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.
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