10-26-2024, rev. 11-6-202, rev. 11-27-202, rev. 12-7-2024
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. Supposedly Gino Morreli designed the rig, but he's a smart guy and he never used this design on any other boat. I'm think it's far more likely this was a John Walton (son of Walmart founder Sam Walton) design choice, one of many that Ian did not approve of. 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. The proportional stretch of the uppers and lowers insures that the mast remains accurately in column, even as it is slightly deflected to the side under wind loads. 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, though larger mast sections and those stabilized with pre-bend typically do not require this extra support.
One the left (F-24) rig there are two sets of diamond wires on each side. The drawing omits the outer set on the left and the inner set on the right for clarity. Those are not loaded when the mast is pressed to the side and forward reaching.
Re. Center of effort. According to calculations based on ABS (American Bureau of Shipping) documents, the force on a sail is primarily concentrated on the head, clew, and tack. A quick look at the reinforcements and the arraignment of fibers in load path sails makes this obvious. Consider how the luff is relaxed when the sail is backwinded. In fact, about 65% of the sail force above the boom is concentrated at the head, with the balance spread along the luff tape. The load is NOT uniformly distributed along the mast, as commonsense might first suggest.
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 they heel less and as a result, the mast is not outboard, contributing to heel. They have a wide shroud base, reducing the need for spreaders to create angle. Often multihulls cannot sustain high mast base compression loads because there either is no center hull, or in the case of trimarans, because it is lightly built, without the need for structure to support a keel. For the same 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 of a thin, aerodynamic wind section. Forward support isn't needed, because the mast is wide in that direction. The diamond spreaders are angles aft to help create some pre-bend, which helps with mast stability (resists inversion and pumping) and with draft control (you can de-power by de-rotating the mast which pulls cloth out of the sail, reducing draft).
The combination of diamond wires, aft-swept spreaders, and aft-angled cap shrouds also helps eliminate the need for a fixed backstay, something that is problematic with a high roach mainsail, the other half of the quick-heel-control formula. It's hard to maintain enough forestay tension without a backstay, and savy multihull sailors quickly learn to compensate with a tight mainsheet.
But the F-24 MKI mast does not rotate, so why the 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. There is also the need for aft-raked cap shrouds to replace the backstay. Of course, many cruising cats have either conventional shrouds or diamond wire stays with aft raked cap shrouds and no second set of diamonds.
There are several problems with the willy-nilly addition of stays without working through the design. When a mast fails, it fails in compression. Like buckling a drinking straw, the tensioned side does not tear, it is always the compressed inner side that buckles inwards. Every additional wire and its associated pretension adds compression, so unless it provides needed support, it actually weakens the mast. The more wires, the more compression. Calculations of buckling strength for this mast sections give a compression working load of 6,500 pounds. Rough calculations based on wind pressure and heeling force (the boat was reefed and very close reaching) and bending moment at the failure point show that the mast failed under a compression load of about 12,000 pounds (if distributed--this reflects the probable stress on the side that failed). This is well over the safe working load of the section, but less than the expected failure strength.
If we total up the compression loads from the shrouds, diamond wires and other rigging ...
- Forestay. 2200 pounds
- Shrouds (only one is under much tension sailing under full load). 1200 pounds
- Mainsheet. 300 pounds
- Halyard, jib. 300 pounds. Note, jib halyard tension does not change forestay tension + halyard tensions at the mast, only between the mast and the anchor point at the furler. The total is fixed by shroud and mainsheet tension contributions.
- Halyard, main. 500 pounds
- Cunningham. Does not count, since it just offsets friction in the mast groove, from head to foot.
- Forward diamonds. 1000 pounds total (but not included at failure point)
- Aft diamonds. 2400 pounds total
... and adjusting for angles (only part of the tension is directed in-column) we get about 6,500 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 don't need; there is very little forward load on the mast, and the center of the force is near the head of the sail (forces perpendicular to the luff tape are very low, except near the head and tack--not the wrinkles, the luffing when backwinded, and the lack of significant reinforcement on the slug grommets).
The rigging guide gives 500 pounds max for the forward diamonds and 1000 pounds 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.
10-29-2024. I checked the tensions on my F-24 MKI.
Wire Observed (pounds) Maxium Spec (pounds) Re-set to (pounds)
Shrouds 800 1500 (adjusted each sail)
Forestay (same as shrouds by geometry, except as increased by mainsheet tension)
Aft Diamond 1400 1000 800
Fore Diamond 800 500 350 (below failure)
Total Diamonds 4400 3000 2300
This represents a (4400-2300)/6500=32% reduction in mast compression, which translates dirrectly into a 32% increase in strength.
Worse, exactly where the reefed main applies it's greatest force (65% of the drive above the boom), just above the forward diamond wire anchors, the lee diamond wire is pulling down and to leeward. And that is where the mast buckled to leeward with a reefed main. The smoking gun. Removing or slacking the forward diamonds also reduces this stress.
What if the mast raising stays were snugged and perhaps reinforced in the deck (Lower shroud)? This is a conventional rigging solutions? This stabilizes the lower mast with less compression in the critical area. It would also help to move the anchor point slightly aft and outboard, while staying inside the jib track.
Takeaways? The forward diamond wires increase compression and pull in the wrong direction without providing any useful improvement in mast stability. Best would be a re-designed mast with only one set of aft-swept spreaders. The less drastic solution is to reduce the tension on the forward diamonds; I judged 300 pounds to be sufficient preload and sailing observations confirmed they do not go fully slack.The aft diamond wires should be just tight enough to create 2.5 inches of pre-bend (as specified by Ian Farier) and no more. These serve a purpose, but extra tension does not help. 800 pounds appears to be sufficient, and I may reduce this, based on further sailing observations in strong conditions.
Will I reinforce the deck and improve the lower shroud? Undecided. No. The wires are too small to bother.
Time will tell, but I'm comfortable with my calculations. The mast should now be 30-35% stronger, no small improvement.
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rev. 12-7-2024
A few more ideas. I could support the center by running a second set of shrouds to the same anchor point. In this way, they would move together. In addition, they would be tensioned along with the cap shrouds. My gut is that fixing the diamond wires makes this unnecessary. I have to address the diamonds anyway.
