12:1 Sprit Scarf

Skorpa does not have a boom at the base of her sails. She uses sprits to hold the clew of the sail out away from the mast.

This photo shows Graham towing Southern Skimmer down to the water for her maiden voyage. You can see the sprit running diagonally from part way up the mast down to the clew of the mainsail.Sprits have some advantages over booms.One of which is that sprits can be smaller and lighter than a comparable boom.

The main sprit is long, almost 14 feet. I did not have any wood that was clear straight grained and that long. I did have some shorter pieces. I used some scarfs to join shorter pieces into the lengths I needed.

On the left is a piece of Douglas Fir joining it on the right is a piece of Sitka Spruce. The pieces are 1 1/2 inches thick. The scarf joint stretches out 12 times that or about 18 inches. The scantlings call for 2 X 2 so I laminated some 1/4 pieces of Spruce I had on hand. This made for a lot of extra gluing and a real sandwich.

On the bottom you can see the main sprit prior to any shaping. Above it is the aft end of the mizzen sprit. The pointy part slips into a webbing loop on the sail. A line goes through that hole and attaches the sheet block.

The plans say the spars may be tapered and rounded towards the ends, but they do not say how much to taper or where to start the taper. I decided to just use my judgement, after all I am practically a boat builder The taper does not remove much weight, but I was amazed at how much nicer they look with a small amount of taper and a little rounding..

Putting a little taper into the rectangular stock is fairly straight forward. I placed a mark where I wanted the taper to start. At the end I chose how much taper I wanted, placed another mark and connected them with a straight edge. This leaves a thin wedge of wood which I removed with a plane.

The mizzen sprit has been tapered and has a 7 degree angle added for the snotter cheek block.


I chose to leave the sprits squared off on their forward ends and to round them toward the aft ends. To go from square to round I use a plane to bring down each of the corners. This transforms four side to eights side or eight to sixteen. Eventually to something that looks pretty round. The further you can go with the plane the better. To do all this evenly I need guide marks on the spar. Here is a link with some more info on this.                 Making a spar gauge

I have used geometry to divide the sides onto the correct ratio, but it is a whole lot quicker to use a spar gauge.

You adjust the angle of the gauge to keep the outer dowels in contact with the tapering spar. The nails leave marks. Everything needs to be pretty precise. A drill press would have been handy.

 Normally the best way to fasten the cheek block to the spar would be to through bolt it. That leaves an ugly nut on the back side and a place for water to get in. The Gougeon brothers suggest that casting machine screws into epoxy can make a durable alternative to through bolting.We will find out. On the main sprit I also cast a nut into the epoxy well. That almost makes the equivalent of a through bolt.

After I got the spar as round as I could with the plane. I used these torture blocks to finish the job.The blocks are pieces of cardboard tube with sandpaper glued on with 3M spray adhesive.

Coating the spars with epoxy would harden and waterproof the surface, but then the spars would need several coats of varnish to protect the epoxy. I decided to go with a penetrating oil finish. I found a can in the cellar hand labeled Linseed Oil and Turps for gutters. Wooden house gutters were not uncommon around here years ago. Turps is spirits of turpentine used to thin the linseed  oil so it penetrates. I put a coat of Gutter Oil followed by a coat of commercial Teak Oil because it has all sorts of fancy uv inhibitors.

Spars are done.