My first spreadsheet was for sheeting geometry, and ever since I've been thinking about my claim that the sheeting on most IOMs is the wrong way around.  Instead of the main boom having a sheeting post which adjusts to snuggle right under the boom, while the jib sheet runs from a deck fairlead to the jib boom at who cares what angle, I remain convinced it is the jib which should have a carefully engineered sheeting post to snuggle under its boom, and the main sheet which should form an equally carefully managed angle to a fairlead close to the deck.

For the main boom, with the sheeting post snuggled under the boom, 2 mm ease of sheet results in 2 mm sideways movement of the boom. With a sheeting fairlead on the deck and the sheet reaching up to the boom 50 mm above it instead, we now have the hypotenuse of a triangle. Here, 2 mm ease of sheet results in somewhere between 6 mm (very closely sheeted) and 3 mm (very freely sheeted) of sideways boom movement.  As we sheet out, a boom with a deck sheet fairlead eases much more quickly than a boom with a sheeting post.  This is what we need the main to do if a gust hits and we sheet out a little, and it helps the main "catch up" and then sheet in parallel with the jib as we sheet out for reaching.

So here is a prototype jib sheeting post, milled and turned out of aluminium rod.

The pivoting points are radiused so they always bear on the deck track or on the post and are thus always "tensioned".  This means the post doesn't flop around as the boat tacks.  The contraption must be able to slide right up close to the mast in order to accommodate the jib sheet attachment on the no.3 boom, which is almost always right at the end of the boom.  The deck track slides are standard SAILSetc fittings, with the eyebolt unscrewed a half-turn.

The only negative aspect of the prototype is that it does not lie flat enough;  I would prefer the jib boom to be able to lie lower and closer to the deck.  OK, so that will be version 2.

Update:  I used the jib sheeting post at the recent UK ranking event, and two changes are needed, both due to line snag.  First, the sheet can wrap around the eye at the end of the post.  Second, the sheet can jam between the deck track and the radiused end of the post.  Version 3 needed even before version 2 is complete!  Fortunately, neither cost me a race, and both have easy work-arounds.  For the first problem, the post just needs to be moved so the eye is forward of the sheet attachment point on the boom.  For the second, a piece of sticky-backed Dacron over the post-to-track fitting keeps the sheet away from jamming.

 2005-12-18

©2024 Lester Gilbert