Thursday, January 21, 2010

The "BLACK FLY" Dinghy 1: from Plans to Framing

Left: a "Black Fly" built by Maury Madden

The Black Fly is a Platt Monfort-designed Geodesic AiroLITE skin-on-frame dinghy that is 8' long and only weighs about 28 pounds.

My present dinghy, an Eastport Pram, weighs about 75 pounds, so this new one should be a lot easier to lift on deck. The Eastport Pram costs about 1/2 knot in boat speed when towing, so maybe the new 'Fly will tow more easily!

There is also a Yahoo Group for Geodesic AiroLITE Boats.

The build:
Before I started to build, I read over the plans and instructions carefully. The plans included full-size layout  half-templates for the building frame stations. The plans were tacked over a 1/8" piece of plwood, and very small holes drilled at each point along each building frame station (and the transoms), then the 'dots' joined, showing the frame outlines.

The frames were sketched in on the back of the plywood also. This gave me a 'left' and 'right' template for each frame, ensuring symetry was kept.

The plywood template was used to layout each frame, left and right from a drawn centerline on pieces of 1/4" plywood. The plans called for using heavy cardboard, and a cardboard / wood box beam 'strong-back' to set up the frames. But as I had lots of old 1/4" plywood I used this for the frames, and an old 6" x 6" fence post for the 'strong-back'.

The 6 x 6 was mounted to two saw horses and the frames and transoms set up according to measurements on the plans. Braces were added as necessary to make the whole rig nice and stiff.

I followed the suggestion in the plans and laminated the transoms over a 1" block, giving them a nice curve.

Next step was to cut and fit the keelson, the stringers, and the gunwales. These were cut from a nice oak board I had 'in stock'. After gluing them to the transoms with epoxy (I use West Systems), the two 1/4" plywood bottom panels were cut, fit, and epoxy'd in place. A bit of trimming made everything ship-shape.

A nice piece of Ash wood was chosen next, and cut up and planned to specified dimensions to be used as the 32 ribs. I don't have a steam box, so cobbled one together from some scrap 2x6 lumber, a canning pot, and a propane camp stove.

This worked amazingly well, as the box was minimal in size, and heated easily. A couple of ribs got overheated and were more like a wet noodle than a piece of wood! The 1/4" x 1/2" x 24" ribs cooled really fast, so it was handy having the box right by the boat. As soon as a rib came out of the box I bent it by hand, then slipped it into place inside the stringers, clamping it in place with 2" pieces of slotted 2" ABS pipe, and some cable ties when the pipe ran out.

After the ribs had dried somewhat, Gorilla Glue was used to glue them to the plywood bottom, the stringers, and the gunwale.

Next morning it was time to remove the frame from the building jig. The little boat was amazingly lightweight and rigid! However, much to my dismay, a couple of the glue joints between stringer and rib had popped open. This made all the joints suspect.

After sleeping on the problem, I decided to put the boat in a warm basement room, and re-glue all the joints ... all 96 of them ... with epoxy thickened with coloidal silica. This was done in 4 shifts by removing/re-setting every 4th rib so as not to disturb boat shape. In all fairness to the Gorilla Glue I'd used on the first try, some of the joints refused to come apart, and damaged the wood when finally forced. It's probably pretty good stuff, but contains some nasty ingredients!

Finally back on track .... the starboard inwale is finished, and the port one in place and the epoxy setting.

Inwales done, and knees are glued in.

Black Fly goes for a ride to the marina.....

...... for a test fit on Sin Tacha's fore deck. I lifted the boat frame onto the car roof with one hand, and carried it along the docks under one arm. I hope it doesn't gain too much weight before completion!

The next step is to clean up all the glue joints, fill any holes, and coat all the wood with epoxy, while I wait for the Kevlar Roving to arrive. As it was only ordered today, there'll be lots of time to get all the detail work done. There're quite a few little bits and pieces that need making too, as well as the seat frames.

Platt's drawing calls for the main thwart a little too far forward for my liking. Comparing the measurements to my Eastport Pram (very similar in size), if I move the thwart aft by one rib it should be perfectly balanced with one person aboard. I'm not sure if I'll install a forward rowing position yet, maybe later. Looking at Maury Maden's 'Fly in the picture at the top of this post it appears he also moved the main thwart aft by one rib.

Gunwale detail, showing spacer block for inwale, and rib notched into block.

Transom knee installed with fillet.

Rib meets bottom, with generous fillet.

Bow area, with hole to fit over Vega fore deck cleat.

Bow knee and fillet along keelson.

...... To part 2: Centerboard slot and case.

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