10-26-2024
The failure of an F-24 mast at the lower diamond wire junction this week (not my boat) started me thinking about the odd rig on this boat. It was not used on any other F-boat, and in fact, no other boat that I can find. Gino Morreli designed the rig, and though ordinarily a smart guy, he never used this design again and I think he was just having a bad day. Oops.
The failure point was just above the lower (forward) diamond wires. Note also that it was sideways (as typical), not in the direction of the forward wires, suggesting to me that they are not helpful.
Mono-hulls are typically rigged with spreaders and shrouds anchored to the deck. This provides side support with no added compression load. Masts, when they buckle are failing in compression. Occasionally a jumper diamond is provided on the front to control the bend of a very slender mast.
The weird F-24 X-spreaders. The forward spreaders hold the shorter wires. The aft spreaders are about the typical rake angle and provide the pre-bend.
F-24 manual image.
Multihull masts are often rigged with big rotating masts for several reasons. A big main is advantageous, as it provides accurate heel control through twist and easing the mainsheet. They can tolerate a heavier mast (rotating masts must be heavier because the staying is less efficient) because it is not outboard. They have a wide shroud base, reducing the need for spreaders to create angle. They cannot typically sustain high mast base compression loads because there either is no center hull or it is very light (no keel). For the converse reasons, rotating masts are nearly non-existent on monohulls. Because there can be no fixed spreaders to carry side loads, rotating masts use diamond wires, and it works out well that they rotate with the mast, supporting the weak side. Forward support isn't needed, because the mast is wide in that direction. The diamond spreaders are angles aft to help create some prebend, which helps with draft control (you can de-power by de-rotating the mast which pulls cloth out of the sail, reducing draft).
Diamonds or aft-swept spreaders can do away with the need for a backstay, which is a problem with high roach mainsails, which are a favorite of multihulls for their ability to adjust heel quickly. Also, it's hard to maintain enough forestay tension for a mashead jib without a backstay; aft spreader sweep and a tight mainsheet help, but only to a point.
But the F-24 MKI mast does not rotate. Why diamond wires instead of fixed shrouds? It's easier to step the mast when trailering, since no tensioning is required. The center hull is relatively narrow and very light, with no good place to anchor the shrouds. But I think it was mostly the ease of rigging and no tuning.
The problem with adding diamond wires is that they add compression. The more wires, the more compression. Rough calculations based on wind pressure and heeling force (the boat was reefed and very close reaching) show that the mast failed under a compression load of about 12,000 pounds. This is well over the safe working load of the section.
If we total up the compression loads from the shrouds, diamond wires and other rigging ...
- Forestay. 2500#
- Shrouds (only one is under much tension sailing under full load). 1500#
- Mainsheet. 300#
- Halyard, jib. 300#
- Halyard, main. 500#
- Forward diamonds. 800# total.
- Aft diamonds. 2000#
... we get 7,900 pounds total. That does not allow a lot for bending moment. Curiously, 36% of this is from the diamond wires and 10% from the forward diamonds, which we probably don't need (most rigs have only the aft swept diamonds or spreaders.
The rigging guide gives 500# max for the forward diamonds and 1000# max for the aft diamonds. But what if they really don't need pretension for stretch, since the mast is not bending? Then the pretension for the aft shrouds needs to be just enough to establish the prebend (0.8% of length), and forward diamonds just enough to keep them from going slack, since we probably do not need them at all. But I'm disinclined to remove them entirely, and going slack might allow them to jump out of their fittings.
Takeaways? The forward diamond wires increase compression and create a hard spot in the mast without providing any useful improvement in mast stability. They should be as loose as possible without them coming out of the spreader tips. 200-300 pounds of pretension should be plenty. Just barely snug. The aft diamond wires should be tight enough to create 2-3 inches of prebend and no more. These serve a purpose, but extra tension does not help. 1000 pounds of pretension is not required to prevent the lee side from going loose.
Time will tell.
