Stemballs are a high performance fitting used to connect stays and shrouds to the masts of yachts. They may be made in stainless steel or bronze to ensure high strength and corrosion resistance in a wide variety of configurations. These include threaded connection, swaging, compression fittings and clevis pins. The fittings are chiefly utilised in offshore and high performance sailing in a range of wire diameters. They are more resistant to fatigue than other attachment methods and provide greater rig stability.
There are no standard dimensions or sizes, each manufacturer selecting his preferred dimensions. If replacing it is necessary to specify the ones for the particular mast/boat. The ball with its cup sits in a domed fitting on the mast, known as a backing shell. These are also manufactured in various shapes and materials.
Stemballs are attached to the shroud or stay wire in a variety of ways, The example shown here has an eye for attachment to a tang by a clevis pin and is known as a stemball eye.
A stemball eye was used as the upper connector of the forestay to the mast on a 40 ft yacht. It fractured while the yacht was sailing but fortunately the mast remained in place, supported by the sail and halyard. The fractured part was recovered for examination,
The failed component as seen, fracture at left. The eye at the right hand end shows evidence of fretting, indicating movement in service. There is some polishing on the shank, right of centre, also indicating movement against another part. The fractured end was examined visually without magnification but the photographs give about x6 magnification.
The photograph shows a rather unusual fatigue fracture. Fatigue occurs when a crack grows through the metal under the influence of alternating stress. In this case the cracks initiated at multiple origins throughout the circumference of the fitting. These origins, known as ratchets, can be seen as small notches all around the perimeter of the section. Two principal cracks have propagated inwards towards the centre, one from each side, creating characteristic marks known as ‘beach marks’. Finally, the stress acting upon the remaining area exceeded the strength of the metal and it fractured in a ductile manner, creating the small, almost vertical mark left of centre. The area of this is small, perhaps 5% of the total cross section area, indicating that the forestay was fairly slack at the time of fracture. The final fracture is aligned with the eye at the end of the fitting, which in service was in line with the axis of the boat, showing the forces to be lateral, due to the load imposed by the foresail.
Fatigue cracks are initiated at stress raisers that may be changes in section, design features such as holes, or service damage such as grooves, scratches or corrosion pits. In this case the cracks have initiated almost simultaneously around a circle that is the circumference of the fitting. Although the ball was not recovered it appears that the fracture occurred at the change in section between the shank and ball where there is a small fillet radius that is normally adequate. It seems likely that a wear groove was formed by repeated contact of the fillet with the cup, due to movement that exceeded the design intention. This groove was sufficiently acute to initiate fatigue. There may also have been a sleeve outside the fitting, accounting for the rubbing marks seen on its shank but this is not currently known.
The conclusion is that the rig was excessively slack when the boat was being sailed. General rig slackness would probably not have resulted in such a perfectly aligned fracture.
It is recommended that forestay tension be increased, keeping movement of the stemball within design limits.