Hi Pierre, Can't tell you but I know of a vertical boiler with a big central tube and on the outside the same coils. Problem was the size of that one and due to the big central tube there was a lot of water and hence a lot of weight quite high up in the boat. Anyone who has a small boat knows this makes things quite instable. Regards Wilfried Vermeiren http://users.skynet.be/modelbouw.wilfried MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.