Friday, January 9, 2009

Wyliecat width of wishbone on Blueberry

Link to earlier blogs in blog order
Links to earlier blogs in chronological order:
Nonsuch 22 Brochure
Forward looking sonar on Blueberry
MarsKeel stability bulb torpedo on Blueberry
Wyliecat width of wishbone on Blueberry

Some 50 years before Mark Ellis designed the Nonsuch, and Tom Wylie designed the Wyliecat, Sidney Herreshoff was designing small sailboats with wishbone booms, sometimes referred to as "wishboom" rigs. Wishbone boom design has flourished among suppliers of wind surfing equipment.

Tom Wylie's racing oriented boat has a wishbone design considerably wider than the wishbone design of the Mark Ellis Nonsuch cruiser, to allow for some light air sail shape racing advantage. So, "It seemed like a good idea at the time." came into play one more time in my life

The drawing below was based on my "back of the envelope" sketch for the estimator at Accurate Tube Bending in Newark CA. My sketch required a new nose piece and a new tail piece of mild steel bent to fit the original Nonsuch aluminum side tubes. Mild steel was selected for this design instead of the carbon fiber nose and tail pieces that the Wyliecat used with their aluminum side rails. A lot of thought went into electrochemical isolation of the aluminum and steel components.

The tail piece casting is shown below. The side tubes are fitted over the tail piece casting, and held in place with screws drilled through the tube and tapped into the casting. The stainless steel screws have to be coated with some dielectric barrier such as Lanocote to prevent the aluminum, in contact with the steel to "rot" into a crumbly white material.

The new tail piece, shown below along with the old one, is not a casting, but is, instead a bent tubing, the same size as the side tubing, with a pair of ferrules slipped inside these tubes to bridge the joint.

The nose piece casting is shown below. As with the tail piece, the side tubes are fitted over the nose piece casting, and held in place with screws drilled through the tube and tapped into the casting.

The new nose piece, shown bekow along with the old one, is also not a casting, but is, like the tail piece, a bent tubing, the same size as the side tubing, with a pair of ferrules slipped inside these tubes to bridge the joint.

Shown below is the new tail piece fitted, but not yet drilled and tapped for the the joint screws.

Shown below is the new nose piece almost fitted, with one of the ferrules visible on the right hand side.

Shown below is the longer front to back extent of the new tail piece compared to the original tail piece casting.

Shown below is the longer front to back extent of the new nose piece compared to the original casting.

Shown below is a side view of the new nose piece.

Shown below is the new wishbone in place on Blueberry. The original Nonsuch rubber bumpers were retained on the side rails to cushion occasional slamming of the wishbone against the mast when all wishbone control lines happen to be slack.

The length of the original mast to wishbone hangers was retained, though provision was made for an alternative attachment position further forward, as in the Wyliecat design. I was concerned about the length of these hangers until I realized two things:

First, the twin topping lifts could be used to effectively lighten the weight of the wishbone on the leech of the sail in light air conditions.

Second, in all but the lightest air conditions, the front of the wishbone was essentially supported by the choker lines, at least on the Wyliecat, leaving the mast to wishbone support lines slack.

There are wishbone hanger length, hanger position, choker line attachment and wishbone angle design details involved here that are yet to be explored.

The picture below shows the double topping lift of port and starboard lines, running from the mast head towards the rear of the wishbone, similar to a pair of running back stays.

The green, starboard topping lift, and red, port topping lift, are each spliced to a shackle. A shorter length of line is spliced to the shackle and continues on to another splice on a shackle at the mid point of the rear of the wishbone. Those two fixed lengths combine to hold the wishbone safely above the cockpit. The lower fixed length lines each have an adjustable length line beside them, to use as a topping lift for raising the wishbone.

Either side of the topping lift may be used to raise the rear of the wishbone for hoisting the sail, or reefing the sail. After each of these procedures, the topping lift is released, with the leech of the sail supporting the wishbone.

In light air sailing conditions, the windward topping lift can be tensioned to ease leech tension enough to allow a desired amount of twist to the sail. When the windward topping lift is tensioned, this moves the wishbone towards the leeward side, providing slightly more room for the draft curve of the sail inside the wishbone in light air sailing.

When tacking in light air, the leeward topping lift is tensioned to match the position of the windward topping lift, the tack is initiated and the windward topping lift is released, to become the slack, leeward topping lift.

Each topping lift has a two part tackle of blocks at the rear side of the wishbone. The topping lift line then runs through hanger blocks along the side of the wishbone to a second two part tackle of blocks running from the wishbone to the deck and back to the cockpit.

The resultant four part tackle makes the level of effort for topping lift adjustment quite reasonable. This is shown in the choker line picture, below this one.

The two choker control lines start at a pad eye on each side of the mast, pass through blocks on either side of the center of the nose piece, and through blocks at the beginning pad eyes, then down through a single block in front of the mast.

The picture below shows a one piece choker line beginning at each side of the mast, running out to blocks at the nose piece of the wishbone, and around a single block supporting a triple block below it. When the choker line is slack, allowing the wishbone to relax to a fully forward of the mast position, the single block pulls the triple block up as high as it ever gets. Pulling on either the port side or starboard side choker line draws that triple down the front of the mast closer to the double block between the two cheek blocks on the mast near the deck.

Having port and starboard choker lines, port and starboard topping lift lines, and a double ended main sheet, with port and starboard winches, makes single single handed sailing rather convenient.

Well, except for one design detail. The double sided topping lift works wonderfully to adjust the twist in the upper part of the main without interfering with the leeward side of the main. What about hoisting the sail or shaking out a reef or two back to a full sail?

The two lines of the double sided topping lift  come together at the top of the mast in a very narrow V. This makes it almost impossible, in any sort of breeze, to hoist the fully battened sail, with its large roach, in between the topping lift lines. The solution to that is a bungee cord, running from one topping lift line around the mast to to the other line. The two end of the bungee cord are spliced around a thimble to allow the bungee to slide up and down to topping lift line as required. the picture below shows both topping lifts pulled tight enough to hold the wishbone in place with the sail down.

In the picture below, the port topping lift is holding up the wishbone while the starboard topping lift being slack, allows the bungee cord to pull the starboard topping lift back toward the mast allowing the sail to be fully hoisted without snagging the sail on the topping lift.

The strategy is to allow the wishbone to blow off to the leeward side of the boat, for this example to the starboard side. Notice that with the starboard topping lift eased, the bungee cord pulls that starboard topping lift back towards the mast. As the sail is hoisted, the headboard car keeps the top of the sail between the two topping lift lines. With leech of the main flapping in the breeze on the starboard  side,  none of the full battens catch on that pulled forward starboard topping lift.

With the main fully hoisted, ease the port topping lift so both topping lifts are pulled back towards the mast until one or the other topping lifts need to be tensioned to hold up the wishbone when the main is to be dropped.