Then cut enough planks of 1/16" - 2" x 1/4" should be fine - to fit round the tube. Mine needed 23, your number may vary as you bevel them to fit snugly.
Here is a simple dummy steam engine cylinder, built into the rectangle we left out.
Paper and balsa construction - it looks rather bare at the moment. We will liven it up with copper wire steam pipes later, and maybe some small hex nuts if I can find any....
And here is the tube, cut to 2" height, with a little rectangle to clear the motor shaft. Check it fits snugly over the metal motor mount.
Then make a paper tube 1 1/4" diameter. I wandered around the house with some calipers until I found a convenient former. UHU glue holds the paper tube together, or balsa cement.
You can see the motor in its mount in the bottom right...
Here is the metal motor mount shape being cut out with a paper pattern from the plan.
And here it is, planked, with a rectangle still cut out.
On top, a 1 1/2" diameter circle of 1/16" is fitted, with a paper chimney, made like the boiler. I made mine 1/2" wide by 2 1/4" tall, because that was the size of the modelling knife I was holding at the time...
And this is how it goes on the boat. Job done!
And here it is being bent to shape. It can be glued or bolted to the launch floor - I decided to bolt it and drilled a couple of mounting holes.
Don't forget the hole for the motor wire to go through!