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2. Mechanical Design

Team <skype>

Piem Wirtz <v2_piem>
Sebastian Muellauer <dr.funkstrahl>
Joshua Updyke <josh.updyke>
Roberto Meléndez <robertomelendez>
Henrik Rudstrøm <lavvotore>
Peter Keen <pwkeen>
Julia Cerrud <julia_cev>
Aaron Gutierrez <amorphica>
Cesar Harada <cesarminoruharada>
Gabriella Levine <gabriellalevine>

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?
 Please add you name if you want to be part of this group. 

 Questions => Decisions

  • Can we drag a long absorbent?

  • Can we sail upwind with an articulated hull?
What is the behavior of an articulated hull?

  • How to maximize pulling power and control of Protei?

  • Articulated (+ springs) / flexible?

  • One mast / multiple masts?

  • How many degrees of freedom?

  • How many channels for the Remote control system?

  • Symetrical / Asymetrical hull?

  • Inflatable / Hard or Inflatable + hard?

  • Number of Actuators required (degrees of freedom)

  • Modelling input to hull design

  • How do we make power transmission? (hydraulic / pneumatic / mecanic - bowden cable)

  • How do we ruggedize the model?

  • What are our preferred materials?
  • Do we have to do some tests of a boat with a heavy tail attached behind as soon as possible, to obtain direct experience?

Intermediary deadlines

0. Introduction

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 : 
  • Develop an oil spill cleaning technology
  • Develop a new family of articulated sailing vessel

20110423 forces in sailing, Mechanical engineering
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)?

1. Pulling Power

That is its end purpose, to pull the load of an oilboom (and potentially other items)

1.1. Masts and sails

  • Hypothesis 1 : 1 mast | 1 sail
  • Hypothesis 2 : 1 mast | 2 sails

  • Hypothesis 3 : 2 masts | 2 sails
  • Hypothesis 4 : 3 masts | 3 sails
  • Hypothesis 5 : 4 masts | 4 sails
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 :)

2. Steering

Able to steer while pulling a big load

2.1. Hull shape

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. 
  1. Test 1 : straight forward, the boat going with a trajectory we could easily anticipate. 
  2. Test 2 : the "S" curve allow the stern and the bow to "compete" to see which will have more influence on the steering.
    in here the stern has more influence, hence when moving forward the vessel goes to the left.

2.2. Velocity and lift effect

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.

2.3. Articulated VS Flexible

2.3.1. Ruggedize

How do we make it sea-worthy ?

2.4. Motor

2.5. Transmission of power

There are 2 main options for control transmission in the body of Protei :
  1. Cables (like bikes cables) : cheap and reliable but high friction, rust in Marine environment.
  2. Pneumatic :
  3. Hydraulic (fluid) transmission : low friction, powerful, expensive and rare. Car brakes uses fluid because it is very powerful and reliable, and does not rust. Hydraulic cylinders are also notorious high-power transmissions. Now in a saline environment, what is the lost durable / affordable option? This choice will completely condition the design and price tag of Protei. 

2.6 Motors

3. Self sufficiency

One aspect is to recover from unfavourable situations: such as grounding, capsizing, entanglement

3.1. Capsizing & Self-righting

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.

3.2. Grounding

A lot of the stability of the vessels depends on its cross-section.

3.3. Mass distribution

3.4. Entanglement

3.5 Emergency procedures for the sail

Points for discussion:
  1. As well as being an emergency procedure this can also be a control procedure allowing the sail area to be set according to the most efficient use of wind conditions in relation to power requirements and vessel stability.  This may help guide us in selection from the list provided above, that is the one that both effectively reefs the sail completely in an emergency situation but still gives graduated control of the sailing surface.  From that perspective F and G appears good candidates.

4. Durability

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)

4.1. Materials

5. Ease of manufacturing

  • Make a desing simple enough to be easy replicable.
  • Keep construction principles/materials simple.
  • Depend on good design rather than good craftmanship -

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. 

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,%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.
Subpages (1): Notes from 07062011