River Rat



I stated in the previous post that the goal was to have the craft somewhat operational before ice up to see how much weight it could carry before it was too heavy to get over the hump on water.  Mother nature won and our lakes are frozen over for a couple weeks now.

As the hull was formed in a single piece, I had to add on a 2 1/2’ section onto the floor of the hull (directly beneath the fan duct), to ensure the back plenum profile was identical to the sides.  This will simplify the skirt construction, keeping the design the same on the sides and back.  Also, the hull extension will provide valuable buoyancy to offset the robust Toyota 22-R engine and drive reduction system.  I was able to make the hull bottom extension by using 2” pink Styrofoam SM.  It cuts and sands easily.  I was able to use some long framing nails and push them in by hand to hold the “form” in place, and then pour “2 lb” part A and B urethane foam into this cavity.  I also learned that polyester resin tends to chew away part of the surface of this foam.  Apparently epoxy resin does not react with the Styrofoam….

A pic showing the fiberglass panels being put into place to complete the plenum.

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I have completed the plenum!  I needed some fiberglass panels to complete the plenum.  I had a 4’x8’ sheet of 1/8 steel plate available that I put on two sawhorses.  I sanded the plate with fine sandpaper to make sure it was very smooth.  I then waxed the plate using automotive “carnuba” wax. 

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I then buffed the surface with a cloth.  I laid down a layer of 4’x8’ of fiberglass matting. 

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I worked in the polyester resin into the matting using a roller, removing tiny bubbles of air.  From what I read, the better resin to use is epoxy resin as compared to polyester resin as it is much stronger and would be lighter in weight as the end material does not have to be as thick.  The cost of epoxy resin is very high so I have opted to use the polyester resin.  I then applied a second layer of matting.   When it cured, it was easy to pry the sheet away from the metal plate.  I was then able to rip the sheets with my table saw to the exact width required to complete the plenum.  At this point, I routed the “push-pull” steering cable through the plenum before I closed it in.  The steering cable is completely enclosed in a sheath so I don’t anticipate problems.  If I have to change the cable in the future, I can use the old one to fish the new one through the plenum.   I used duct tape to hold the sheets in place while I used 3” strips of chopped matt to attach the panels onto the craft.  I put the smooth side to the inside of the plenum as it will provide little resistance to air flow and the wax residue left on the panel would have to be sanded if I wanted fiberglass to stick to it.  I then applied another layer of chopped matt over all the panels and overlapped edges.

Next I drilled holes in the plenum for each individual segment.  My hull will use 90 finger segments, each having a width of 6”.  From what I read from various forums, the hole size should vary from the back segments to the sides and front.  I used a 1¼” hole size on the back, then 2” on the back corners and 3” on the sides and front.  I may have to even elongate the holes on the front to ensure there is plenty of air that can get to the front segments to prevent plow-in.

I also attached the P clips to the plenum wall using special rivets that will “peel” rather than bulge to ensure the rivets don’t pull out of the fiberglass.  I was very impressed how firm they attach the P clips.

I needed some skids for the bottom.  I originally was trying to track down some aluminum channel, similar to what a step ladder uses.  I was concerned about the expansion difference between aluminum and fiberglass and how that might break the bond between the fiberglass and aluminum.  I decided to construct fiberglass skids.   A friend had some 14’ 2”x6” planks.  I wrapped them in a layer of poly plastic sheeting and used deck screws to put them together in an U configuration to make a mold.  I applied a layer of wax on the plastic.  I used a layer of chopped strand matt.  When it cured, I was able to pry the fiberglass skid away from the mold.  I fiber-glassed the three skids onto the hull bottom.  I drilled holes in the top of the skid and poured some A-B urethane foam.  The foam expanded and filled the skid, which added quite a bit of stiffness.  I sanded the skid slightly and then added a second layer of chopped matt to provide more strength.  I then sanded the surface lightly with a belt sander to provide a smoother surface for the aluminum rails.  I drilled holes in the 1/8” by 1” wide aluminum flat stock.  I used a larger bit to countersink the holes so the screws would be flush to the rails.  I applied silicone to the skids and underside of the rails and screwed the rails to the skids.  I would anticipate that most of the bond will come from the silicone and not the screws over the long-term.  I would expect the aluminum to expand and contract differently than the fiberglass but the silicone should give enough to keep the bond.

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I designed four scupper valves and printed them on a 3D printer.  They will help get rid of water that should make its way into the plenum while keeping water from entering the plenum.   


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My wife and I have been getting some help in the design of the skirt.  Special thanks to Ian from Rebel Hovercraft for sending me some sample skirt segments.  They really helped us understand how the different finger types were constructed.   We also want to thank Kevin Foster from HCGB for his guidance on skirt design.  We settled on following the design from the HCGB Constructors Guide.  We will have a unique profile for the front, front corners, sides, rear corners and rear of the craft.  We obtained a 50 yard roll of urethane coated nylon that has a special coating that repels water on both sides of the fabric.  The constructors guide is very detailed on how to make the curved segments.


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I wanted to make a false floor on the bottom of the craft.  The floor will be filled with foam to provide buoyancy for the craft as well as channel any rainwater into a sump area that will be pumped out with a sump pump.  I used three lengths of 2” PVC piping (Central vacuum pipe) down the middle of the craft.  In one, I will have the throttle cable and the fuel lines.  The second will be for the electrical and the third will be for the heater hoses as I want to have some heat in the front as the climate is very cool here.  I made a lattice out of some of the left over fiberglass panels.  I am using strips of fiberglass cloth to adhere them onto the bottom of the hull.  It is providing strength to the hull and is stopping some minor flexing of the hull.  I will lay a fiberglass panel over the whole lattice and pour expanding foam into the cells which should stiffen them up further.  In the photo below you can see the joystick in the right side.

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This is how the lower shaft and pulley look:

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I lowered the engine onto the fiberglass “pillars” of the hull.  The bottom of the craft had some flex so I made a bulkhead out of lightweight rectangular tubing and a fiberglass panel.  The engine is very secure.  I am presently working on the drive reduction mechanism mount.  The fan looks nicely centered into the duct!

Here is a pic of the bulkhead support.


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A pic of the fan mount and upper sheave.  The mount is not complete as of yet and will need some cross bracing.

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I have been working on the rudders as well...

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I am using a 3D printer to make bearing holders for the upper and lower rudder mounts.


Well, that's where I am right now!  I have been dreaming about the project and each day get more exciting as it develops.  Stay tuned.  Robert



# Gone7 2015-11-24 09:27
Holy cow's really coming along! This is a completely different craft than what we're used to seeing here and it certainly adds to the interest. You're also using some unique techniques to get things done...impressi ve! I like how you've incorporated your 3D printer as you have. I may have to go out and buy one myself! I like how you've used your metal table as a fiberglassing mold...very cool stuff!

Those "scupper valves" are ingenious, you may be able to market those. I'm a little confused though about what their purpose really is though.
They will help get rid of water that should make its way into the plenum while keeping water from entering the plenum.
From my understanding of the word "plenum," this is the airspace encapsulated beneath your craft and contained by the skirt. This is what basically makes you hover. It appears to me that you're wanting to drain water from inside the hull to the outside?

Also, I can see that your skirt must be filling from an air scoop coming off of your thrust propeller, but from the photos showing the bottom of your hull I can't see where this fill actually goes from the hull into the skirt. Is that just not shown, or? Like I said, this is a completely new craft to me, I know nothing about it and I'm really curious.

Nice job!
# tymorobe 2015-11-27 08:03
Thanks for the comments! When I used the term plenum, I meant the pathway for air to flow between the inner and outer hull. When one stops on water, there is likely some seepage of water coming in through the rivet holes that hold the P clips for the lower skirt attachment points. The scupper valves, although not perfect, will help drain any water that should make its way in while preventing a uptake of water through the valve when the craft is stopped on water. The alternative would be to have some holes drilled in the outer hull to purge the water accumulated but without a valve, the holes will allow water in quickly (so after a short stop on water, some time would be needed at hover to purge the water out). Regarding where the air goes into the "plenum", right behind the fan, the lower third will have a splitter plate that directs that air in. I am considering making a variable splitter plate so the amount of lift verses thrust air can be changed on the fly.

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Author / Member : tymorobe

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