"Twins installed in the same boat are not independent units when they share the same electrical, fuel, and water system." Dr. Al, Your statement is correct. My statements were also correct. Two engines need not share electrical, fuel, and cooling system components, although in most boats they do. If they don't share such components, they may be considered independent and the estimated probability of simultaneous failure may be reduced. (Not to mention the math being simpler.) By the way, it wasn't my intention to advocate twins as Al P. seemed to interpret my remarks. I was attempting to clarify the math for Patrick G. Back to lurking (I hope), Joe Joseph J. Blass, Ph.D., ... Oak Ridge, Tennessee <jjblass@ispchannel.com> P.S. Those bored with this thread might as well skip it without comment. I've at least glanced at every post since 1 July 1997 and observed that complaints don't stop the beating of dead horses. The beaters eventually tire of the activity. Flames beget flames. Patience is a virtue. Reading this stuff might help to take your mind off what's happening in the world. ... No, I don't believe it will.