Didn’t realize that the Michigan Vortex series included 3B props, as the one I bought was a 4B.
Perhaps it would be useful to share my experience with 4B props. I had never experimented with 4B props before I replaced my transom-mounted 31 year old 300 lb V-4 with a ~130 lb heavier V-6, set back 30” on a bracket. I knew the extra setback and heavier motor would move the CG back a lot, which is why I stayed with a 2-stroke to minimize the weight increase when I repowered. However I was NOT prepared for the dramatic change it created in the boats planing performance! A boat I had run for 31 years that would plane easily at 12-13 mph and run comfortably thru the 2’ square waves on the Bahama Bank at 15 mph while sitting down, would no longer hang on plane below 22-23 mph! Initial tests with a dealer-provided aluminum 3B 17” pitch OMC prop (I think about 14.5” diameter) gave exactly the right WOT rpm (and would probably be about right for your boat), but it’s low speed planing performance was unacceptable and it was so sensitive to throttle setting that it was very difficult to even maintain a steady 22 mph! I tried another 3B aluminum 19” pitch OMC prop but WOT rpm was too low, although it did seem to have less throttle sensitivity.
I concluded that more blade area would better absorb the E-Tec’s awesome mid-range torque. (The torque curve is nearly vertical in the 2000-3000 rpm range, after the injectors switch from a very lean stratified charge mode at about 1800 rpm, to a much richer mixture to keep from overheating the pistons!)
Not only did the 4B prop completely eliminate the throttle sensitivity issue, the extra stern lift also dramatically reduced min planing speed, down to about 15 mph! The addition of a fin on the AV plate got min planing speed down further to the 12-13 mph that I had previously found to be so useful.
So I would suggest that you try a 4B prop if possible . . . You may be pleasantly surprised at the results! But before you do that, I’d bite the bullet and install an accurate digital tach! If you don’t know what your WOT rpm is, any prop testing you do is a complete waste of time!
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