boat boom dia. and length

I know you're not going to want to hear this but the answer is - It depends.

On the particular ship class, time frame and navy. I assume you're talking US Navy? Based on my limited recollections of crawling around a couple of WWII-era battleships, the boat booms were hinged metal poles that were about 10 inches in diameter and - going on memory here - about 25-feet long.

On smaller ships, the boat booms are going to be shorter and perhaps not as large a diameter. When you get down to the smallest craft, such as destroyer escorts, corvettes, minesweepers, etc., many did not have boat booms at all.

Be sure to rig it.   It is not jyst a stick glued in place, perpendicular to the hull.

Image extraced from the US Navy's Basic Seaman NRTC,  Chapter 5;  Boat Seamanship

http://www.globalsecurity.org/military/library/policy/navy/nrtc/14067_ch5.pdf

mfsob wrote:

the boat booms were hinged metal poles that were about 10 inches in diameter and - going on memory here - about 25-feet long. When you get down to the smallest craft, such as destroyer escorts, corvettes, minesweepers, etc., many did not have boat booms at all.

Too true.  Some boat booms are tapered, too (which, at certain scales does not matter that much).  Most USN booms are metal, some war-time small-craft booms are wood.

Really, the best answer is to go back to your documentation for the ship being modeled (and the when/where of that, too).