July 5, 2015
Description
##My entry into the "Science: Make It Float challenge"
See the Instructions for a narrative of the iterative discoveries, and design modifications that resulted.
See Updates below to see changes since first published.
This is definitely a "work in progress". Things may still change based on the ongoing printing, testing, and experimentation.
I normally don't publish my things until after I successfully print one myself and verify everything prints and works as designed. However, for this challenge, you get to "come along for the ride", and follow the progress, setbacks, redesigns, and hopefully, the ultimate success of this project. A chronicle of the process can be found in the Narrative section under the Instructions tab.
The first thing that came to mind when I read the challenge goal — to optimize the amount of weight your boat can support using coins in your local currency — was a container ship, having the containers hold rolls of pennies.
An object floats based on Archimedes' principle:
TimeFramedAny object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
— Archimedes of Syracuse
Therefore, in order for a ship to float, it must displace a volume of water that weighs more that the ship and its cargo.
So, the first thing I did was weigh a roll of pennies to determine the size that a container would need to be in order to displace a volume of water having the same weight.
The idea behind that, is that if each container itself is at least (neutrally) buoyant, then a ship's hull that would surround the containers should have enough buoyancy to float the hull and the containers in it.
That conclusion is based on the fact that the hull would displace the volume of all the containers in its hold, plus the additional volume of the space around the containers (weighing only as much as the air in it), and the thickness of the hull itself. Since the hull will be made of plastic, and not solid, the volume of water equal to the volume of just the hull itself should be enough to float the hull. i.e., If I filled the hull with water it shouldn't sink.
This would be difficult to calculate ahead of time (although, using the slicer's "filament required", and approximating the weight of that much plastic would probably be close enough. Note: I proved this to be the case experimentally once the hull was printed.
Penny rolls ranged from 127g to 133g. I looked up the weight of water — 1g/1000mm³ (it varies with temperature, and of course salt content in the ocean, neither of which should be factors for this model...). From that, I determined a "container" size -- having dimensions to surround a roll of pennies, and enough volume that the container itself should float with a roll of pennies inside. If each container is buoyant, then by definition, the ship itself should be buoyant if the containers all fit inside the hull, since the ship's total water displacement would be even greater. It should, in fact, be greater by enough to add a layer or two of containers on the deck, and still be buoyant. I didn't try to pre-compute that; I figure that will be done by simple experimentation once the ship is loaded and floating by adding containers to the deck. Of course the weight of the water displaced by the hull must be greater than the weight of the ship, and it's cargo — by enough to raise the deck of the ship high enough above the water's surface to avoid flooding the hull.
The design I came up with is modular so that
The bow and stern make it actually look somewhat like a ship (vs. a square barge), and add some buoyancy since they are empty, which should allow for on-deck containers.
Another design goal is to only use printed parts (other than the penny rolls), without the need for gluing, and to have no parts that require support material (which, for instance, influenced the shape of the stern away from being relatively flat to be somewhat pointed (it prints on end).
The hull part is sized to fit a dozen containers (4 wide × 3 high) based the dimensions described above.
I also incorporated a keel into the hull that can hold a roll of pennies to add weight at a the lowest point to aid in lowering the center of gravity, in an effort to increase lateral stability (to prevent the ship from rolling over).
In order for form a (reasonably) water-tight seal between the modules, there is a groove running the length of the joining edge of each piece. I employed a "gasket", printed with flexible filament, that is forced into the grooves, and the modules are then clipped together using 12 clips (ABS has enough "flex" to allow them to snap into place, while being rigid enough to hold tight).
##Updates
Bow_108.stl — it fits within required build volume limits. I left the original, Bow_68.stl for reference, and an option if your vertical print volume is not limited to 150mm. Printing Progress:
License:
Creative Commons - Attribution - Non-Commercial
