Wet exhaust through aluminum hull

it is better not to use ss, it hates warm salt water esp if there are welds in the pipe that have not been saturated with argon during the weld process Unless you can find 2205
You can use any metal as long as there is isolation before the hull
On sailing boats you will need check valve or loop well above WL
You need also a valve to shut of the outlet as boats with exhausts open to sea air, lose engine valves fast
when all said and done the heavy rubber steam hose will outlast all, but it costs
worth a read
http://highseasyachtservice.com/wp-content/uploads/2011/10/Marine-Shaft-Crevice-Corrosion1.pdf

 

Deering, you are assuming that there will be an unacceptable rate of corrosion if you just weld in the aluminum sleeve.
Every outboard, and inboard outboard runs their exhaust out through aluminum housings without issue, and many in a salt water environment.

Leave enough room for a double clamp, Stainless steel clamps of course and ensure that the worm screw is also stainless (as compared to automotive which often uses a stainless band but a plated worm which is subject to rust)
Minimum band width 1/2 inch

You might have to align the sleeve so it is concentric and parallel with your engine exhaust manifold so the hose does not have to be kinked

The hose has to be marine/exhaust rated or at least in 1993 specs equal to SAE J2006 or UL1129.


ABYC does not seem to offer specs for the thickness of any aluminum thru-hull though they recommend sched 80 for exhaust piping.

You did not mention if it was gas or diesel. Evidently the sulfur in diesel is more corrosive than a gas exhaust but that being said Volvo diesel inboard outboards run their exhaust through aluminum housings

 

Simplest and also likely durable would be a round hole with thick wall rubber hose (non wire wound hose, just reinforced rubber) shoved in tight fit and sealed inside and outside with a polyurethane adhesive like 5200. Done straight out the transom. Only issue might be if you whacked something, but say you have a transom platform extending outwards, then would be better. And it is also paintable the rubber hose and the 5200. 5200 comes in white or black color.
And the hose end can be ground- sanded smooth, even the edge can radiused so it looks better. Rubber through hull will not corrode.