On Kala Sona I have both log and depth sounder through the hull and they are both plastic. Six or Seven years ago I had to replace the log and found that the hole was too big so I turned some mahogany to reduce the hole to the correct size, glued it in with epoxy and followed NASA's instructions for fitting. I have had no problems at that end with either of them. The unit in the cockpit is a different matter as I did not seal the log unit properly and the consequent ingress of water ruined the head. More expense!!