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You need to do several things:
Take the mast down or simply flip the boat onto its side on the launching trolley. Remove the topping lift and spinnaker halyard sheave boxes. If your topping lift is already just under the jib halyard, leave it alone. Keep the spinnaker halyard sheave box, you will reuse it.
Measure from the top of the deck band up 5955 mm (about 19 feet 6.4488 inches). That is the maximum height of the new spinnaker halyard. The way that is measured is to hold the spinnaker halyard straight out from the sheave box, perpendicular to the mast, and measure to the bottom of the halyard (so we ignore the thickness of the halyard). This is essentially the same as measuring to the surface of the sheave. Mark that point on the mast. Go a couple of mm lower so you don't have a measurement problem. Create the hole for the sheave cage. I drill out several holes and then file from one into the other with a round file, then flatten the sides and shape to fit. I like to tap the mounting holes so I can easily remove the fitting.
Take a break and go buy a single hole eye strap, and a Harken air series single block, and a small Ronstan screw head shackle, Shackle the block to the single hole eystrap (a "P" in cross section).
Mount the eye strap, shackle and Harken air block from the lower mounting hole of your jib halyard sheave box. Remove the fastener and then replace it with the mounting hole of the eyestrap included. Use a corrosion barrier such as Lanocote between stainless steel and aluminum.
File the old topping lift sheave hole, smoothing the inside bottom and the outside top, so that when your run the topping lift outside the mast through the old fitting hole, it does not chafe on the edges.
Run the topping lift line outside through the old sheave box hole up to the Harken air block and then down outside the mast. The new position is a little more than a foot above the old US-standard position, so you may need a longer topping lift. I use the old spinnaker halyard for this.
Fish the main halyard out of the old spinnaker halyard sheave box hole, tape the spinnaker halyard to it, pull the main halyard up so that the spinnaker halyard can be seen at the new spinnaker halyard sheave box hole. Now Fish the spinnaker halyard out and remove the tape. You can do this lots of other ways too. What you want to do is get the spinnaker halyard to emerge from the mast at the new hole, rather than the old hole, and you don't want to twist the spinnaker halyard around the main halyard while doing it. Sew your new spinnaker halyard to the old one, and pull the new one through into the mast.
I drill and tap number 10x24 and then use short 10x24 bolts to attach the spinnaker halyard sheave box to the mast. Use a corrosion barrier such as Lanocote between the stainless steel sheave box and the mast, and between the stainless bolts and the mast.
Make sure both the topping lift and spinnaker halyard are where you will be able to reach them when the mast is upright. Re step the mast, or flip the boat upright.
Reattach topping lift and spinnaker halyard, and...
You're done. Now go sailing!