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Older and Wiser (or Older and Easily Fooled)

Posted by Tom Benedict on 27/05/2012

Older and wiser, or maybe just older and totally hoodwinked by my own design work. Time will tell. In any case, it was a little less than a week ago that Rydra and I managed to dismast our Pacific Catamaran while trying to raise the mast. Now, a week later, the boat is fitted out with new parts and a new mast raising system that should keep us from a repeat performance. The jib still isn’t back from Sailcare, so essentially we lost no time on the water. Not bad!

But let me backtrack…

When we broke the mast hinge, I really didn’t know if it would be possible to replace it. Spare parts for the Pacific Cat are almost impossible to come by. I do mechanical design and machining for a living, so making a new part wasn’t out of the question. But judging by the color of the mast hinge, I figured it was made out of bronze. I have scrap material of almost every kind of metal that comes across a machine shop floor, but no bronze.

When designing replacement parts, it’s always important to see what was there in the first place. If you replace a stainless steel part with an identical one made out of aluminum, it will break well before the old one did. Likewise if you replace all your high tensile strength bolts with stainless bolts, the stainless ones will fail long before the old bolts did. If the mast hinge truly was made out of bronze, I had two choices: go out and buy some bronze bar stock, or find a chunk of stainless. Neither prospect was very appealing.

Turns out it was never an issue anyway. I took the bits in to work to show to one of my co-workers. “Cast aluminum, huh? Colored to look like bronze?” I couldn’t believe it! The surface of the crack was clearly a white metal, not the more golden color of bronze. The coloring was only a paper thin layer near the surface. Part of me was a little peeved that the mast step had been colored like that, but the rest of me was pretty happy with the news. Aluminum I can deal with.

So over the weekend I made a new mast step hinge.

Broken Mast Step Hinge

No CNC on this one. It was all manual machining. Unfortunately it’s a lot uglier than it should be, because I tried to replicate the broken part rather than produce something that would do the broken part’s job. Note to self: Don’t trace ugly stuff onto metal prior to saw cutting, and expect to make anything but an ugly part. But the lesson is learned: If I ever have to replace this hinge, I’ll design the new one from scratch.

The new mast step hinge is made from 6061-T651 aluminum; common fare in a machine shop, and a lot tougher than cast aluminum. Cast metals tend to be brittle, and will crack before they bend. Metals that are ductile tend to bend before they will crack. 6061 is an alloy of aluminum that includes other metals to increase its mechanical characteristics (tensile strength being one of the more important ones). T651 heat treatment increases hardness, but also increases how brittle the aluminum is. 6061-T651 will bend, but only up to a point. Then it cracks. But it bends a heckuvalot more than cast aluminum! Structurally, this was a win.

The original hinge pin was a stainless tube, which was peened into place once it was installed. This was a major PITA to get out. The tube had been so deformed over the years, it wouldn’t just slide out. I had to pound it out 1/8″ at a time, cut that section off, then pound it back the other way to cut that side off. After going back and forth about twenty times it finally gave up the ghost and the last little nubby of tube popped out. I replaced it with a solid steel rod, held in place with split rings. This is how the Prindle mast step pivots. Good enough for me.

So we were back in business, but no better off than before. Rather than risk a repeat performance of the downed mast, I sought to remedy the root cause. So during my spare time over the week I designed a new doodad for the boat:

Mast Step Assist - CAD

It’s a contraption that creates a virtual pivot axis for the mast that will constrain the mast to the fore-aft plane. It’s strapped down to the boat just behind the mast step hinge. The geometry of the side plates locates the eyebolts along the axis of the mast hinge. Once the thing is strapped down to the deck of the boat, side stays are attached between the mast and the eyebolts. This prevents the mast from swinging off to one side the way it did during our last disastrous attempt to raise it.

The width of the crossbeam is critical. It places the eyebolts so the straps coming up from the trailer attach to the eyebolts at a 10 degree angle off vertical. This is the same angle the side stays come off the mast. This keeps the side plates from being subject to any torquing motion. All the tension forces acting along the stays should go straight down the straps to the trailer.

I had most of the material to make this in the scrap box. But as with any scrap box project, the design evolved to match the materials available. The feet at the base went from 1″x1/4″ to 1″x1/2″ because I had a good sized bar on hand. The side plates went from 1/4″ to 3/16″ thick, and were clipped a little deeper on the rear corners. The cross tube and eyebolts came from Ace Hardware. All in all I had less than $30 rolled into it. After a test fit on the boat I added some split lengths of 1/2″ fuel line to the bases to make rubber feet so it wouldn’t mar the deck any more than it already is. Here it is installed and ready for use:

Mast Step Assist Installed

The wind is still too gusty to try to use it, but it was nice to see it in place and ready to go. Since our boat is rigged with two pairs of trapeze wires, we plan to use one pair as the baby side stays for raising the mast. Once the mast is up and the real stays are all locked in place, the trapeze wires are removed from the doodad, and are clipped back onto their shock cords for use on the water.

The only job left is to give this thing a name other than “doodad”, “contraption”, or “thingy”. Mechanical design and machining I can handle. Names? Er…. HELP!

– Tom

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