The vicious spiral
Damp wood fuel will cause the stove to run at a lower temperature as the heat of the fire will be producing steam and so the stove needs much more fuel to keep the boat warm.
Secondly the steam dissolves-out flammable, acidic tars which will cling to and block up, as well as inevitably damage, the stove and its chimney.
As well as the early and costly damage to the stove installation brings with it an increased risk as combustion gases are likely to leak into the cabin space
The situation will be made worse still because of incomplete combustion as the fuel is damp, those gases are more likely to contain carbon monoxide (CO) – giving the cycle that could see a highly toxic atmosphere build up in the boat.
And although the downsides of using insufficiently dry wood, such as, providing additional fuel to stay warm, spending money on the early replacement of the stove and flue pipes and running the risk of carbon monoxide poisoning, are bad enough; you need to be aware that stove flues lined with tar could also lead to a chimney fire.
In that context it is worth noting that fires on boats often develop quickly, leaving occupants seconds to escape and regularly resulting in total loss of boat and belongings.
The size of the issue
The respected Soliftec website publishes this information about unseasoned wood:
‘Almost all problems associated with burning wood are caused by damp fuel. A 1kg log of fresh wood will contain about a tea mug's worth of water.
‘To burn effectively, wood needs to be dried out, 'seasoned', to a maximum of 15 to 20% water content.
‘The difference is huge. A fresh 1kg log with 60% moisture may be able to give out just under 2kW of heat energy, a 1kg log dried to only 25% roughly doubles the heat per kW to about 4kW.’
Wood that burns easily and cleanly with good heat will feel dry and have a hollow sound when tapped.
The bark will either have fallen off, or will fall away easily and good wood often has cracks in the end where it has dried out.
Wood that feels damp and dense, or has the leaves attached should be avoided. The other indicators unseasoned fuel that needs drying before use are if you see it has green or white mould on the surface and doesn’t have any radial cracks.
There is much more information on stove fuels at http://www.soliftec.com/woodfuel.htm