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
regarding the question you asked on http://engineering.stackexchange.com
The simplest self-locking rotating actuator is one that uses a worm gear, like windshield wiper motor does. If that is too strong for rotating only the wind vane mast, there can be found smaller motors with the same worm gear that does self-locking in position.
The weight is not an such importanr issue, because the wind vane assembly is not fixed to the rudder but to the stern of the boat. The movement between the windvane and rudder tab is transmitted through the linkage figured in the top-right circle in this picture: http://www.faymarine.com/plansite/images/4_SS_vert_vane_to_TT.GIF (the one that captions "vane stock", "pin", "slot" and "trim tab stock")
So everything above this linkage (windvane, turning worm-gear motor) does not weigh on the rudder but on the stern of the boat.
Regarding Ceconite - it in itself is woven polyester. Of course UV can degrade it and water pass through if it werent a resin (or dope) and paint applied to it. The resin seals it and the paint protects agains UV. The polyester fabric itself isnt affected by salt water anyway.
Many aircrafts are parked outdoors, in the sun and rain for years. And many kayaks or canoe hulls are covered with the same kind of fabric (heavier however) but being soaked in some sort of resin (like yacht polyester paint) they have no problem keeping the boat structure dry. If water gets in, it does through the openings above not through the skin.
The fabric in itself does provide only strength, much like fiberglass fabric does in a fiberglass-resin boat skin.
Need only to make sure the resin you use for soaking the fabric blocks UV, some paints/laquers/paints are formulated for long UV exposure or you can use an UV-blocking additive in the last layers of resin. A couple good additives are zinc and titanium oxides - in my place of the world these can be found cheaply as white powder paint additives in construction stores.
Zinc and titanium oxides are so good they-re also used in sun protection creams formulated for human skin, a very thin, barely visible layer of which reduces the UV exposure by 10 to 50 times.
It starts to look like a proa :) The tail vane, made of transparent polycarbonate is barely visible. It has a red trailing edge.
prouta17feb2.jpg
here-s a page with info about DIY wind-vane. I guess the expensive ones are anyway sized for much bigger boats.
http://www.faymarine.com/plansite/fay_marine_yacht_pl.htm
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In one of the variants, the wind vane moves a trim-tab on the rudder. http://www.faymarine.com/plansite/images/1_SS_aux_rudder.GIF
From the description I understand the rudder is rotating freely around its pivot axis (except when locked), and steering is done by the windvane moving the tab on the leading edge. That means a much lower force is needed to rotate the tab.
I guess that we can use a servo or linear actuator to move the trimtab instead of the whole rudder. That will require a much lightweight actuator. And less wear on it, without the complication and risks of using a wind vane
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For satellite messaging, I think there (still) are satellites that can be used for short messaging by licensed amateur radio operators. These are free sattelite messaging relay services. Considering the drone is far away from land, it should have little problems with congestion traffic encountered on land with such systems, so it is quite possible to reliably use this network to transmit/receive message packets. The system also has internet gateways, so one can communicate with the drone via internet - gateway - satelite - drone. I don't know the technical and legal details, but it's worth asking a knowledgeable radio amateur, or a forum.
There is needed a software to predict when/which satellites are in sight and there are several passes a day, that is enough to receive a status/position update from the drone and - eventually - transmit back a change course command.
Here are some links:
http://www.aprs.org/astars.html
http://en.wikipedia.org/wiki/OSCAR
http://en.wikipedia.org/wiki/Amateur_radio_satellite
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Here-s a document about designing a rudder trim tab http://www.svsarana.com/selfsteering%20gear/sarana%20trim%20tab.pdf
It is not attached to the leading edge, but farther aft like an airplaine's horizontal tail instead. Much more effective since there-s a much longer torque arm.
Yesterday I headed to the coast to find some boats with wind-vanes and to find somewhere to build a boat.
No luck with finding wind-vanes, but I may have found two places to build a boat. And topped-up my sun tan :)
I have a couple of hours free today so will read the links you provided and hopefully some follow-up.
whatever method is used to rotate the rudder (or windvane) there is still needed a method to read its actual position.
An encoder.
One method is to have the encoder on the motor shaft (or the screw shaft like they use on tiller-pilots), and count/track numbers of rotations left-right of the shaft and for each rotation there-s a slight movement of the rudder, depending on the overall reduction ratio.
Another method is to have the rotary encoder attached directly to the ruder shaft, that will read directly position of the rudder.
I personaly like this second method, since in case of an unwanted reset of the ardupilot board/software, in the first method it loses the counting that provides the actual position of the rudder.
Another reason I like it is that there is no need to hack the actuator in order to mount an encoder on the actuator itself to know the rudder position, so it allows to swap it if during testing it prooves inadequate - too weak, too strong (and power hungry) or unreliable.
Here-s one encoder that can tell angular position of a shaft by using only one magnet mounted on it, and the encoder suspended closely above it. http://www.amazon.com/AMS-AS5145-ADAPTER-EXTERNAL-ENCODER/dp/B00GOO...
Similar function can be obtained with two linear hall sensors, a small magnet, and APM's or Pixhawk's two A/D converters to read rudder position. The sensors dont move, dont need to touch the rotating magnet, so they can be sealed in hot glue or epoxy for weatherproofing.
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Since both linear actuators and windshield wiper motors can use an encoder externally, and have the desired feature of locking their position (no power used when not moving the rudder), I see no reason not to test a windshield wiper motor. A friend gave me one for free, now I need the circuitry to power it, hang some hefty weight on its arm and test longer time for wear.
After reading some more things and writing to people, I'm less convinced that a linear actuator directly controlling the rudder will be the right tool for the job. It's probably too many actuations. The idea you suggested earlier, re: self-steering wind vane, may prove to be the right answer.
Indeed, this is what the author uses:
http://www.tor.cc/articles/steering.htm (section 'Tiller Pilot to Windvane Self Steering')
So, I will do some more reading about self-steering rigs. If a rudder tab has to be moved less frequently and with much less force, then the lifetime on the bearings will be longer.
Windscreen wipers are good, although I trust the engineers will have selected the best motor for the task. I've seen home-built actuators, but I prefer an off-the-shelf one. Just one would be nice, but two would be OK. You are right that I would needd to know when it is failing. The issue is that they don't always fail and stop working completely. As the bearings begin to die, the amount of force the actuators provide or the speed they move at gets progressively lower - and this is not so easy to measure.
The screw-type actuator are very strong in static mode. There is a little back-movement, but very little. Knowing when the rudder is on the centerline would be worth keeping track of via a Hall sensor.
Testing will probably be me sat on the beach with an alcoholic beverage in hand, putting it through its paces, maybe with a motorboat to collect it if needed. A long anchor is a good idea for rough weather and endurance testing.
You mentioned that Saildrone may have been made to go slower to reduce the stress. This may be the case although in the first few days of the trip it was average knots in the teens when the wind was perfect. I assume that they did not deliberately slow it down.
If you wanted to go inflatable, take a look at this craft:
http://www.carpictures.com/pics/full/07IDB474832735A/WAM-V-Proteus-...
Not sure I would want to trust the air staying in the tubes for several months though :)
Yeah, I was also looking more into weather vanes and tiller pilot steering.
From what I found about tiller pilots they fail from various reasons, there are few failure modes:
1 - sealer failure and water getting inside - this one can be prevented with careful design and building.
2 - handling - some of them do not have adequate end-of-movement sensing and the motor/gears fail trying to push agains dead end. That can also be prevented by design.
3 - electric motor failure due to aging - this one is surprisingly infrequent considering the use of cheap brushed motors, so my worries about their life could have been exagerated.
4 - most important one - gear/motor wear due to frequent movements at high stress. e.g the tiller is fine steerin a 2 ton displacement boat in calm weather, things change in wave/stormy weather when rudder forces, number of movements and electric currents increase abruptly.
What I can observe about this last one, is that often, even small sailboats might need relatively high steering torque from rudder's center of pressure being far away from its own pivoting axis. Mabe small boats are intentionally designed so when hand steered the human pilot feels feedback force from the rudder.
But in a lightweight robot boat, there is no reason not to have a pivoting arm (distance between rudder's center of pressure and its pivoting axis) as low as 8% of the rudder chord. That's 1-1.5cm, half an inch for a 4m boat. This together with the boat being an order of magnitude lighter than manned ocean sailing boats, makes me confident that the smaller boat could use these tiller pilots (or their motor&mechanics) with much lower rates of failure.
What would be cool is to source a cheap one with electronics failed (your auto pilot will supply logic), but with gears in good condition. Even a motor replacement isnt that expensive nor difficult, provided the gears ar fine. There are a few pages and youtube clips showing their insides, and what fails... they look reasonably simple to work within.
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Now, still remains the reason that you want to reduce the number of times the rudder actuator needs to engage, and a wind-steering mechanics would cut down these movements (and increase its life) with at least an order of magnitude. And also the redundancy problem, you want to be able to steer the boat when a steering actuator fails.
The windvanes are a choice, what bothers me about them is that they have to be very fine ballanced and sensitive, low friction to function in light winds and also to resist not only in storms, but what scares me is how they could handle a breaking wave in a storm, one that washes all over the deck since I dont see how the windvane could be mounted more than a couple feet above the water line.
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So here is another idea - to have a second "helm-and-steering" wing - much smaller than the main wingsail (30-40% its height and 10-15% its area) placed far-aft from it. It would be turned around its mast by a strong servo, it will also be made strong, and its purpose will be to act as a wind-vane that keeps a certain angle of the boat relative to the wind (its helm) very much as the wingsail tail controls the angle of the main wing.
This way the boat will have:
- a means to maintain constant helm (wind relative) without needing frequent rudder updates.
- redundant steering method in case main rudder fails.
- in case the "helm-wing" fails it will be steerable with the main rudder
- a means to tack at low boat speed, using the "helm-wing" to slowly turn the boat out of the irons.
- a means to hold the boat's bow towards the wind (and breaking waves) in storms, when main wing must be depowered and main rudder becomes inneffective
See the attached drawing I made, sizes are only descriptive.
Oh, unrelated to the steering, I figured there a "tangle prevention line" between the rudder's tip (its lowest rotation axle point) and the keel, to prevent any debris or lines passing by the swept-back keel to float up onto the rudder's foil. Not sure if it works, or it wont induce some other problems, for sure it adds some extra drag (not much since it's horizontal)
helm-wing-boat.jpg
I found this rig configuration is called yawl:
"On a ketch, the principal purpose of the mizzen sail is to help propel the vessel as part of the working sail, the sail area being split up between two masts to ease handling. On a yawl, the smaller mizzen mainly serves the purposes of trim and balance, working more as an "air rudder" or trim tab than as a substantial part of the working sail"
http://en.wikipedia.org/wiki/Yawl
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