Original Year
# of pieces
Other:  Watercraft
not including crane
Rotating dual propellors, thrust  vectoring,
Slewing and lifting boom, lifting jib
Other Views

8839 Supply Ship

iso revolve
Click the image to download the LDraw file of this model.
Model by Benjamin Wendl

The 8839 Supply Ship, released in 1992, was the first and only large boat ever released in the Technic line.  The fact that the subject matter was never repeated is unfortunate since this is an excellent model.  This boat is packed with features including rotating and vectoring propellers, and a crane which slews and raises.

Even though the construction of the hull is all done with straight beams, it produces the impression of a curved hull.  This is partly done with simple hinges, and partly by the fact that the hull width tapers from the top to the bottom at the bow.  The flex system is used extensively, both for control of the steering system and even as simulated rigging on the mast.  It is a great looking model.  There's even a set of control panels on the bridge.


Directional Control
The ship has multiple means of direction control: one real and one simulated.

The simulated system is based on thrust vectoring.  An overhead "Hand of God" wheel drives a pinion gear meshing with a 24 tooth crown gear.  On this second axle is a liftarm used as a crank which drives a flex cable.  This flex cable runs aft to the steering assembly.  It drives an inverted steering arm used as a crank.   The opposite pin of the steering arm is used for another flex cable.  This cable is attached to a ball joint on one of the propeller housing axes.  Yet another flex cable attaches the housings on the port and starboard sides.  Movement of the cables causes both housings to pivot together.  Like you would expect with this type of steering, the rotation of the housings is opposite in direction to the turning direction of the ship.  Note that the propellers themselves do not pivot.

The real steering system is based on the fact that this model has 3 wheels hidden inside the hull.  The two in the front are not steerable, but the real wheel bogey can pivot.  The same steering arm which was mentioned above for use with the flex cables also turns the steering axle.  In this way, movement of the steering input moves the prop housings and also turns the wheels to make the ship really turn on solid ground.

steering1     steering2

Click for an animation of the steering linkage in motion.
Click for an animation of the thrust vectoring in motion.
The hidden front wheels share an axle which drives through a pair of 14 tooth bevel gears.  After passing through a pair of universal joints, torque is transmitted to a 24 tooth spur gear.  There are 2-8 tooth pinions on either side as can be seen in the computer image.  The final outboard gears turn drive shafts which rotate the twin 3-blade propellers.

There's no differential thrust here; both props always rotate at the same speed.
props     props
Slewing Crane
The loading crane is mounted to a turntable.  A crank on the port side turns a pair of 14 tooth bevel gears.  An 8 tooth pinion then drives the outer ring gear of the turntable. The crane cannot turn 360 degrees because the bridge is in the way.
slew    slew
Click for an animation of the crane slewing.
Luffing Boom
The main boom of the crane raises via a crank on the port side of the hull.  An 8 tooth pinion and a 24 tooth crown gear come first.  The a set of 14 tooth bevel gears pass torque up through the center of the turntable to a worm screw.  The worm screw drives an 8 tooth which in turn drives another meshed with a 24 tooth spur gear.  This spur gear luffs the boom via liftarms which act as a 4-bar linkage as seen in the computer image.  Total gear reduction is (24:8)*(14:14)*(8:1)*(24:8) = 48:1.

Use of a worm gear means that the system cannot be backdriven because the axial friction is higher than the backdriving torque due to the screw pitch angle.
boom    boom
Click for an animation of the boom in motion.
Luffing Jib
The jib can be pivoted through a small angle through use of a crank on the side on the crane base.  Two sets of 8 and 24 tooth gears result in a gear reduction of 9:1.  The final gear turns a pair of liftarms which have a moment arm of 3L.  A pair of long tapered arms then reach out to the jib.  Since the jib has a moment arm of 5L, it does not rotate very far but has a large lifting capacity.  The jib liftarms have a hard stop at either end when they contact the top of the crane housing.
jib    jib
Click for an animation of the jib in motion.

Other Views

Side View
side view
Top View
top view
Front View
front view
Back View
back view
Computer Render

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