Amel Mainsail Foil Repair

This was an issue we discovered on Harmonie last fall, just as we were getting ready to sail out the Chesapeake down to Florida. You are looking at the base of the mainsail foil, the part that rotates to furl the sail. The lines that secure the tack of the sail are loose to give a better view.

The bottom piece with the hole the lashing lines go through is the shaft at the top of the manual furling gearbox. The visible part where the bolts go through is the coupling, and the part going up the top of the photo is the actual foil that extends to the top of the mast. The lower bolt goes through the gearbox shaft, and the upper bolt through the foil.

There has been some bad advice given about how to repair this recently, and having done this repair, I hope I can prevent someone from investing a lot of time and/or money into what could prove to be a very short-lived repair. This is a very important part, that is under high loads, and there is quite a bit of detail to the repair that needs to be attended to correctly.

When repairing this do NOT just drill a hole in the foil and put a bolt through! If you have a rigger do the repair, do not allow him to do it that way! This would concentrate all the turning torque on the thin walls of the foil extrusion. This WILL fail very quickly.

Amel’s original design for the foil drive bolt consisted of a piece of tubing through the foil that had an inner diameter matching the bolt (8mm) and an outer diameter of 10mm. Inside the foil on each side was a reenforcing plate that distributed the torque of the bolt and tube to the concave parts of the foil extrusion, greatly distributing the torque load. Over time, with many back and forth loadings, the soft aluminum of the backing plates and foil extrusion begins to wear, the bolt begins to bear on the thin sides of the extrusion, and then the cracks start.

It is worth noting that the failure here is not the result of a sudden loading of force, but a long term wearing of the parts. Boats that sail a lot, and furl/unfurl the sail often will see earlier failure. Reducing the load on the system by being sure the sail is luffing when furling will certainly slow the damage, but will not eliminate it.

Some people recommend just cutting the foil and allowing it to drop down. That works. We went a different route, and had a new coupling made that is long enough to replace the cutoff parts of the foil so the top of the foil still sits at the same height as Amel originally designed. The foil’s internal backing plates had been worn and distorted quite a bit on ours, and were not reusable. Instead of machining new ones, we set up a better system for distributing the torque loads on the foil.

Do this:

  • Using the coupling as a guide, drill a 10mm hole in the foil to accept the tubing. Tap the tube in place, it should be a good tight fit. If it is at all sloppy, tape around it to be sure that epoxy does not leak out. If your tubing is lost or damaged, aluminum tube with 1/4″ ID–3/8″ OD will work and be easier to source in the USA. The length of the tube should be equal to the OD of the foil.
  • Carefully tape over the bottom of the foil. I used blue painters tape, but the type isn’t important.
  • Just above the height of the top of the coupling when it is installed, drill a 1/8″ (3mm) hole in the foil.
  • Mix a batch of epoxy and thicken it with high strength filler to a thickness where it sill still flow, but just barely.
  • Using a syringe, inject the epoxy into the hole you drilled in the side of the foil. Be sure you spread plastic or other protective material sufficient to catch and contain ALL the epoxy in case of a spill!
  • Fill the bottom of the foil up to, but NOT blocking the small fill hole. It must be there to allow water to drain, very important in freezing climates!
  • Once the epoxy cures, bolt all the pieces together. Use Tef-gel and nylon washers to isolate the stainless steel bolt from the aluminum parts.
  • Use silicon to fill the concave parts of the foil where they go down into the coupling just to be sure that water can’t stand in here, freeze, and crack the metal parts.

The bottom of the foil is now one solid piece, with no point loadings where cracks can start, and it will not wear with use. It is not often a repair on an Amel can be said to be better than new, but this one is.

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