Aluminum melts if you even look at it with anger in your eyes, which really limits the processes for bonding it to other metals. Welding aluminum to steel is not possible. Even brazing of aluminum to steel is “practically” impossible (it can be done, but a quick perusal of this topic online leaves me with the impression that it’s only when using extremely specialized products and methods). Soldering would be more doable, but I doubt if it’s worth the trouble when bolting will do the job.
Also, lets not get too carried away in estimating what stresses from sailing will be. Try to envision how the mast of the sail will transfer stress to the boat. That mast will act as a lever, with the wind exerting the force on the long end of the lever, and the resultant levering action being between a thwart-like brace across the gunwales and a fitting on the keel line. It might seem possible to generate all kinds of destructive force this way, but it’s not, as I’ll try to explain below.
This setup will be most susceptible to lateral force, since that’s the weakest orientation for stress applied to the gunwales. Doesn’t this mean that the gunwales will have to be really strong to tolerate sailing in a cross wind? No. Consider what the force is that resists lateral “levering action” of the mast when subject to a cross wind. That resisting force is almost nothing, because before the lateral force on the mast increases much at all, the boat will simply roll over. The leeboard won’t provide any actual resistance to rolling over, even if it will slightly slow-down the process (the leeboard’s only purpose is to enable the sail to propel the boat forward in a cross wind because skidding sideways inline with the force of the wind is prevented). The only resistance to rollover is provided by the off-center leaning of the passengers onboard, and how much force does that generate? Not enough to break the gunwales.
The sail will be able to generate more force in the longitudinal direction, but that’s no big deal for a similar reason. It can be pointed out that the gunwales are far less susceptible to damage when stressed in a longitudinal direction, but even here, there’s a limit to how great that force can become, since the amount of resistance to a longitudinal force is limited. Picture this: If you attempted to theoretically increase the longitudinal force to an “impossible extreme”, what would happen? The boat is not anchored to the water, but easily moves through the water instead, so the boat would simply move forward at an “impossibly fast” speed. You may have even seen this be demonstrated. If you were to attach a pair of ropes to opposite thwart connections and pull with a biggest, fastest motorboat you could get your hands on, would those thwart connections break? No. The canoe would simply be towed as fast as you could make the towing vessel go, and the canoe would never be in any structural danger (towing methods that allow the canoe to broach are not applicable for this comparison). The same principle applies for the thwart-like brace supporting the mast. The longitudinal force of holding up the mast when the sail is pushed by a tailwind will never reach a point where the gunwale failure occurs, but instead, the boat will simply go faster as the force from the wind increases. Winds do occur which could break these connections, but you won’t be out sailing at those times, and if you were, you’d tip over before having the chance to break anything.
I am quite sure that even if a crappy job of repairing the broken gunwales wouldn’t likely tolerate the forces of holding up a mast for sailing, any decent repair job will be good enough (though as has been pointed out, a decent job won’t exactly be easy).
Oh! And lets not lose sight of the fact that the breaks in the gunwales are not even at the location where a mast would be installed.
