The J dimension


There's an article of its own here. On the one hand because it is relatively extensive. On the other hand because of the importance.

The J dimension regulates the leech length of the jib. It can be 1,1xJ. That's why we have a 110% jib, because of 110% of J. The exception would be the self-tacking jib, which may only have 95% of J.

Furthermore, with the mast case you can influence the pressure point in the mainsail.

When I tilt the mast backwards, it affects the distance between the mast and forestay at deck height - i.e. the J dimension.

In order to have a well-balanced lateral plan, we all want the sail pressure point of the jib to be as far forward as possible. At the same time, at least when there is more wind, the sail's pressure point should be as far back as possible.

So it is clear that there must be a definition of where the jib should begin and where it should end.

We want everyone to sail the same size.

And that is exactly what will be discussed here.

Point F.6.1 (a) of the ERS applies

The rear boundary is formed by the intersection of the deck with the front edge of the mast.

The front edge of the jib is, in terms of measurement technology, equated with the central axis of the forestay. So that's where our J measurement starts.


We pull the mast (with the mast controllers) as far back as possible. Now we start the eighth day slightly. Then we enforce the forestay. We have thus ensured that the forestay comes down at the “right angle”. The mast should now touch a stop aft. This stop must be retrofitted to older boats that still have a long, evenly wide mast passage. How to do this is explained at the end.

If the mast is not yet at the stop, loosen the forestay and further tilt the mast on the backstay until there is contact. Then enforce forestay again.

We take the central axis of the forestay and align the rear edge of a ruler with it. Now we slide the ruler down until we touch the deck. We check again whether we really have the central axis of the previous day and draw a line where our ruler touches the deck. The longer the ruler, the easier it is.

There are two problems here:

  • We have to be careful when extending the central axis, the pulleys are a bit of a hindrance.
  • We have to touch "the deck". The deck, not any point in the extension of the deck.

The bow is rounded to a hemisphere. This raises the question of where the deck merges into the hull.

A popular assumption is that the radius curve is divided into three equal parts, the lower one is assigned to the hull, the upper one to the deck and the middle one remains undefined.

Ergänzender Hinweis:

Anyone who has repaired a damage and was particularly generous when rounding off now naturally has additional “problems”.

What you can then do is quite simple: We tinker 1-2 mm deck and you're done.

Unscrew the forestay fitting and restore the deck height with gelcoat.

Attention: Restoring the deck height means restoring the deck to its delivery state. 6-7 mm rounding should remain.

Of course, it can also be the case that the fitting with the deflection rollers is simply too far forward.

The easiest way is to put the wheels a hole backwards. Then we gave away a few mm.

So check exactly how much we are too far forward and use a round file to make the hole in the forestay pulley 1-2 mm to the rear to form an elongated hole.

When we have now marked the intersection of the forestay center axis with the deck, we measure from there to the leading edge of the mast at deck height. Since the wave deflector is in the way here, things get complicated again. I haven't come up with a simple home remedy that anyone can measure themselves without long preparation. We measurers use a template, basically a huge slide gauge with fixed stops.

So much for pure measurement.

To fix a mismatch, we need to consider our trim options. Therefore, there are only suggestions here on what to look for.

Let us assume that we had measured and that the J dimension was too high. Then what is to be done?

At this point it proves that "sailing" more precisely "right trim" is a total work of art. It depends on the type of our artwork, at which point we change.

What about the mast position?

Do I drive with so much fattening at all? If we never actually pull the mast back so far that it touches the stop, the obvious solution would be to move the stop forward.

What about the greediness?

If we always have the feeling that the boat is too windy anyway, the mast is probably too far back. Here both problems (windiness and J-overlength) would be solved if we put the mast a hole forward. And then change the stop.

Where is the intersection of the previous day with the deck?

In the case of OD, it must not be more than 10 mm behind the leading edge of the deck. For non-OD boats, it's 80 mm.

If the undersize is measured, there are in principle the same considerations. Only that we are compliant and do not have to change anything. But can.

Installation of a J-dimension limiter:

What conditions must this stop meet for the rearmost mast position?

It must be a firm stop that the sailor cannot manipulate while driving.


  • A string goes below deck from the beginning of the mast-to-front-pull mast controller around the mast to the pulley on the other side. The length is precisely matched to the J dimension and the knotting cannot be undone without tools.
  • In the elongated hole for the mast passage, there are surfaces on the sides to which the mast can be attached at max. Leaning on the mast case.

With newer boats they are in the factory. Of course, you only vote for the mm if the deck has also been glued to the mm.

In old boats there is still an elongated hole without these stop surfaces. Here the position must be determined and then a stop must be in.

Basically, the problem can also be solved with the backstay: If the backstay is just long enough that the connecting shackle between the backstay and the backstay tensioner is on the deck passage when the J dimension is reached, you cannot pull any further than J.


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