Keeping stoves in overnight is something I don't recommend to land based customers as a chimney sweep, due to tar and creosote condensing in the cooling chimney. However I appreciate that a boat is a different thing and generally the flues are short enough that any deposits can be burned off when the stove is cranked up again.
I'm not sure what brand of eco-logs the OP is referring to - but you'll find that any using paper as the base, or as an ingredient will tend to be ashy. Same goes for some of the more recent biomass briquettes such as straw and rice husk. Rice husk is about the worst at around 18% ash.
There are also big differences between the other briquettes on the market. In general, the solid cylindrical ones will tend to expand when burning, and also to collapse into a pile of hot sawdust if disturbed or poked. The expansion results in a greater surface area and a reduction in burn time. You may be lucky to get these to stay in overnight.
The briquettes I supply (and use myself at home) are produced in a different type of machine from hardwood exclusively, and are probably about the densest briquettes on the market. Many competitors struggle to get a tonne on a full sized pallet - we can get it on a Euro pallet! They don't expand or do the collapsing thing, but form proper embers instead which continue to give out good levels of heat for a considerable time after the flames have died down. This is in contrast to most competing products, which do have a good output to begin with, but which tails off fairly rapidly after the flames have gone. Quite a number of our boat based customers use our briquettes to keep their stoves in overnight, and we find at home that there is generally enough life left in the stove in the morning to re-light without starting from scratch, though we don't stoke up or close the stove down at bed time. It just gets left at whatever setting it was running at through the evening. They are also made in the UK rather than being imported from eastern Europe or beyond like a number of the competing products.
As far as stove size goes, this calculator may help. Stove calculator
It may be that a large open space such as a boat will benefit from some sort of help with circulating the heat, such as an Eco-fan.
Anything I can help with - please give me a shout!
Cheers,
Andy