Burning wood that is not properly seasoned will cause a tar like substance to form and run down the inside of the chimney and leak at the point it sits on the roof collar.(picture right) The picture below shows the collar with the pipe coming up from the stove. The dark substance around the edge is high temp. sealant plus some of the tar like substance that has run down the roof chimney.
This tar will form as creosote inside the chimney like a honeycomb and will block the chimney. Where do the fumes go?....correct, have you smoke and CO2 alarms in your boat? There is a good article to read here. The chimney blocks because the pipe from the stove is hot but as the gasses exit through the roof into the outside chimney they cool and condense. If as was the case on Nb valerie many years ago you only have a single skin outer chimney the cooled gasses in the form of creosote run down the inside of the chimney and leak where it fits on the collar.
The answer is a double skinned chimney available at chandlers for loads of money. The inner skin must be a good fit or the problem of tar will persist. The gasses remain hot inside the inner skin and have very little cold surface to condense on. I will go on to show my own version that is cheap and seals perfectly BUT be warned the tar can still become a honeycomb blockage whatever method you choose, less likely but possible so keep an eye on your chimney, push a chimney brush down it to keep it clear. So if you have a mess on the roof check the inside of the chimney, I care about my blog readers.
On the left is the old home made inner chimney I have just removed. At the top a hole has appeared (3yrs.old) and it was here the gasses were escaping and condensing on the cold outer chimney.
The Blue next to the chimney is the off cut from my home made version. It was in a past life a printers lithographic plate 25" x 29" aluminium in composition. Find a printers and ask them for an old plate or buy a sheet from perhaps a diy store. On the right the new inner is in place. I rolled the sheet into a tube pushed it down into the stove pipe in the collar. Let it open itself for a tight fit, put tape around to keep the shape when you remove it. Now just drill holes and use pop rivets to hold it all together and what you now have is a fairly strong tube. Insert tube and outer chimney and mark the inner so you can remove excess length.
I have left mine a bit high (marking error) and will reduce it later. It needs to be slightly higher to make sure the hot gasses don`t easily come into contact with the cold outer chimney. If you look carefully you can see the the tar stain on the roof to the right of the roof collar although I have just noticed my closer picture at the top and i have no intention of changing this post layout. So after 24hrs i can wipe my hand all around the bottom of the chimney without a trace of tar to be seen. Boaters who continuously cruise have no space to store wood long enough for it to season so the tar problem is common when we burn wood with to much moisture. So even if you have the tar problem and choose to ignore it PLEASE at least make sure the chimney is not becoming a honeycomb killer.