Thrust Duct

I will be using a 54 inch prop, therefore I will cut the disc former 54 1/4. This is greater than the width of a sheet of plywood, so here you can see were the extra wood was attached to complete the disc.
Used the router and jig to make the cut. ** Make sure of your measurements. ** Cut one disc incorrectly and had to add 1/8 ply to outer diameter to correct the cut.
Made some separators out of scrap 2x6, ripped them to 5 inch width

This shows the finished disc former on a 1 inch shaft, so that I can rotate it while constructing the duct.
front view

Here I used a piece of wire and pen to mark the thrust duct for cutting, be very careful with your measurements

The thrust duct is all cut out and I put down 4 oz cloth, notice the 1 1/2 inch portion on the top that was not glassed
Just another view
Here I have cut slots into the non glassed side so that the foam will bend easy

The foam is tapered from the leading edge to the trailing edge, I made a hot wire jig to do this. What you see here is the leading edge end, notice the wire going through the wood at 4 inch then taper down
The wire then runs through the other end at the correct angle, a spring is used to correct wire tension
The jig is hooked up to a battery charger and the cut is made. This turned out very nice
Completed foam, needs a light sanding
This picture shows the plywood on the disk former, I will glass between the tie down straps using 6 oz tape.
This is a picture of the foam attached to the disk former, I ran into some trouble here, not sure were I went wrong but the trailing edge did not meet up with the former exactly, I had to cut slots into the foam to force the foam down to meet the former. I was very careful in my measurements, I t seems that the angle that was cut into the foam was two much. I have a gap of about 1/8 all around the disk. It is a lot of work to redo the duct, so I will try to fix it.
Here is a picture of the straps holding the foam to the former , If you look closely you can see the slots that I had to cut in the trailing edge to make the fit.
A picture of the disk after the straps have been removed. The gap at the trailing edge is still there in places, I will need to do some jiggery pokery later to clean this up.
I rough cut the inner wall of the duct. This should be a 6 inch radius. I used a auto body rasp to do this. I then rough cut the outer wall of the duct to a 3 inch radius. After that was completed I sanded the areas smooth. I checked both curves with a template that I cut in some plywood.
I now needed to fill all those cuts in the foam that I had made so that the foam would bend easy. I did not know what to use for this so after some investigation I used a water resistant light weight spakle??"Not sure of the spelling". Here I have just smeared it into the cuts with my hand.
This picture shows the finished Leading edge. It turned out good.
I am putting 3 steering rudders on the thrust duct. This is the cut that I made in the duct to accept the center rudder support.
Here I have removed the foam, and leveled the leading edge of the duct. I will be making a second cut in the duct much like this one to accept the steering linkage.
I am dry fitting the duct on the craft, I am running the steering linkage under the duct.
This is a close-up of the base of the duct. Notice the two slots that were cut in the duct to accept steering and rudder supports.
Time to add supports to the side of the duct and foam the areas inside for support. I cut 1/8 ply 18X5.5X8.5 for the supports, then used duct tape to close the front and back areas, leaving a space to pour the 2 part foam in.
This is a picture of the back
I poured the foam in and let harden, 2 part foam will expand and harden in about 1 hour, The sloppy stuff you see is the areas that I used Great stuff expanding foam.
A picture of the back. If you are wondering how I centered the duct, I used the 1 inch drive shaft with a level and a square. A word of caution with two part foam, it will lift the duct if not weighted down. After the foam hardened I was out of square about a quarter of a bubble in the level, Duct pointing slightly up, I am hoping this will not matter.
I have cleaned up the thrust duct support and have enclosed it with plywood. I will be glassing all seams and corners so that this is water tight.
I used some string to position the supports for the duct. I also cut a template out of cardboard, you can see this on the right of the picture
Here is a close up of the engine mount connection for the support.I sandwiched the 1/4 inch ply between two pieces of aluminum that I bent to the proper angle. The other end will be epoxied into the duct.
This is another view of the support. I also cut a slot in some doweling and will be gluing it to the leading edge of the support. This is for two purposes. 1) It looks better, and 2) I will be able to hide any wiring for a light in it. This is necessary because were I live I understand that I must register this as a boat, and all boats must have a light placed at the highest point on the stern.

This picture is to show the duct supports. I will not be adding the braces that the plans indicate to have between the hull and the duct. I do not think I need them because of the amount of area that I foamed in.
I rolled on a layer of epoxy to the inside of the duct to water proof it.
This picture shows a spread of epoxy/microballoons mixture used to fill in imperfections and fibre weaves from the cloth. once sanded out the duct intake should be smooth.

mailto:bruce.einarson@mts.net