aXatlantic is about building an ocean going drone. We are aiming at setting the world record for the first solar powered and autonomous crossing of an ocean.
You can follow us making progress in our build log.
A technology like this can be used as a payload platform capable of carrying multiple sensors and communication technology essential for environmental and geophysical research as well as marine life observation. Compared to traditional methods the availability of a solar powered, autonomous drone provides a very safe solution and can be easily scaled and equipped according the specific needs of the given application.
We are not the first to try this and there are already a lot of different solutions for autonomous ocean going vessels. However, our main concerns, given the capability of undertaking long lasting autonomous missions running on solar power, are that the boat could get entangled in floating debris or keel over due to external forces. Both could mean the loss of the boat and are adressed by our unique design featuring a self righting hull without the use of a conventional weighted keel and a custom propulsion and steering unit.
Still in dry dock…
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Using Friendship Systems CAESES Software we were able to optimize the boats stability and self-righting ability. In order to do so, 5 design variables were introduced into the parametric hull design.
- The wall thickness of the “wings” was set to an interval of 1 mm to 5 mm.
Since the thickness of the wings defines how much buoyancy they provide in the flooded stage, this parameter has direct influence on the self-righting ability of the hull.
- and 3. The weights of 2 different points in the NURBS-curve defining the hulls section were allowed to change in a given range.
This mainly changes the position of the center of buoyancy when the boat is heeled over and therefore affects the initial and large angle stability, but also influences the shape of the inner deck which is of great importance when it comes to self-righting.
- The weight of the center point of the NURBS curve defining the inner deck was set to an interval of 0.5 to 2.
This changes the stability (or better: instability) of the keeled over boat in order enhance self-righting performance.
- The “wings” were allowed to thicken at their outer edge up to 50 mm from their originally crisp corners.
This has a similar affect as the change in wall thickness and therefore should not be overdone, but it also provides a huge amount of secondary stability in the upright position as long as the boat doesn’t heel over for a long time which would cause the wings to fill partially with water.
The objectives in the optimization process were to maximize the righting arm which needs to be as high as possible and to move the center of area (COA) of the righting arm curve (RAC) as low as possible in order to improve initial stability. At the same time the minimum value of the RAC was observed and all designs leading to a negative value were sorted out because a negative value means that they would not right themselves up after keeling over.
First step of the optimization progress was to run an algorithm to explore the design space wich was done using a Sobol algorithm and a given starting point and then analyzing 100 different designs spread out evenly over the entire design space.
Based on this data a multi objective simulated annealing (MOSA) algorithm was used to find the best possible design for our needs.
This means that the hull has to be only just self-righting (no negative values in the RAC), but at the same time be as stable as possible for small and larger heeling angles (small COA of RAC and high maximum value of RAC).
Calculations of another 2000 designs were done and here it is: our perfect design (of course limited to the restrictions by the parametric model)
It should also be noted that there is no way of optimizing both objectives at the same time. However, there are self-righting designs with a very high maximum righting arm and at the same time an acceptable low COA of the RAC. We chose those over the ones with the smallest COA and therefore lower maximum righting arm, because we rate the first-mentioned higher.
The video below shows graphically what happened during the exploration and optimization process that led us to our final design.
It is highly unlikely for a boat of only 2.8 m in length and 0.6 m in width to operate in an environment as harsh and unpredictable as the open seas without being able to right itself up after keeling over. Usually that kind of behavior is achieved by adding weight at the very bottom of a long keel protruding from the ships hull. One of our main concerns is that such a keel is the ideal way to catch as much seagrass, rope, fishing line and other floating objects that might float around the surface of an ocean as possible. At the same time we need quite a big surface on top of the boat to be able to collect enough solar energy to power all the hardware and propel the boat. Such a big and almost flat surface would lead to a perfectly stable position of the hull once flipped over.
The solution we came up with is quite unique but can be found in modified versions in similar applications:
Some coastguard boats flood parts of their hull to lower their freeboard in order to take people on board swimming in the water. When accelerating again the boat starts to plane and the flooded parts empty themselves due to gravity and the change in trim. Also some RC-speedboats use floodable chambers to at least partially right themselves after flipping over. Once hitting full throttle they start planing and the chamber empties out while the boat returns to its fully upright position.
In our case a large, round-shaped, transparent cover on top of the solar panels would be the easiest way of getting the boat to right itself without using a keel (similar to the way free-fall liveboats work and look like). This however would make for a perfect sail in high winds and lead to the boat drifting around without control.
So, we changed perspective a little bit and decided to hide the deck we need right below the solar panel. That way we can keep the large surface of the solar deck and at the same time lower the profile to a minimum to avoid drifting to much in high winds. In order for the inner deck to have any effect on the self-righting performance, it has to get in contact with the water somehow. By drilling many small holes through the “wings” at the top edges and right above the waterline, they will simply fill up with water once the boat keels over until there is only the inner deck providing buoyancy anymore. Given a center of gravity which is located low enough in the upright hull, this means that the boat will start to right itself again. As soon as one of the wings gets lifted out of the water it will start to empty itself out through the holes and the boat will continue to right itself up until it returns to its upright position. This works whether or not there are wings at all (just a flat solar panel on top of a lifeboat shaped hull). The wings however, are needed to add structural strength to the solar panel and provide buoyancy similar to an outrigger once the boat is heeling.
Sounds a little bit confusing and hard to imagine to you? Just take a look at the animation below.
The video shows the center of gravity CG and the center of buoyancy CB while the boat heels over. As long as the forces acting on those two points create a righting moment on the hull it will return to its upright position. If however the momentum becomes negative due to the forces changing sides the boat will continue to capsize and not right itself back up.
Still not convinced? Take a look at our first prototype righting itself up in the kitchen sink.
Although we are working on this project self-paced it is not too far from what we have learned.
Johannes used to study IT a couple of years ago and is pretty much constantly procrastinating by building almost anything that flys – preferably autonomous and carrying cameras for aerial mapping and 3D landscape scans but sometimes also just a bottle of beer wich needs to be delivered in time. At the time he is doing his masters in Architecture at the Bauhaus University in Weimar, Germany with focus on virtual environments and rapid prototyping.
Heinrich is studying Physical Engineering Sciences at TU Berlin with focus on Fluid Dynamics and Numerical Computation wich is of course a huge advantage when it comes to hull design and hydrodynamical computations. In the last couple of years he has gained a lot of experience in building boats of similar sizes by building a skin on frame Greenland Kayak and most recently a cold molded canoe made from mahogany veneer with a thin core of carbon-olefin composite working together with a friend who is a professional boatbuilder.
Support & Donate
Since we are working on this project self paced and with very limited budget we need your support. If you like the idea and are willing to help us out to get this project done please feel free to leave a donation and watch it become reality!
If you even consider to get involved as a sponsor don’t hesitate to get in touch with us.
We would like to thank our sponsors for their generous support in getting the aXatlantic drone to launch!