Verify Emergency Spare Propeller Fits Rebuilt Taper
Since the taper on the propeller shaft had been "humped" and had to be recut, it was important to verify that the two bladed backup propeller still fit correctly prior to re-installing the MaxProp. Since both the MaxProp hub and the shaft had been sent to Wilmington Propeller I knew they were matched to each other but it was not clear if the new taper was identical to the original taper. If I had been thinking quickly enough the spare two bladed propeller would have gone down to Wilmington Propeller when I sent the rest of the hardware down for inspection. At some point in the future I will send this propeller in for inspection and have it cleaned up and checked for correct pitch / taper. Here the spare two bladed propeller I found aboard is installed on the new taper and the prop nut installed lightly. The fit was in fact good. Note that this is before we had to cut 2 threads off the shaft for the MaxProp as recutting the taper involved moving the propeller forward on the shaft a couple of threads. The propeller was not torqued down so it will move forward a bit. However, there is definitely not room for a second nut on the propeller shaft if this propeller is being used. Thus, if in the water and installing this propeller the technician at Windward Yacht Center recommended using 3M5200 as a thread locking compound in a pinch. If the boat is out of the water he recommended pinning the nut with a cotter pin and hole drilled through the shaft. However, if the boat is out of the water it is likely that there is time to repair or replace the MaxProp and so not a significant issue. The propeller is marked 22x18 which is 22 inches of diameter and 18 inches of pitch with right handed rotation. A close up of the tip clearance, the distance to the hull appears to be about a half inch closer to the hull to so the MaxProp appears to be a 21 inch propeller. This is the tip clearance to the hull, it is about 1.3 inches which is quite close. Normally one would like about half of the blade length which would be 11/2 = 5.5 inches to keep tip noise down. If the MaxProp is 21 inches then 10.5/2 = 5.25 inches of clearance desired and 1.75 inches of clearance in reality. If the MaxProp is 20 inches then 10/2 = 5 inches with 2.25 inches of clearance. Interestingly the entire aft area of the boat around the prop has non-structural coring material and a thin (non-structural) fiberglass layer over it to help keep the noise of the propeller tip vortices interacting with the hull to a minimum. I thought the tip clearance was the major source of noise in the aft cabin at higher throttle settings but it turned out that the slightly bent shaft and highly worn cutless bearings was the primary source. If I had to replace the MaxProp now I would probably select their new 4 bladed propeller which would allow using a propeller about an inch smaller with lower blade loading which would further reduce the noise. However, at this point the noise is not objectionable and so I will continue to run the drive train as it is now..