Tuesday, 11 March 2014

Adding an inner forestay - Part 1

To prepare Auriga for ocean sailing, we want to make the boat as reliable and adaptable as possible. A common modification for offshore-going yachts is the addition of an inner forestay. A small staysail can be flown from it in heavy weather, being easier and safer to set than a storm jib and giving the crew additional options in most conditions.
Storm staysail in heavy weather © Carlo Borlenghi / Rolex
This requires a deck fitting of substantial strength, and we had to face a host of challenges in modifying Auriga accordingly.
The deck on our yacht is foam-cored and can hardly take the stress of a tensioned stay. Luckily, the location we chose for the fitting is directly above the bulkhead separating the anchor chain locker from the forecabin (this is also were the primary anchor/mooring cleat is located). The old bulkhead was merely decorative, and we decided to replace it with a structural one, laminated from plywood and glass-fiber cloth.
Location of the watertight bulkhead and the inner forestay fitting
This solution provides additional strength and stiffness in the bow, and offers some protection in case of collision with a UFO (Unidentified Floating Object). Most of the load on the through-deck U-bolt will be taken by stainless steel brackets, which in turn transfer the load to the bulkhead and thus the sides and bottom of the hull. The overall design took several iterations to finialize. Leaving nothing to chance, we sized the brackets to support the full breaking load of the stay wire, while trying to reduce the weight to a minium.
Early design concept for the backing bracket
Simulating the brackets' behaviour under load allowed us to make some changes, in particular adding oversized washers of at least 5mm thickness seemed necessary to eliminate any chance of damage around the boltholes. Later, I sighted similarly thick washers supporting deck blocks on a Challenger 72 - a known weak spot of any backing plate, I guess?
Stress analysis on the staysail backing
The brackets were made from a 40x40x3mm 316 stainless steel angle, and extend about 30cm down the bulkhead to distribute the load. Welding stainless is best left to the pros, and so the good Samaritans at the Physics Department's workshop provided invaluable help.
Almost finished! Lots of driling left
Drilling the mounting holes in the welded parts required some cutting oil, sharp drill bits, a low-speed pillar drill and lots of patience - the result, however, inspires confidence with its heft! Maybe some weight-saving cutouts will not hurt...

In the upcoming Part 2, we will describe fitting, laminating and reinforcing the bulkhead - stay tuned!

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