Thrust Duct

The former for the duct is constructed from two sheets of 3/8″ plywood.
2×6 lumber is used to separate the two 3/8″ plywood sheets. This gives the sheets good separation, which eventually makes the 1/8″ plywood that wil form the inside of the duct easier to wrap around the former. The downside is the increased weight, which can become significant on a 60″ duct.
This is a dolly built to hold the thrust duct and allow it to be freely rotated during shaping and fiberglassing.
A custom circle cutting jig is used with the router to cut a perfect circle from both the top and bottom plywood sheets
The completed former on the dolly. I added casters so that the duct could easily be wheeled around the shop to make room for other projects when necessary.
Glassing the seam on the 1/8″ birch plywood that will form the inside of the thrust duct.
Preparing to nail the 1/8″ plywood to the duct former.
Once the plywood iss wrapped around the former, the final finishing nails are hammered into the ends. Finishing nails are perfect for this job because they have very small heads, and can be pulley through the plywood from the inside of the duct after it’s mounted to the hovercraft.
The first layer of 1″ EPS foam has been wrapped around the duct former. I used gorilla glue to bond the first layer.
An additional two layers. The clamped board is helping to support the ends of the EPS foam sheet, which tended to pop up while the glue was drying. For all but the first layer of EPS foam, I used 3M spray adhesive to bond the foam.
Now all 4 layers of 1″ EPS have been wrapped around the duct.
A closeup of the wrapped EPS sheets. I also added a thinner piece (~1/2″) around the inside of the duct adjacent to the inner plywood to be sanded to a smooth transition between the foam and yplwood. If not for this, there would be a 1/8″ height difference between the plywood and foam that is difficult to fiberglass over.
The inside of the duct mouth has now been sanded to a smooth contour. Bulk material removal was done with a belt sander and an 80 grit belt, final sanding was done by hand.
This is a quick and dirty hot wire setup to cut the contour on the outside of the thrust duct.
After hotwiring, the finish is fairly rough, as the hotwire has to go over bumps in the gorilla glue squeezeout at the back of the duct.
A little work with the belt sander cleans the outside contour up nicely.
After a while, the spray adhesive holding the EPS sheets together started to let go, so I quickly began fiberglassing around the inlet lip to hold the foam together.
Light weight spackling compound is used to fill depressions and irregularities on the outside of the duct before fiberglassing.
The duct now has three layers of 6 oz fiberglass, and is feeling very strong. If you press hard, you can slightly deform the outside of the duct, but the curved inner lip is very rigid. I sawed off a flat area where the duct will mount to the hull.
The top of the hull where the duct will contact was scuffed is 80 grit sandpaper for better epoxy adhesion. Epoxy with microspheres is used to bond the duct to the hull. I poured a thick layer where the flat area of the duct would contact, hoping to get uniform squeezeout all around.
The thick epoxy/microspheres mixture worked perfectly. There’s a nice bead of squeezeout epoxy all around, meaning that the two surfaces being joined should have a good contact.
1/4″ plywood is used to seal off the joint. This completes the thrust duct installation! After bracing is installed, it’ll be time to remove the former.