Trim: Schultz sail
This article was originally written in German by Gabriel Wicke.
David Ashburner(USA-14) and Ollie Booth(GER-432) have cooperated to present
this translation. The original article is on the web at:
Article written by Gabriel Wicke about rig tuning.
The mast trim requirements are completely opposite for different conditions. Good boat speed over the entire wind range will depend on how well you understand these contrasts.
At the start of the article I will briefly describe the background with trim guidance at the end.
In medium winds we need a stiff mast: As soon as you can trapeze straight out, you can use the pressure of a relatively deep sail and a closed leech. Even with an extremely stiff mast (for example Proctor Epsilon) a little bend of the mast cannot be prevented.
In very light winds the sail must not be too full, - we have to prevent the sail from being too baggy despite the missing mastbend.
In strong winds we need a particularly soft mast: We are already completely hiking out in the trapeze and cannot use the additional pressure of a gust. Already with kicker and Cunningham on, the leech of the flattened sails must open further in the gusts, the mast must be as flexible as possible now.
Here we need the ability to make pressure with a deep sail. In very light winds we might flatten the sail with the outhaul slightly. In very strong winds we can depower easily with the control-lines set tight.
In light winds the projected surface is the only thing that matters: is outhaul very tight, everything else is irrelevant. The angeballerte Unterliek raises the boom and twists the sail at the top. In stronger wind and waves one should ease the outhaul ever further, the kicker is in use again now. The mast bend is irrelevant downwind.
Straight trimmed mast, sail with only a flat luffcurve (this points on the particular sailcut. Olli) (e.g. Wavelength)
A straight trimmed mast will always bend in a medium wind. In very light winds the bend disappears however, the sail is much too deep now. More rig tension (over 170 kg on the wanten(shrouds)) will stiffen a straight trimmed mast in strong winds. The shroud-tension changes just a little due to the little swept spreaders with increasing mastbend. Like this, we already loose badly asked for pressure at medium windspeeds.
Pre-bent trimmed mast, Schultz sail with a full luff-curve.
A pre-bent mast is achieved with higher rig tension (over 190kg) and a relatively strong Salingpfeilung (spreaders angled further aft) (approx. 15cm to the sailtrack). Thus we avoid the problem of too full a sail in very light wind.
On the one hand side the pre-bend mast takes the kicker-load in medium wind speeds without further bending. (As if you try to bend a bow with a high loaded string). On the other hand the high rig tension stabilizes the mast particularly laterally. The consequence is more pressure on the beat in the right moment. Particularly in waves this induces a clear speed advantage.
The behavior of the pre-bent mast is very interesting in strong wind: The tension disappears with increasing mast bend like the string getting loose, when the bow itself is bent, so that mast can freely bend more! The moment, at which this effect occurs, can be adjusted with the Cunningham precisely. A little Cunningham tension is sufficient to depower the rig (the fabric between Masthead and Cunningham eye has a much larger lever for bending the mast due to the pre-bend).
The effects on the reaches are interesting too: The pre-bent mast is held straight at right angles and in addition stiffens due to the greater rig tension. In combination with the increased luff curve it provides much more power while reaching.
Downwind the sail area is a little smaller due to the rounder luff-curve (a strip 2-3cm wide), that is however not so dramatic. My Wm sail was missing approx. 3cm of width anyway. The bigger depth apparently has a compensating effect.
How does the spar become bent?
Very simply, it depends on rig tension and spreader angle. The spreaders should be swept between 14 and 16 cm. In order to measure this, you put on a sail batten or the like on the spreader at the shroud and measure the distance from the batten to the aft edge of the sailtrack.
To prevent the loss of rake when combined with the greater rig tension, it is sensible to shorten the shrouds by fastening them a hole closer to the chain plate. We place the mast on the boat and loosely connect the forestay and shrouds; the lower shrouds are not attached. We fasten and push the kicker the boom on the gooseneck. We place the boom end padded (close to the gunwale) on the afterdeck. With the kicker on we stretch the shrouds, which causes the forestay to become loose. We can easily retighten the forestay now.
After closing the folding lever (highfield lever) we can control rig tension and pre-bend now. Pre-bend should amount to 5-6 cm, with rig tension over 190 kg. You can determine the bend very easily, by stretching the halyard along the aft edge of the mast.
The mast rake should be about 654 cm (from the Mast top to the upper edge of the transom). Without a long tape measure the mast rake can be determined: You stretch the halyard along the rear edge of the mast and mark the position of the black band (upper edge), now swivel the halyard to the transom. The distance between the marking and the upper edge of the transom should be approx. 70 cm. If you find the boom is too low, you might set it up a little higher washing your sail while tacking will reduce speed even more.
With everything measuring in tolerance, we can fix the lowers. With the rig under tension they should be just taut without tension.