Saturday, June 20, 2009

Replacing the mast head sheave

After 18 months of 2 to 3 times a week racing on San Francisco Bay, the main halyard sheave on Blueberry, my Nonsuch 22, sail # 48, gave notice of quitting. A call to Mike Quill Rigging (a specialist in Nonsuch rigging and the source of many Nonsuch replacement parts) outlined for me what I was up against in the replacement of this sheave.

With three races in the next week, I was reluctant to pull the mast. Pulling the mast was estimated to be cheaper than having the same pair of riggers (one operating the crane, the other at the mast head) doing the repair without pulling the mast for work at the ground level. If I could source a replacement sheave, before beginning the surgery (the mast head has to come off to get at the sheave) I might be able (if lucky) to keep my racing schedule intact.

Mike explained that some of the 22's had single sheaves and some had dual sheaves. Mine is a single sheave. Mike would need to know the dimensions of my single sheave to see if he has a sheave that might fit. A classic Catch 22 for my Nonsuch 22

So the call went out: Anyone have the dimensions of the single mast head sheave for a Nonsuch 22? The consensus was that another source of custom stainless steel parts was Murray Cressman. Murray is a jewel of a resource, and he is highly recommended by many as well as by me after this project.

Given the choice of sheaves from various suppliers in various sizes, Murray took his best guess, based on sheaves supplied to other Nonsuch owners, and arranged for a Garhauer sheave to be drop shipped from southern California. With a great looking sheave in hand, then came arranging to get the mast pulled. Wonderful service by Murray and I thank him for that.

In the San Francisco Bay area, only a very small percentage of the boats on the water year round, (even those which are dry sailed) have their masts pulled each year. The quotes from the three nearby yards, to my yacht club, included the cost of two riggers at $90.00 each per hour, and crane charges each time for a total out and back in the next day, of an estimated $600.00 US. about another approach?

Since I am a Green Card Canadian, with the first 30 of my 62 years of boat owning in the annual boat haul out great white north, mast pulling was familiar enough for me to use a couple of family and friends. With their help I was able to use my local yacht club dry sail haul out crane, at lower low tide. We were able to pick up the mast at the balance point, at a crane and salary cost of gratitude to those involved.

The base of the haul out crane is just above the word Encinal. The tide is much higher at the time of this picture than during the mast haul out.

Same location at Lower Low Water, Large tides ( mast haul out conditions) the next day.

The picture below shows the mast head with the mast head cap removed. The piece of paper taped to the saw horse shows the tidal heights for the day, and a note explaining when the mast would be put back in.

The picture below is a view looking up from the tube of the mast into the mast head cap. The stainless steel pin, through the two flanges of the aluminum casting holds the mast head sheave in place.

The picture below is a view showing one end of the pin in the aluminum casting flange.

The picture below shows the other end of the pin, with a small stainless steel plate welded to the pin and held in place with a flat head screw tapped into the flange. The sheave is resting in the gap between the flanges. So the plan was to remove the old sheave, replace it with the new one, insert the pin, secure the pin in place, and put the cap back into the mast. Easily done eh? Mmmmm....maybe not.....

This shows the elegant Garhauer sheave, the favorite of myself and the choice of Murray Cressman, and a second sheave, sourced locally at the last minute, on the right. And the reason for that would be....shown in the picture below this one......sigh.....

Remember the comment that some Nonsuch masts had single sheaves and others had double sheaves? Well it seems there was some variety in castings as well. My favorite Garhauer could not be fitted even if I had the casting milled out enough to fit into the slot as that would leave the flanges too thin. This was a situation neither Murray nor I had imagined might be the case.

OK, if we are not going to fit the Garhauer, what is Ken Blake doing with the milling machine? Ken Blake runs a small custom metal parts manufacturing company with machine tools ranging in age from the 1940's vintage milling machine being used in the picture to a modern 5 axix CNC machine used for longer runs of small parts production. Ken can be reached at 510-258-3592. Over the years I have used Ken's ability to visualize what has to be done, without a drawing, to create a number of boat related metal parts.

The locally sourced sheave, that would fit betwen the flanges, would not fit far enough into the slot between the flanges because the slot was tapered slightly when cast. The original sheave apparently just barely fit, for a while, but as the bronze bushing wore in use, the sheave tried to drop further into the slot, and in the effort to do so milled, or at least tried to mill a wider slot for itself until it jammed. The jam got my attention, from some 30 feet below the mast head, when pulling up the main sail.

So Ken milled out the slot to the width at the top of the taper, all the way to the bottom of the slot. This allowed us to position the sheave on the stainless steel axis pin, with three thin mylar washers on each side as low friction spacers.

Great, all done now?.....maybe not.... It seems that the stainless pin was not quite square to the stainless plate holding it in between the flanges. Tighten the screw, the sheave jammed....loosen the screw, the sheave spun as it should. OK, some shim stock under one end of the plate, tighten the screw, and all was happy again. Oh yeah, all of the stainless screws threaded into aluminum on this casting were well coated with Forespar Lanocote to prevent electrolytic corrosion in the salt atmosphere.

At the next lower low tide, the spar was picked up just above its balance point, and reinstalled in Blueberry. The next couple of hours were spent re-rigging the wishbone, bending on the sail, and cleaning up the clutter for a beer can race that evening.

After going to all the trouble to source a Garhauer sheave that would fit the size of the line at full hoist of the sail, why was I able to settle for a narrower sheave instead? The high tech halyard had a wonderfully strong mast head shackle spliced into the line. The bury of the splice, tapering from 1/2 inch at the shackle down to the 3/8 inch line beyond the burried tail of the splice, was all on the sheave . So the splice was cut off, the rope end whipped and the dinghy method of fastening the halyard was used.

A loop of the halyard is pushed through the ring in the top of the sail. The short end of the loop tail, with a stopper knot tied in the end, is passed over the top of the sail and pushed down through the loop, the loop is pulled tight. Now only the 3/8 inch line is loading the mast head sheave instead of the fatter spliced section of line. Elegant solution, eh?