This is for the actual design and construction of the Protei hull and sails. What will it be made of? How will it look? Where will everything fit?
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It is a good idea to start by a glossary, so everyone has the basis and understanding of the forces that are in presence in the act of sailing.
In several ways, Protei is challenging these factors mainly because it is an unmanned vessel, and because its hull can change shape.
The development of Protei has two main goals :
From "Principles of yacht design - Larsson, Eliasson" Download whole book (64 MB).
> How many degrees of freedom?
> How many channels of control (on the RC command)?
That is its end purpose, to pull the load of an oilboom (and potentially other items)
The main advantage of having multiple sails is that it reduces the chance of one sail shadowing the other's sail. It is not unusual for a boat to have sails that slightly shadow each others, and ... it works! (This is of course in a non articulated hull where all the sails get the wind from the same side. The big question is when the hull is articulated and one set of sail get the wind from one side, while another set will get the wind from the other side.)
The disadvantage is this creates another level of complexity in the fabrication, number of sensor and mathematical model. As a first experiment, the simplest we stay, the better - yet great discoveries may just lay before us if we dare :)
Able to steer while pulling a big load
The hull being also the centerboard and the rudder, the way we design it is absolutely crucial.
The way we proceeded for research is to design a hull that we can crop - yes literally carve - , rectangular at the beginning (fig. A), than progressively cropped to a circle (fig. C), than with a dominant stern (fig. E). We invented a dominance test, called the "S test" which can help make an approximate judgment of which of the articulated stern (back) and the bow (front) have the most influence on the steering.
If Protei reaches high-speed (which may not be counter efficient to absorb oil) we may reach that point where the shape of the hull creates a lateral lift. As represented in the diagram, this means that the hull behaves like a wing (but vertically). If this hypothesis is accurate, than we might witness the opposition of 2 lateral forces : the sail (air wing) VS the articulated hull (water foil). This is not an emergency since we do not know yet what kind of speed we can achieve with Protei, but just anticipating what happen in extreme conditions. If this is verified by testing, that means that we may use this effect to go closer to the wind. The world fastest boat - hydroptere - uses these foil to reach dramatic speed. You need to watch the video! vehicle going at much modest speed - the waterbird - can also use this effect and support a lot of weight.
Interesting article about stall, and "post-stall" dynamics. Stall diagram.
Interesting article about "Center of effort VS center of lateral plane".
In classical yachts the the keel works as a wing underwater, which provides much more lateral resistance than weight and water resistance alone. To control the lift and get a predictable trajectory, the rudder is used as a stabilizer, just like a typical aeroplane wing+stabilizer.
explained very well in this article (fourth section "The wing/stabiliser system") How this would translate to the protei which in its current form only has one deformable wing, is an interesting question.
How do we make it sea-worthy ?
One aspect is to recover from unfavourable situations: such as grounding, capsizing, entanglement
Too high winds and unfavourable waves can cause the boat to roll over. the weight of water distributed over the length of the mast can over power the momentum of the keel and the boat may be out of action.
A lot of the stability of the vessels depends on its cross-section.
Points for discussion:
Another challenge is to simply sustain the strain of the ocean environment without human supervision.
Peak forces due to sailing maneuvers
Repetetive strain from constantly changing forces due to wave motion, changing winds and sailing maneuvers. This can cause fatigue and eventual failure.
Hull damage due to collision and debris. Able to absorb damage or in worse case limit the effect of the damage (redundancy/compartimentalized etc)
Some thoughts on the idea of an articulated hull (Henrik):
non rigid hull necessary to articulate a hull
rigid hull necessary to absorb to balance the forces from the sailing rig and the lateral reaction forces from the hull.
https://docs0.google.com/drawings/edit?id=1eeD7ejbg0KHxJ4k7lcKUKURoAaZnZzJyeflZaBbdhn8&hl=nl&authkey=CNmS_qAJ (very naive sketch, feel free to edit).
Either the hull has some rigidity to resist the forces of the rig/hull passively, that means the actuator mechanism will have to provide force to deform the hull.
Or the hull has minimum riditiy and the actuator structure has to bear the load of keeping the hull straight.
See red lines in the sketch above.
The tricky thing is that the dimensions and magnitudes forces of a sailing boat do not scale linearly with length of the boat (L), sail area scales with L^2, while volume scales around L^3. (see http://protei.org/download/20110417Principles%20of%20yacht%20design%20-%20Larsson,%20Eliasson.pdf page 11-12 (Main dimensions). So what seems feasible in terms of actuator power on a 1/10 scale model might need to be 100-1000 times more on a 1/1 scale model.
Can the actuator sustain the constant changing forces applied to it without failing prematurely?
Can we balance the hull resistance(keel shape/positions) with the forces from the sailing rig to reduce the moment on the hull is reduced?
Can we design a articulated hull that is robust enough to sustain a longer ocean trip, fatigue due to repetetive forces is often a source of failure in a sailing boat, and an articulated hull will inevitably add complexity/new sources of failure to the design.
Maybe most important questions: What are the benefits of an articulated hull?
- Better steering ? - possible, but will it be so much better that it outweighs all the disadvantages it brings?
- Able to catch wind two ways during tack ? - Questionable, sails will shadow each other unless they are spaced far away from each other.
Some old sketches _random (sebastian):
look at the boom to be the hull, and several independent sails as drive units.
6 A new thought about the tow point of the long heavy tail (by Qiuyang)
Just an idea, I don' know if you considered this before or not, feel free to edit&comment
From my point of view, the difficulty for Protei is the maneuver ability when a heavy and long boom has been attached to the stern of the
boat. It is known that the boom will add huge rotating inertial on the boat, thus it is difficult to tack or jib.
What I am thinking is to put the point of the tow force at the inertial axis of the boat. Then the drag force commits ZERO rotate torque to the boat when the boat starts to tack. (Of course the drag force is still there) . Check the follow diagram.
The boat can rotate around the red frame when tacking, which means when tacking, the boat turn first to catch the wind as fast as possible (not influenced by the tail, the tail still move to its original direction). As the boat moves out of the NO-GO zone, the pulling force can be assured the then the tail will turn progressively.
Theoretically the drag force from the boom will not influence the rotate inertial if we can find the inertial axis exactly. In practice this is impossible, but move the point of drag force closer to the inertial axis will definitely decrease the added inertial.