On Water Setup and Tuning


  • Bending the mast is the main mechanism to flatten a mainsail.
  • Its not just bending the mast that matters. It needs to bend by the right amount, in the right places at the right time.
  • The effect of rig tension on mast bend will depend on spreader configurations AND the relative position of the shroud bases.
  • The mast needs to bend to suit the luff round cut into the mainsail for the sail to perform as intended.
  • As wind increases we aim to flatten the top of a sail first so we should bend the top of the mast first.

All boats have some controls that allow you to flatten the sails. In the case of the 5O5, the Class rules allow you to pretty much have anything you like so the 5O5 sailor has more control over their sail shape and therefore power than most boats without needing to change rigs or sails. This is why the 5O5 can be sailed effectively in all conditions and with large variations in crew weight. The 5O5 is a very powerful boat that heavy teams can sail but the controls allow a lot of control over the power so lighter teams can also be competitive.

For newcomers or less experienced sailors the number of controls we have on a 5O5 to change sail shape can be daunting and it takes time to work out what each one does and which is best to use in any given circumstance.

Tuning grids will help but you will sail faster if you can learn more about what the grid is trying to achieve. That means getting used to what your sails should look like and what controls will have what effect.


A mast ram or chocks at deck level are used to control the amount of bend in the mast down low (and therefore how full or flat the mainsail is down low). More ram down or more chocks will straighten the bottom of the mast.

When a mast is raked aft, the ram will generally need to be adjusted as well to maintain the same prebend profile. In fact if you don’t adjust a mast ram down when you rake and this allows the bottom of the mast to bend, the result is that the mast gets straighter higher up. This may flatten the mainsail a little down low but it makes it fuller up higher which is the opposite of what you need to do.

Don’t ease the ram to depower. It may flatten the bottom of a mainsail but it makes the sail fuller higher up.

Pull ram on when you rake to stop the mast bending too much down low.

Try it on shore. With the boat on its gunwale, pull rig tension on so the mast bends and look from the mast tip to the deck when someone adjusts the ram for you. Observe what happens to the ‘bubble’ where the most bend is. PHOTO


Also referred to as rig tension, it serves two key purposes.

By the compression[1] it puts on the spreaders, rig tension controls how much the mast bends in a fore and aft plane (and therefore how flat the mainsail is). More rig tension will generally bend the mast more at the area around the spreader bracket. picture

Because the spreaders also support the mast sideways, more rig tension will also support the mast more in the middle and stop the mast bending sideways as much. (need a section on side bend. Work in Progress)

More rig tension will generally bend the mast more and therefore flatten the mainsail more.

More rig tension also tightens the forestay and reduces sag in the luff of the Jib. 


If you pull it hard enough Cunningham also bends the mast and therefore flattens the mainsail. When Cunningham is tightened it pulls the luff curve cut into the mainsail into a straight line and effectively eliminates that bit of the sail… THIS NEEDS WORK? And a photo


Pulling the main foot outhaul on will flatten the bottom panel but does little to change anything in the rest of the sail. (is this true?) However reducing the depth of the mainsail in the foot area can allow better flow of air through the jib/main slot. A bit more here?


The primary purpose of the vang is to control the amount of twist in the mainsail leach. However, particularly with masts rigged like those in a 5O5 ( a lot of unsupported mast above the hounds), the vang also bends the mast. If the mast ram is  stopping the mast bending down low and there is sufficient rig tension to control mast bend at the spreader band, then the vang will bend the top of the mast and therefore flatten the top part of the mainsail. More vang may flatten out the mainsail to depower but it also reduces twist which can be counterproductive.   See article TO TWIST OR NOT TO TWIST. Work in Progress.


Despite the fact that many boats rake their mast as it gets windier, mast rake itself does little to depower a boat.

Flatter and or more twisted sails depower a boat. Not Rake.

The main reason we rake is to re-balance the boat as wind increases and we flatten the sails. We tend to flatten a mainsail relatively more than the jib. So we progressively reduce the power in the mainsail proportionally a lot more than we reduce the jib power.  We therefore need to rake the mast so the whole sail plan moves aft and re-balances the boat to compensate for the relatively greater reduction in power in the back half of the rig.

In boats like a 505, when the mast is raked, the jib sheeting angle changes to pull more along the foot than down the leach. This is the equivalent of moving the jib sheet leads aft in boats that do not have rake as an on-water control (mainly yachts). This change in jib sheet angle flattens the bottom of the jib and allows the top leach of the jib to twist more – effectively reducing power in the jib.  See more under Jib Sheeting.

Another effect of rake is that the mast o