Ok, I'm getting real close to pulling the trigger on putting shaft tubes on my boat. I know that Paul Spencer is using them almost exclusively. He claims a 10% increase in efficency, ie, if at 1800 you are running 70% load, after tubes it will be 60% load. That may be exaggeration. The guy I'm talking to about putting them on claims that maybe I'll pick up a knot, but the main plus is a savings of 3 gal or so per hour per engine at cruise. He also says that if you can't cruise 30 kts, they don't help as much if any. My cruise is 30kts at 1900 - 1950 depending on sea state. I have C-18 CATs. If I save 3 gal per hour per engine, the tubes pay for themselves in one year. Anyone with experience with shaft tubes? Thanks for any input.