sealant for thru hull chimney fitting

Fire cement is not flexible in any way so not a good choice of sealer for your coachroof.
Silaflex do make a heat resistant silicone but you might want to check to what temperatures they say it can be used.
Not sure as to flue temps on a boat heater.

Just checked- Sika 15LM seems to be the kiddy.
 
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