From the Wallas Manual, bold emphasis by me.
Exhaust pipe
The longest permissible exhaust gas pipe is 13ft. Exhaust runs over 8’ should not have more than 360 degrees of turns. Turns should be as long radius as the installation allows.
The exhaust through hull fitting may be installed into the hull, the transom or the deck, depending on fitting type, hull design and other factors. If a deck fitting is desired and heavy water on deck is expected, then a closeable deck fitting (2460) with an extension (2069) should be used. On heeling sailboat through hull applications, the through hull fitting should be installed as far aft as possible, preferably at the stern. Recommended distance from the through hull fitting to the surface of the water is at least 12 inches.
Exhaust gas is hot! The exhaust pipe and fittings must be insulated well and installed away from flammable materials. Air must be allowed to flow freely past either the through hull or deck fitting. Please contact Scan Marine for information.
Here is a link that provides one owners install.
My sailboat did not come with a heater, and Wallas has one of the best you could install. You can do it yourself as well.
seabits.com