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A SWATH Model for the Charles River Meg Hendry-Brogan1 and Sheila Saroglou2

ABSTRACT Pipe Dream, a model SWATH (Small Waterplane Area Twin Hull), is the product of two semesters worth of design and construction. The work was done in an effort to satisfy a challenge that required first, a working SWATH whose design addressed those fundamental aspects of naval architecture, marine engineering, and ocean engineering which pertained to the boat. Second, the challenge required a demonstrated method for measuring and correcting for pitch instability, while the third part involved quantifying the performance of the model boat in rejecting wave disturbances. Using a detailed analysis of the Charles River �sea state� and the traditional engineering theory associated with ship design, Pipe Dream was physically designed to operate at natural periods in roll and heave which allowed for sufficient stability in its intended environment and also to withstand the structural stresses imposed thereby. Focusing more on the implementation than the design process, the construction of Pipe Dream involved the development of the SWATH structure, propulsion and steering systems, pitch control scheme, and the electronics used to read the sensors and drive the vehicle. Complex machining was required in many areas of the construction for waterproofing and design requirements, and an even more complex electronics layout was engineered in order to meet the challenge. In the end, pool and river testing and associated analyses were completed to a limited extent. Pipe Dream has the potential to be used for an extremely interesting and useful series of tests which would further the understanding of stress states in SWATH structures, stability characteristics of this type of surface vessel, and the extent to which the model design was sufficiently matched to the Charles River.

1 MIT OE Class of �03; SNAME Member since 2001 2 MIT OE Class of �03; SNAME Member since 2001

INTRODUCTION SWATH, or Small Waterplane Area Twin Hull, vessels are becoming increasingly popular in many areas of the marine transport industry. Their superb characteristics in sea keeping, overall stability, ride comfort and quality are unique and attractive to those industry professionals who seek to maintain cruise speed and comfort in rough seas. Serious SWATH designs have existed since the mid

1960's. Currently there are approximately 50 SWATH ships in existence world wide, with the majority being built in the U.S. In 2002, an addition was made to that family of vessels through two semesters� worth of work at the Massachusetts Institute of Technology. In response to the very detailed challenge, outlined below, the model SWATH, Pipe Dream, was designed and constructed. This vessel was

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specifically designed to operate in the Charles River, Cambridge MA. Pipe Dream addresses each aspect of the challenge, some more successfully than others, and was christened in the Charles on November 24, 2002. The Challenge: 1. Build a remotely controlled SWATH, whose design addresses: a) Structural Design for Strength b) Dynamic Response in Waves c) Static Trim d) Electronics and Power Systems e) Water proofing f) Propulsion and Steering Control 2. Develop and demonstrate methods for measuring and correcting Pitch instability 3. Quantify the performance of the boat in rejecting wave disturbances SCALING AND CHARLES RIVER TEST Pipe Dream was modeled after an existing SWATH, MBARI's Western Flyer. The Western Flyer, 35 m long, was chosen as the model because it represented an acceptable balance of length, transit speed, and operability. The three main factors used to scale down the Western Flyer to an appropriate size for operation on the Charles River were Froude number, significant wave height-to-length, and wavelength-to-length. Froude number measures the relative importance of surface wave generation, and scales with length and target speed, while the other two factors scale with operating conditions. In order to analyze the operating conditions, it was necessary to collect data from the Charles River. A wave probe was installed near the side of the river where the vehicle testing would occur. Data was collected for two twenty-four hour periods and then analyzed. The data showed the waves in the Charles to have short periods of around one second. The Western Flyer is designed to operate in sea state 5 in the North Pacific, ( H1/3= 3.25 m) so Pipedream was designed to operate in a similar "sea state" on the Charles River. The mean significant wave height, H1/3, for this similar sea state on the Charles River was estimated to be 14 cm. Setting the mean H1/3 /Length ratios for Pipedream and the Western Flyer equal, it was determined that the Pipedream should be 1.5 m in length. Figure 1 shows the major spectral results of the Charles River Testing. When the wavelength-to-length ratios of Western Flyer and Pipedream were compared, it was determined that the length of Pipedream should be

0.6 m. This number was drastically different from the 1.5 m length determined when using the H1/3 /Length ratio.

Figure 1 � Wave Spectra

After much consideration, the value of 1.5 m was chosen because it closely corresponded to Froude number scaling. The most likely reason that the wavelength-to-length ratio did not give a value similar to that of the H1/3/L ratio is the fact that Charles River waves do not directly scale with ocean spectra. Due to small river size, as well as constantly changing wind speed and direction (see Figure 2), the waves in the Charles do not form regular swell patterns or fully developed seas as in the open ocean.

12 10 8 6 4 2 12-5

0

5

10

15

20

Significant height and wind vectors for 4/22/02-4/24/02 starting at 5:00pm

time of day

significant height

N W E S

(arrows represent wind mag. and direction)

5:00pm 4/22 12:00am 4/23 12:00am 4/24 6:00pm 4/24

Sea State 5

Figure 2 � Wind and Current Direction

The wave data was just as important in designing for dynamic loading as it was essential for scaling the boat. In addition to designing a feedback system to control motions in pitch, and rudders to control motion in yaw, it was important to examine the motions in heave and roll. In order to design so that the model would be stable in heave and roll, the natural frequency of the waves must not correspond

0 5 1 0 1 5 2 0 2 50

5

1 0

1 5

f r e q u e n c y, w , r a d /s

S(w

) cm

2/s

C h a r le s W a v e S p e c t r a fo r 4 /2 2 /0 2 to 4 /2 4 /0 2

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to the natural frequency of the SWATH in these motions. The natural frequency in heave for Pipedream was designed to be 1.91 rad/s. This value is much lower than the wave excitation frequencies. The roll motion for Pipedream was designed to be 1.39 rad/s, also much lower than the measured natural frequency of the waves. DESIGN PROCESS, HULL GEOMETRY, AND HYDROSTATICS The design process can be characterized by an emphasis on simplicity, off-the-shelf components, an effort to minimize machining, and similitude with existing SWATH. Once length and beam dimensions were generated via the Charles River wave data and scale determinations, and given a rough idea of desired maximum speed, hull geometry characteristics were further outlined and an initial stability and seakeeping analysis was completed. At this point of the process, the iterative nature of naval architectural design began to become apparent. As hull geometry and stability parameters were generated, so were thrust requirements and propulsion plant size. If the plant size was simply unrealistic, it became necessary to iterate back through and either streamline the vessel more, i.e. reduce drag, or simply decrease the overall size/scale. Structural considerations also contributed to the iterative nature of the design process. For example, structural integrity requirements may have called for larger struts, but this would increase waterplane area and therefore sacrifice stability or shorten natural periods in roll, etc. This is where optimization via iterative design convergence was, again, proven necessary.

Figure 3 � Vehicle Geometry/ Solid Model

The process of hull form analysis with respect to geometry, stability, and dynamics started with a study and comparison of existing SWATH and was followed by the Charles River wave research. At this point, the basic geometric parameters were

generated as a result of the first two steps and based on a practical upper size limit and a target speed of 2.5 m/s. Then, the basic coefficients of form, e.g. Cw, Awp, CB, IL/T, were found. The hydrostatics of the vessel were analyzed next; meaning, the centers of gravity and buoyancy, the Metacentric Radius (BM), and the distance from the keel to the Metacenter (KM) were determined in both the transverse and longitudinal planes (see Figure 4 below).

Figure 4 � Hull Cross Section

Finally, if these hydrostatic characteristics produced a satisfactory GM, Metacentric Height (i.e. it was greater than zero) then the process did not need to be reproduced. The natural periods in roll, pitch and heave come directly from the longitudinal and transverse values of GM. Table 1 outlines the basic geometric and hydrostatic characteristics of Pipe Dream as shown in Figure 3. Parameter: Value: Units: Comments: L 1.75 m Length on Waterline B 0.751 m Beam L/B 2.33 v 2.5 m/sec Ship Velocity

rp 0.083 m Radius of Pontoon

Lp 1.75 m Length of Pontoon S 2.083 m2 Wetted Surface Area

W 56 kg Based on Preliminary Approx.

DWL 36.7 cm Design Waterline V 0.088 m3 submerged volume

bb 55 cm Transverse Strut Spacing

ll 145 cm Longitudinal Strut Spacing

∆ 90.2 kg Displacement

Aw 0.033 m2 Area of Waterplane Table 1 � Main Vehicle Parameters

STRUCTURAL INTEGRITY

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Both static and dynamic loading analyses were completed to a first approximation. The SWATH cross section was modeled as an open channel beam and the stress observations were made as such. The global loading on the deck of the vessel implies that maximum axial stresses will occur at the corners of the cross section, where the struts interface with the deck. This is congruent with the research publicly available on SWATH structural limit states. The very SWATH that Pipe Dream was modeled after, Western Flyer, experienced cracking in its haunch (strut-deck interface) sections shortly after being put into service, and it had to be strengthened. The maximum shear stresses occur at the neutral axis of the cross section which, although not exactly at the same interface, is relatively close, and contributes considerably to a prying moment on that particular joint. Figure 5 outlines these stresses. The design of Pipe Dream addressed these issues via the incorporation of angled, PVC reinforcements at the strut-deck intersection. However, as will be described in full later, even with this degree of structural reinforcement, the vessel experienced near failures at these joints.

Figure 5 � Channel Cross Section Shear Stress Diagram

The dynamic loading of a SWATH, or model SWATH, is unique from that of a traditional monohull in that the depth-to-length ratio is much larger and helps to eliminate the vertical, longitudinal bending moment as the most critical effect on the structure. The vulnerability of the underside of the superstructure, or �wet-deck�, to slamming impacts from waves is also distinct from the dynamic loading situation on a monohull. These differences provide for the redefinition of the primary and secondary stresses on the hull form. The primary wave-induced loading on a SWATH is the prying force resulting from waves in beam seas, creating a transverse bending moment in the superstructure or in the cross members. In beam seas and for a sea wavelength equal to approximately twice the centerline spacing between the two pontoons, the wave forces on the two pontoons tend to pry them apart at the instant

that the crest is about at the ships centerline. This effect was clearly observed during the river trials of Pipe Dream. The secondary wave-induced loads are the slamming forces on the wet-deck. Wave induced loads on SWATH consist of five components. First is the body mass inertia force, which can be found by basic, Newtonian F=ma. The second is the incident wave or Froude-Krylov force, third is the diffracted wave force, fourth, the hydrodynamic force due to body motions, and fifth, the hydrostatic restoring force due to vertical displacement. The last four force determinations require integrated hydrodynamic pressure forces over the wetted surface of the hull. Reily et al. [1] predicts, through computational analysis confirmed by model testing, that the critical loads on a SWATH are the �side force� or �prying moment� in beam seas, and the �splitting yaw moment� in bow quartering sea. Finite element analyses show that the maximum stress levels (in some cases up to 60% of the yield stress) occur at the upper part of the vertical web of the strut intersecting the tapered haunch section, just as was observed with Pipe Dream. SEAKEEPING SWATH seakeeping theory in general rests largely on the decoupling of the heave motions of the ship from the sea surface in an effort to reduce the effect of waves. The waterplane area is small, the displaced volume is deeply submerged below the water surface, and the center of gravity is high because the underside of the deckhouse, the �wet-deck�, needs to have a certain clearance above the design waterline. SWATHs often have issues with small TPI (Tons per inch immersion) values and small metacentric heights. Increasing the spacing between the hulls provides a modicum of relief; if not enough, then the only resort is to increase the waterplane area, which is against the SWATH design philosophy and generally has the unwanted effect of reducing the heave natural period [2].

NA

5

Figure 6 � Waterplane Area

Transverse stability depends on an adequate second moment of inertia of the waterplane about the ship centerline (see Figure 6). Variation of the spacing between the twin hulls gives the designer control over the crucial considerations of hydrostatic stability. The transverse spacing of the hull mass also increases the roll radius of gyration, contributing to a relatively long natural period in roll. These long natural periods enable the vessel to avoid resonance with the waves in most sea states. [3] A SWATH design in small vertical motion can be thought of as a linear oscillator, where the amplitude of response depends directly on how much force is applied, inversely on the static restoration coefficient, and the dynamic magnification factor. The general way for describing seakeeping dynamics is by a ships natural period in a particular degree of freedom. The natural periods of oscillation for a �boat� are:

Roll

TR2πkT

g GMT⋅:=

Pitch

TP2πkL

g GML⋅:=

Heave TH 2π

Vg Aw⋅

⋅:=

Within kT/L, GMT/L, and Awp we find the geometric dependence of these formulas, thus they should apply to monohulls as well as ships like SWATH. In the heave period equation, one can observe that the smaller the waterplane area the longer the natural period. This is exactly the behavior that makes a

SWATH a SWATH. Pipe Dream, through the first round of dynamics calculations, was found to have a natural period in roll of 4.5 seconds. Tpitch was 3.5 sec while Theave was 3.3 sec. Initial issues with the roll period were encountered but determined to be based on an analytical error rather than an instability intrinsic to the design.

TROLL TPITCH THEAVE

4.507 sec 3.469 sec 3.286 sec ωroll ωpitch ωheave

1.39 rad/sec 1.811 rad/sec 1.91 rad/sec Table 2 � Dynamic Stability Parameters

One of the stumbling blocks to the stability analysis of Pipe Dream, and SWATH in general, is that very little is understood about the hydrodynamic interference between the two submerged hulls. It is necessary to decouple the system and employ the extensive descriptions available for slender bodies moving through water. These descriptions imply the use of yaw stabilizers (analogous to the feathers on an arrow). Most ships operate at a marginally unstable, open-loop, condition in yaw. This gives the ships better maneuverability and allows for a tighter turning radius. However, with any forward velocity and uncontrolled heading, the vessel becomes unpredictable and possibly spins out of control. Thus, a helmsman is generally required to �close� the loop. In our case, there is an individual at moderate range with a remote control. Although this should imply real-time control over the rudder, there are issues with reflex time, line of sight, etc., that prompt the desire to design the system so that is actually slightly open-loop stable. Too stable would make the vessel difficult to turn, so the ideal operating point is just below the point of marginal stability. Pipe Dream eventually displayed a large, but satisfactory, turning radius, with a slight instability in yaw when moving relatively "fast", meaning above 1 m/s, roughly. RESISTANCE AND PROPULSION In order to choose a motor and propeller combination to advance the vessel at the desired speed (2.5 m/s), it was necessary to first calculate the total resistance. The total resistance coefficient CT was taken from a graph found in a paper on high-speed SWATH (see Figure 7) [3]. According to the graph, CT is approximately 6 * 10-3. Using the previously determined wetted surface area of 2.08 m3, the total drag resistance was found to be about 44 N. About half of the total drag, 24 N, was due to

y, longitudinal

ll x,transverse

rs

bb

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frictional drag, while the rest results from residual resistance. The total drag resistance value of 44 N means that each thruster would need to be capable of producing 22 N of thrust force.

Figure 7 � Total Resistance Coefficient vs. Froude Number [3]

To choose the motor and propeller combination, the desired thrust force for each motor was set equal to 27 N. This value, 22 % higher than the calculated thrust, was chosen to offset any errors made in the drag calculation. Using this desired thrust force value, the desired speed, and propeller curves for various propeller sizes and geometries, a 5 inch diameter by 5 inch pitch propeller was chosen. According to the propeller curves, this propeller could provide the required 27 N of thrust if spun at 1,575 RPM. At this speed, the propeller would be operating slightly below its peak efficiency, as shown in Figure 8. The propeller curves also showed that the torque on the motor would be 80 ounce-inches. A DC Baldor motor was found which could provide the necessary 80 ounce-inches of torque and a speed of 1,575 RPM when operating at 24 Volts.

Figure 8 � Propeller Efficiency Curve

PITCH CONTROL AND STEERING Due to their unique geometry, SWATH boats can be unstable in pitch at high speeds. This instability is generally the result of lift moment acting on the submerged hulls as the vessel cuts through the water. To stabilize this movement, Pipe Dream was designed to include a feedback controlled pitch system. The system included two fins attached to the pontoons, Figure 9, as well as electronic sensors to monitor the ship's movement.

Figure 9 � Pitch Control System

A servomotor, controlled by a central computer onboard, dictated the angle of attack of each fin. The data from the tilt sensor, speed sensor, and accelerometer were used as inputs to the feedback loop which would control the position at which the servomotor held the fin. A pair of rudders, one behind each pontoon, was chosen as the steering system for Pipe Dream, as most existing full-sized SWATHs feature a two-rudder system. Equation 1, was used to determine the minimum surface area for each rudder.

Ap = [(LT)/100] * [1+25(B/L)2] [EQ.1] Where: Ap = Projected area of Lifting Surface;

L=vessel length; B=vessel beam; T=draft (Gilmer and Johnson, 1982)

This equation is generally used to determine the rudder size for a monohull, so for Pipe Dream, each pontoon was assumed to be a monohull. The total wetted surface area for each rudder was determined to be 20.2 in2. The rudders actually installed on Pipe Dream were about three times larger than the calculated wetted surface area. Over-sizing the rudders was done to compensate for any errors incurred while using the monohull equation for a SWATH. The rudders were foil shaped, to minimize

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drag, and are positioned three inches behind each propeller. MECHANICAL DESIGN AND FABRICATION Modularity, ease of fabrication, and low cost were three of the major factors considered when designing parts for Pipe Dream. Ease of assembly and disassembly were important not only to facilitate transportation, but also to make the vehicle more adaptable to change. As most of the parts were to be made by fairly inexperienced students, avoiding hard-to-fabricate, complex designs was an important consideration. Choosing inexpensive materials, as well as keeping outsourcing of fabrication of parts to a minimum, were both important in staying within scope of the prescribed budget and fine constraints. The overall structure of Pipe Dream can be broken down into three major parts, the pontoons, the wet deck, and the PVC frame. There are two pontoons, Figure 10, each

Figure 10 � Pontoon Assembly

comprised of three bulkheads (front, mid, rear), and two hollow tubes placed between the bulkheads. The tubes were designed to be hollow, not only so they would be buoyant, but also so that they could contain electronic equipment, such as sensors, and the batteries needed to power the boat. Each of the tubes was made from 0.25 in. thick, 6 in. OD PVC. PVC was chosen because it was waterproof, close to the density of water, easy to work with, and fairly inexpensive. While the front pontoon tubes contained the batteries and other electronics, the rear pontoon tubes housed the pitch control surface motors. The control surface motors were housed in hollow Lexan tubes with waterproofed endcaps. The shaft of the motor extruded from one end of the housing and was shaft coupled to the fin of the control surface. This whole housing was inserted into a transverse hole drilled through the rear pontoon and then glued into place. The completed assembly can be seen in FIGURE BLAH above.

The front, mid, and rear bulkheads are all connected to the tubes by a pressure fit O-ring seal. Made from solid PVC, each bulkhead to pontoon connection was made watertight by two O-Ring seals at each joint. The mid bulkheads contained holes through the middle to act as a wire pass-through, while the front bulkheads had holes drilled from the end to the strut to act as path for air to flow. The rear bulkhead, Figure 11,

Figure 11 � Rear Pontoon Assembly

served as housing for the thrust motor. Teflon shaft seals were used to allow the shaft to spin while still denying the entrance of water into the rear bulkhead. A PVC flange was added to the top of the rear bulkhead to increase the strength of the strut/bulkhead interface. Each bulkhead was attached to a strut, a hollow 2 in. OD, 29 in. long piece of hollow PVC. The struts were designed to be hollow to allow both air and wire to pass through. PVC was chosen because it was lightweight, cheap, and easy to interface with the PVC bulkheads. The struts connect to the wet deck, a 0.25 in x 31 in. x 76 in. piece of sealed plywood, through a PVC frame, which is U-bolted to the wet deck. The PVC frame, is made of miscellaneous pieces of PVC tubing, joints, and unions held together with PVC glue. To facilitate easy assembly, the front and rear struts were connected to the PVC frame through the use of a PVC union. Each union contained an O-ring seal as its waterproofing mechanism. The mid strut connected to a joint attached to the wet deck, and was held in place with a bolt. Angled pieces were added to the PVC frame to help the boat resist prying moments. It was important that the wet deck and PVC frame be relatively lightweight, as increasing topside weight would make the boat less stable.

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The most challenging aspect of designing the rudder system was choosing the mechanism for turning the rudders. The original design used one motor and a sprocket and chain system. After many attempts to build this system, the design was changed to instead have two separate servo motors control each rudder, (Figure 12).

Figure 12 � Rudder System

Modularity was an important aspect of rudder design, as the rudder system would need to be removed in transport. The rudders, made from polyethylene were created using existing molds from the MIT Towing Tank. Each rudder was fin shaped, with a metal shaft extruding from one end. This extrusion was shaft-coupled to a longer, 1/4 in diameter stainless steel shaft that passed up through a hole in the wet deck and attached to the shaft of the motor. To constrain the motion of the shaft in directions other than rotational, a "whale back" was added to each strut. The "whale back" consisted of a rigid piece of plastic with a hole that the shaft passed through. WEIGHT AND TRIM The distribution of weight on any structure is directly linked to its static and dynamic stability. With SWATH design, both physical accessibility constraints and the high premium on buoyant volume, require that accurate weight and trim predictions be carried out. With Pipe Dream this process was even more imperative because of the amount of �splash-proof� electronics located on deck. A miscalculation with respect to buoyancy could have been very expensive. As with any floating body, a low center of gravity and high center of buoyancy were sought from the beginning. Initial issues with a low center of buoyancy, prompted the incorporation of buoyant foam pieces to fill the, otherwise free-flooded, volumes within the strut members. These foam pieces also increased our waterplane area slightly, which helped to ameliorate transverse

stability. Because the vessel was inherently bilaterally symmetric, weight was also distributed evenly in the transverse direction to the extent possible. Where some sensors, e.g. the paddlewheel speed sensor, were present only in one pontoon, the other was compensated with lead weights of comparable measure. A spreadsheet outlining the mass and position of all structural members, propulsion plant components, and instrumentation pieces was generated and kept as accurate as possible, with error being introduced by the inability to accurately weigh some members and to determine their exact centers of gravity. Aided with the predictions of the weight and trim spreadsheet, the preliminary pool tests were focused primarily on observing satisfactory trim and static pitch and roll stabilities. ELECTRONICS AND POWER SYSTEMS A complex network of microcontrollers, sensors, and motors was constructed to facilitate the completion of the challenge. Most of the instrumentation decisions were made with the goal of maximizing simplicity and exposure. For this reason, TT8 microcontrollers were used as the main �brain� of the SWATH. This computer had been used many times before by the team, and it was also the most readily available. The overall electronics system is segmented based on position on board the vessel, i.e. topside and submerged. On deck, there are two servo-motors for the rudders, the KVH compass, a 12V (combination of four 6V nickel-metal-hydride packs) power source for the hotel load, and an electronics box (E-box) which contained the bulk of the electronics. The E-box served as a junction for all of the other electronic components except for the water detection system which was completely autonomous. The E-Box contained the two Tattletale Model 8 (TT8) microcontrollers, the Crossbow tilt sensor and accelerometer, the frequency to voltage circuit for the thruster system, the RF receiver, antenna and PIC microcontroller for the interface circuit, and the PIC microcontroller for the paddlewheel sensor. Via these connections, the main TT8 had centralized control over all of the sensors and propulsion systems. Inside each of the pontoons is a 24V power source (two sealed 12V lead acid batteries), the thruster motor servo-amplifier, the Baldor thruster motor, the Pittman control surface motor and PIC-servo unit, the cooling fan, and in the port pontoon, the paddlewheel speed sensor. Due to the complex structural design and sheer size of Pipe Dream,

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interfacing all of the electronic systems with each other was quite a challenge. Most of the interconnect cables were in excess of 130 inches, or 10.5 feet. The modular logistics on the wet deck and in the pontoons were organized in an attempt to optimize cabling simplicity. Pipe Dream has quite a demanding power requirement of 250 Watts. In order to quench this demand for power, the supplies were split between the hotel and propulsion loads. Table X.X shows the power breakdown for the vessel. As was previously mentioned, the topside or hotel load power was provided by a 12V, nickel-metal-hydride, source. The two TT8's and other subsystems requiring 12V were powered directly from this source. All topside electronics that ran off 5V received this from the +5V regulated source on the TT8 expansion board. The TT8 sends 5V to the RC receiver PIC, the paddlewheel PIC, and the rudder servo motors. �On-Off� capabilities were present via two switches, which power cycled the 12V topside source (which essentially power-cycled the computers) and the 24V sources in the pontoons. Item Power - Watts Source Stbd/ Port Propulsion

100 ea 24V Sealed Lead Acid-Stbd

Stbd/Port Control Surface

10 ea 24V Sealed Lead Acid- Port

Hotel Load 12 12V NiMH Rudders 18 12V Sealed

Lead Acid Total 250 Watts

Table 3 � Power Budget The most obvious weakness to the electronic design of Pipe Dream was an underestimation of the overall power requirement of the system. The servo motors controlling the rudder positions turned out to be an exorbitant draw on the top side, 12V power source. The voltage sags which resulted caused the computer to crash on multiple occasions and forced an additional power source to be added in the field. Additionally, the system could have been better planned with respect to cabling, meaning quick disconnects and an on-off switch should have been incorporated in the design from the beginning. Overall, however, the vehicles electronics system turned out to be a well integrated network of controllers, sensors and power systems.

SOFTWARE AND CONTROL INFRASTRUCTURE The control of Pipe Dream can be divided into two main segments, the first being the basic motion control, which involved translating the radio commands to thruster speeds and rudder angles. The second portion of vehicle control involved the pitch control and data logging. There are two TT8 computers. The primary one controls movement and data logging, and the second, when told by the primary TT8, can initiate a pitch control algorithm to help stabilize the boat. A six-channel controller and receiver combination was used to control the vessel: the Airtronics VG600/322Z. The RF receiver consists of a battery-powered, six-channel transmitter (operating at 75.79 MHz) with antenna, and a small receiver module, also with an antenna. The commands from the remote control are transmitted via radio frequency to the RF receiver, which picks up the 50 Hz, square-wave signals from its antenna and sends it to the PIC microcontroller in series. The PIC helps to decrease any time lags in command readings from the receiver because it communicates with the TT8 via a 9600 baud serial line. This PIC keeps a continuous update of the values of the six RC lines and sends them to the main TT8 when queried. From the PIC, the TT8 receives twelve bytes of data which represent the high and low bytes for each of the six channels of the remote. These values scaled linearly with the displacement of the RC transmitter joysticks. The main TT8 uses the information transmitted via the PIC to update the settings for thruster power, rudder directions, data logging and pitch correction. The order of the programming on the main TT8 was organized with respect to command priority, i.e. changing direction is considered of higher importance than data logging. Figure 13 delineates the flow of the main TT8's control code. The second TT8, which controls the pitch stabilizers only, is separate from the main computer because with accurate PID (proportional, integral, derivative) pitch control it is necessary to read and update the commands very quickly which would have been bottle-necked by the other sensors. The pitch control program (Figure 14), when enabled, loops continuously independently of the main TT8's routine.

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Request signal update from RC

Interpret values, assign variables

Send directional command to rudders

Send speed command to thrusters

Determine if pitch control should be on

Compare with current pitch setting.

different

Send new pitchsetting to TT8(2) same

yesno

Logging data?

Read sensorsWrite data to disk

Figure 13 - Main Vehicle Control Logistics

Pitch control TT8(2)

Read pitch sensor

Determine anglefor control surfaces

Send commands to PICservo

Check status of pitchcontrol (on/off)

onoff

Figure 14 � Secondary TT8 Control Algorithm

The ability to log data was imperative with respect to the third portion of the challenge which required the vehicle to be able to quantify the performance of the boat in rejecting wave disturbances. The fastest way for the main TT8 to log data would be for it to write to an array and then save that array to disk after every control loop, however, initially it was feared that the TT8's 256 kilobytes of RAM would be a limiting factor. This turned out to

be an erroneous assumption. Instead, the TT8 wrote the data to a Persistor card which had 16 megabytes of memory. This method was slower than saving to an array but insured sufficient memory space. TESTING: POOL AND RIVER After the main frame of the vehicle was constructed, Pipe Dream was placed in the MIT Towing Tank to test for water proofing and stability. To determine the location of the leaks, sensors were installed in each pontoon and attached to a LED panel to create a leak detection system. This system would emit a sound and the LEDs illuminate when water reached the sensors. The result of the initial tests showed that there was some leaking in the pontoons and that the boat was very sensitive to loading (i.e. small TPI). During the first trial, the boat was very unstable. This was not unexpected, as the pontoons were empty, and thus very buoyant. The boat was very sensitive to the addition of weight to the wet deck, and this did cause some concern, so some calculations were done and it was determined that with the current design, Pipe Dream had a 0.1 lbs/in immersion. This was far too sensitive for the amount of materials that were allocated to be placed on the wet deck so water plane area pieces were designed and added to decrease the value of lbs/in immersion from 1.2 lb/in to 10 lb/in. The waterplane area pieces are made from large blocks of foam that are surrounded by aluminum sheeting (see Figure 15). These pieces are attached to the struts through the use of hose clamps. The rear waterplane area piece on each side is faired for hydrodynamic reasons.

Figure 15 � Photo of Waterplane Area Pieces

After the initial leak tests were completed, multiple pool tests were performed. The first sets of pool tests consisted of placing the boat in a shallow pool (3ft. deep) and checking for leaks. There were

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still some leaking problems, but these were solved with the addition of stiffeners to the mid bulkhead, and the addition of a ratchet strap around the struts to aid in maintaining the integrity of the O-ring seals. Once it was determined that the pontoons did not leak, the payload was added and weights were placed on various lengths along the pontoons to trim the boat. At the time of the final pool test, there were problems with the remote control, so it was difficult to drive the boat around the pool as planned. Despite this problem, thrust tests were still run. The thrusters were set to full speed through a link to a laptop, and the boat moved forward under its own power. As the pool was short in relation to the boat, about 15 body-lengths long, the boat was never able to reach its maximum speed. To test the effectiveness of the rudder system, the rudders were set by hand to an angle of about forty-five degrees, and the thrusters were turned on full blast. The boat proceeded to turn, so the rudders were deemed acceptable.

Figure 16 � Charles River Testing

The final test occurred in the Charles River at the MIT Sailing Pavilion. Pipe Dream was assembled on a dock and dry tests were run. While on the dock, the thruster motors and rudder motors worked with both the remote control and laptop interface. The boat was placed in the water and a short test was done using the remote control. The boat appeared to be fairly stable in response to the waves on the river. The PVC frame was not holding up very well against the prying moments due to the waves, and the pontoons were swinging in and out. Eventually the prying moments caused one of the angled stiffening joints to crack, so the boat was removed from the water and two additional stiffeners were added. These stiffeners were made from faired Aluminum pieces and attached to the struts using hose clamps. Pipe Dream was returned to the water and showed a major improvement in resisting prying

moments. In the meantime, electronics problems arose with the remote control, so further tests were limited. The final test was to tow the boat in open water and see how it would respond to the waves. Even without a closed loop feedback control system controlling the pitch control fins, visual observation showed that Pipe Dream appeared much more stable in pitch than the 6m monohull skiff towing it. Due to electronic problems logging sensor data, the motions of the boat could not be analyzed quantitatively; however the hardware is in place for further tests to occur. CONCLUSION Although Pipe Dream was not used to the full potential of its system capabilities, it remains an ideal vehicle for studying SWATH hull characteristics, strength issues in design of the haunch area, stability characteristics in pitch and active control systems, and many other areas of naval architecture and ocean engineering. Extensive strength and sea keeping analyses could be conducted using the vehicle in a tow tank setting, outfitted with accelerometers which interface with the existing sensors. Also, given its size, logistics, and stability it would make a perfect platform for instrumentation and power system testing. Currently, there are plans to enhance the software capabilities and prove the sensor reading and data collection abilities of the electronics package. Pipe Dream, at the very least, addressed each aspect of the challenge. The vehicle, as it currently exists, is most certainly a working, remote control SWATH model, which is water-proof, fully instrumented, maneuverable, and with strength enough to withstand the prying forces of a moderately rough day on the Charles River. Its design and construction both further corroborated those tenets of SWATH operation that are available in the literature, as well as expanded the information available on Charles River �sea states� and strength and stability issues with scaled down SWATH. ACKNOWLEDGEMENTS This paper is a condensed representation of the work done by the entire MIT Ocean Engineering class of 2003, including Daniel Sura, John Dise, Kai McDonald, Stephanie Fried, Sheila Saroglou, and Meg Hendry-Brogan. The design and construction of Pipe Dream would not have occurred without the help of Dr. Tom Consi and Dr. Franz Hover, the instructors of the senior design course. It is also

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necessary to thank the MIT Ocean Engineering Department as a whole for their continued support and financial assistance to the course. References 1 Reilly E., Shin Y., Kotte. �A Prediction of Structural Load and Response of a SWATH Ship in Waves�. Naval Engineers Journal: May 1988. pp.251-264 2 Tupper, Eric. Introduction to Naval Architecture. Third Edition. Oxford: Reed Educational and Professional Publishing, 1996. pp.100-104 3 Papanikolau A., Zaraphonitis G., Androulakakis M. �Preliminary Design of a High-Speed SWATH Passenger/Car Ferry�. Marine Technology: Vol. 28, No.3, May 1991. pp. 129-141