Self-tacking jib

Copyright 2008 Applied Engineering Services Ltd.

A broad description of the self-tacking jib and why the track is normally curved and how you might work out the correct amount of curve.

Self-tacking jibs offer a simple system of headsail sheeting (trimming) for short-handed sailing and for simplicity. The headsail is made with a 92% to 85% LP measurement so that the foot of the sail finishes in front of the mast and can tack from one side of the yacht to the other without hitting the mast and getting snagged on it. Sheeting is transverse and so fore/aft control or so-called lead angle is usually manipulated by having a clew board with a series of holes so that lead angle can be manually adjusted up and down by moving a shackle. Once fixed these are rarely moved.

The sheeting of the sail normally takes one of three forms. Here I will limit myself to systems where there is a transverse track across the vessel and where there is a single car riding on that track.

1. The sheet is dead ended at one end of the track, goes to the car, goes up to a block on the sail, goes back to the car and then goes to the other end of the track, around a sheave and off to a cylinder or winch. This system is also often used on mainsheet systems too. There is only one winch used.

2. The yacht may have a headsail sheeting winch on both side of the yacht. The lines run forwards on each side to a point forwards of the mast actually not far from the bow. The lines then each go to the car and up to the sail. The sail has a block on the clew and the system is continuous. A kind of sub-set of this method is where only half this system is installed thus line runs from clew to car to bow and then aft to a winch. This is simple, but it sure can make a mess if you tack and accidentally leave a hatch open in the centre of the foredeck. The system above and below have less impact on other uses for the deck such as crew access and hatches.

3. The sheet is attached to the clew of the headsail, it goes to the jib car and then up vertically to a sheave in the front of the mast probably at 40% of the height of the mast. The line then runs inside the mast. It can be attached to a hydraulic cylinder inside the mast, or it can come out of the mast like a normal halyard and run to a winch.

The systems all have their merits. Our office favours the first one if at all possible for most applications. The continuous systems have a block on the sail, the 1:1 systems are dead-ended on the sail. Generally the clew block is not a problem because unlike an overlapping headsail it never hits the mast during a tack.

A commonly asked question is "does my track really NEED to have a curve in it; what should the curve be, up and down, canted, or a flat curve. The answer to this question does depend on a great many things. Firstly it depends on which of the three systems you have and the respective geometry of each system.

What on earth does that mean?? Well, let's start at the beginning. Firstly, each of the three systems has a control line. This control line runs from the car to the centreline of the vessel and it controls the distance that the car can move each side of centreline; this controls the so called foot sheeting angle. Depending on track curve, the sheet tension will alter as you adjust this control line. Get the curve right and the tension does not alter as you trim sheeting angle; thus you don't have to adjust the sheet during a change of course or during a tack.

Now in real life, as the car passes from port tack to starboard tack, the distances of sheeting and sail may alter. You can imagine that if the sheet goes up the mast to the second spreader or forwards to the bow then the various lengths must all change. Now this is not a problem provided the net or overall length stays constant. So, if the overall length stays constant then the sheeting tension will not alter and indeed you can tack and if you want to adjust the control line (at any time) you can and theoretically the sheet wont need adjustment.

Fore/aft sheeting is kept constant because effectively the track is inclined perpindicular to the forestay. The curve is there so that the sheet stays the same length, which should hopefully keep the tension in the sheet constant.

Some versions do not have a control line they just have an end stop on each side. This works fine, but most cruising boats don't sail much close hauled and there are times when you want/need/should move the end stop further off centreline. Given un-controlled end-stop system then no curve is necessary since the sail is only sheeted in one position at a time. It may however still be sensible to incline the track perpindicular to the sheet lead, according to the track/car design. The clew load will generally be directed to a point half way along the forestay or even a little higher.

For any given system we (you) can usually achieve this with curve in the track. The track can sit flat and have curve in only one plane. The track can be inclined and have bend in only one plane. Or the track can have bend in both planes with/without inclination. There may in fact be an infinite number of bends that can achieve the right result in some of these (3) cases.

Well, I know this is sounding hard, but it need not be. Say we take system 1. The port and starboard leads will more or less cancel. The clew of the sail will swing in an arc perpindicular to the forestay. So, the track must be curved in both planes. BUT, if we incline the track so that the track is perpindicular to the forestay then we only need to bend the track in one plane. Not only that but it is the normal 'thin' direction of common RCB (recirculating ball) tracks.

This is in fact pretty easy. The radius of the bend is simply the perpindicular distance from track to forestay. The load from the sheet will NOT be perpindicular to the forestay it will probably be more vertical toward the middle of the forestay, but provided the car can accept the load at this angle there is no problem. With system 2 and 3 the sheet load and the clew load combine to give an overall car load whose direction must be found by adding 'vectorially'. Generally you could say that system 2 should generate a lower load direction lower therefore closer to the bend plane or LP line and that system 3 will generate a load direction which is higher and closer to vertical suiting a flat non-inclined track bent in the thick (wide) plane of the track.

Moving along then let us discuss the sheeting tension. Probably as you go head to wind during a tack the sheeting tension will naturally drop off since there is no pressure in the sail, so the car will probably roll easily when unloaded. In any case there will be plenty of pressure to push it through once on the new tack. If you had a straight piece of track in the middle then this would be a short cut forwards of the curve and sheet would go even slacker. So, if you have to have a join or are limited by length of track to bend then a straight bit in the middle where you would never sheet is ok.

Extra for experts: I suppose if you want to maximise the foot length of the headsail toward 95% LP you could actually put in a modest reverse bend in the middle such that the self tacking track would have passed through the mast if it had a continuous curve.

Warning for everyone: Make sure your car will go around the curves without sticking. Make sure the manufacturer's working load and warranty also apply to the radius of curve you are going to use. Make sure any track joins use proper joiners and that the track joins match PERFECTLY and will do so under load too.

Outside the critical sheeting regions on either side, you may determine that at such times as you sheet really wide that sheet needs to be eased to create a deeper sail. To save yourself making this trim adjustment you could slightly increase curve to slacken the sheet. It might still require adjustment but may be you can improve things. In a heavy boat you might sail a little below your proper course at a wider tacking angle with slightly eased sheet to build up speed again before tightening back up to close hauled. Actually, you might do this on every yacht.

So, overall, we have a situation where you can split up the jib track into a series of regions of sheeting angles from the bow.

from zero to 6 degrees a straight track would be ok.

from 6 degrees to 12 degrees you probably want sheeting constant.

from 12 degrees out to say 18 degrees you probably want the sheet to ease.

It will do no harm to calibrate your track and write degrees of angle from the tack fitting; with big numbers on the track/deck support so you can read these from the cockpit.

Lastly, you might put a padeye on the bulwark so that you can hook up a broad reaching sheet using MPS sheet for those occassions when the self tacking track is simply not long enough or where you want to run goose-winged.......