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[malcf]

I had a lucky escape while testing my brushless motors with 4s lipo,s for the first time , I had been using 12 volt sla batteries previously,6 months ago I tripped up and fell on the floor holding my 1/72 scale Uss John Finn and landed heavily on top of it, the model sustained damage to the stbd prop which I straightened up again using pliers etc and the model went on to sail fine afterwards, the props are homemade with the brass blades slotted into a turned brass boss fitted to the hub of the prop and all silver soldered in place, i also had connected an Action electronics mixer everything was ok until I turned the rudder to stbd and the prop went into reverse and a blade snapped off at the base just above the groove and silver soldered joint, this hit the bottom of the hull creating quite a deep groove and then the blade hit the wall and rebounded past me at a great speed and I still have to find out where it landed, it could easily been a lot worse and hit me in the face and blinded me so it is a lesson learnt as by straightening the blade I must have started off the metal fatigue at the root, also the A frame sustained some damage most likely from the imbalance following the loss of the blade or possible collision of the blades with each other Btw these props are 75mm in diameter.

[derekwarner]

Malc........you mention fatigue, however you could question original manufacture as the root cause as this failure would appear to be more related to the earlier impact

1. silver soldering brass components [5 individual blades into one hub] requires an understanding of post soldering heat treatment of the propeller assembly if the blades were soldered individually
2. an acknowledges post soldering process for such a component could be to constrain the hub on axis and the blades over their diameter [a machined aluminium bush], then heat the complete propeller assembly....'hub & blades'' to say 450 degrees C.......this being ~~ 70% of the silver solder melting point......then let cool naturally
3. this post heat treatment will normalize or remove any heating stresses induced at the interface between the  silver soldered joint between the hub and the individual blades

The only other question is the image you present shows no real fusion   between the parent material of the hub slot to the propeller blade .....ie., the blade that became dislodged ....

[what type or grade of silvers solder produces these results ??

Derek

PS....

1. for home made propellers, they look excellent   
2. from the damage to the stdb A frame, I believe an emergency docking will be required......for the replacement of the A frame & line check of the stdb drive shaft 

PPS......

I also see you have incorporated the hydrodynamic curvature modification to the inboard vertical faces of each propeller

[rnli12]

Hi,
 
I would consider putting a taper on the damaged blade and getting a deeper root penetration for the solder.
 
Regards,
 
Rich

[malcf]

Thank you for your comments this is the silver solder I used a    55% silver solder   
silver solder 55% is a cadmium free silver brazing filler metal. It combines a low brazing temperature with a short melting range. This silver solder is very free flowing and produces neat joints with small fillets. These characteristics make this product an excellent general-purpose silver brazing filler metal. In many countries it is accepted as the best cadmium-free alternative to the low-melting cadmium containing filler metals. For example, it is used as a substitute for alloys such as Easy-flo? and Easy-flo? No.2. Joints made with Silver-flo? 55 are suitable for seawater applications, being resistant to dezincification. The optimum joint gap for this filler metal at brazing temperature is normally 0.05-0.15mm. 
General icon Ag Cu Zn Sn Si Melting Range ?C EN1044:1999   cdfree 55 21 22 2   630-660 AG103   
Product uses: Can be used to join all the common engineering materials (excluding aluminium) such as copper, copper alloys (including, brasses, bronzes, gun metal, nickel silvers, aluminium bronze, copper nickel), steels including, mild, carbon, tool steel, stainless steels, low alloy steel), tungsten carbide and PCD segments. On large or difficult to wet tungsten carbide pieces specialised silver brazing filler metals containing nickel / manganese are often preferred.

Conditions for use: This filler metal is mainly used for brazing in air with a flux. In most cases Easy-flo flux can be used in conjunction with silver solder 55%. However, flux selection is application dependent. Please refer to JM technical department or flux product datasheets before selecting a flux.

As cadmium based silver solder is no longer available here in the Uk which I do prefer as it will form a larger fillet, the location grooves are quite deep cut with the aid of a slitting saw and tapering the edge of the blade would not help as this grade of silver solder does not like slack joints, my photo's are not the best but it is the blade that has fractured not a breakdown of the soldered joint, with hindsight I should have gone up a gauge with the blades they are 1mm next time it will be either 1.2 or 1.5mm with some tapering towards the outer edges, and yes the blades where normalised after the soldering process ,These blades have been tweaked a bit to give them some shape and I believe that this twisting not to mention the damaged blades being straightened has caused the fatigue at the root of the blade as this section is solid and will not twist like the blade plus the brass is in an annealed state after the soldering and is quite malleable.

[malcf]

Hi Rich I wont be re using the old blade I will cut another one and heat the prop up to hopefully remove the bit that is still in the slot and solder the new one in, I also found one other one which is weak on the same prop so it looks like it stems from the earlier damage, its like bending a piece of metal back and thro until it breaks, I should have replaced them rather than straightening them.
Just glad that it missed me when it broke loose.

[jarvo]

Hi Macf, thank god your eyes were out of the way when the prop sheared, had this happen with model aircraft props at 20,000 plus rpm, not nice, might i suggest a complete new prop,  how much unknown damage is there to anymore of the blades???


Mark

[malcf]

Good point Jarvo for what extra work is involved I reckon Mk2 revised props are the way to go with thicker blades and putting the curve in before assembly.

[nemesis]

Why not try and put a fillet of soft solder at the root of the blades/ hub, get a nice radius up the blades. Never know, it might work and save you a lot of work. If not, back to plan A. nemesis

[malcf]

Recently I have had some work done at home that involved removing an old copper hot water cylinder from the modelling  room thus giving me more room to fill with boats etc,  while re- decorating the room I found the missing blade from my prop in a corner all bent and twisted, while painting the ceiling I noticed a cut in the wallpaper on the ceiling directly above  where the boat was! about the same size as the prop blade, this cut extended into the plaster as well so I was lucky that it did not hit me instead.
At last after to many hours devoted to DIY  I have remade the blades using 1.5mm brass and cutting the slot wider, they have been silver soldered in again, I managed to cadge some cadmium based silver solder off a friend of mine and this has enabled me to obtain some nice fillets at the root of the blades, the blades have been tapered using a small angle sander and various files to create the radius on the leading edges and also the tapered trailing edge, I am now looking for advice and techniques on prop balancing bearing in mind the hub is only threaded on one side M8 the other side is blind?.

[The Wizard]

Hi Malcf,


I set two steel rulers on edge on a flat level surface. The prop and shaft was set up on these . the shaft was heavy enough to counterbalance the prop.
The prop will turn heavy side down. This may sound crude but is accurate.My prop is 90mm diameter. I have used this method on props as small as 35mm.


Wizard

[malcf]

Hi Wizard thank you for the info , did you find after balancing that the prop would then stop at random positions or did it still have a bias towards a particular position?.

[malcf]

A few pics of the reworked props and the repaired A frame.

[RAAArtyGunner]

An engineering question?

Would that style of prop, 'long' leading edges, exert extreme loads to the boss connections?

[malcf]

I cannot really answer that question other than to say that they where drawn up from photos of the real ships and the blades are set at not to accute an angle, the blades are set in a groove machined into the boss so as to give a bit more support and contact area for the silver solder, now they have been remade out of thicker material they should be fine only time will tell.