There are 4 different types of bend we look for in setting up a mast.

• Mast Rake.
  This is not technically bend, but is often misunderstood as bend.  Mast Rake is the angle aft that your mast sits compared to the waterline.   Without bending the spar itself and the relationship to the step of the mast.  You can have a lot of rake with a very straight mast.
• Pre-Bend.  This is the amount of bend in your mast from the gooseneck to the masthead measured from a straight line.  This is measured without sails up and backstay lose.
• Max Bend.  This is the amount of mast bend you get when you put the backstay all the way on.  We measure bend while sailing as a percentage of pre-bend versus max-bend.
• Side Bend or Sag. Sag is the amount your mast sags to leeward when sailing.  In some cases, you have middle sag.  This means that the middle of the mast goes to leeward and the top goes to weather.  Other cases you have tip sag.  This is when the top of the mast sags to leeward.

Using these definitions I will explain what each one does to the sail shape and boat performance.

Mast Rake effects the center of effort for the sail plan over the keel.  Generally the more rake you have the more weather helm you have.  In boats that sail with heal, it is usually faster upwind to have more rake in light air and less in heavy.  This is based on the amount of helm you can feel in each condition.  Many one design boats have a maximum forestay length for controlling rake and it usually is fastest to be at the maximum number.

Pre-Bend is difficult to generalize because on some boats the straighter the mast the better and in others, they carry a lot of pre-bend.  This has a lot to do with the cut of the sails and the amount of forestay tension your boat needs to perform.  With a lot of prebend it is usually a little harder to control the sail shape.  This usually requires check stays and other mast controls.

Generally you want to set prebend up so the mainsail is at maximum fullness without any backstay on.  This will be your light air set up.  This is controlled by mast butt position along with mast position at the deck and spreader angle along with shroud tension.

Max Bend.  Max Bend is when you have all of your backstay on and you are trying to flatten the main.  When a sail is cut correctly you will be all the way flat at the max bend.  If the sail has too little luff curve you will get overbend wrinkles (wrinkles from clew to middle of the mast).  If you get overbend wrinkles you can add length to your backstay, start with less pre-bend, or have the sail re-cut with more luff curve.

Sideways Bend or Sag.  This is one of the most important and most overlooked tunning that can be done to most rigs.  Many one-design classes have tuning guides written usually by the sailmakers.  These guides give shroud tensions for different wind velocity.  What this controls are the sideways bend of the rig and sometimes the forestay tension.

Many sloop rigged boats use the spreaders in light winds to create “negative” side bend, i.e. to push the middle of the mast to leeward. This introduces a negative twist in the middle of the sail (the mid-section are at a higher angle to the wind) and closes the slot – a very efficient way to add power in the light stuff.

There are a couple of things that happen with mast sag.  When the middle of the mast sags to leeward it powers up the mainsail and reduces twist in the leech.

When the tip sags to leeward it does the opposite.  This forces the middle of the mast to windward, As the middle of the mast moves to windward, the slot between the main and the jib is opened, de-powering the rig efficiently.

the main the leech opens up

By sailing upwind and looking at the rig you can see if the mast is doing what you want it to do.  When you take the sails down check your rig tension and log these numbers.  This way you can duplicate the setting the next time you have the same conditions.

Chuck Skewes

Ullman Sails