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 obviously moves aft. Since this includes the spreader tips, the effective spreader length of the spreaders increase so they push on the mast harder and cause the mast to bend more at the spreader height. This can cause the mast to bend more which will flatten the mainsail (although we would have to tighten the shrouds to maintain rig tension for the mast to bend). Boats with adjustable shroud bases (picture) can move the shroud bases aft as well as rake to stop the spreaders causing too much bend in the middle of the mast (why is this bad? See Where It All Goes Wrong)

Of course when the mast is raked the rig tension is less and the side stays become loose which can mean there is less load on the spreaders and an opposite tendency for the mast to bend less. VIDEO and diagram of spreader triangle superimposed on chain plates. So when the mast is raked we generally have to also tighten the shrouds if we want to keep the same rig tension.

Rake also has the effect of allowing the mast to bend low down if the mast ram is not adjusted proportionally.

Raking the mast effectively changes the settings for a whole range of other controls and changes the Prebend  in the mast which may result in the bend no longer matching the luff round cut into the mainsail.

So there are a number of other adjustments that generally must also be made if you want to keep the Pre-bend constant.

Done properly the prebend in the mast as we rake can be kept fairly constant to suit the luff round built into the mainsail.

The Tuning Grids attempt to give you the position these other controls need to be set at to maintain the proper mast bend and therefore the effectiveness of the sail. It is true we will want to make small changes to the mast bend to power the mainsail up and down (make it flatter or deeper) but the changes that would happen when we rake without the corresponding adjustments in ram, rig tension, etc would be far more than required and generally distort the sail beyond its design capacity.

[1] See section on what spreaders do


  • The harder you pull on the jib sheet the flatter the sail will become. If you pull the jib sheet hard enough you will pull the shape out of a jib completely and creases will appear. If you ease the jib sheet the sail becomes fuller.
  • What part of the jib is affected most when the jib sheet is pulled or eased depends on the Barber Hauler lead. If the lead is aft (or up) the sheet is pulling more along the foot of the jib and you will flatten out the bottom of the jib as you sheet harder. If the barber hauler lead is forward or down the sheeting angel pulls more down the leach and the jib will tend to be fuller for the same jib tension.

Typical Barber Hauler lead on a 5O5. When pulled down the sheeting angle moves forward so sail becomes fuller for the same sheet tension.

  • If the leach tell-tale is not streaming most of the time, the jib leach is too tight and the jib should be eased. If this means the luff tell tales don’t stream as they should (ie the top one breaks too early) the Barber Hauler will also need adjusting.
  • If the jib is too full, move the jib barber hauler up or back. You will now be able to pull the jib sheet on harder before closing the leach too much and causing the leach tell-tale to stall. The jib will now be flatter.
  • If the jib is too flat, move the barber hauler down or forward. You will now have to sheet less hard to close the leach so the sail will be fuller.
  • A jib sheet can be temporarily eased a little in gusts to allow the jib leach to twist more and spill power. HOWEVER the ease must be only a few millimetres. Anymore and the bottom of the jib gets fuller and therefore more powerful which is the opposite of what you want in the gusts.
  • Allowing a sail to twist more will depower it. A well set up boat should have the jib twist matching the mainsail twist. If you allow the mainsail to twist more you should also allow the jib to twist more. Ease the jib sheet to allow the jib to twist more. However, if that makes the jib too full, move the barber hauler aft or up and sheet the jib on harder. This will flatten the sail and allow more twist.

Note that the jib twist is parallel to the mainsail so bot sails work well together.

Important Note.

When we adjust for very windy conditions by raking the mast, the whole set up of the jib changes. Here are some crucial things to look out for and fix when you rake the mast.

  • As you rake, the jib sheeting angle changes a lot. The further you rake, the more the jib sheeting angle is along the foot and the more the jib leach will twist open. So as a mast is raked the jib lead may have to move forward if the ib is twisting too much (top luff telltale breaking well before the lower ones).
  • Most 5O5 Barber Hauler configurations mean that as you move the lead block outboard it also moves UP. This will change the sheeting angle so when you move the block in or out you will often have to also move it down or forward. See Where It All Goes Wrong for more.