The Ugly Duckling: Redundant By Design

by Web FishAug 30, 2012 @ 12:21pm

So we spent all this time on this section of the blog talking about everything else BUT Pilot Fish. It's time to unveil or design. Before we do, let's summarize everything we've covered so far:

  • Possible energy sources: solar-driven photovoltaics, solar-driven heat engine, wind turbine, sail rig, a combination of any two or more of the above;
  • Design objectives: structural integrity capable of withstanding the wave and weather systems energy, maintaining correct vessel orientation at any time (flip issue), corrosion, surface deposits above and below waterline, foreign objects in the water;
  • Primary design principle: redundancy;

After a long deliberation, the decision was made to go with solar power as a primary source of energy and electric motor(s) as a primary propulsion system. It's been very much a battle of attrition - it's not so much about what we like about this solution, it's more about what we dislike about all other solutions:

  • Moving parts: This solution provides the least number of moving parts (with the solid wing system coming at close second). Less moving parts - less chance for failure;
  • Efficiency: Hard to get close to 100% energy harvest rates while satisfying all design objectives, making the Power Budget issue that much tougher;
  • Availability: All other options require various levels of custom fabrication;
  • Fit with the rest of the design philosophy: Probably most important of all (as you will see below).

Since we are using photovoltaics (solar panel) as our energy source, we now need to ensure we capture as much sun-light as possible within the limitations of our 1m3 virual cube. The sun is (generally) shining from above, so we need to maximize our harvest size in the horizontal plane (for the time being we will not worry about strictly following the sun's direction). A typical boat hull looks something like this:

 

 

Producing a structurally sound hull would not be a major challenge. With the proper keel in place, we also solve the flip-over issue. But... the only area guaranteed to face the sun at any time is the deck. To achieve any reasonable speed, the Length/Width ratio should be no less than 6:1. This leaves us with ~1/6th of the usable energy. If we take a look at our budget again, we can see we are way below our goals.

How do we extract more surface area from our hull? First thing that comes to mind is mount a horizontal plane above the hull extending to the boundaries of our virtual cube (think hellipad on a very small mega-yacht :):

 

 

In addition to raising the center of gravity really high and increasing the listing (tilting) and pitching (yawing) forces to unreasonable levels, we'll and up with a 1 square meter of plowing surface (not necessarily the thing you want when trying to maintain speed with minimal power reserve). And... we just made our self-righting task MUCH tougher - flipping this monstrosity back would require a deeper mounted and much heavier ballast. Plus we are not helping our redundancy.

A platform that would have most of the benefits of the above design while minimizing the negatives is a catamaran:

   

 

We do keep all of the surface area, avoid the erratically pitching deck and add a solid dose of redundancy. The issue with this design: its very stable on the surface of the ocean, requiring more force to flip over. Wait, this is a good thing, right? Well.... not unless you are designing a self-righting solution. Wave forces will have no problem flipping it over. But designing a self-righting single-keel catamaran is (very) tricky. A quick back-of-the-envelope calculation shows that it will be next to impossible to design something like this while both utilizing all of the surface area (maximum solar power) and fitting the whole system within 1 meter vertically (righting effect is proportional to the mass of the keel bulb and the square of the keel depth, plus the angles just don't work).

At this point we spent the obligatory time to research the various self-righting solutions for catamarans. Then we spent triple that designing our own crazy ideas (computer-controlled balloon inflation, articulating arms with ballast, etc.). Not much seems to work.

As we were getting ready to give up and go back to the single-hull idea (with all its negatives it could still be a workable solution - just not a very good one) there was one final thought: instead of fighting the flip-over and spend time, effort (and boat weight/displacement!), why not... EMBRACE IT!

That's right. The main reason for us trying to solve the flip-over issue was to keep our solar panels pointing towards the sun and keeping our propeller(s) in the water. But what if we don't need to do that? Getting the sun to shine from below could be a problem... But getting solar panels on both sides of our deck would be trivial! Getting the propellers to work in the air would be a problem as well... No, not really. Easy to move the whole drive line up or down under gravity. Or even better, how about we replicate the propeller and rudder setup on both sides of the vessel and intelligently decide which ones to activate! So, we end up with something like this:

  

 

Now, we should admit, this is will not be the best looking vessel to sail the ocean... And there are still many ways things can fail. But this approach solves MOST of our design objectives:

  • Structural integrity (no small / fragile /moving parts sticking out);
  • Maintaining vessel orientation (or not!);
  • Corrosion-resistant (again - nothing that cannot be built using composites and/or aluminum alloys);
  • Foreign objects (no big keel collecting sea-weed as we go);
  • Redundancy (on so many levels!):
    • Redundant solar power;
    • Redundant rudders (as long as a rudder on one of the hulls is functioning, we are in decent shape);
    • Redundant propulsion left/right (we could ride on one motor as long as we have a functional rudder to compensate);
    • Redundant steering (steer both with rudders and left/right motor on/off);
    • Redundant  flotation (breaching a compartment in one of the hulls should have less impact);
    • And all this is fully redundant by flipping the vessel over. We almost WANT the waves to flip the vessel every once in a while if we have a problem;

We still have quite a few left to tackle, but those are now engineering challenges. The foundation is set. Time to start drawing!

See you on the PilotFish Anatomy design blog!

Fighting the Elements

by Web FishAug 15, 2012 @ 04:26pm

 

So what does a model boat crossing the Pacific ocean face? Short answer: A LOT! Here's a quick summary of the environment factor's we'll need to figure in when making the final decision on energy source(s), configuration and propulsion system:

  • Waves

If you've ever witnessed first-hand the power of ocean waves you'll agree that this is quite possibly the biggest challenge both from structural and navigational point of view. Structurally the vessel will have to withstand the dynamic loads associated with wave breaks. Even though waves usually break in shallow water near the coast, a wave break can occur anywhere where the amplitude is sufficient. This will add excess torsional and flexural loads (twisting and flexing) that we have to account for.

Another aspect of ocean waves on a small vessel (1 meter or less) is the rotational forces around the horizontal plane (flipping force). Unlike a larger vessel, a boat of this size will be very easy to flip during the course of a normal passage even by a medium size wave. Since pretty much every energy source we are considering above is strictly directional (available only above the water surface and only with the proper orientation of the harvesting element) maintaining the proper orientation for the boat will be critical. After all, a sail, wind turbine or solar panel will not do much good if they are submerged under the capsized hull. There are many self-righting designs (a very nice deep keel & asymmetrical deck solution by the SCOUT team here), almost all of them having one major limitation - being applicable to single hull vessels only. Solving the flip-over problem will be right at the top of the engineering tasks during the final configuration consideration.

  • Weather systems

The vessel will need to be designed to withstand any weather system it may encounter during the passage. In addition to significantly increasing the impact of the wave factors above, a special consideration will be needed for wind-based propulsion. A vessel rigging designed for 10 knots wind might exhibit some problems in a gale.

  • Corrosive effect of ocean water

Almost all model boats are designed for use in fresh water basins. In addition to the basic corrosive effect of the water itself, mixing metals with different electrode potentials (e.g. steel and zinc or copper) leads to accelerated galvanic corrosion.

  • Salt / hard water deposits on dry surfaces

This is mostly a solar panel issue - a thin film of salt deposits will significantly decrease the efficiency of the solar power harvest.

  • Biological deposits 

Barnacles are a threat to any size ship.

  • Small size biological / foreign objects in the water

In addition to the various sea weed species floating near the surface, there is (unfortunately!) an increasing amount of man-made debris circulating the world oceans. According to some studies, the amount of man made (mostly plastic) content in some spots of the Great North Pacific Garbage Patch exceeds up to 6 times the normal plankton in the same volume of water. While plastic bottles and (especially) fishing line might not present real danger for a 300 foot boat, they can be deadly for a model boat.

  • Large birds / fish / animals

Jaws anyone? 

  • Navigational obstacles

Land masses are fairly static and can (usually) be avoided with proper planning and navigation. Commercial and recreational ship traffic - not so much.

 

Energy Density and Power Budget

by Web FishAug 12, 2012 @ 01:51pm

 

From an energy density perspective, here is a (approximate) comparison of our energy source finalists and their currently available harvesting methods:

  • Photovoltaics: 100 Watts/sq.m. (~10% efficiency);
  • Solar thermal: 100 Watts/sq.m. (~10% efficiency);
  • Wind turbine to electric and/or mechanical (from extrapolated data from here and here): 60 Watts/sq.m. (assuming overall 20% efficiency and the energy levels at 10 m altitude. In reality - probably lower);
  • Wind sail soft/rigid (assumed - cannot find proper data): 100-250 Watts/sq.m. (same wind assumptions as above);

Those are the theoretical maximums we could derive from each energy source type if we were to use 100% of the available area of our virtual cube. In reality, the harvested energy will be lower (due to partial surface utilization, variable angle towards the energy source, environment factors, etc.). This doesn't really leave us with a lot to work with. So we need to be frugal :)

We have a few "fixed needs" that will have to be addressed before we can even start expending energy for propulsion:

  • Navionics: computer guidance system, sensors, servo control (minimized). Allocated budget: 5 Watts; 
  • Comm: we will need to phone home. Only required in short burst mode. Average allocated budget: 1 Watt;
  • Navigation lights: Optional. Average allocated budget: 1 Watt;
  • Image acquisition: camera(s) / storage. Optional. Only required in short burst mode. Average allocated budget: 1 Watt;

So yes, we DEFINITELY need to be frugal.

Assuming a displacement hull (and at these energy levels that is the only type that we can figure out how to build and get moving) our 1 meter maximum length will give us a maximum displacement speed of 2.4 to 2.7 knots (4.5 to 5.0 km/h).

From past experience two 6V Decaperm motors will propel a 4 foot "fat" hull (tug-boat scale model - ~12 kg. displacement) to approx. 3 knots drawing roughly 5 Amps each. 5 Amps * 6 Volt * 2 = 60 Watt total. This is under ideal conditions and in calm water. But we are in the ballpark! Even with the fixed needs, each of the energy sources above are still in the game. And for now this is without combining two or more of them.

The engineering considerations will be the deciding factor.

 

Energy Sources and Propulsion Choices

by Web FishAug 11, 2012 @ 08:18am

Warning: scientific content. If you are easily bored, please scroll down for final conclusions.

Now that we've established the rules, first order of business would be to figure out our energy source and the associated propulsion system. Not being able to rely on stored energy for propulsion (at first read) automatically discard several obvious sources:

  • Any exothermic chemical reaction based process - internal combustion engine, most forms of jet propulsion (fuel-powered jet engine), most forms of steam engine (where fuel is used for heat generation);
  • Any direct chemical reaction energy conversion for which at least one of the reagents is stored on board - chemical battery bank, classic fuel cell, pseudocapacitor;
  • Any direct electric energy storage system - pre-charged super capacitor;
  • Any mechanical energy storage - flywheels, etc.;
  • Any nuclear power source (not that our rules are the main limiting factor there :) 

What is left then? Actually, plenty (not necessarily in order of practicality):

  • Harvesting electromagnetic wave energy (to simplify things we'll refer to it as sunlight, even though any EM waves could do):
    • Direct conversion of  into mechanical energy. Theoretically (and somewhat practically - this, this or this) possible, nothing available yet that will work with diffused sunlight and produce the levels we need;
    • Conversion into electric current and from there on into mechanical energy for propulsion and (optionally) into other types of energy for storage. Plenty of variations of propulsion and storage methods are available. Solar panels driving electric motor and/or charging chemical battery bank can be achieved with mainstream components;
    • Conversion of sunlight into chemical energy and from there on into electric current and so forth. Various forms of photocatalysis with 100% external reagents (chemicals from ocean water?), photosynthesis. Not much available within our budget and size;
    • Conversion of sunlight into thermal energy and from there on into mechanical, electrical and/or chemical energy. Heating water by (concentrated) solar energy and powering steam engine, turbine, jet propulsion, etc.;
  • Harvesting mechanical energy from the surrounding ocean water:
    • Direct use of mechanical energy of ocean currents for propulsion. All forms of ocean gliders; 
    • Direct use of the mechanical energy of ocean waves for propulsion. Wave gliders;
    • Conversion of mechanical wave energy into electric current and so forth. Wave power generation - certain possibility but energy density is minimal within 1 cubic meter under the surface;
  • Harvesting mechanical energy from the surrounding air (wind):
    • Direct use of wind energy for propulsion. All types of sails
    • Conversion of mechanical wind energy into other forms of mechanical energy (i.e. driving a propeller by a wind generator). Complete decoupling of wind direction from vessel direction;
    • Conversion of mechanical wind energy into electric current and so forth. All types of Wind power generation;
    • Conversion of mechanical wind energy into other forms of energy - thermal, chemical, etc. Mentioned for completeness;
  • Harvesting chemical energy from the surrounding ocean water:
    • Harvesting and converting energy of phototrophic organisms;
    • Exothermic chemical reaction triggered by catalyst carried on board but with reagents derived 100% from the surrounding environment. Way out there... but still a cool idea :)

Not too bad. We will revisit some of the options above in the (hopefully not too distant) future, but for the time being, based on the other two factors in the rule book (volume constraints and budget considerations) our finalists are:

  • Harvesting sunlight energy via photovoltaics;
  • Harvesting sunlight energy via heat engine;
  • All forms of wind energy harvesting;

Time to see if anyone of these can actually get us across the ocean.