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DUKW “ A long journey”
During the Second World War the allies required a vehicle to be used during invasions from the sea,
both in the Pacific and European theatres of war and would be able to move equipment from ships
across the beach to supply dumps behind the beach-heads. One of the vehicles developed was the
DUKW.
The DUKW prototype is based on the US army CCKW 2.5 ton army truck, which was already in
service. The DUKW utilised the same 6 wheel drive and chassis arrangement as the original truck but
instead of a standard body it was fitted with a boat like body and a propeller drive system.
DUKW is actually the letter codes used by the manufacturer to denote individual features of the
vehicle.
D denotes
Year of first production (1942)
U denotes
Utility or Universal
K denotes
All-wheel drive
W denotes
Tandem mounted driven rear axles
Today you can still see the DUKW in operation, brightly painted as tourist vehicles in places as far
afield as London, England to Rotarua, New Zealand.
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Conception
It’s a long time now since I first got interested in building an amphibious model which would be able
to move between land and water under its own traction. I looked at many different options ranging
from the Stalwart six wheel drive truck to the WWII Buffalo tracked vehicle. Eventually after seeing
the DUKW in Rotarua I decided on the WWII DUKW Amphibious truck.
After some further research, looking at the Deans mouldings, and models of the DUKW built by
others as shown on the internet I decided I would build the hull myself from wood and utilise
‘Robbe’ Axles and wheels for the road running gear. As a base for all initial measurements I used the
‘Italeri’ 1/35th scale plastic kit number 6392.
I also wanted my model to be as realistic and faithful to scale as I could. During my research I had
found some modellers had made their models extra large in some areas. As an example I found one
model which had a higher bonnet (about 25mm higher) and no dropped load deck (load deck level
with main deck). This had been done to accommodate the drive motors and batteries that the
builder had fitted. I knew that trying to be faithful to the original would be difficult but did not
realise how long the challenge would take.
Mechanics
The final scale of my model is approximately 1/11th, which is a bit of an odd scale but this was
dictated by the ‘Robbe’ axle track width and the diameter of the Robbe Tyres used.
The model is entirely built from scratch and initially I based all measurements on the Italeri plastic
kit.
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The photo above shows the wooden hull after being fitted with the axles and wheels. The
suspension system is made so as to follow the original prototype in the design and operation as far
as was possible. I used brass blocks and plates to mount the axle springs to the wooden hull. The
tandem rear axles swing on blocks which swivel on brass mountings fitted to the brass plates. The
springs are mounted above the axles as per the original and tie bars hold the bottom of the axle in
place. This allows full movement of each axle. PTFE bushes are fitted between the spring end loops
on all 3 axles and the fixed mounting screws to allow their free movement without wear.
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Once I had the road wheels in place I could then mount the propeller drive system and rudder. The
above photo shows the external end of the water drive system and rudder.
Internally the next stage was to make up both the road drive mechanism which would couple with
the axles and the water drive system which would couple to the propeller shaft.
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The road drive system consists of a lay shaft mounted in 2 bearing blocks, (each fitted with 2 ball
races and PTFE seals) mounted centrally in the hull. Each end of the lay shaft then drives the front
and rear axles. The centre of the lay shaft which is internal to the boat is fitted with a tooth belt
pulley. The motor is a 540 size motor fitted to a planetary gearbox fitted with another tooth belt
pulley. A tooth belt couples the drive to the lay shaft.
The road drive system and space below the load deck dictated that the only logical space left to fit
the water drive motor would be in the bows. Again I used a standard 540 size motor and a second
planetary gearbox since the propeller would be 50mm in diameter, but the bow location of the
motor was a long way from the end of the fitted propeller shaft. The problem was solved by fitting a
second propeller shaft within the hull coupled to the gearbox and the external shaft by two universal
couplings.
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Problems
Once I had progressed to having the hull nearly complete I needed more information on the detailed
fitting around the model so decided to visit the Imperial War Museum at RAF Duxford . However on
seeing their original DUKW I realised to my horror that the ‘Italeri’ kit did not show some of the
details correctly.
The picture at RAF Duxford shows the solid tarpaulin hoops which were not a part of the’ Italeri ‘kit I
had, but more concerning was the winch arrangement at the rear, The ‘Italeri ‘kit shows the winch
and its mounting brackets mounted parallel with the main deck. However inspection of the real
thing showed that in fact it is mounted at an angle to the main deck. I realised this was to allow the
drive from the main gearbox power take-off to the winch to couple up without undue misalignment.
As I had already made a set of winch mounting brackets , which were now wrong this set me back
until I could get the winch arrangement correct.
Armed with the fact that the Italeri kit was not 100% accurate I set about gathering a lot more
information from text books, museum visits and the internet as well. This all took time and the
project was on the shelf for quite a while before I was happy to proceed. After quite a while doing
research I also realised that the exhibit at Duxford was not fitted with lights in the same place as
they would have been for wartime use. They had been modified for modern road use.
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Detailing
The picture shows the model with the winch now mounted correctly at the right angle to the deck.
To assemble it I used tiny brass 10BA & 12BA (about 1mm diameter) nuts & bolts (some of these
required threads tapping carefully into other components).
The light housing to the left of the winch in this picture is for one of the two rear lights. The diameter
of this rear light housing is 10mm with an 8mm diameter internal diameter. If I were to fit working
lights into the housings I would need really small lights since on one side there are 3 lights (Blackout
brake and 2 blackout convoy lights) and on the other 4 lights ( Service marker & Service brake & 2
blackout convoy markers), more on this later.
Also in this picture you can see the channels which form part of the hatch cover. These were cut
from plastic mouldings and glued to the wooden hatch cover with Super-Acrylic glue, which I must
say is excellent if you want to glue dissimilar materials like these.
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The above picture shows the bonnet and front lights. Again I used plastic channels stuck to the wood
with Super acrylic. The light housings were all machined on my ‘Cowells’ 90 lathe from brass and are,
from left to right, Blackout marker, Main Headlight, Main Headlight , Blackout marker, Blackout
headlight. The blackout markers have a 4mm diameter internal diameter and the main headlights
are 13mm.
Luckily the markers only require a single light source as does the blackout headlight. The main
headlights require two, (one for main and one for dip).
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This picture shows the main driving cab with the dashboard, driving levers and steering wheel. The
dashboard was made with wood, fitted with circular thin ply cut outs glued on for the dials, and
brass components for the switches & knobs. The steering wheel and driving levers were also made
from brass.
The curved cab sides protecting the air intake louvers at each side of the cab were made by
laminating three layers of very thin ply around a former and then carefully cutting the shape out.
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Lighting.
I wanted to try and make the lighting system work as per the original and so set about trying to find
small light sources which I might be able to use. I could have used wheat-germ filament lamps but
the problem with these would be, if they blew, they would not be easy to replace. I have always
used LEDs for lighting as these are much more able to withstand rough treatment than a filament
lamp. The problem I had was that the smallest I could find were still too large to be able to fit four of
them within an 8mm diameter hole for the rear lights. Then after much searching I found the
answer, NANO LEDs.
Above is a picture of a NANO LED shown on 5mm grid lined paper. Also in the picture are the LED
dropping resistor and a PTFE mounting which I machined to fit the LED into. This was to allow it to
be insulated and fitted into the 4mm Blackout marker brass lamp housing.
NANO LEDs were marketed by a company in Freemantle, Australia and they come fitted with thin
enamelled copper wires already soldered onto the 1mm diameter (YES you did read right ... 1mm
diameter) LEDs. These were ideal for what I wanted as they came in several different colours,
Prototype White, Sunny White, Red and Amber. You can tint the white lamps with paint if you
require any other colour. Here is a link to the supplier’s website page.
http://www.dccconcepts.com/index_files/Prewired08mm0603SMTLEDs.htm.
They are available in the UK and I got mine from ‘Gaugemasters’ at Ford near Arundel who have a
mail order website as well as a shop.
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I also machined PTFE housings for the blackout headlight and the two rear light housings. For the
front headlights I machined some clear acrylic rod to the required shape. Each machined item of
PTFE or acrylic was then carefully drilled with a 1mm hole and each LED was then glued into position
with clear epoxy. The enamel wires were sleeved with small heat shrink tubing to protect them
from any scraping which might remove the insulating enamel covering of the wire.
On the rear lights there was a problem with the LEDs shining through the white PTFE when either
the upper service/brake or lower blackout marker pairs were on. The Blackout & Service lamps are
never on together on the prototype and this would not look right. The answer was to carefully cut a
slot with a ‘Dremel’ mini cutting disc horizontally across the PTFE housing and glue in a plasticard
shroud with Super acrylic between the upper and lower halves of the housing. This solved the
problem very well.
While I now had all the LEDs fitted they still required their individual dropping resistors to be wired
in and a switching system to control them to be made up. I decided to make up a Vero-board with
the dropping resisters fitted to them together with any additional protection diodes and reverse
current flow diodes. I made up one board for the bow lights and another for the stern lights. The LED
wires were not long enough to reach as far as I needed so I had to solder extension enamel wires
onto the existing and then fit additional heat shrink tubing to protect the extensions.
The ends of the enamel wires were then soldered to the appropriate connection post of the Veroboards. Each Vero-board was fitted with a DIL pin connector and fitted into a homemade plasticard
case.
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The picture shows the completed bow resistance board wired to the enamel wire ends. To the left of
the board are the pins for the 6 pin DIL connection which will interface with the switching relay
system.
To control the LEDs I used five radio controlled switching relays from Active Robots. After spending
time engineering how they would need to be wired I fitted five of the Servo relay 1Amp switches
into a plasticard box that I made up. This also had DIL pin connections fitted inside the end of the
box so that it would be easy to connect them with the resistance boxes using prewired 6 way ribbon
jumper cables from ‘ Technobots’ in Totton.
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Relay box showing the DIL connection pins can be seen to the right of the box
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Photo of the service headlights on main beam
Batteries.
Since space inside the model is at a premium I needed to choose batteries that would give me a
reasonable duration on both water and land drive systems and also fit within the available space. I
decided to use a 7.2 volt system on both the water and land drive and bought twelve 3800mah NiMh
7/5A size cells from ‘Cellpack solutions’. These were made into two 6 cell packs and fitted to the left
side of the hull between the front and rear wheel wells.
The lighting also required a small battery and for this I used a 6V pack of AAA 800maH Eneloop cells
from ‘Overlander’.
The radio is also powered from a 4.8V AA pack of 1000maH Eneloop cells,
Motors
Both the water drive and land drive systems were fitted originally with 3 pole light wound standard
540 motors.
Speed controllers
Both the water drive and land drive systems are fitted with Microgyros FR40 Electronic speed
controllers.
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Radio.
I have fitted an 8 channel ‘Spektrum’ 2.4Ghz. radio to the model. The transmitter is fully
programmable and this allows me to fine tune and mix all the individual channel settings to get the
correct performance from the systems fitted. My Radio is set up for Mode 4 operation.
Channels are allocated as follows;
Stick or channel
To ESC, Servo or relay function
Left Vertical (Throttle)
Water drive motor
Left Horizontal (Aileron)
Right Vertical (Elevator)
Land Drive motor
Right Horizontal (Rudder)
Water Rudder
Gear
Land Steering
Aux 1
Service/Blackout lights selection
Aux 2
Main headlight Dip/Main selection
Aux 3
Brake lights
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Testing on the water
After checking the balance of the model in the bath and after adding some additional ballast the
model was finally ready for its maiden trip on the water in 2013.
The full size maximum water speed for the DUKW was 6.3MPH and the model has more than
enough performance to meet a scale speed.
Steering of the full size original was done using the steering wheel which operated both the front
road wheel steering and the rudder in tandem. The model follows the same principal for its
steering.
The model seems to be able to steer better in one direction than the other and this is probably due
to the dual effect of the torque from the large propeller and the tunnel in which it is mounted. The
original probably suffered the same problem.
Testing on the road
The full size maximum road speed for the DUKW was 50MPH and the model is able to reach a
reasonable speed to match the scale.
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One problem found with the model was that with the motor I had originally fitted it would not climb
any reasonable incline, so I decided to fit a more powerful motor.
First I tried a Spirit 600 30turn motor but this still seemed insufficient, so I then researched higher
torque motors and have now fitted a Novak 55turn Rock Crawler motor. The 55turn motor gave a
much better climbing speed but I was experiencing problems with some strange noise in the drive
system. I found that one of the axles appeared to have a damaged gear within the differential so I
removed the axle and dismantled and inspected it. I found that two of the differential bevel gears
were metal and two were plastic. The plastic gears had some teeth stripped. I bought a new
differential from the ‘Robbe ‘website and when it arrive it had 4 metal bevel gears. The plastic gears
had been upgraded in the time since I first purchased the original axles so I ordered some more for
the other two axles and changed them too. The noise has disappeared and performance was better
but after a short while of road running the system would not pull away or climb very well.
I decided to test the pushing power with a pair of scales mounted against a wall and slowly
increasing the motor power when in contact with the scales. With an ammeter fitted to the wiring
and watching both the scales and the ammeter I found that after a while of pushing at full stick
output the power started to drop away. The battery supply system seemed to be limiting the
output available. I changed the cables in the system to a larger size of 2.5mm to reduce any undue
resistance in the system but the power still dropped off after initially being OK. I checked all the
connectors in the power system but these were not causing the power loss, so I decided to try a
different set of batteries. I used a 7.2V sub C pack that I use for my model tanks and this proved to
be OK with no power loss, in fact the power output went up and the maximum speed went up.
So it seemed that the original batteries I had chosen were the problem, so I went back to the
internet and researched the internal resistance of the NiMh 7/5A cells. I found that the internal
resistance was typically 13-24 milli-ohms with a maximum rated output of 11.4Amp.
The sub C cells however had an internal resistance of only 5-8milli-ohms with a rated output at
30Amp.
So I had to replace the road drive batteries and have managed to just fit 6 new sub C cells into the
model. Further testing will be required, but it looks much more promising since speed pickup has
increased and I have now had to reduce the maximum throttle output to keep the top speed down.
Much more to do
There is still more work that can be done on the model although it looks the part and operates OK as
it is now.
A canvas cover could be made for the load area, with another slightly different hoop still being
required for the back of the cab area and further fittings and detailing to the deck and side would
add to the scale appearance.
Conclusion
I had to carry out all research, design and building from scratch. The majority of the model is made
from wood, with brass for mechanical & other items, PTFE for bushings and a small amount of plastic
strip for detailing where required.
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In summary, this model has taken a long while from conception and through the building process
but I am very happy to have managed to get the model as far as I have. Hhowever I know that when
I look at it there are still items that I know need to be completed.