There is value in welding the cast with nickel rod ,the crack will not continue and is strong . Also nickel will not corrode. I have welded tons of castings over the years ,gearbox flanges, bearing housings and once a head off a chevy 350 . No issues if you know what you are doing pre heating the cast before welding and cooling slowly.A couple responders have mentioned welding onto the cast iron keel to effect some sort of a repair. I don't see what value there would be in adding weld metal to a cast iron keel. From the size of the crack, there does not appear to be any structural change to the keel, just some surface rust at most, or the coating on the keel has been cracked open by water expanding when freezing. It is worth mentioning that when iron or steel rusts it expands. Either ice or rust could expand and create a crack like the shown. It also seems possible that fairing compound and epoxy coating could have been built up in that area during construction and broken down over time.
I don’t think the concern is that welding would do any harm. As you say it should be possible to weld without hurting anything. But what is the advantage of trying to weld something on that spot? Unlike a structural component, this part of the keel is just there to provide a smooth surface and a little bit of weight. Welding seems like overkill for the fix, rather than just cleaning up whatever’s there and smoothing it out with some fairing compound.There is value in welding the cast with nickel rod ,the crack will not continue and is strong . Also nickel will not corrode. I have welded tons of castings over the years ,gearbox flanges, bearing housings and once a head off a chevy 350 . No issues if you know what you are doing pre heating the cast before welding and cooling slowly.
Where we live in the winter we gets days of -30 C or -22 F .If he gets more water in a crack in cast iron it will and or crack right offI don’t think the concern is that welding would do any harm. As you say it should be possible to weld without hurting anything. But what is the advantage of trying to weld something on that spot? Unlike a structural component, this part of the keel is just there to provide a smooth surface and a little bit of weight. Welding seems like overkill for the fix, rather than just cleaning up whatever’s there and smoothing it out with some fairing compound.
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Look I repair 30 million dollar machines for 40 years , I look at doing a repair to put it back to or better than original . Fill it, fair it , I don't really care , the proper repair if it is cracked cast iron is to weld it with nickel rod ,to prevent the crack from spreading and give it back its structure integrity period.