No I am not talking about Brent here, even though he was a popular rugby player. The Beast on Impi is our affectionate name for our 156sqm asymmetric sail. Prompted by a recent question and as a forerunner to this weekend’s new movie, this blog will look at the asymmetric sail and how it is different to a symmetric spinnaker plus how it is rigged on Impi. It is aimed at people who are considering buying an asymmetric sail for a cruising catamaran.
Our asymmetric sail was constructed by Ullman Sails South Africa and it is a good compromise between quality built, quality material and price. After 6 1/2 years of cruising on Impi we are currently on Beast number 2 with Beast number 1 haven giving up the ghost due to UV damage of the stitching.
What is the difference between an asymmetric sail and a spinnaker?
An asymmetrical has a distinct luff and leech.
The cross-sectional shaping in an asymmetrical sail (above) is not uniform and semi-circular – as it would be with a spinnaker – but asymmetrical. A spinnaker (on the left) is symmetric in shape, material and cut, about a line joining the head to the center of the foot.
Asymmetrical sails can be built for different points of sail, ours is built for a broad reach. In a broad reach, the wind is coming from behind the sailing craft at an angle. This represents a range of wind angles between beam reach and running downwind.
The problem for our asymmetrical comes in lighter 10-14 knots of breeze when trying to sail at broader angles (more than 135 degrees apparent). At these angles and velocities, the sail has trouble remaining full when the main sail is up. Our asymmetrical sail flies well between 16 and 20 knots. It is constructed with reinforced luff and leech as to cope with winds up to 24 knots.
What do you need to rig an asymmetric sail for ocean cruising?
You will need:
A spinnaker halyard that is above the forestay.
We have a sock with potty around the asymmetric sail and use the spinnaker halyard to hoist the sail inside the sock.
Two control lines or guys attached to the tack and led through two blocks mounted forward of the headstay. Then they can be lead aft to a convenient cleat. However on Impi, we use a Harken snatch block at the foot of the mast and a low friction ring, then a multiple sheave Harken deck organizer, then jammers to bring the lines back to the winches on the starboard side.
A sheet (approximately twice the length of the boat) led aft to a block just forward of the stern. Some people prefer to have two sheets as to easily gybe the asymmetric but this is not necessary in ocean cruising as one tends to have a steady course. It reduces the number of lines on the deck.
We would advise you to have a Tylaska fitting to help bring the sail down. These come in various sizes and for our boat we use a T12. You will also need a punch to release the shackle. We attach the tack of the asymmetric to the fitting and then attach the control lines to the other side of the fitting. Watch our new movie to learn how to do this.
How do I hoist an asymmetrical sail on a cruising cat?
Secure the sail in its sock near the middle of the foredeck.
Attach the sheet to the clew, and lead the control lines or guys through the blocks around the front of the forestay. Attach the Tylaska fitting to the tack and tie bowlines to secure the control lines to the Tylaska. Our lines come back to the starboard winches through jammers and a Harken snatch block and a low friction ring attached to the foot of the mast.
Attach the spinnaker halyard to the top of the sail. Make sure the halyard is outside your genoa sheets.
Have a look at all your lines and make sure they are running correctly. Imagine the sail is flying and then look at your lines. They must be for the sheet outside of the side stays and for the control lines ahead of your forestay, for the halyard, outside of your genoa sheets.
When you are confident that all the lines are correct, check your course is on a broad reach.
Hoist the halyard. Make sure your sail is not twisted inside the sock and that the sock control lines attached to the potty run smoothly. This is easier done with one person operating the winches and another person controlling the sock and its control lines.
Pre-pull the tack control lines so that the tack is a bit off the deck. Pre-pull the clew to the shrouds.
Pull the sock control lines so that the sail releases bit by bit.
When the sail is fully hoisted, trim the sheet.
Attach the sock control lines to the forward anchor cleat.
Now it is up how do I take The Beast down?
Release the tack by punching the Tylaska fitting.
The sail now is like a flag in the wind and totally depowered.
Untie the sock control lines and pull the sock down, lowering the potty.
Lower the halyard and undo the clew line.
Stuff the sail in its sock in its storage place.
That’s it guys. It sounds a bit scary as this sail has massive power but through the use of a Tylaska fitting, there is absolutely no danger as you can quickly de-power the sail and lower it.
Catamaran Impi 