Marlin spikes are used extensively in sailing for tying and untying knots. There is a fairly detailed section in the Ashley Book of Knots on how to use them to tie turk's head knots, there are plenty of online resources on how to use them to tighten and loosen knots. I make a variety of sizes, many of which are custom orders and don't show up on my web site. If there is a marlin spike pattern that isn't shown (baring copying another makers work) please let me know and I should be able to make on for you.
Each Marlin Spike is hand machined, using various grades of titanium, brass, bronze, copper, and other materials.