Re-Pitching the David B's Propeller
This past week has been the David B’s annual spring haul out. As part of this year’s yard work we’re once again sending the propeller out for some more tuning. This will be our third time altering it, because the wheel that we have is not the original and it has never worked quite right. The story we’ve been told is that the original prop was removed from the boat sometime between 1951 and 1981 when the boat was laid up at the Libby cannery in Ekuk, Alaska. Then, at some point in the 1980s the boat’s owners got a “new” propeller: a bronze fifty inch by forty-one inch three-blade. It had been salvaged from a boat called Aleutian Native. The reasoning for using this propeller was that the Aleutian Native had the same size four-inch shaft. It also happened to have a Washington Iron Works diesel with cylinders the same size as the David B. The difference is that the Aleutian Native had six cylinders and the David B only has three. Consequently, it was way too much propeller for the David B, and our engine just can't get it spinning fast enough. It's so slow that it's dangerously close to lugging (like when you drive uphill in too low of a gear).
Christine and Jeffrey with the David B's propeller before sending it to be re-pitched. Photo: Chris Wallace
It didn’t take us long after we bought the boat, to start asking the old-timers about the process of re-pitching the propeller, and we quickly realized that there was as much science and math involved as there was art, intuition and voodoo. While each person we talked to started with the same ideas about mathematical calculations based on RPM, horsepower and theoretical hull speed, they all had differing ideas as to how much pitch we should remove to get the most speed out of the boat and to run the engine more efficiently. A couple of them even suggested removing blade area or diameter. I liked how they all talked about how the guys from “back in the day” had the feel for how to tune a propeller to a boat just right. That was when there was art and craft in working on a propeller and not just plugging some numbers into a computer.
After talking with the old-timers, Jeffrey called two local propeller shops to ask what should be done. They happily entered our information into their computer programs, and both places came up with different answers. We chose the shop that was closest to what the most reputable old guys had said.
That first year we were timid and only took our pitch down from forty-one to thirty-nine. We got a little bit of speed, and the engine ran a little bit cleaner, but what was most noticeable was how much quieter it was in the back of the boat and the lazarette.
The second time, we were a little braver and had the shop remove a one-inch strip off the leading and trailing edge of each of the blades. We hoped this would change the blade area to be more like the David B’s original propeller. We once again got a little more speed so that we are running closer to seven knots, and the engine ran even cooler and cleaner.
This year will be the third and hopefully last time we work on the propeller. When Jeffrey phoned the propeller shop the other day, it was decided that we’d need take the pitch down to thirty-six. This, by the way, was the original recommendation of one of the old-timers. The Prop Shop guy even complimented us on the beauty of the David B’s propeller. I was surprised, since I was unaware that we had an unusually nice-looking propeller, but with that comment, I realized that the art and craft of repitching propellers still lives on, even in the computerized shops of today.
Watch the David B being hauled out at Seaview North Boatyard in Bellingham, WA:
Check out Christine's book about rebuilding the David B- More Faster Backwards.
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