No problem in the slightest and your input here is valuable.. I am making up some stuff here so could be wrong and other opinions are good to add.
Regarding the use of the term "displacement hull" for the 1990 Macgregor 26S (this is the old style Mac - before the power sailboats), I didnt look up the exact definition of displacement hull but I put a close to 10 hp outboard at 450 ft elevation wide open throttle and achieved a speed 3% higher than the theoretical hull speed number from the equation. I believe that puts it right in the ballpark of operating where the wave making causes a the curve of speed vs drag to get steep. I thought that would be called operating in displacement mode.. could be wrong. Noted that all this data is specifically for a 1990 Mac 26S and not necessarily for any displacement hull
Another point was brought up that Ive heard elsewhere on the internet. With a prop, you can have a condition of ventilation or cavitation that will limit the thrust. Ventilation is air getting sucked down from the surface and I can get this to occur by doing a very sharp turn in choppy conditions with the outboard on the high side of the boat during the turn - prop gets near the water surface. When the prop ventilates, there is a very ABRUPT loss of thrust and with the loss of thrust comes an increase in RPM - since the torque on the prop just dropped way off. Ventilation was absolutely not occurring for the data I took. You can also see this in the data, there are no abrupt changes in rpm for any data points.
So another question is "do these under 10 hp outboard props cavitate". I have convinced myself that neither of the two props I tested in this thread have any issue at all with cavitation. Cavitation occurs where you are really stressing pressure boundaries and it seems to me that for our small under 10 hp outboards, we are just not operating with enough HP for this to occur..
Also, it seems to me that the onset of cavitation would result in a loss of blade lift and a resulting loss of torque to the motor. So at the onset of cavitation, you would feel a loss of speed (or no more speed) and also a "non linear" change (increase) in prop rpm.
I took the Mac 26S out yesterday and ran the outboard a bunch (also sailed a bunch) trying to note any indication of something resembling cavitation. This is with the three blade 8 pitch prop and I just don't think there is any cavitation going on that I can either see or feel. Also please take a look at the data and curves early in this thread. The speed vs rpm curves are smooth. I think cavitatation would have caused an obvious discontinuity in the curves - but there isn't any.
Yesterday I took some pictures of the prop wash (and thanks again for making it so nice to post pictures here!!) These pictures are for the three blade 8 pitch prop but the four blade 5 pitch prop looked and felt similar (just with a lot more noise and vibration on the boat for the same speed).
The picture below is the prop wash at about 3.5 mph (3 knots). You can see the thrust exiting the back of the prop but you can also see a bubble stream from the through the hub exhaust.
The picture below is at wide open throttle doing mid 7's mph range and up againts the hull speed limit curve. You can see more output - since its higher rpm/ thrust and also see the bubbles - which once again I think are just the through the hub exhaust.
