Thrust Assembly Design

Below are some pictures of a more accurate and detailed thrust assembly on the hull model.

side view:




top view:




isometric view:




I decided to make the beam spanning the prop cage a 2 inch square box beam of aluminum, but I need to discuss its suitability with my solid mechanics professor to make sure it doesn't fail. In theory, all the thrust loads are transferred via the shaft to the forward bearing and then out to the diagonal braces, but a) I don't know if this would actually happen in real life, and b) there are still some very weird gyroscopic loads to contend with.

The other challenge was to devise a way to brace the forward inverted "U" without taking up a lot of cockpit space as my previous design did. On an outside suggestion, I tried bracing it to the top of the prop cage, but the angle was too steep. I decided to combine the lateral and axial braces into one, which sends them off at a strange angle, but gets them out of the way while still extending forward quite a bit. The modeled braces are a bit rough because of how I built them up, but in actuality they will be blended into the corners and the deck.

Except for the aluminum box beam, all of the thrust structure is made out of wood. The advantages of wood are that a) it's cheap, b) it's easy to shape, and c) the joints can be epoxied and fiberglassed, making them ridiculously strong. While aluminum is stronger per pound than wood, to fasten members together you either have to use bolts, or weld them. Both of these joining methods produce stress concentrations, which reduces your overall strength.

You may notice the prop cage is a lot bigger than the prop I have sketched in, and there's a good reason for this. Initially I will be running a prop I already have, which is pitched to be run directly on the engine with no reduction (36" dia, 18 degree pitch). At some point in the future when I have more time, I will fabricate a larger diameter prop with a higher pitch and adjust the pulley ratio to reduce the prop shaft rpm. The prop cage is sized to this future prop (48" dia).

You may also notice that there is no bottom on the prop cage: this is on purpose. The side legs will be glassed directly into the deck. Putting a bottom on the prop cage on the last craft caused debris to build up in the cage that would eventually get sucked through the prop, damaging it. This design allows a free drainage path to prevent this.

I am still working on how to mount the engine to the platform (3/4" plywood). The most elegant solution would be to drill and counterbore holes in the plywood, so that the bolt heads sat flush from the underside, and then epoxy them in place when the plywood was laid down on the hull. However, there are 2 main problems: 1) they have to be in EXACTLY the right spot the first time. From personal experience, I can tell you this is a lot harder than it sounds - it normally takes 3 iterations of drilling and "adjusting" the bolt holes in the mounting substrate before they are right. And 2) if the bolts separate from the epoxy later from vibrations or what-have-you, it will be very, very, very difficult to fix. I'll have to sleep on this problem some more.

That's all for now, folks.

Hull re-design

On the advice on some very experienced hovercraft designers, I have modified the hull design to maintain the full width for 7 feet of the length. This will increase payload capacity and increase the craft's lateral stability.

This does complicate the fabrication of the hull somewhat, as now the sides and their plow planes must be made in 2 cuts, and the walls must also be fabricated in 2 pieces. I am becoming concerned about the amount of 1" foam the design will need for the walls, bow panel, and top of the bow. If necessary, the top of the bow could be fabricated from 1/4" foam, since it is not structural.

I have also decided I'm going to need 2 more gallons of epoxy ($200), and most likely another roll of 10 oz. fiberglass ($7.xx / yd.). Many of the components could be fabricated with a lighter weight glass, (probably around 6Oz.), but purchasing a roll of 6 oz. in addition to a roll of 10 oz. would eat up a large portion of my already cramped budget. This weekend (easter weekend) I will visit home, so I will have the opportunity to ascertain exactly how much fiberglass and skirt material I have left over.

Anyway, renders of the re-design, courtesy of SolidWorks as usual (click for full resolution).

Side view:




Top view:




Underside view:




Isometric view:



Happily, modifying the hull allows me to take a much more conventional approach to constructing the lift duct. This will decrease construction time and angst quite a bit.

There is still a lot of design work to be done, even though it looks pretty finished. For instance, I'm going to need to install a tow hook in the bow in the event that the craft ever needs to be towed. This will require routing a hole in the forward section of the hull, epoxying in a large block of wood, and cutting another hole in the bow panel to allow the hook to pass through, then sealing the entire thing up. Little details like this are almost not worth the trouble to put on the plans, because all the fitting is done to other components, so any dimensions you give in SolidWorks are useless. The main thing is not forgetting to put the tow hook in.

As a second example, the back of the craft will not be left open: there will be small panels on either side of the prop cage that prevent water from entering the cockpit (for lack of a better word) when off cushion. In the bottom corners of those panels will need to be some drain holes that allow any water that enters the cockpit to drain out. After the panels are glassed in place, I will need to take a drill to them, and then seal the holes with epoxy.

As a third example, on my last craft I had issues with sand and other debris being carried by water into corners near the prop's plane of rotation. The debris would then get sucked into the prop, tearing up the tip tape. To prevent the same from happening on this craft, I'm going to make two low "dams" to direct any water and debris around the prop cage and engine mount area. But you won't find these on the renders, because they're a minor detail.

As a fourth example, (by now it should be obvious that I'm cataloging these here so I don't forget them), I have not designed the lift engine mounts that must be embedded in the lift duct. To be perfectly blunt, I have not given much thought to how these should be fabricated.

So as I said, a lot of little details remain to be planned out. Almost all of them can be dealt with "on the fly" as I construct the craft, but the challenge is to design to a high enough level of detail so that build time is minimized.

Pictures

Here are some renders from SolidWorks of my rough ideas. The model is low precision and only serves to get the basic ideas down and make sure it all fits together; it is not precise enough to actually start spec'ing out materials such as lumber.

Top view:




Side view:




Underside view:




Isometric view:




I need to do a post about the lift duct, which is probably the most complicated piece to fabric on this entire project.

Starting up

Hello and welcome to my humble bit of cyberspace. This blog will (hopefully) chronicle the design and construction of my second hovercraft. I'll post some links in the sidebar that will explain what a hovercraft is, how it works, and the log for my first craft.

Currently I am in the design phase - the idea is still very young but so far appears doable. The goals for this craft are to simply get something put together as quickly and cheaply as possible. The current design is 11 feet long, 3 feet wide at the bow, and 6 feet wide at the stern. The hull will be constructed out of extruded polystyrene foam; a.k.a. blue/pink/yellow foam; the base will be 3 inches thick and the walls and other panels will be 1 inch foam of the same type. The hull will get 2 layers of 10 oz. glass on the top with the underside left bare except wood for mounting the landing feet. While this means the hull is not a true composite, all Universal Hovercraft designs are spec'd out this way and there are no systemic structural issues that I know of.

To save cost, I will re-use the powerplants from my previous craft: 6 hp vertical shaft lift engine driving a 24" 4 bladed fan, and a 12.5 hp horizontal shaft thrust engine spinning (initially) a 36-18 prop via a belt drive with a 1:1 pulley ratio. I broke 2 crankshafts with the prop directly mounted to the engine, so I'm not going that route again!

I have already done a rough design of the thrust engine reduction stand (originally I intended to modify my first craft, but it didn't live long enough...), so there is relatively little design work to be done.

I am shooting to assemble the hull in 2 weeks while working a full time job this summer. Depending on what job I end up with, this may or may not be feasible. Rough first cut estimations put the total cost at about $600.