PilotFish Mk.II - Le roi est mort, vive le roi!

by Web FishApr 17, 2014 @ 12:33pm

The king is dead, long live the king! Our last post was more than 6 months ago and some of our followers have been asking what is happening at PilotFish headquarters. The short answer is: A LOT! After we assembled the basic frame of the boat (4 floats and a deck structure) it became evident that building a craft which is strong enough to withstand the challenges of an open ocean voyage, yet light enough to stay afloat and move under 50 watts of power or less, while fitting in a 1m3 cube has its challenges. In retrospect, hoping to get it right the first time was a bit optimistic. Here is a summary of the issues identified during the build and initial testing:

  • Windage factor. This should have been obvious from the get go, but somehow it wasn't. The symmetrical design approach called for two-sided hulls stacked on top of each other. The result was a massive "superstructure" towering above water, combined with relatively shallow draft. Increasing the draft is not really an option, as every inch of draft adds an inch of height above the waterline as well. Once on the water it became clear that in fresh breeze the boat would drift significantly and require constant course correction (wasting energy in the process). This was one of the items discovered early enough in the process to addressed as we went along (by splitting the original hulls into separate top and bottom floats) but the required changes ultimately caused some of the other issues listed below;

  • Build method. The original build plan called for two solid vertically-symmetrical hulls built out of fiberglass and mounted on a solid deck structure. As the build progressed this approach was morphed into a honeycomb composite float with glue-on fiberglass skin. While this produced relatively strong floats, it further complicated mounting of the components;

  • Overall strength. The hull mounting points were originally designed as an integral part of the hull frame in order to ensure structural integrity and torsional rigidity. As the build transitioned to the four-float design, the mounting points ended up being attached to the float deck (and through that to the float honeycomb structure). The deck structure itself was based on three 0.75" aluminium tubes with limited cross-bracing. In addition, extra reinforcement was needed on the mounting point assembly to ensure the design allowed disassembly while still withstanding the forces in the joints. Although there is no clear evidence that it would have failed under normal conditions, the design just didn't "feel" robust enough;
  • Limited adjustability. The horizontal mounting points of the floats predetermined the deck position, the clearance between the floats and the overall free board. This further complicated some of the other issues as it was difficult to make adjustments to address them;

  • Battery chemistry. The initial design assumed the use of Ni-MH batteries (same type used in RC car/boat models before the advent of Li-Po packs). At the end the Ni-MH charging model (and the Li-Po one for that matter) combined with solar panels as a power source and the requirement for on-line charging turned out a bigger challenge than expected. Adding to that the incurred power losses, as well as the limited longevity of these battery packs in deep-cycle applications pushed the implementation to a different chemistry - sealed lead-acid (SLA). These battery packs check all the boxes with one added nuisance: relative weight. More on this below;

  • Navionics and battery bays. The original plan was to store main battery banks in the hull structures and the navionics bay and antennas in the deck structure. With the changes of the design and the build process the payload had to be relocated in the inter-float space. This partially blocked the wind slot between the floats, thus negating its design purpose, while leaving components exposed to the elements;

  • Propulsion pod design / mounting. The original design design called for traditional motor-shaft-prop arrangement. With the change to the float build the approach became impractical. This item warrants its own series of posts, but in short: building a miniature, geared, salt-water rated propulsion pod is not as easy as it sounds;

Each of these problems is bad enough in itself. Yet probably none of them is insurmountable. The one issue that resulted from all of them and tipped the scale was overall WEIGHT. Our original weight target for the solid hull design was < 45 lb. Switching to the four float model looked like a great way to save weight. Unfortunately, with the added weight from the new battery packs and all the reinforcements needed to make the new design sea-worthy the overall weight was creeping up towards the 35lb. mark even before the final deck structure was in place. With the reduced buoyancy of the new floats we were looking at scaringly low freeboard. 

After countless hours of calculations, numerous back-of-the-envelope sketches and proper amounts of coffee and pizza, in one swift act of bravery the most important decision of the project was reached:


Give Up and Move On with Our Lives!


And so we did. The final result: PilotFish Mk.II. The next series of posts will detail how The New Bigger and Better Vessel (OK, maybe not bigger - 1m3 rule still applies) came to be and why sometimes going full circle is the best thing that can happen to a project.

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