The cure for FBS (Floppy Boom Syndrome)

Late last year we started noticing that the boom on Mobert was sagging a bit when the mainsail was furled.  When moored it would hang down and touch the bimini and scratch the eisenglass skylights we have over the cockpit.  This of course contrasts with the boom staying nicely up in mid-air when we first got the boat.   In addition, when we released the boom vang control line, the boom used to pop up a bit to help with furling the main.  This is something it no longer does.  It took a while to work out in my mind what had happened but I finally concluded that our rigid boom vang was no longer rigid and we were suffering from FBS.  I’ve since temporarily rigged a topping lift line to keep the boom up off of the bimini but it’s a pain to manage when you actually want to sail.

For background, a rigid boom vang is a tubular strut-like device that connects from the bottom of the mast to about the middle of the boom.  The “strut” applies outward pressure to hold the end of the boom up in the air when nothing else is supporting  it.  The rigid boom vang is usually combined with a line that is used to compress it, ie: shorten the vang.  Compressing the vang, which lowers the boom, is done under certain sailing conditions to flatten the mainsail, while easing the vang line and allowing the boom to rise, can help in other sailing conditions.

Last week while taking down the mainsail for maintenance we removed the rigid boom vang and took it home to inspect.  Mobert’s mast, boom, and vang are all made by Z-Spars in France.  Looking at the cast end pieces of the boom vang and comparing to US Spar’s (The US division of Z-Spar) website, I determined that the unit that came on our Jeanneau Sun Odyssey 45 was the “Medium” vang.

Once I got it home I happily discovered that it was ridiculously easy to disassemble.  Just remove one screw to release the plastic ring and the two tubes come apart.  Inside is a large spring that floats free and provides the power to hold the boom up in the air..  or did anyway.

I ordered a new spring from US Spars (part#1800) and when I got it I was able to compare the difference between our old used one and the new one.  Note that without any pressure, the old spring is nearly an inch and a half shorter than the new one, despite having the same number of coils.   So it’s not broken, it’s simply tired of holding up the boom.

With the new spring dropped inside the large tube and the whole unit reassembled, it’s ready to be installed and I can stop relying on the topping lift all the time.

That said, I found this line on page 23 of the Z-Spar Mast Operational Manual — “When boat is at rest at its berth, attach the boom lift and release pressure from the vang.”  I suppose 9 years of holding up the boom with no help will do it in.  I’ll try and make sure I attach the topping lift when we leave the boat for any period of time.

I’m happy that I was able to just replace the spring.   A new boom vang would cost at least $400 whereas the replacement spring was less than $100.

A couple more photos from when I reassembled the unit.

Next it’s time to reinstall it on the boat and secure it with new cotter pins.