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by Charles Darley

Construction of the Bow Thruster Part 1

What is a bow thruster?

This is a form of propulsion but is used to move the bow one way or another by forcing water through a tube, called the thruster tunnel (tub), that connects both sides of the hull. Such a unit help with the control of the ship when operating in confined spaces. The description which follows is based upon designs of a model bow thrusters enlarged to a scale sizes that equates to the full sized ship, and from information received in emails.

The operation of a bow thruster relies upon the thruster tunnel (tub) being full of water, and therefore must be fully submerged. I am to understand that even the smallest amount of air in the cross tube will render the thruster ineffective. The model ,as with the real ship ,will need to be ballasted to ensure the cross tunnel remains below the water line.

The side of the hull must be marked with the logo for a bow thruster so that any small craft passing are aware of the possibility of a flow of water in its location.

The model bow thruster will have a vertically rotating paddle set half way across the thruster tunnel, rather than propeller(s) use on the actual ship.

This is the bow thruster base unit in its starter stage of construction. To this will be added ply to the side to reinforce the timber in an effort to avoid splitting. GRP resin will be coated inside to make water tight.. I hope !!! Then when the assembly proceeds the outside will also be coated in resin.

This gives you some ideas as to how the tubes are to fit into this centre section. The vertical tube will be used to hole the motor arrangement. The paddle on the motor will be a flat blade that will turn at about 5000rpm, controlled by a PWM (pulse width modulated) Speed controller to give a water transfer in either direction as required. The two sets of tube intersect each other by 50%.

Cutting the holes in the side of the bow was a daunting task. We needed to be square to the centre line of the model and at the same time square to the base of the hull. A 32mm drill was used and it complained when it broke through the timber and stopped the electric drill!!! This 32mm drill was then followed by a 34mm drill more used to cutting holes for fitting flush door hinges.

After all the drilling, easing out the size of the holes was still required. Here a rough rasp is bing used to enlarge the hole all round by about 1mm.

Whilst this look as if we a grossly out of alignment the two holes lined up surprisingly well, there being an error of less than 1mm, quite within the allowable tolerance level and nothing the a little use of the rasp was not able the rectify and resin will be able to fill.

Here is a well protruding tube which has yet to be cut to size. On the full size ship the thruster tube (tub) is flush with the bow flare and shaped to the hull's side. This will be similar on the model.

The centre block makes good contact with the two tubes and upon completion would sit neatly in the centre of the hull and not to one side as shown in the picture. This part will eventually be surrounded by a water tight bulkhead so that should any leaks occur they will be restricted to a confined space. Electronic monitoring will also be fitted so that upon a leak being discovered the craft can be quickly brough back to the quay side and any problem rectified.

Now that the hard work of cutting the hull has taken place the remainer of the construction of the bow thruster will take place. Eventually the whole assembly of tubes will be set in GRP.


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