Stern thruster – Prototype I

Max-Power R300

The original yacht has an retractable stern thruster from MaxPower, type R300 with an 25 hp hydraulic drive. This type is used when you have a vessel where it is not possible to incorporate a tunnel thruster due to too narrow hull or flat bottom.

The model kit from Graupner lacks an stern thruster. I do not know why, but I guess it was decided to be too complex (and too expensive) for a model kit. One major reason is that the model is in 1:20 scale and the stern thruster is also retractable.

Such complicated items in 1:20 are hard to make operational, but since I have a desire to make true-to-original as well as to have as many functions as possible I will try to realize a 1:20 scale replica of this stern thruster.

Brass material

In order to get an idea if it is feasible I decided to start with a first prototype. To accomplish a first propulsion test I decided to use some brass rod, pipe and sheet metal. Since I did not have any drawings of the propeller I just made a sketch based on the picture above and printed it in 1:20 scale. From this I cut, sheared and grinded until I had all small parts needed.

To put everything together I tried “welding” the prop-blades onto the shaft by using soldering. This showed to be a bigger challenge than expected. When welding prop-blades in full size you only get heating up close to the weld, but when trying to weld a prop-blade that is only 0,5 mm thick onto a shaft with 2 mm diameter the heat will distribute around the entire propeller. For the first prop-blade this is not a problem, but when trying to solder the second blade the heating up will dissolve the tin on the first prop-blade and it was continously falling off.

Welding jig

To manage the dissolving tin I decided to make a small welding fixture and “weld” all prop-blades in one setup. I had to use several clamps to achieve fixing of all parts at the same time. In the picture the jaw vice under the plywood is holding the shaft and for each blade I used two clamps.

One for holding the blade and a second to hold the blade clamp in correct angle to shaft. I realize that the accuracy in this jig is low and I propably have a blade deviation of several millimeter pitch, but for my purpose it is sufficient.

Prototype I

What I wanted to find out, was if it is possible to get thrusting power from such an small thruster. You might say that even if the thruster is 1:20 in scale the power would be equivalent, but you need to keep in mind that many things can’t be made in the same manner after scaling down the original 20 times. Due to its size it is necessary to find new approaches  to solve the build. Although the dissolving tin was making some creative thinking, the final result was to my satisfaction. Compare the prototype thruster to the match and keep in mind that the thruster has three blades “welded” to a shaft that is inserted into a shaft bushing that keeps it centred inside the thruster tunnel.

Test run was made in the kitchen sink by using a power drill connected to the shaft and holding both tunnel and videocamera with my second hand. The thrusting power was not very high but it was easy to determine that all parts moved in a satisfactory way. The prototype confirmed the feasability at least regarding thrusting. For next propeller prototype I will try to make it from one piece of material and cut away openings, thereby I will avoid the problems with “welding”. See video on youtube.

Another protype will be needed to find out how to get the power transmission to work. Due to its size I believe a bevel gear is impossible, but I have an idea for hydraulic-like transmission.