Particularly when it's a big 4 stroke! I remember what I was thinking when I had my new YS.91 (the largest and most powerful engine I had ever owned at the time) mounted to a piece of wood and clamped in my work-stand........"Man, that prop sure looks a whole lot more scary now!" I made sure I stayed *way* out of the way of the prop on that thing when I did the test runs!! I've got some photos of the dodgy 'test rig' arrangement I made on my site somewhere......basically, a piece of wood (with a slot cut in the end), some rubber bands to told the tank down and a length of piano wire for throttle control