Tuning Guide - F-18 (Diam)


Mast Set up – Spreader rake/ diamond tension

The mast set up is crucial to tuning the Formula 18. The combination of spreader rake and diamond tension has a major influence over the power of the rig. The Diam mast has quite a stiff section, so the spreaders will be raked slightly more than some of the other F18s.

Body Weight
3 -7 knots
Diamond Tension
8 - 14 knots
Diamond Tension
15+ knots
Diamond Tension
 < 140  60 - 65mm  37 - 39  39 - 40  41 1/2
 140 - 150  55mm  37 - 39  39 - 40  41 1/2
 150+  50mm  37 - 39  39 - 40  41 1/2

Note: Diamond tensions measured on the black Loos gauge do vary slightly and will read higher with age. Rotate the wheels periodically to change the wear point. 

As a general rule, set the mast up according to the guidelines above. Then adjust the
settings according to your body weight and how you like to sail. There will be a slight
variation in the stiffness of different masts, so again use the above as a guide and then go from there.

Ideally go tuning against another boat in just twin wiring conditions, and assess the amount of power that you have. If you generally feel overpowered, the boat is healing and not going forward and you are wanting to downhaul very early then it is likely that the spreaders are too far forward. Try increasing the diamond tension within reason (1-2 turns), and test again. If you still feel overpowered increase the spreader rake, go at least 6mm at a time or it will be hard to notice a difference.

On the otherhand if you feel underpowered, you are slow to fly a hull, ease the diamonds first, then if necessary rake the spreaders forward. 

Different teams alter the way they set their sails up, some use quite a large range on the diamond tension. Other teams like to leave it much the same, optimised for max. power (spreader and tension combination) and then depowering predominantly with downhaul and rotation.

Mast Rake

This is measured by swinging the trapeze wire forward to the bridle and then back to the transom. Many teams set the rake and leave it. However a few of the teams will increase rake in strong wind by a couple of inches. Increasing rake will increase the feeling on the rudder, moving the mast forward makes the rudder lighter. As a guide, half way between transom and back of hatch.

Rig Tension

As the wind increases, increase the rig tension as below. If the tension is too high in the light air it will interfere with the mast rotation.

 Wind Speed  3 - 9 knots  10 - 17 knots  18+ knots
 Rig Tension  ~ 26  ~ 28  ~ 30


The rotation changes the aerodynamics/entry angle of the sail and the mast bend
characteristics. The range of movement is from 2cm in front of the shroud (light wind), to shroud (most conditions), 5 cm behind shroud (overpowered conditions).


The downhaul is a critical control, the windier it gets the more you pull! However when
the wind is very up and down it is crucial to quickly adjust the downhaul, especially in the lulls. Make sure the mainsail moves freely in the track so that the downhaul will release quickly in the lulls. Cleaning the mast track and ‘Mclubing’ the luff of the sail will help this.

In very light winds (sub 6 knots) it is important to flatten the sail to help the air flow, a little downhaul can help this. Pull the downhaul so the horizontal creases are removed and then 3cm more. Write a scale on the mast, mark every 3cm. Once fast settings are found, make a note so that they can be reproduced.


In the maximum power condition, when you are just hull popping, you can ease the outhaul so that there is approximately a 10cm gap between the foot of the sail and the boom to give more power.

In very light winds a 3 - 4cm gap is fine.

In breeze a 3-4 cm gap will suffice, as the downhaul is pulled on, the outhaul is
effectively tightened so that this gap will reduce to about 2cm.


The North sail is engineered to gradually twist and depower as more downhaul is
applied. However to make sure that there is maximum power in the marginal conditions the head of the mainsail has a reasonable amount of shape. 15 knots plus, when a lot of downhaul is being applied is the point when you should be thinking about the stiffer battens. It depends somewhat on your body weight and the conditions you want to optimise for.

Pull the battens to remove the creases and then a little bit more to allow for the knots setting in. It is not necessary to overtighten the battens.

If you struggle to pop the battens at the top of the sail in tacks and gybes, it is because of the amount of shape in the head, this has been engineered to give extra hull popping and performance in marginal conditions. Either apply a little downhaul in very light conditions before manoeuvres or practice giving the mainsail a good flick.

Jib Settings

The jib set up is crucial to the speed and balance of the boat. Even though the sail is
small relative to the main it is crucial to get it right.
Jib car position
The jib car should be moved out as the wind increases from around 36cm from the
centreline in light, 38cm wiring and 41cm very windy.
Jib sheet angle
In light winds you want to sheet more vertically on the jib, giving a deeper and more
powerful jib. As the wind increases start moving down on the sheeting angle. If the boat feels very stalled and difficult to sail, quite often it will be because the jib is set up too full.
Jib downhaul
Just as with the mainsail, increase the jib downhaul as the wind increases, again this
has a surprising effect on the feel of the boat. The North jib is engineered to depower
significantly as the jib downhaul is pulled on. In light winds pull just enough downhaul to remove the horizontal creases. As the wind increases increase the downhaul tension, trying to keep the draft of the jib at about 40%.


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