This would be in line with what most of the J-105 racers believe the difference is in boat speed with an aligned set prop vs. one that's left to free wheel. (A couple tenths).
How many J 105 racers are using a fixed prop? As far as I rememebr these boats were delivered from the factory with a folding 15" Martec..? Most that I know use the Martec.
Some thoughts on this. I am not being defensive but I would like to see you win some races.
On my old Cape Dory 27 if I locked the prop behind the keel or locked it not in line at 90 degrees I could decipher NO discernible difference what so ever and tried many times. If I unlocked the prop from in line with the keel it started to spin naturally!
This tells me that even being about 5" behind a rather fat Carl Alberg designed dead wood that the turbulence still did not prevent the prop from wanting to spin on it's own. The prop was taking the path of least resistance naturally. Locking it is causing drag but in this situation it is probably not noticible between fixed and freewheeling much because these boats are already slow and cumbersome when compared to fin keelers.
We know from the the three scientific studies that a locked prop in the standard Michigan Wheel shape causes more drag when locked than if allowed to free wheel. The only exception to this would be a prop like the Campbell Sailor which has virtually no difference between locked and freewheeling due to it's foil shape at least in my tests.
So if a two balde Michigan prop, even hidden behind a large, wide dead wood wants to spin, that tells me the turbulence factor would be next to nil on a fin keeler with the prop four to six feet behind the very hydrodynamically designed fin keel with a very smooth tail end taper.
On my old C-36 I had a two blade fixed Michigan wheel. The boat was faster with the prop freewheeling vs. locked in any position. I experimented with this for a full afternoon with some racing buddies who doubted this. In the end they agreed the boat was faster freewheeling sometimes by as much as almost a half knot in lighter winds when we were not already near hull speed.
All of the boats I currently race on have feathering of folding props.
With that test jig I made the jig accounted for roughly 12 pounds of drag at WOT with no prop attached. The jig bearings were adjusted for friction to closely match that of my CS-36 and the prop began to freewheel at .8-1.2 knots just as it does on my CS.
If you subtract 12 pounds of jig drag from 25 pounds of total drag in freewheeing mode you have a net prop drag of about 13 pounds when freewheeling.
If you subtract 12 pounds of jig drag from 50 pounds of total drag in locked mode you have a net prop drag of 38 pounds when locked.
In this case with a three blade Michigan Wheel the locked prop caused 2.92 times MORE drag than the same exact prop when allowed to free wheel. 38 pounds of drag can also be looked at as 292% more drag than 13 pounds. In this case with a similar friction to my own boat I am causing nearly 3 times more drag when I lock the prop. I still lock the prop when cruising unless of course someone wants a gentlmens run at which point I would unlock it for a quick little impromptu race..
Even if a two blade was only 2 time more drag locked I would still find it next to impossible to believe that any "turbulence" from fin keel, multiple feet away from it, would mitigate a doubling of drag between fixed and locked. With a three blade it was 2.92 times more.
There is a very easy test for this on your own boat. Before you launch mark the shaft with two pieces of tape one at 90 degrees and one vertical. When under sail and with the shaft locked mount a clamp on lever to the prop shaft inside the engine compartment and put a fish scale on it then put the boat in neutral and read the side load. Now repeat this with the prop at 90 degrees and note the readings. The boat should be sailing at the same exact speed to be a fair assessment. I still say to let it freewheel if you have a fin keel. Every ounce of credible scientific data suggests that a freewheeling Michigan Wheel style prop causes more drag locked.
If it wanted to spin naturally on my old Cape Dory 5" from a wide and fat keel.....