Forestay is different from other stays. Firstly, it likely has a jib attached to it. Secondly, because of the jib, it sags.
Regarding jib trim, this sag is important. This is because the more the forestay sags, the deeper the jib gets. This is why some sailors attach load cells to the forestay: they measure its load and correlate it to the sag. But there is a major flaw in this logic.
The forestay is affected by three things: the hull, the mast and the sail. The hull and mast just pull on the forestay axially. The jib, however, pulls on it radially. This is a two-way battle: hull and mast try to make the stay straight and the jib makes it bend.
The problem arises from the fact that the jib’s effect does not necessarily increase the tension in the forestay — it just makes it sag more. We know that depending on windspeed, angle of attack and sheeting, the forces on the jib vary tremendously. Thus under the same or very similar tension, the sag of the forestay can vary a huge amount.
This is why measuring tension should not be equated to measuring sag.
Additionally, we find sag to be a poor metric, much like how we previously discussed mast bend.
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