Why masts can fail in a collision

Why masts can fail in collisions: Hylas 70

From an engineering standpoint, masts are considered to be columns in compression. All the loads from the sails are directed by the standing rigging into compressive loads along the length of the mast. These loads typically run into the tens of thousands of pounds. For instance, the Hylas 70 carries 2,419 sq/ft of sail area between its main sail and genoa. In a 12 knot breeze this generates 4900 lbs of horizontal load, and a compressive load in the mast to exceed 135,000 pounds!

So, masts are incredibly strong in compression. However, columns under compression will fail when the column is bent in excess of their safety margin. If they are bent far enough, they become unstable as the compressive force just tends to bend them more and more out of column until they snap.

The rig design takes into account the normal forces generated when the boat is pounding into a headsea, or even in a blow-over situation, then adds a safety factor. However, pounding into water is much more gentle than pounding into granite. When you hit rock, the boat stops abruptly. The stopping distance can be within say the 3” of deformation of the lead in the keel. Hitting a brick wall like this translates into high acceleration forces on everything on the boat.

The mast is very susceptible to damage from this kind of force because it is only supported at the top of the mast and the deck. As the boat stops suddenly, the center of the mast tries to continue moving forward. On the Hylas 70 the mast is 86’ above the deck. In a collision, the acceleration forces amount to about 4500 pounds pushing the mast forward in the middle. This causes the mast to bow by about 27” or twice the design limit. Once bent this much, the compressive force in the mast will then cause it to come down.

There has been some discussion about this being an in-mast furling spar, and how the open cavity might be to blame for the failure. Furling masts have been in use for a long time, and their structural properties are well known. A furling mast will have the same safety factor designed into it as a conventional mast. Sometimes though, in an accident, the forces involved simply become greater than what were accounted for in the design of the rig.

One could argue that this should not be, and all masts should be able to withstand a hard grounding with sails up. There must be a balance between what a rig should reasonably be able to with-stand, and what the cost and weight of the rig should be. As with much in boating, it’s a compromise to reach the most desired goal.