You should use a copper based stainless steel. The alloy is commonly used
for heat exchangers and oil coolers on large commercial marine engines. It
goes by several names, but I think cunifer(sp) is one of them. Be careful
with your construction and avoid exposing it to unsupported vibration as it
is prone to fatigue cracking. However, it is very resistant to salt water
corrosion.
Steve

"MarshallE" wrote in message
...
Hi,

What is the normal metal tubing used for keel coolers and how long should
it
last?

I am having a heck of a time locating a seawater condenser and have about
decided to make from scratch.

I want to A/C using 5 tons for my boat.

thanks for help and opinions,

Marshall