How to Change an Inboard Prop

Tips for pulling an inboard propeller.

March 13, 2015
1. Remove the cotter pin and loosen the nuts — a short piece of wooden 4-by-4 between a propeller blade and the boat hull keeps the prop from spinning. Leave one nut where it isn’t quite contacting the hub — about a quarter-inch gap — to stop the prop once it comes loose. We left a larger gap to illustrate.
2. Slip one arm of the Algonac over the hub close to the strut. The other arm fits over the tail end of the shaft. Snug the upper screw to hold the Algonac in place and then tighten the screw at the end of the Algonac farthest from the prop. This pressures the propeller by “pinching” it toward the end of the shaft.
3. The prop should pop off the shaft taper with a clear ring and stop at that nut you left close to the prop. The Algonac will likely fall to the ground. Remove the nut and retrieve the shaft key as the prop comes off. If the prop won’t budge, “leave the Algonac tight and give the propeller hub a little heat from a torch,” Fay says. “It will usually pop right off.” A solid tap with a two-pound sledge­hammer on the end of the Algonac closest to the rudder might also jar the prop loose.
4. To reinstall the prop, first make sure the key slides easily in the keyway — freshly reconditioned props nearly always require filing right at the end of the keyway. Clean and dry both the hub and shaft, and never use grease.
5. Slide the prop on the shaft without the key and mark the forward edge of the prop. Slide the prop on the shaft again, this time with the key. Make sure that key stays in the keyway. If not, the prop will tighten against the wedged key, not the shaft.
6. Tighten the larger nut until the prop reaches that mark on the shaft. “If you bind it on the key, the boat will vibrate like crazy, like the prop is way out of balance,” Fay says. A key-bound prop can also crack a shaft. Once it’s tight, propeller experts suggest, swap the nuts so that the thin one (not shown) goes closest to the prop. Use a fresh cotter pin and recheck that nuts are tight after the first time the boat is used.
Dialing the Shaft
To check for a bent shaft, use a dial indicator clamped to the rudder or strut. Shafts up to a 2-inch diameter shouldn’t deflect the needle more than two-thousandths of an inch at the tail of the shaft, and not more than five-thousandths halfway between the prop and the transmission.

Pulling an inboard propeller isn’t hard with the right equipment and some know-how. Here we change a prop with Jeff Fay of Fay’s Boat Yard on Lake Winnipesaukee, New Hampshire, and an Algonac prop puller — a tool good for most props smaller than about 24 inches. Scroll through the gallery to see the steps.


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