Although I do not have a lot of experience with the copper coil kind of engine, I can point you to some very good instructions by others.
This gentleman from Belgium, Yvon Masyn, has done a wonderful job of creating a simple design that young kids can participate with. http://www.instructables.com/id/Pop-pop-or-put-put-steamboat-made-easy-for-childre/ Check out his other high- quality projects that have been featured on Instructables here.http://www.instructables.com/member/masynmachien/
We can thank Marc Horovitz for these high-quality, illustrated (and revised 2 08!) instructions for making a complete coil-type boat with recycled tin sheet. http://www.nmia.com/%7Evrbass/pop-pop/buildpop.htm
My friend from the Netherlands Dr. Guus Flogel has written the ultimate guide for building a boat with a coil engine out of a wooden Dutch shoe! He has also done the experimental work http://www.sciencetoymaker.org/boat/flogel Instructions.htm to determine what size works best and how many coils should be made for best results.
One of the biggest hassles has been finding the right size of tubing. Bob Jay (bobjayx67 at netzero.net--his e-mail is written like this to avoid spambots) seems to have solved the problem for people who have a PepBoys (automotive store) nearby. As he wrote me,
"Here's the skinny on the copper tubing . At two local Pep Boys there is a large section of an isle dedicated to various auto parts called " HELP ". They are on red cardboard with the part in plastic blister heat shrink. The red pack has the word " HELP!" in white letters. The tubing that I used is 1/8" x 8'. The HELP number is 55134 and the SKU # 37495 55134 2. I hope this helps ya out
Thanks Bob. Another thing I think is worth pointing out is that copper can be annealed (softened). This is important because bending hard tube results in kinks (which block the tube) rather than smooth bends. Strangely, copper is softened in exactly the way that steel is hardened--by heating to red hot and quickly quenching in water. As copper is worked (bent, for instance) it once more starts to harden. It might have to be annealed again. The procedure is explained in both Marc and Guus' articles.