The Lift Duct
The following  are construction images of the lift duct. The images and disciptive notes should give you an idea on how the lift duct of my hovercraft was constructed.
The images above show locating the lift duct center, scribing a line with a compass, and then cutting out or removing the plywood where the bottom of the duct is to be located. I used 2" masking tape to allow better visibility of the scribed line, and to protect the hull.
After the plwood was removed, and a couple of coats of epoxy resin applied, I laid down a strip of peel-ply around the diameter of the duct hole. Once the epoxy  resin has set, the peel-ply will be removed. This will create a good bonding surface for the fiberglass work that will take place when the duct is constructed.
The hover was flipped over to skin the top.The top skin was rough cut,  install with bronze boat nails and epoxy resin, then clamped inplace.
After the epoxy resin set, I removed the clamps. The next step was to find the center where the top duct hole was to be located.I used a chauk line to mark the exact spot.. I drilled a small hole in the center of the duct, then used a router with and extesion bar to cut the hole which turned out almost perfect.
The image on the far right is the lift fan, and the 3/4" plywood disk that will be used to fabricate the duct form.
The lift duct form was pretty easy to fabricate. I used a router to cut out the disks from sheets of 3/4" plywood. Attaching a extesion bar to the router, and bolting  he bar to the center of the disk, the router cut an almost perfect circle. The 1/8"  lift duct wall sections were spliced together. I used 2"x4" to spaced the disks, then wrapped the duct wall around the disks and tacked it in place. I installed the duct form in the hull to check the fit.
Not shown is the method I used to center the duct form in the hull. I obtained a section of 3/8" threaded rod inserted through the cernter of the disks and used nuts and washers on both top and bottom of the form. I cut a 2" x 4" long enough to span the distance between ribs #1 & #2 for support and to obtain the correct angle that the duct needed to be placed in.  I drilled a 3/8" hole in the center of the 2" x 4", installed the threaded rod through the 2" x 4" and used nuts and washers on both sides of the 2" x 4"  to allow raising and lowering the duct form.
After I centered the duct, turning  the nuts on the 2" x 4" allowed me to make micro adjustments to the lift duct form height. Once set to the proper height, I was ready to pour the 2 part foam.
To keep the foam from leaking out of the bottom of the form, peices of cardboard was taped to the bottom of the hull, far left pic.
A hack saw blade was used to cut the foam after it cured.
Shaping the foam to obtain the proper radius was really easy, in fact you have to be careful not to remove to much material. These images show the foam sanded to the correct or nearly correct radius.
The left image is the fwd and the right is the aft portions of the duct inlet lip which are sanded to different radius.
I actually began fiberglassing the bottom of the craft first, so the pictures are a little out of order in relation to the construction sequence.
As mention above, I used cardboard to retain the foam as it expanded. Sanded and shaped the foam, then fiberglassed the bottom of the duct.
Once the last coat of epoxy resin set, I sanded the freshly fiberglassed duct and surrounding area one last time.
Here are a couple of "not so good images" of the bottom after I painted it.
The camera flash failed, I didn't realize this until the film was developed.
I used a two part epoxy paint which turned out pretty good. 
A trial fit of the lift engine and fan. The engine mounts holes have to be located and the mounts install, the fan drilled and mounted to the fan hub.
The lift engine has been stripped of it cowlings, bead blasted to clean her up a bit. The lift engine cradled has been completed, and painted.  Lift engine mounts are installed including the fwd engine mount support blocks. The center image of the lift duct is showing the electrical leads, engine control, and fuel line access ports. There located  on the outboard sides of the duct about 3" to 4" below the top hull, 4 each. The last image shows the fuel line, and the engine choke cable routed. The top hull skins are  in the lay-out process.
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