Model Steam Tug

I constructed a model steam engine and boiler, and then a model of the steam tug 'Lyttelton II', to use the model steam engine.

By powering the model tug with the steam engine, I faced a major problem in controlling the speed and also providing reverse direction for the model tug, as the steam engine has no speed control, it only rotates in one direction and has to be hand started. Obviously in the middle of a model boat pond this could cause a few difficulties!

The simplest solution to these difficulties appeared to be the use of a variable pitch marine propeller which would enable the steam engine to be started on the bank and the variable pitch propeller giving the speed control and stopping and reversing capabilities to the model tug. After searching local model shops, catalogues and the internet, I was unable to locate a suitable variable pitch propeller. I then remembered a magazine I had purchased and kept from years ago that contained plans for a variable pitch propeller. It proved to be just what I was looking for, and I built the variable pitch propeller following the plans in the magazine, except I used simple brass bearings instead of the ballraces that the plan specified. I could not see the need to use ballraces, as they are prone to corrosion, construction was more difficult, and a suitable ballrace was difficult to obtain.

My brass bearing alternative has worked really well, with radio control servos having no difficulty moving the control lever to control the pitch of the propeller. I recommend this variation to the design.

Plans are in the September 1964 edition of 'Model Maker and Model Boats', published by the Model Aeronautical Press Ltd, 38 Clarendon Road, Watford, Herts. England.

As I have no knowledge of whether the Model Aeronautical Press Ltd or the magazine still exist, I have included a copy of the plans and the construction instructions, thus avoiding difficulties in trying to locate a copy of a now obscure publication.

Variable Pitch Marine Propeller

By E.T.J. PETERS

"A Variable Pitch Marine Propeller is the simplest method of acquiring complete control of a vessel, without the use of complicated gear boxes, clutches, etc.

This particular propeller is quite easy to construct using a propeller shaft and stern tube only, no control tube being necessary. The propeller shaft is coupled to the engine by a sliding coupling. The propeller can be synchronised to the throttle by using a common servo, Fig. 9.

Construction is described from the propeller end.

The 1/4 in. bore stern tube is fitted with bushes bored to 3/16 in. and the propeller end bush is extended past the stem tube to hold the 1/4 in. bore by 3/4 in. ball race, as in Fig. 1. The outside diameter of the ball race supports the propeller shell Fig. 2 which, therefore, spins on the stern tube.

The propeller boss runs in this shell, being supported by the propeller shaft. The boss itself is 3/4 in. in diameter for 5/16 in. of its length then shapes off to a point. Fig. 3. Looking at the flat end of the boss, the centre is drilled and tapped 3/16 in. B.S.F. to a depth of 1/2 in., then midway, and to either side of this hole, slots are milled - this operation can be carried out by inserting three blades into a hacksaw - to give slots wide enough to clear 4 B.A. rod. Fig. 4. Note that these slots are cut at approx. 30 deg. to the flat end of the boss, from opposite sides, Fig. 5 will clarify this. The next step is to drill a hole 1/4 in. in diameter right through the boss, at right angles to the slots, at a distance of 3/16 in. from the centre to the flat edge 3 in Fig. 5.

The boss shell is ¾ in. in length and approx 13/16 in. outside diameter and is first bored through its length to a diameter of 5/8 in. It is then bored to within 1/16 in. of one end, this bore to be a tight press fit for the 3/4 in. ball race. The shell is now bored again to within 5/16in. of the same end, to clear 3/4 in. This is to allow the boss to slide freely in the shell:. At X in Fig. 2 a 1/4 in. hole is drilled right through the shell, these holes then being slotted out to the edge.

The boss is now threaded on to its shaft, and the shell passed over the end of the shaft and on to the boss. It will be noticed that the slotted holes in the shell coincide with the 1/4 in. holes bored through the boss and although the propeller shaft now passes through this hole, a piece of 1/4 in. stock brass or stainless, is inserted into one side, its inner end touching the shaft. This 1/4 in. material is cut off 3/32 in. outside the slotted hole in the shell, the same procedure being carried out for the other side; these two pins carry the propeller blades, after being slotted with a hacksaw on the outer ends Fig. 6.

The boss is now removed from the shell with the blade pins still in place. They will be seen to pass through the two slots cut in the flat end of the boss and each pin is now marked in the centre of its slot. The pins are now removed from the boss, and drilled, tapped 4 B.A. on these marks and parallel to blade slot as in Figs. 6 and 7. A stud is fitted into this hole, facing the deepest end of the slot in the propeller boss. The remaining process at this end is to drill a clearance hole in the shell. to allow the 4 B.A. stud to pass through and into the blade pin. Enough clearance must be left here to enable the boss to slide freely. Another ballrace is used in the control gear; this race, Fig. 8, has a bush inserted in the centre, with an extension which, by means of two grub screws, holds the race to the propeller shaft. The outer diameter of this race is enclosed by a brass cage. Two screws in this cage support the fork (and also allow it to swivel) which in turn is connected to the servo, as is shown in Fig. 10. Also shown in Fig 10 is the sliding coupling. This is simply a tube fitted with a rod which has a pin riding in slots, cut into the tube. These slots need be no longer than 1/2 in. for the total movement of the propeller shaft is only about 1/4 in."

Click on plans for an enlarged copy

This is my steam powered 1:38 scale model of the steam tug 'Lyttelton II'. The hull is constructed of wooden boards, laminated together and then finished to the correct shape of the hull. The model has a length of 960 m.m. with a beam of 230 m.m. The deck and wheel house are constructed mainly from marine ply wood. The model is fitted with an O.B. Bolton #5 open column single cylinder double acting steam engine, which has a 16 m.m. bore and a 16 m.m. stroke. I chose to construct a model of the 'Lyttelton II' to be powered by the model steam engine, as the tug 'Lyttelton II' is powered by steam, I had seen the real tug several times and had taken photographs of it in Lyttelton Harbour. I also had available a copy of the plans of the tug 'Lyttelton II'.

Castings and construction details of the steam engine I made are available from:

I found E & J Winter supplied an excellent product, with the steam engine arriving as a kit set containing all the necessary castings, with clear detailed instructions for constructing the engine, complete with lots of good practical advice. While construction was quite time consuming, I did not find it to be particularly difficult. ( I am not an engineer by profession) Access to a lathe is essential to construct this engine from the kitset provided.

The steam engine, boiler and control mechanism for the variable pitch propeller.

A photo I took of the 'Lyttelton II' steaming in the inner harbour at Lyttelton, Banks Peninsula, Canterbury, New Zealand, just prior to being sold to Australian interests and being relocated to Melbourne Australia.