Building an IOM rig.

The starting point for a really good rig is ensuring that you set up the mast to match the sails as far as possible. To halp you achieve this, there are some general guidelines on this page. Most boat builders supply a rigging guide with their hulls, and it is worth getting your hands on the guide for your particular boat. Prior to commencing the rig building process, make sure you have a plan to follow.



Why put pre-bend into an IOM mast? The purpose of using pre-bend is to increase the amount of forestay pressure that can be generated. Without pre-bend, the end result will be a forestay which sags.


The amount of pre-bend and the distance over which it is placed, depends on the luff curve of the mainsail. It is not dependant on the design of boat – often one hears skippers talking about the amount of pre-bend for a particular hull design, but this is not the defining factor. When ordering sails, you should ask you sailmaker how much pre-bend they recommend for the luff curve of their mainsails. The final outcome that is required is a mast curve which matches the sail’s luff curve, with good forestay pressure.


To achieve this, the following factors all need to be right:

  • The type of mast – diameter, style (groovy or round) and material

  • The amount of pre-bend

  • The distance over which the pre-bend curve is bent

  • The height of the shrouds

  • The backstay crane length

Some typical examples can be found in the rigging guides for the various hull designs – although these assume you are using sails from the hull builder. Most recommend using 11.1mm high tensile aluminium masts.


Most new masts will not be perfectly straight. Find the existing curve in the mast and use that to assist you to achieve your desired pre-bend. Pre-bend around a curve of 700mm radius works pretty well. You will need to go slowly at first, as some masts bend easily, and only require one or two bends around the jig, whereas others may take quite a few.


The top 200mm of the mast does not bend uniformly, so it is worth chopping that amount off after the bending is complete.


It is wise to pre-bend the mast prior to drilling any holes.


Make sure that the mast remains in exactly the same orientation each time you bend it around the curve, otherwise the result will look really wobbly!



If you can get your hands on one of these drilling blocks, they are worth their weight in gold! The block on the end of the mast holds the mast in the same orientation all the time and allows for the mast to be rotated through 90 degrees or 180 degrees. The drilling block ensures that the hole will finish up in the centre of the mast every time!


Follow your rigging plan to place the holes in the position you want. for the shrouds, consider drilling just a single hole in the front of the mast, rather than two holes either side. As a general guide, shroud height higher up the mast will provide better sideways control of the mast, but allow for less forestay tension. Conversely, lower shroud positions will allow you to generate better forestay tension at the expense of sideways bend.


Run your shrouds through the hole, and pull them out the top of the mast. Run both shrouds through a plastic ball, and crimp them off. Once that's done, pull the shrouds back down, so that the plastic ball sits up against the front edge of the mast. This will spread the load of the shrouds over a larger area.





The mast plug can be adjusted so the small metal arm which the top of the sail is tied to, rotates really easily. Tap a thread into the bottom of the mast plug’s smaller hole, and screw a small screw with a flattened end into the hole. Sharpen the bottom of the wire arm which swivels, so that it is a point. When the wire arm rests on the screw, it will rotate easily. The method is shown in the Obsession rigging guide from Craig Smith.


The mast crane should be long enough so that the backstay only just clears the leech of the sail. If the crane is too long, it will put too much mast bend into the top of the mast when you pull on backstay tension. A shorter "lever" is better.






Many skippers have their spreaders too long. The spreaders should deflect the shrouds by only 10 to 12mm at most. When the rig is loaded up, the windward shroud will be under much more tension that the leeward one. Longer spreaders will have the effect of pushing the mast to leeward at that point when the shrouds are loaded up. The result is that the jib slot closes and the mainsail backwinds when under pressure.


The outer end of the spreaders should not snag the jib topping lift. The method is shown nicely in the Obsession Rigging guide drawn by Craig Smith


In addition, you may wish to retain the jib topping lift with some elastic at the bottom, so that if the jib boom lifts, the topping lift remains taut.



The invention of variable geometry goosenecks has been an important development in ensuring more correct mainsail leech angles at all points of sailing. There are a number of these around – notably the goosenecks from AA Parts and from David Potter at Ullman Sails. Fitting these to the mast requires accuracy, so that the shaft of the gooseneck is exactly parallel to the mast when viewed from astern.


The small locking screw at the bottom of the gooseneck allows the bottom end of the gooseneck barrel to move aft when the main boom is sheeted in. This screw can be adjusted independently of the boom vang, so that mainsail leech angles can be set up for upwind and downwind independently of one another.


If using a parallel gooseneck - for example a ball raced gooseneck from Radio Yacht Supplies Australia (Bantock), then you can achieve the same effect by wrapping sticky back sail cloth three times around the mast at the bottom of the gooseneck. This offsets the axis of the gooseneck about the right amount.


when you attach the downhaul on the mainsail, it should pull to a point aft of the axis of rotation of the main boom. In this alignment, the downhaul will become tighter when the boom sheets in, and will loosen when you sheet out.



The aim is to get your jib boom as low to the deck as practical. If the jib boom is too high off the deck, when a gust hits, the jib boom will move to leeward first and forestay sag will develop, resulting in loss of pointing ability and lee helm.


There is a good knot that you can use to attach string to the jib boom. There is a You Tube video which shows this knot, called “How to tie a “tonys knot” - a great video from Tony Gonsalves.


The placement of the pivot point is critical to the functioning of the jib. Too far forward, and you won’t be able to generate sufficient forestay tension. Too far aft, and the jib boom will not lift when a gust hits, and the angle of attack of the jib will be too wide, reducing pointing ability. As a guide, the jib boom should lift with about 320 to 350 grams of pressure.



If your jib boom is low enough, you will find that the jib counterweight may hit the foredeck of your boat. To counteract this, try placing two right angled bends in the shaft of the counterweight, so that the weight sits higher than the line of the jib boom.


The first bend sends the shaft upwards and the second bend returns the counterweight to a direction parallel to the jib boom again. You will need to tie this in place, so that the counterweight cannot rotate downwards.


When setting up any bowsie, try to have the bowsie pull in the same direction as the control you are pulling when you are tightening the string. This is especially important on the backstay. Try to get the bowsie puling downwards when the backstay is being tightened. It makes it much easier to get adjustments right.


Bowsie threader – you can make a simple bowsie threader which will save you hours of time and frustration by simply bending thin wire over in two. Push the bend through the bowsie hole, and thread the string through the loop in the piece of wire. Pull the wire back through the bowsie hole.


A Very Useful Knot

You can watch a short video about how to tie a really useful knot by clicking on the "Play" button.....................


Instructions about how the knot can be made as follows:

  • Pass the piece of string through the hole that you want to tie to.

  • Tie a double overhand knot in the end of the string. You can burn the end.

  • Using the double overhand knot end of the string, make a single overhand knot around the string that passes through the fitting or loop. Pull tight. This knot will not undo unless you want it to, and will hold tighter the more pressure placed on the string.