Mk.II Floats - Week 3

by Web FishMay 10, 2014 @ 12:40pm

Final finish. An external coat of epoxy applied and finished to a polish. We are now ready for mounting on the vacuum base plate:

 

With combined weight of float model and ballast of 19.5 lb. we still have 2" of free board. In a catamaran configuration this gives us a target total weight of 40 lb:

Mk.II Floats - Week 2

by Web FishMay 6, 2014 @ 11:26am

More hand plane, sanding, filler, sanding. The float plug takes shape:





 

Mk.II Floats - Back to Square 1

by Web FishMay 2, 2014 @ 01:22pm

As we mentioned in our previous post, the new design called for increased displacement and lower weight. Going back to the early days of the project, the original plan was to have the float shells thermoformed / extruded from polycarbonate plastic. This plan proved to be too ambitious and was abandoned early in favor of fiberglass shells (and later on composite wood/foam/fiberglass construction). With the new weight target in place we decided it was worth revisiting the plastic shells idea. Instead of polycarbonate (which, even though very strong, turns out to be less than ideal for thermoforming) it was decided to stick with more traditional polystyrene or ABS floats. The two materials have similar properties with the ABS route adding extra flexibility to the structure (which might or might not be a desire-able feature). Both materials lend themselves very nicely to vacuum forming.

There are two major hurdles when going the vacuum forming route: the cost of the process makes it impractical to run really short series of pulls (we really only need 4 floats for now) and you need a solid (and strong!) plug (positive model). There isn't really much one can do about the former (other than run a larger batch and store the extra shells for future use ;) so we focused on the latter.

Week one results:





 

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.

Building the floats - Step 6

by Web FishMar 19, 2013 @ 01:59pm

 

The fiberglass skin on the floats is now done. Ready for the final finish. Before we do that, we should put the frame on water to validate our math (and make sure Archimedes was right :)

 





Building the floats - Step 5

by Web FishMar 13, 2013 @ 08:30am

 

Fiberglass week at the shipyard. Floats should stay under 3.5 lb. each to to keep within the overall weight budget:

Building the floats - Step 4

by Web FishMar 9, 2013 @ 07:34am

 

Final layer of body filler on the floats. Sanding halfway done:

Building the floats - Step 3

by Web FishFeb 26, 2013 @ 10:37am

 

Time to put some "meat" on the bones of this fish. Polyurethane foam is light, sticky and easy to shape with basic tools:

     

Once the foam is set, its cut down to size and further shaped to produce the final float core:

Building the Floats - Step 2

by Web FishFeb 22, 2013 @ 07:55pm

The sun is finally out and we have three more float frames:

Building the Floats - Step 1

by Web FishFeb 21, 2013 @ 01:57pm

Bulting the floats: Part 1

After spending a good portion of the last month experimenting with various build techniques (composite skin with minimum glue-on frame, one-off float core cast from two-part polyurethane foam, even considering CNC machining of the foam cores) the decision was finally made to go with a structural cross-frame built of 1/8" plywood, polyurethane foam cast inside the frame cells and a fiberglass skin finish. This will give us almost 100% flotation preservation in the event of float breach (OK, it is probably less than 100% as the polyurethane foam will never be 100% closed cell and as such it will soak water in the event of a skin breach, but it's way better than a hollow float), manage-able cost and reasonably reproducible results (shape/weight) using simple tooling (we will need at least four of those floats). On the flip side, this is a bit more labor intensive than simple foam casting / machining, and the solid hulls mean that we'll have to fit the navionics bay and all batteries within the deck structure. Under other circumstances this would have been a less-than-desirable solution (high center of gravity) but considering our overall design (what is up today might actually be down tomorrow) this works ideally to contain the weight in a balanced way.

 

  • The float frame starts as 4 separate elements: upper deck, lower deck, upper keel, lower keel:

 

  • The upper deck with the upper keel section:

 

  • The lower deck attached to the upper keel section:

 

  • The lower keel section attached to the lower deck:

 

  • The extra cut outs (keeping the overall float frame structure under 18oz.):

 

  • The motor pod tunnel:

 

  • Finally, the deck frame mount points are bolted on / glued and float frame is ready for the foam treatment: