How Fuel-Efficient Are Pods, Exactly?
Comparing the Sabre 42 Hardtop Express, one with pods and one with shafts: At wide-open throttle, the pods’ horizontal thrust and contrarotating propellers increase economy (0.8 nmg for pods, 0.63 nmg shafts). But downward-pointing conventional shafts lift sterns, negating, or even reversing, that fuel-economy advantage at slower speeds (0.92 nmg shafts @ 15.9 kts. vs. 0.8 nmg pods @ 16.2 kts.). Most pod boats burn roughly the same per mile through a wide range of cruising speeds; conventional shaft boats typically have a narrow sweet spot. Savings on a long haul might disappear when slowing to navigate a tricky channel or speeding up to avoid a storm. Naval architects have also gotten better at maximizing the efficiencies of pods at all speeds.
When pods shear off, their recovery might cost a huge percentage of their value. “Mark their location any way you can, with a waypoint or a buoy,” said Steve Little, president of TowBoatU.S. Charleston. “If we can go straight to them and safely lift them into the boat, most tow boat operators are going to charge an hourly fee or a flat fee, and not claim salvage. If we have to go searching or put divers in the water, that’s another story.” To prevent later claims of piracy, “come to an agreement beforehand,” Little said.
When pods vibrate, propellers or jack shaft bearings are the likely causes. To determine which, run the throttle up slowly on one engine and then the other. If one engine vibrates but the other doesn’t while in neutral, the problem is likely in the always-turning jack shaft. If one engine vibrates in gear and the other doesn’t, it’s probably caused by fouled or damaged props.
Do pods make a boat easier to handle in all conditions and situations? Weigh in with your experiences here.
To read more about pods and see side-by-side comparisons, click here.