Heating Skeg with a Torch to Bend

[Oldslowandugly]

Most advice about straightening a bent skeg on an outboard engine involves banging the skeg back into shape with a big hammer. Some say first to heat the skeg with a torch; some say to never heat the skeg.

Q1: is heating a skeg with a torch to help straighten a bend a correct method?

Q2: would using a 12-ton press to straighten a bend in the skeg of an outboard engine be a better way of applying force than using a hammer?

I know aluminum doesn't like to bend.

The worst case [in trying to straighten the skeg of an outboard engine] would be to break the skeg.

BACKSTORY: I took the hard-working Evinrude 30-HP outboard engine off of my 15 footer. I replaced it with the original Evinrude 48-HP engine that had the water in the lower unit. That motor's lower unit is a total loss from corrosion. I traveled to Baltimore to get another 48-HP engine for its lower unit. It is in great shape except that the skeg is bent.

[Phil T]

How bent is the skeg?

Post a photo, or send one to me to post on your behalf.

[jimh]

I know aluminum doesn't like to bend.

The ability of a metal to be formed by bending under pressure is known as the malleability of the metal. Every website I can find giving advice on the malleability of aluminum rates it as highly malleable. However, I suspect that the malleability varies with the alloy.

The gear case assembly is an aluminum casting. Cast metals may not be as malleable. I suspect that the melting and cooling that occurs in casting may result in the metal structure become more crystalline. Also, the uneven cooling of the casting results in variation in the metal atomic composition in different areas of the cast part.

That metals will become more ductile upon heating is well known. Heating generally makes metals softer, weaker, and more ductile. The process called annealing is a method of heat treatment that will cause metals to become more workable and have greater ductility. However, in annealing the metal must become quite hot and then be allowed to slowly cool for any significant change to occur in its microstructure.

On this basis, there is every reason to heat the aluminum skeg before attempting to apply a bending force to it. I suspect that even a moderate increase in temperature will have useful effects. On that basis heating to the range of 150-degrees ought to be a moderate temperature.

Advice to avoid using a torch to heat the skeg of an outboard engine may be given as a caution against heating the lubrication in the gear case, causing expansion, and possibly damaging a seal. On that basis, removal of the lubricant from the gear case may be prudent before heating it. On the other hand, the presence of the lubricant may help spread the heat throughout the gear case. Heating can also cause expansion of components like shafts, gears, and bearing. Excessive heating could cause too much expansion and possibly cause harm.

Using a slow, steady, and variable application of force to bend the metal of a gear case sounds to me like a better approach than using an sharp high-force impact.

[Oldslowandugly]

In my research I have come across the dilemma of heating and not-heating quite often.

We know that aluminum is a terrific heat sink. Some say that leaving the oil in the gearcase is the best way since it disperses the heat over a large area. It also saves the seals.

One source said an infrared thermometer showed that if the moderate were heat and were directed at the bent section of the skeg and not the gearcase, the heat doesn't travel very far and the parts are safe.

I agree that applying heat is probably the best route to follow.

If the 12 ton press is the better bending force, applying heat safely while doing so can be a challenge. I can monitor the heat with an infrared thermometer so things don't get out of hand.

I also agree that swinging a big hammer at an aluminum part is probably not the best approach.

The illustrations below show the bend and a missing chunk. I can repair the missing chunk. The bend is my problem.

Fig. 1. A bent skeg.

skeg1.jpg (17.47 KiB) Viewed 5737 times

Fig. 2. A missing chunk.

skeg2.png (42.73 KiB) Viewed 5737 times

[Phil T]

Going out on a limb:
--given the bend is on the bottom half, I would say try using compression or a hammer with a little heat;
--worst case, cut off the skeg and install a skeg guard.

[jimh]

Put the skeg between two pieces of wood. Put the wood and skeg in a large bench vice--hey, every good workshop should have a large bench vice. Slowly tighten the vice.

[jimh]

Use of a torch to heat the skeg will scorch the paint, and the heat of the flame is much too great. A better approach to heating the aluminum in the skeg will be to use a hot air gun, but not one designed to melt off paint. Use a moderate heat setting. I expect that even a moderate increase in the temperature of the aluminum above ambient air temperature will be useful in improving the ductility and will make the corrective bend easier to impart to the skeg.

[Oldslowandugly]

The bend is a strange bend as usually the whole skeg is bent to one side or the other, which really doesn't affect the flow of water past the skeg. But this bend is like it was caused by going in reverse at a high rate and hitting something solid. This bend would act similar to the adjustable fin on the cavitation plate. That would explain the missing chunks too.

I do have a large vise on my workbench. My plan was to use the press and clamp the skeg between two press plates and apply force that way.

Q3: does aluminum bend and stop where it was bent, or does it bend and rebound, like steel does?

[jimh]

Q3: does aluminum bend and stop where it was bent, or does it bend and rebound, like steel does?

I don't know of any method to predict the elasticity of aluminum once it is formed into a finished part in a casting process. A great influence on the ability of any part of the casting to be deformed by bending then to spring back to a prior shape (elasticity) will likely be a function of the shape and thickness of the casting part you are trying to bend.

I believe you will be able to discover the elasticity of cast aluminum after you make the first attempt to bend it to the desired shape.

Regarding a comparison between the metals aluminum and steel for their elasticity, one could compare the metric called the modulus of elasticity. The more stiff or inelastic the material, the higher its modulus of elasticity. Values for the modulus of elasticity (or Young's modulus) of steel and aluminum are given in a Wikipedia article of this topic as follows:

Steel = 200
Aluminum = 68

The inference to be drawn from these values is that aluminum is three times easier to bend than steel.

[Oldslowandugly]

The skeg bent easily with the 12-ton press.

Fig. 3. The bent skeg after straightening.

IMG_0248.JPG (52.09 KiB) Viewed 5688 times

I was right about the rebound. It did not stay straight when pressed flush with a press plate. I had to take a piece of 1/4-inch bar stock and position that past the bend point. Then I pressed it again going slightly past flush. It rebounded back to perfectly straight.

I then massaged it as flat as I could with a small machinist hammer. Much easier than steel.

I had tested the gearcase for leaks in my slop sink before work. The propeller shaft and shifter shaft seals leaked at 20-PSI.

Now I am taking the whole thing apart for new seals. After that I will fix the missing chunks.

Onward, ever onward.

[jimh]

The propeller shaft and shifter shaft seals leaked at 20-PSI.

That amount of pressure on the seals seems high. Your test may have caused the leak.

Q4: Is there a test procedure that specifies using 20-PSI as the test pressure for the gear case---of a 40-year-old engine?

Q5: did you heat the skeg before bending it?

[Oldslowandugly]

No heat was used at any time [prior to bending the skeg].

The skeg bent very easily.

I tested [the gear case seals] for leaks before I started any work. A pressure of 20-PSI was as close as I could get with a bicycle air pump to the recommended 18-PSI from [the factory service manual]. If that held then [the factory service manual required a repeat test with] a 15-inch vacuum applied.

Also, when I drained the oil in the gear case I saw some water come out--not a lot, and not emulsified like butterscotch that you normally find. First a small amount of clear water, then clean oil came out. I have never seen that before.

[Jefecinco]

I'm not surprised you were able to straighten the skeg without heating it. I've had to do this twice and never had to use heat.

The skeg on a previously owned Verado suffered the loss of a large chip on the leading edge. I was able to file the skeg to eliminate the chip and restore the approximate original shape. I was not able to detect any changes to the handling of the boat after the repair.

[jimh]

Water and oil are generally immiscible. They can form an emulsion. That is what you see when you drain the oil after the engine has been run. The action of the gears turning in the gear case mix the water and oil into an emulsion. If the gear case had been allowed to sit without any turning or agitation, the water and oil could have returned to their unemulsified separate states.

[Oldslowandugly]

I always drain and change the lower unit oil at the end of the boating season. That is so in case any water got in it can't freeze over the winter and split the gearcase. Normally if there is water it gets emulsified and looks like butterscotch pudding when it comes out. I then take that water/oil mixture and store it in a jar to see how long it takes to separate. It takes literally years for that to happen. I have never seen it separate into clear water and oil. It is usually an emulsion that lays at the bottom and the oil floats on top. I was thinking that the type of oil I found was some kind that separated the water easily. It was a very dark green and thick like honey. I normally use OMC Hi-Vis oil and it stays emulsified a very long time. When I saw about a shot-glass amount of clear water come out I was amazed.

[jimh]

Thanks for the further observations about the oil and water from the gear case. Perhaps the oil in the gear case was some special blend of oil that was highly resistant to any emulsification with oil. Or perhaps the oil had NEVER been mixed with water be the gears because the gear were never run. I would expect that if someone had just changed the gear case oil and the engine had then never been run since, the oil could have stayed completely unmixed with any water that came into the gear case or some water that could have remained in the gear case and not been drained out before the oil was added.

I just mixed some olive oil and water in a small dish. After 15-minutes I can see the water is coalescing back into three or four very large droplets in the oil from the hundreds of very small droplet state of the emulsion. The rate that this occurs is probably related to the viscosity of the oil. Olive oil is rather light oil, maybe 10W. Gear case oil is 90W, so much more viscous. After an hour or two the oil and water were almost completely separated.

[Oldslowandugly]

What is the best method to fix the missing chunk in the skeg?

I normally use Marine Tex to fill small holes and crevices. There is no heating involved, and Marine Tex is a very strong repair.

I can have a welding shop use TIG to fill the damaged area.

There is a process called aluminum brazing that used melted aluminum, a torch, and filling with TIG rods.

Once I have the gearcase empty, heat is no longer a concern.

[jimh]

What is the best method to fix the missing chunk in the skeg?

Obtain some aluminum sheet of the same thickness as the missing portion of the skeg. Cut and shape the aluminum sheet material to the shape of the missing portion of the skeg. Weld the replacement material into the skeg using the appropriate welding technique. File and fair the weldment to create a smooth transition.

[Don SSDD]

For bending thin metals like an aluminum skeg, I have a very large--at least 16-inches-long--adjustable wrench. Adjusted tight against a skeg, you have excellent control and good leverage. I move it along the length of the skeg at an inch at a time, bending as I go. Doing this slowly and passing over the bent section several times gets the skeg straight without risk of damage. Much better than a hammer, and the work can be done without removing the lower unit from the boat.

[Oldslowandugly]

DON--your method of bending the skeg [with an adjustable wrench] makes a lot of sense.

I have used big adjustable wrenches for other things, but never considered an adjustable wrench for straightening a bent a skeg.

[I think if trying to straighten a bent skeg with an adjustable wrench] an outboard engine gear case is probably better supported if left attached to the outboard engine. [The gear case I was working on was already] off [the engine] for other service.