Hi all,
I am interested in long-distance ocean-going drones.
To date, I am aware of only 2 craft that have made it across the Pacific; both WaveGliders. They are the Papa Mau and the Benjamin. Both released at the same time from San Francisco, it took 1 year for the Papa Mau and another 3 months for the Benjamin. They were both solely under wave-power using the SV2 version.
However, 2 more WaveGliders released at the same time failed to make it from SF to Japan.
Another boat, the Honey Badger, is a sailboat and made it from SF to Hawaii in 34 days. This design is different to a regular sailboat, but clearly it works.
Scout was an attempt to go across the Atlantic using solar power and a propeller but got lost about half way across.
The new SV3 version of the WaveGlider is now more of a boat-hull design than a surfboard and now possesses a solar-powered propeller pushing the underwater glider.
BlueRobotics are beginning to produce the T100 and soon the T200 thrusters, with 5lb and 10lb thrust respectively. Two T100s were used to power their SolarSurfer in an attempt to travel up the Californian coast but some weed fouled one of the thrusters (possibly a prop spur would be the right solution?). Plus, when the sun went down, there appeared to be just as much drift during the night than forward motion during the day.
An underwater torpedo glider like SLOCUM or Seaglider needs very little energy, but working at depths complicates things immensely.
And here is my question:
"If you wanted to go from one side of the ocean to the other, which design would you choose? And which route would you choose?"
If I wanted to go westwards across the Pacific, I could potentially use the equatorial currents and use OSCAR (Ocean Surface Current Analyses - Real time) to help catch the best currents but then a couple of days ago I saw a beautiful map at Global Fishing Watch (something Google are involved in) and saw that those same currents are highly fished, and so probably best avoided due to the danger of nets.
I like the SailDrone idea as there is less underneath to catch upon compared with the SolarSurfer and WaveGliders, but the guidance tech on the SailDrone is probably harder to master. Other sailboats don't appear to have much success.
To date, thrusters and props for this size of boat have not proven themselves of this type of endurance. I would like to see how well the T200s fair out at sea.
A good design might include:
- A hull that cuts through the water like the SV3 rather than gets bumped around like a surfboard.
- A glider with a thruster on the glider like the SV3, but with a tether that can be detached in case of emergencies e.g. fishing net entanglement which seems to be the biggest risk.
- A self-righting hull.
- Two or three T200 thrusters attached to the hull of the boat, but with only one in operation at a time as the other two are redundant until one thruster fails (problem: the glider below should be in front of the boat and pulling it along)
Thoughts?
Replies
hmm, I guess that is somehow wrong, the aft wing will stabilise only when sailing upwind, with wind from back it will unbalance the helm.. that's annoying
mabe a two rudder configuration, main rudder then a wind vane actuated auxiliary rudder farther aft would work: http://selfsteer.com/windvanes101/images/trimTabOnAux2.gif
having two separated rudders will help with redundancy, but still needs a way to detect rudder failure and decouple its servo.
One certain way to detect rudder failure is to notice it doesnt steer. Others could be sensing the currents of actuator motor, that can be done easily by measuring overall current consumption and compare it with no power at the actuator currents. (the ardupilot BEC module can do that).
------------------
I managed to sew a skin on the wing. I had some light (40-50g/sqm) kite fabric, it stretched well over the skeleton. Acceptable, not the nicest. The stretched fabric also improved the torsion strength.
Now, with the mast (1/2 the height), servo wires through the mast and tail boom it weighs 247 grams. The tail, servo and forward counterweight I calculated they will add 150 grams, most of which will be forward ballast.
Not too happy with a total 400grams, that will be 1.6kg/sqm (mast included) and I don't feel it's so strong to resist heavy storms, even when feathered. It will be almost a quarter of total boat weight.
Yet it spins quite freely around its small joint on the top of the mast (center of the wing). That's fine considering there are no roller bearings.
So, the problems with a wind-vane is that it requires careful setting, and it is susceptible to water coming over the bow and hitting it (this latter problem obviously does not affect yachts as the wind vane is several feet higher).
And the problem with a mizzen is that it works badly with a following wind.
Potentially, the mizzen could also be placed in neutral, although this would require disengaging the wing from the servo, to allow the wing to swing freely. Also, the mizzen would need to be aft of the rudder, correct?
I will write on a couple of boat forums to see what they suggest. For long distance crossings, a wind-vane appears to be the apparatus of choice, and so I would like to determine why. They appear to be very expensive for the amount of materials involved in making them, but that sailors are willing to pay for the relief that they provide.
If the problems of sensitive adjustment and possible damage by waves can be overcome, then a linear actuator connected to a wind-vane still seems a sensible solution to me.
Good effort on the wing. I'd favor a strong wing able to last many months and accept the extra weight. Saildrone looks like it has a carbon-fiber sail vs. the fabric/plastic covering for the back of the wing for the Wind+Wing Technologies boat. But, your boat will probably be faster in the water than mine :)
http://sailinganarchy.com/wp-content/uploads/2013/11/Hawaii_dep.jpg
http://pressure-drop.us/imagehost/images/99970372167949697031.jpg
"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world." :)
I am hearing a few bad stories about tiller pilots failing, and I am trying to work out why as they are very similar to what I intend to build. I don't know if they have improved over the years and are much better now.
I couldn't find any good figures for how long they are supposed to last i.e. hours at sea.
If you take a look at some Youtube videos, some of them are in motion a lot of the time, but this can of course be damped down.
I did look at the wind vane idea but you are right in that it adds extra complexity.
The major thing with an actuator appears to be not to place the maximum load on it. I intend to have the rudder somewhat swept back so that it doesn't catch on weed and nets. Because of this, the force needed to turn the rudder is higher. To counteract this, I may need to build a balanced rudder instead.
The rudder actuator failing is critical. I am trying to think of ways to build in redundancy. If the electricity or motor fail, this will lock the shaft in place so maybe connecting two together in a straight line could work (so that when one actuator extends, it also pushed the other one). There have to be other ways. I will try to think of some as I go for an evening stroll...
Mabe that complexity isnt that big, I mean instead of a big actuator moving the rudder there is a small one moving the vane that acts on a rudder tab. Both have the same number of electric parts.
The rudder tab "trick" can be used even without a wind vane, I mean actuator moving the tab instead of the whole rudder.
Also in a very low displacement, long hull, the forces required for steering remain small, specially if the boat speed doesnt get too high.
Two actuators on the same rudder should either have some elastic linkages/arms or have a means to decouple each other. And electric/logic level to detect actuator failure (=sensor) in order to engage the decoupling of the failed actuator and coupling of the "good" one. That means instead of one actuator you need at least three - actuators themselves and a third one to engage only one of them.
The screw-type actuators, either linear or rotating (like in a windshield wiper motor) tend to have a strong holding force even when their motor is not powered.
An elastic arm or chord between rudder and actuator will also give it some protection against excessive forces, with the penalty of reduced rudder authority at high speeds.
-------------------
If you design for electric redundancy then the rudder actuator isnt the only component that can fail. Solar pannel, charger, batteries, wingsail tab actuator, and pilot itself or any of its sensors can and eventually will fail.
I think tiller pilots got some failing reputation only because they were the most used (stressed) piece of electric equipment used on boats. Other parts fail too - electric or hydraulic winches, outboards, gensets, radios or whatever.
Another form of steering redundancy is to have a shooner rig, with two wingsails. The steering can be made with either both wingsails or with rudder and one wingsail. Still, if one of wingsail actuator fails you have to have a method to both sense the actuator isnt working as it should then get that wing in depower/zero trim, to disengage the actuator. For each wing.
---------------------------------------------
You will allways have to choose between not having redundancy (= pick/make good reliable parts), or design for redundancy but the costs, complexity and development time build abruptly.
I'd rather go with a good, oversized windshield wiper motor for rudder and use a magnet/hall sensor for rudder position, to know when to power the motor, how much power to give it, and when to stop.
Then have the boat on a long light (dyneema?) anchor somwhere at deeper water and let it manouver there within a hundred meter radius for several weeks, or months, that means ability to check if, what and why fails, including the chance the floating anchor line to tangle into rudder or daggerboard or what else.
Not sure it's the best strategy, but what I feel is that simplicity + good testing beats complexity, since complexity, even for redundancy, increases the number of parts that can possibly fail and numbers of ways the parts can interact in order to produce failure.
---------------------------------------------
A skeg a little deeper than the rudder would protect it enough?
I wondered why saildrone being capable of 10 (or 14?) knots, it averaged 3 knots. One reason might be they wanted to reduce both stress on actuators :) and, more importantly, the damage produced in case of impact with a heavy log or other debris.
Forces on lifting foils increase with the square of speed. Actuator life decreases abruptly with applied forces (currents in electric motors being proportional with force/torque they produce)
Energy of impacting a large object also increases with the square of speed.
A 6 kts (aprox 3m/sec) speed impact of hull with a large solid floating obstacle, is equivalent with dropping the same hull (with attached wing sail, other equipment and amas) bow down from 45cm or 1.5ft. Doesnt sound much, but a CF boat it's much heavyer and almost as rigid as a sledge hammer.
A 2m/sec (4kts) impact is equivalent 20cm drop, less than half.
Makes me wonder about virtues of inflatables :)
------------------------
A 30x10cm naca 0015 airfoil rudder, at 6kts, max lift in water can reach 16kg or 35 lbs of lateral (bending) force. 64kg at 12kts. At 0.5 lifting coefficient, max recomended for rudders, it's only 6.6kg at 3m/sec.
--------------------
Finally I attached a picture of a 1.05m long kind-of skin on frame vaka waiting for the second linseed oil coat to dry.
vakalinseed.jpg
Are you still researching design concepts for long distance, and are you open to sub-surface concepts?
I have been tinkering with an idea for a few years now, based on the same principle as the Aereon 26 from the early 1970's...
Hello LoopZilla,
There are some interesting underwater drones out there, mostly torpedo shaped. I think one of the most promising is one that uses changing bouyancy to glide through the water at about a 45 degree angle, both down and up.
My main concern would that any underwater drone require more power to overcome water currents.
A different concept uses a sail which can be lowered and submersed whne needed, I presume mainly for getting a drone into a location suitable for survellience.
Power for any underwater craft would be an issue. Some newer solar panels are better at collecting the light that penetrates into water, but they are far less efficient than above the water. I suspect that you would need a physically large array and would struggle to reliably propel the craft along.
You are right about the lifetime of the actuator being longer if the peak load is not applied each time, but yes, a description of how much longer is hard to find.
I wouldn't be sure about building it from parts. I have heard that windscreen wipers have very long lives but I wouldn't be able to sleep nights if the boat was relying on my knowledge of motors :)
I thought that this was interesting: http://www.simrad-yachting.com/Root/TillerPilots/TP10-22-32_OM_EN_9...
and a write-up of two models here:
http://www.practical-sailor.com/issues/35_6/features/Entry-Level-Ti...
The 'Steer by GPS' is of course very similar to what the PixHawk will do. Of course the PixHawk wins because it is more open and is being actively developed, plus there is a wing to manage, but it is interesting to consider. The tiller pilots aren't very reliable and of course when they go wrong are expensive to repair.
reading some tiller pilot specs I noticed that a 57kg thrust actuator can steer a 3 ton yacht. If you scale it down to a 100kg displacement robot boat, then a 2kg thrust rudder actuator should be sufficient.
I bet there are some serious life actuators - e.g. these one specs 10million cycle life, but they-re too small. http://www.intelligentactuator.com/rcd-mini-electric-cylinder/
-
1
-
2
-
3
-
4
-
5
of 8 Next