View Full Version : Need to straighten bronze rudder
04-13-2012, 04:12 PM
I need to straighten my rudder don't know how it bent but it's like a thirty degree bought boat this way.....from what I can tell I should heat to dull cherry red quench it in water and bend. I hear if I'm to hit it should be few hard hits instead of many light ones... I have use of a large press it is slow any thoughts about heat quench then press it flat in one pass??? One of the last things to do before getting wet and if I screw up dont think she goes in this year..funny how we do so many aspects of boat repair yet something always comes up to scratch ones head....
I'd go with the press in one pass. Different bronzes work somewhat differently. Hope the bent rudder lowered the price of the boat.
04-13-2012, 07:55 PM
Bronzes contain tin. Tin has a low melting point. If you get bronze too hot the tin will not only melt but will vaporize and what left will be unusable. What you describe may be workable but I woud think there has to be a rock solid way of keeping the heat from climbing to the point of destruction.
You had best contact somebody who knows the drill like someone who repairs bronze boat props.
BTW, what you described, heating to red heat and quenching in water, is the way copper is annealed.
04-13-2012, 08:03 PM
It was Garrett, I believe who had a recent thread about straightening out a chock, which he mistakenly called a cleat, IIRC. There was good info on that thread about the working properties of bronze, from Peter Sibley, Canoeyawl, Jim Ledger and other experts. It would be worth you while trying to find it. I seem to recall mild heat, but no discussion of quenching, though.
A photo would help.
04-13-2012, 09:06 PM
My meager experience w/ working bronze is that under heat it will go from workable to crumbly in a heartbeat.
I'd definately get expert advice before trying or maybe better yet give the job to someone with experience.
04-14-2012, 11:40 PM
Some bronze alloys contain tin, many do not. Statuary bronzes were generally tin-bronzes. Silicon bronze, a common alloy used in marine applications does not contain tin. Manganese bronze, a copper alloy used for propellors, is ~3% manganese and ~25% zinc (plus varying amounts of aluminum and iron).
As to tin evaporating from a tin-bronze: tin melts at 450 F but boils at 4716 F, copper boils at 4643 so the tin will not boil away. Molten tin is also resistant to oxidation - plate glass is made by floating molten glass on a bed of molten tin in open air, the tin stays bright and shiney and imparts its smooth surface to the glass as it cools.
As previously mentioned some bronzes will crumble as the "hot short" (a foundry term) temperature is reached. The hot short temperature is near the melting point. So while you want to want to heat the part to soften it you must avoid getting it too hot. You will probably be all right if you heat to no more than a dull red heat. Don't do this in sunlight because you won't be able to see the color before you get the part too hot.
/// Frank ///
What I learned about bronze is that it is what's called "hot short" which means that you don't try to heat it and bend it the way you would steel.
The very good metal guy I once worked for taught me to use an acetylene torch with the oxygen turned off, and coat the workpiece with a coating of black soot. Then turn the up the oxygen, hopefully on a big rosebud tip, and heat the piece until the soot burns off, which indicates the critical temperature. Then let the piece cool. He did not quench them, but other people who seem to know what they're talking about say to do that.
In any event, once the metal is cool, bend it. When it starts to get "stiff," it's displaying work-hardening, and might break. Repeat the heat (anneal) process, and bend it some more.
I have straightened some pretty bent-up bronze and brass pieces this way, and broken a couple, too. Patience. As far as I know, it's better to anneal too often than not enough.
I'd suggest that you hold the rudder in a vice, and bend it with a lever, rather than a hydraulic press. That way, it'll be easier to feel when the metal is stiffening (work hardening). Unless you have a press with a strain gauge...
04-15-2012, 11:44 PM
I vote for the prop shop approach.
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