Gary, it is normal practice to weld a flange around the exiting tube. The
flange is then scewed or bolted to the transom. It is the flange to transom
that is sealed, not the tube. That solution eliminates significant heat to
the seal. The seal should be polysulphide, not RTV. RTV or any other
silicone sealer should be used with great caution on a boat. It is not
compatible to paint or any coating that I am aware of.
Steve

"Gary Warner" wrote in message
...

Where the exhaust pipe goes through the transom
I'm going to pack some heat-resistant fiberglass
rope around it. But it also needs some calk to
keep the water out. I don't imagine it can be
*that* hot but the cooling water flowing through
will heat the pipe some.

What type of calk can I use that will be OK
in that heat?

Thanks,
Gary