just over three fingers between the mast and jib boom should suffice unless you have fingers the size of fish fingers.

Jib Foot depth

This is the distance between the side of the boom and the sail and should be somewhere between 15 and 15mm.

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.  After this I put the boat on the water and watch how it sails and adjust each accordingly to balance the boat and how the sails look.

The Advice

With experience, setting up a rig is easy but it takes a long time to fully understand how everything interacts.  Always start with any guide you have and experiment from there.  It is less complicated than it sounds and you simplify matters by fixing and documenting as many variables as you can so you can replicate a set up next time you go sailing .