Self-propelled Model Boat
From MarineWiki
WORKING MODEL of a 46,000 TDW OIL / CHEMICAL TANKER
The Aim of this project is to make a 1:200 scaled down working model of a 46,000 TDW Oil / Chemical Tanker. This ships is one of the eight ships being built by Hyundai Mipo Dockyard, S.Korea for our company OMI. The options for propulsion for the model are:a propeller connected to a motor; or a propeller connected to a small I.C engine; or water jet driven by a small gear pump. In all the three cases the propulsion system and the manoeuvring to be controlled by a Radio Control Unit (remote). For this part I would need the help of my friends working in electrical and electronics field.
On the right are the snaps of the two ships JEANETTE(on the left) and LAUREN(on the right), sailing way from the yard!. For further details about the ship, refer to the Q88 available on OMI web site.
Basic dimensions: Length(O.A) = 183.20 m, Breadth (extreme) = 32.47 m, Depth(extreme) = 18.83 m
PROJECT KEY DATES
Project commencement: 15 Jan '05; Model tank test: 7 Aug '05
PROJECT PROCEDURE
The first stage is the hull fabrication. The material used for this GRP (Glass Reinforced Plastic).
Basic dimensions: Length = 91.6 cm, Breadth = 16.1 cm, Depth = 9.4 cm.
FIRST WEEK (16th Jan 2005)
(i) Wooden Block Fabrication
A rectangular wooden block of the above dimensions is made, either as a single unit or joining smaller blocks. Now the lines plan of the ship is taken and scaled to 1:200. From the water line plan, the upper deck line is marked with pencil on the wooden block. The frames are marked on the top of the wooden block with pencil. Then from the body plan, the template of various fames section is cut in acrylic sheet (by placing the body plan on top and cutting). A chisel and file is used to remove the wood, so as to take the final shape of the ship; and at each section the template is placed to check the shape at various stages. Finally the wooden block takes the shape of the ship (refer snap).
(ii) GRP Mould Fabrication
Once the wooden block is ready, then we use this to make the GRP mould. One important thing to note is that the mould is in two parts, and are bolted in place. This is achieved by having a frame in the center of the ship on either sides, with holes for bolting. Also the mold must have frame on the upper side(where the deck is) so as to prevent buckling of the mould. The GRP mould follows the same procedure as the hull fabrication, described below.
GRP Hull Fabrication
The GRP mould (two parts) is joined by bolts. It is then cleaned thoroughly. Now we take a piece of cloth, dip the mould release wax (silver wax burnish), and apply on the inner surfaces of the mould to give a smooth finish. Alternately corn oil can also be used. After this the mould is cleaned with a clean white cloth.
I used the EPOVIA TG-165 W, a brand name of CRAY VALLEY CHEMICALS for the first layer. This liquid is mixed with its curing agent MEKPO (comes together in a separate bottle) in the ratio 10:1. The mixture is applied quickly on the inner surfaces of the mould using a paint brush. This is then allowed to keep for drying (1-2 hrs). Do not forget to remove the mixture from the brush using acetone or paint thinner.
Once the mould is dry, the next mixture is to be prepared. I used EPOVIA G-650 ABY, also a CRAY VALLEY CHEMICAL product. The liquid is mixed with its curing agent in the ratio 10:1. Apply the first layer on the inner surface of the mould quickly. Now put the GRP chopped strand mat on the surface and apply the liquid mixture on top and press it properly. If one needs good strength, apply second layer of GRP followed by the liquid layer. Leave it for drying (2-3 hrs).
Once it is dry, remove the bolt, and using a knife separate the GRP from the mould. Slide out the mould from both the ends and now we have the GRP hull form (as shown in the snap).
SECOND WEEK (23nd Jan 2005)
(i) GRP Hull Reinforcement
The GRP hull needs to be reinforced, both for strength and to attiain the proper shape of the final ship hull. This is done by providing longitudinal stiffening using wooden (plywood) strip, in its parallel body (straight region, before the curving in the ends)region at the upper deck level. The wooden strip is attached on both the sides on the inner surface at a distance of approximately 10 mm. This distance is kept, as would be evident later, to spread the epoxy putty in the gap after fitting the upper deck. The strip is attached using strong adhesive. Now it has to be supported in the transverse direction. Measured sizes of the strip, corresponding to the extreme breadth of the model, 16.1 cm, is taken (4 no.s) and attached as shown in the snap.
The next reinforcement is the upper deck. The material used is 5mm thick acrylic. The upper deck shape is marked on the acrylic sheet and cut out. It must be noted that although the upper deck ends at the starting point of the forecastle deck, for better strenght it is advisable to make a continous deck from fore end to aft. The forecastle deck can be fabricated above this. Once the exact shape is cut out (need not worry about the gaps due to inaccuracy, since it can be filled with epoxy putty), cut out 10 cm x 15 cm hole near where the propulsion system will go in. Now stick horixontal strips of acrylic on the sides to level the deck acrylic sheet) with the ship side (GRP). On the forward and the aft inner sides, where the wooden strip is not there (note that it is there only in the straight region), bend a wire along the curved shape, and stick on the inner surface inline with the wooden strip.
Now take the epoxy putty; I used KCC AUTO REFINISH PC-2900-GREY, and mixed with its curing agent in the ratio 10:1. Apply the paste (now yellow in color) above the wooden strip and the wire upto the edge of the GRP hull. Now place the acrylic sheet (upper deck) and level using the rectangular strips. Make sure that the paste covers all the gap. Allow it to dry. After it is dried, chistle out the extra putty and level the deck with the edges to give a smooth finish.
(ii) Deckhouse Fabrication
Meanwhile the deck house needs to be fabricated. The material used is 1 mm thick acrylic. The dimensions are noted from the deck house construction drawings. Care must be taken to reduce the dimensions corresponding to the thickness of the acrylic sheet: 1 or 2 mm depending on the location. I decided to make the deckhouse minus the wheel house. That will be fabricated later.
(iii) Propulsion & manoeuvering system
My original plan was to go for water jet propulsion. But I changed my mind. Now I plan to have an electric motor driven popeller as the main propulsion. Two motors would drive the propeller through a reduction gear and shaft. The motor speed can be controlled by a variable resistor which is inturn controlled by a servo motor.
For the manoeuvering, rudder would be used as in the original ship. The rudder turning will be controlled by a servo motor. I decided to try out a bow thruster also in this model (this is not there in the original ship). The water jet drive would be used for the bow thruster, but getting a suitable water jet system was not easy. The motor was easily available but the pump was the difficult part. I got a gear pump from the damaged automatic toilet flushing unit. The assembling was also tough, since the two shafts(motor and the pump) had to be coupled directly. A plastic mould as a sleeve between the two shafts was inserted and fixed by quick-fix glue. In addition the pump casing was fixed to the motor casing after proper alignment. This alignment required the use of strips of wire and quickfix. Finally the unit looked like this.
(iv) Pump Capacity Test
Once the pump was decided, I decided to check the capcaity at the design condition: draught - 55 mm, suction head - 25 mm. The motor/pump unit was fitted on an wooden base. A wooden tank was taken as the storage tank, a wine bottle (750 ml) as the measuring jar and 6V battery as the power source was used. Refer the photo for the arrangement.
Results
The capacity was tested for five times and at an average 22 sec to fill up the bottle (750 ml). So, 29.33 sec for 1 liter.
Flow capacity (Q)= 34 ml/sec = 34 x 10E-06 cu.m/sec ; mass (m) = 34 x 10E-03 Kg/sec
Area (A) of the nozzle (tube of internal dia 6 mm) = 28.27 sq.mm = 28.27 x 10E-06 sq.m
Velocity of flow (v)= Q / A = 1.2 m/sec
THIRD WEEK (30th Jan 2005)
(i) Rudder and Propeller fabrication
The rudder mould was fabricated from 5mm thick acrylic. The rudder dimension(scaled) was marked on the thick sheet on both sides and grinded out to shape. Also the aerodynamic profile of the rudder had to be maintained. For this, the section drawing of the rudder was refered to.
The propeller was made out of brass sheet. This ship had four bladed propeller. The sheets were cut out the shape, then rolled to give a good profile as the original propeller(refer the propeller drawing). These four blades were then joined to a mould and sealed with quick fix. This propeller is for testing purposes and cannot withstand the load of a high speed turning motor. For that these blades have to be soldered to a brass boss and then joined to the shaft. That will be fabricated later. Refer the photo below left.
(ii) Hull surface preparation===
The hull surface need very good surface preparation before painting. This is done by using sandpaper (resin coated aluminium oxide cloth abrasive 150) and after wetting the hull surface with water. Rub hard untill the surface is very smooth. Feel with the hand for confirmation.
FOURTH WEEK (6th Feb2005)
(i) Motor foundation fabrication
The next important step is the motor(main propulsion) foundation. I had to attach a small gear box with the motor to reduce the height of the connecting shaft. The motor is assembled with the gear box and a universal coupling made so that the propeller shaft can be attached. Various materials were tried for foundation: thermocole, wood; finally the best was found to be foamex. It is hard but can be easily cut in to shapes. The foundation not only needed to be simple but also must prevent any linear or rotation movement of the motor. Refer snap on the left. Note a small projection on the right block. That goes inside the oval hole in the motor to prevent its movement. Lots of designs had to be rejected before this was finalized. The final assembly looks like the snap on the bottom right.
(i) Propeller shaft tunnel installation
Once the motor foundation is finished, the height of the propeller shaft is known. With this we can fit the shaft tunnel in place. Before that a hole had to be drilled at the location. The shaft tunnel is inserted and kept in position(height is measured) with temporary pieces and fixed or sealed on the outside using epoxy. The preparation is explained below.
I used KCC AUTO REFINISH epoxy PC-2900-GREY and mixed with its curing agent in ratio 10:1. After mixing the color of themixture is pale yellow. Apply it around t he shaft tunnel on the outside closing all the gaps. The epoxy can be smoothed to shape later.
(ii) Hull surface smoothness
The same epoxy mixture is used to apply on the hull surface where there is damage or rough surface and the upper deck edges (sheer strake). Leave it for drying. After that it is sand papered to shape as described in the hull surface preparation section.
(iii) Remote control assembling
Unlike my original plan to fabricate a radio control from my own, I decided to go for a commercially available one, due to lack of time and data. But the original plan is not entirely shelved. For the present I got a two channel remote operation at 27 MHz. One channel will be used for operating main propulsion motor, including varying its speed with a resistor. The other channel will be used for maneuvering (rudder/thruster control). For varying the speed of the motor and controlling the resistor a servo motor is used in the Channel 1.
Transmitter, Receiver, Servo motors, Resistor
(iv) Testing tank
I got a testing tank made of glass to test the speed and maneuvering characteristics of the model boat. I had to purchase it from a fish-tank maker and modify to suit my needs. Main dimensions: 2500 mm x 600 mm x 300 mm. It is excellent one, except that it cost me $300.
After a long break of four months, which includes three months in India & Singapore, I am back in Korea. The first six weeks in Korea were busy time with the deliveries of TEVERE and THAMES. Now getting back to the model work.
TANK TESTING (7th Aug 2005)
Earlier the shaft tunnel was fixed on the hull body. Now the motor (propulsion drive), shaft and the propeller had to be assembled. The joining was done by screw. The result is in the snap on the right.
After that it was assembling the controls.
It was a great sense of achievement to see the ship model run forward and astern by the press of the remote button in our hand. It was the result of design and re-design by Remitha and me. We tried lot of methods to get the motor move in two directions (reversing the polarity) by the motion of the servo motors through the remote. Finally we succeeded. Our arrangement was like this. The motor terminals were connected to the the two blades(red. see photo below) connected on the servo motor from the top. From the bottom three poles were made, with the center one (common) connected to the positive terminal of the battery and the other two to the negative.
If the forward direction is activated in the remote, the servo motor moves clockwise and a pair of poles touch the conncetion to motor through the blade on servo motor. With the circuit complete the propulsion motor and the propeller rotates moving the ship forward.
In the reverse activation, the servo motor rotates anti-clockwise and the other two poles are activated and through them to the motor. In this case the polarity is reversed and the propulsion motor and the propeller turns in opposite direction and the ship moves aft.
Check out the video of the first tank test.
TANK TESTING with SPEED CONTROL (8th Aug 2005)
We realised that the ahead speed is very high (atleast we felt in our testing tank) and is constant. For better maneuvering, we thought that we need a low speed ahead motion also. To achieve this we decided to put a 6 ohm resistor across the ahead circuit. The servo motor have a 45 degree motion in the ahead mode for the full pressing of the trigger in the remote control.
We decided to put another pole at 20 degrees with this resistor across so that when the trigger of the remote is half pressed the ship has a low speed and when fully pressed it achieves full speed. The testing was done in the testing tank.
Check out the video with dual speed.
TANK TESTING with BOWTHRUSTER (13th Aug 2005)
Once we were successful with the ahead and astern motion of the ship, we decided to incorporate the port and s tarboard side turning (that is left and right). We decided to use the water jet driven bow thrusuter. In the earlier section I had done the pump assembling and capacity tests. A proper foundation for the pump and its motor was also made. We also decided to lower the remote receivers, the servo motors and polarity reversing mechanism below deck. For that we cut open a hatch in the forward side.
We decided the position of the thruster and drilled a hole through the hull. Next we inserted the tubes (suction and discharge). The pump with the motor was lowered below the upper deck and conneted to the tubes. A similar reversing mechanism, as mentioned above, with the servo motor was made for this pump. See the photo on the left.
Now with one switch on the remote the pump sucks the water from the port side and discharges to the starboard; the net result is a thrust towards the port and the ships turns port side. With the other swich on the remote, the servo activates the reverse polarity and the pump suciton and discharge is reversed and thereby thrust and therfore the ship moves starboard.
Half way through the test our Ni-Cd batteries got drained and we continued the test with a bulky battery (see the second part of the video).
Check out the video of of the tank test with a bow thruster in action.
We noticed some observations. The ahead full speed was lower than the astern speed. The bow thruster (pump) was more effective in the port side turning. After checking the resistances, using a multi-meter, in the various modes the results were as follows: Full Ahead: 3.2 Ω, Slow Ahead: 6.2 Ω, Astern: 1.5 Ω, Bow thruster (Port): 1.4 Ω, Bow thruster (Stbd): 2.8 Ω. We need to check and or modify the wiring to correct this anomaly. More on that later.
