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 where the lashing lines go through is the shaft from 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 very bad advice given about how to repair this, and having done this fix, 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 solution. 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 for a permenant solution.

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 did not build the part that way, and for very good reason!

Amel’s original design for the foil drive bolt connection consisted of a piece of tubing horizontally through the foil that had an inner diameter matching the OD of the bolt (8mm) and an outer diameter of 10mm. Inside the foil on each side the tubing and bolt past through 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 with full torque 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 have recommended just cutting the broken end of the foil and allowing it to drop down. That works, and is appropriate if you know that this repair has not been done before, and you have the extra length of foil to do it. We went a different route, and had a new coupling machined that is just 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.

Our foil’s internal backing plates had been worn and distorted quite a bit, and were not reusable. That wear was actually the beginning of the failure of the entire system. Instead of machining new ones, we set up a much better way of 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 has been 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. This will contain the epoxy while it cures.
• 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 will still flow, but just barely. Thicker than mayonnaise, thinner than peanut butter.
• Using a syringe, inject the epoxy into the 1/8″ 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 with the epoxy mixture up to, but NOT blocking, the small fill hole. It must remain open to allow any water to drain. This is very important in freezing climates! If you have any doubts, run a drill bit through this hole after the epoxy has cured to be sure you have a working drain.
• Once the epoxy cures, bolt all the pieces together. Use Tef-gel and nylon washers to isolate the stainless steel from the aluminum parts.
• Use silicone to fill the concave parts of the foil where they go down into the coupling 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 because none of the parts move relative to each other under load.

It is not often a repair on an Amel can be said to be “better than new,” but, I believe this one is.

Update: This repair is now three years old, and continues to function perfectly.

© 2019, 2022 William Kinney. All rights reserved.