E-TEC M2 Gear Case Propeller Aperture

Optimizing the performance of Boston Whaler boats
jimh
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E-TEC M2 Gear Case Propeller Aperture

Postby jimh » Thu Sep 09, 2021 1:41 pm

Several years ago I checked the propeller aperture on the M2 gearcase on my model year 2010 E-TEC 3.3-liter V6 225-HP engine. The method I used was to presume that the propeller in use (a Mercury MIRAGEplus 17-pitch) had the diameter specified by Mercury, 15-1/2-inches. Using a drill of 1/4-inch-diameter, I measured the clearance from each blade tip of the three blades to the bottom of the anti-ventilation plate. I found that a 1/4-inch drill would just fit in the gap between blade tip and engine, with perhaps a few hundredths of an inch of additional space, which varied only slightly from blade to blade.

On this basis I think the propeller aperture is 16-inches. I also note that Evinrude's largest diameter propeller recommended for this engine has a diameter of 15-3/4-inches (the Rebel TBX™ 3-blade 15-3/4 diameter 15-pitch).

References:
Evinrude Propeller Selection Guide
https://www.operatorsguides.brp.com/Ope ... 5%20EN.pdf

ASIDE: The asymmetry in blade clearance could be due to perhaps three causes:
  1. the propeller blades could be very slightly non-uniform, which would be quite reasonable for a production propeller, as this propeller was not a lab-finished propeller sample;
  2. a very small misalignment of the propeller shaft and the anti-ventilation plate so they were not precisely parallel (but unlikely as I will explain below); or,
  3. the propeller hub kit introduced some asymmetry in the mounting of the propeller on the propeller shaft.
I think that blade asymmetry (explanation 1) is the most likely. Propeller shaft and plate not being parallel (explanation 2) seems unlikely because the noticed asymmetry involved only one blade; if the propeller shaft were not precisely parallel to the bottom of the plate then all blades would exhibit the same clearance, assuming the propeller blades were uniform. Variation due to the hub kit (explanation 3) also seems possible, but due to the somewhat pliable nature of the Delrin insert in the Flo-Torq hub, I would expect that upon tightening the propeller nut to 55-foot-pounds the Delrin would give a little to uniformly conform to the cast-in hub cavity.