Asymmetrical Sprit Conversion

So, Stew Harris was here today (15 June 2014), and gave me hell for not posting the photographs that I have of last August's conversion of a 1989 Melges M-20 to an I-20, i.e., the installation of an asymmetrical sprit.  This IS meant to be a bit of a how-to exercise; if material information is missing, please e-mail with your questions.

The patient at the beginning of the process.

Before starting work, with a copy of the Class Scantling Rules in hand, verify that the boat conforms to the class scantling rules.  This boat was not only being converted to an asymmetrical spinnaker, it was also being converted to a load bearing headstay.  The dimensions were spot on with this hull.  18'8" is the dimension from the plane of the transom.

For the flat decks of an M-20/I-20, I make up a 'pseudo' sprit, a 2 x 4 ripped down to a 2.5" wide timber, the same as the nominal width of an I-20 pole (carbon sprits are about 2.42" OD, aluminum is 2.5") Here the 'pole' is laid on the deck, lined up with the centerline of the boat, a piece of mason's twine strung from the centerline of the transom, over the centerline of the mast base, and out to the centerline of the end of the 'pole'.  The 'pole' as it runs across the deck shows you exactly where the hole in the deck must be cut before mounting the sprit hood.

How to tape a bit of mason's twine to the end of a ripped 2 x 4.  We really didn't need to show this.

The 'pseudo pole' is marked with the limit of the extension of the 8 foot pole from the front of the hull.  In the case of the I-20, this dimension is 46.5".  Note the chalk line down the center of the hull, which is not necessarily where the manufacturer mounted the hardware.  Boatbuilders are human, and they screw up...all the time.

A close look at the back end of the 'pseudo pole', or as I call it, the 'story' pole. The blue masking tape isn't necessary, it's just in here to show contrast, pencil marks alone will do.  The demarcations show the minimum that the deck cutout must be.

The mason's twine through the chain plate, which was centered.  The other pencil line, forward of the 18'8" tack limit, is the forward limit of the sprit hood, 15-5/8" aft of the bow.

A 'dry fit' of the new sprit hood.  Note centerline marks, the hood slides around until one places those 5 pound lead weights on it.  Note center chalk line, tape measure, scantling rule printout, speed square, and of course, the ubiquitous pencil.

An easy way to cut a radius is just to use a hole saw, then connect the resultant holes with a jigsaw.  Note the athwartships pencil line, showing the forward limit of the front of the sprit hood, which the rule controls at 15-5/8" aft of the bow as the maximum forward limit.

We love cutting holes in boats.

Glassing in the aft bulkhead which carries the aft end of the sprit.

Tabbing in the bulkhead is a very simple task.  The machine nuts are on the backside of a deck strap which holds the single block which pulls the pole forward.  Install this first before installing the bulkhead...much easier.

This is the 'pattern' to size the bulkhead...dimensionally every boat is slightly different.  Here two pieces of cardboard, with their top and bottom edges conforming to the boat's shape, are slid up and down to fill the space between hull and deck; it is then taped to hold that dimension, removed from the boat and then the shape and length transferred to the actual bulkhead laminate, and cut out.

Where the rubber meets the road: after the hole is drilled in the bulkhead (note how close it is to the underside of the deck), insert the actual pole, and put shims under the front of the pole.  Adjust the shims to vary the height up and down to conform with the rule as to tack height; this is where one uses the speed square, note the blue masking tape to highlight that tape height. This exercise will tell you how high to drill the hole in the front of the sprit hood.  Note the trouble light inside the hull.

The aft end, showing the tack line.  The line exits the aft end of the pole, turns at a floating block, and up to yet another block to turn it back into a cam cleat at the front of the cockpit.  The blue radiator hose limits the extension of the pole to conform with the rule, and the shock cord retracts the pole automatically, also required by the rule.  This whole exercise would be much easier if the rule permitted the tack line to run outside the pole.

The finished pole, just before installing the sprit hood.  Note the inspection port installed opposite the pole, which permits holding the 3 nuts under the front of the sprit hood...the after fasteners holding the sprit hood are just self-tapping screws.  The inspection port also permits maintenance of the forward downhaul and forestay sheaves, probably a first for this hull.  The red color is optional.

I was looking for a final shot of the sprit hood bolted to the deck, and couldn't find one, so here is posted what I found: a shot of a turtle hanging out on the lawn.  Think of the similarities.