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The design of a propeller is complex and I do not begin to understand the benefits or otherwise of one shape over another. I have built made propellers and have never made comparisons one to another and they have all been one off and for different sized ships. I base my designs upon what I have to hand at the time, and what looks right.
From looking at the drawing and from simple calculations for the 1/31 scale model I will need a propeller of 3 1/2" diameter. This is how I set about construction the propeller to meet that design criteria.
Having bought a 50mm diameter propeller for another project I decided that I would use it as a starting point for the construction of the ship's propeller. I first scanned the image and then enlarged it to full size for the craft. This gave me the basic idea as to the shape is wanted to achieve. The photo show the forward side looking at you.
The profile is however distorted due to the bend in the propeller and the representation of the 3 dimensional object in the 2 dimensional picture. The black line indicates the proposed profile outline of the propeller, and in fact I have made it even bolder to take account of the length of the hub.
The ship's propeller is to be 90mm diameter. The main hub onto which the root of the blades of the propeller are silver soldered was a piece of hexagon brass bar.
The length of the bar equates approximately to the length of the root of the blade where it attaches to the hub. You can just see, in the the photo above, a saw cut which was made across the flat of one of the hexagon's sides to a depth of about 2mm. With hexagon bar (6 sided) is is easy to mark out for a three bladed propeller as progress round the bar leaves a flat out and mark the next etc. When all three flats had been cut to similar depth, judged by eye, the bar was placed in the lathe and turned round. The blades were then filed at their root to be a tight fit to the saw cut. An application of Silver Solder flux powder was then placed on the each side of each joint, the work piece brought up to temperature and silver solder flowed into place.
This is the resulting propeller with its boss placed on top and therefore you are looking at the rear of the propeller.The boss was a simple turning and filing exercise. You will note the rather chopped off outer edges of the blades. The reason for this is that the propeller is to be use in the kort nozzle and needs to be a relatively tight fit to the sides of the nozzle.
The same propeller in side profile. I could take the trouble to clean it all up but as soon as it goes into salt water it will soon change colour again, and it is likely that I shall want to paint it in any event !!!
The main hub has been tapped 1/4 x 32tpi as has the locking end piece. The shaft has been threaded to mate with it.
The propeller blades are at the moment still quite pliable due to the softening effect of the application of heat to the brass sheet. However ate a few hours running in water they will "work harden" and hold their shape. Initial trials will not therefore be carried out at full speed!!!
Here the propeller has been fitted to the propeller shaft ready to progress to the design of the kort nozzle arrangement which is not used on the actual ship.