What is the problem with a welded Al skeg? As far as I can find on the net, the original Mercury drives are made of XR360, which is almost the same as AlSi10Mg with less than 0.2% Cu.
In theory, AlSi10Mg should weld perfectly and the material only loses about 10% of its strength at the weld. So why does it fatigue and crack, since the original skeg has a lot of strength reserves?
Even if it is 30% weaker, it shouldn't be a problem at, say, 75 mph.

Does anyone know if the lateral forces during turning are the largest lateral forces acting on the skeg, or are the pulsating loads due to cavitation stronger?

Please for explanation.

skegs are welded all the time. even our #6 drives that were running over 160mph were welded and never had an issue. so long as the welder is good and knows what he is doing there should be no problems.

I read a lot of topics on forums where writers claim that skeg cannot be welded properly and flies off. In general, the prevailing opinion in the posts is that for fast surface drives you should not weld skeg at all. When calculating the tensions at the weld surface, I was werry on the safe side even with a lift of 3 degrees of the skegs angle of attack, which is certainly far beyond the realistic value during fast turns and would throw the passengers out of the speedboat during such a jerky turn. Since I had no smart explanation where the problem is, I asked here.
We are professionally engaged in welding aluminum alloys and it was not clear to me how this is possible. The only thing that came to my mind is that the welders did not take care into the materials properties, or they made surface welds, which are usually very beautiful, but do not hold.

Thanks for sharing your experiences.

Whilst the skeg IS a relatively important part of the outdrive when it comes to steering, it's always been my assumption that another primary purpose is to simply protect the prop, to be the first thing to contact an underwater object and generate the kick-up protection feature. We've all seen outdrives or outboards with significantly compromised skegs, with some missing most or all of it. So I don't think there'd be TOO much concern in the event it did come off following a poor weld. There simply isn't much surface area present on a skeg.

What outdrive are you seeing the need to repair...or is this more of a general question?

Lasr year we actually extended the skeg on the MSA 300 SXR drive MSA Marine Systems because the original one is only 5.5 inches high and with the drive optimally trimmed, the skeg "loss grip" in moderate waves and the speedboat made an uncontrolled turn. Not sharp, but I need aprox 70-90 degrees of turn to hold him back. When we extended it to 8.5 inches, these problems are gone. Our drive has an AlSi10Mn housing and I didn't see any technical problems with the extension, but subsequently I read all the hell about the welds cracking.
The more I calculated the forces on the skeg, the less clear it was to me what the problem was and that's why I asked here to find out if there might be some unknown loads during the ride, like flutter on an airplane wing, but I don't quite understand why this could happen.
The simplest explanation is that the welds were done by incompetent welders.

Biggest risk is where the weld ends at the trailing edge, is it at or does it create a high stress point. It is also best to have generous radiused "zig zags" or "fingers" in the joint between the two pieces to maximize the strength, not to create stress along the full length of a straight weld. As the drive runs higher, the load transitions to the lower/lowest and most rear point of the skeg giving very high load potential. It is of my opinion that when a high speed boat spins and a skeg is found broken, the spin and sudden high load, possibly on only one skeg is what usually breaks it, not that the skeg broke and caused the spin. Of course there could be exceptions to this.
Another issue can be that welding adds stress to an already annealed/normalized and heat treated part with low internal stresses. It can also cause internal bores to go slightly out of round. Another fact is that in machining or grinding the skeg back to shape, it is generally thinner than original.
I have welded skegs successfully, but there are trade-offs.

OK, by high speed boat spins there are definitely enormous loads on the skeg, because then the lift coefficient come to Cx=1.1, if the skeg is in good grip at that moment. By the way, with a severe spin the force reaches several thousand pounds. This breaks everything.
Has anyone done research, or does anyone know what lateral dynamic pressures occur on the skeg while driving at a certain speed (Surface or classic drive)?

It appears to me you know way more than any of us on this subject lol ... my skegs are welded, if you ask Teague he will tell you it`s not a good thing, that being said there are a lot of welded skegs running around without issues.

Nearly every #6 boat I have ever seen has welded skegs. When they leave the poker runs it looks like a shark took a bite out of them if there is any trash in the water at all.

I have had an idea for a few years about making easily replaceable skegs for Bravo and #6 drives. I have even consulted a machine shop about it, but they weren't interested. You would be able to change the length or shape of a skeg by simply sliding out the current one and sliding in another. I personally don't have the tooling to do it, but would be happy to share my idea with the right person.