thank you for all the the trouble you have taken by answering my letter to the forum. Unfortunately I cannot follow you.
You say that the Autoprop is dumb.
Maybe it is me that is dumb instead because this is how I understand the action of an Autoprop:
I consider the Autoprop quite intelligent. A very limited intelligence of course but it does what is expected.
Namely it self-pitches in a preprogrammed way and the pitch is the result of the water pressure on the propeller blade which makes it swing into position.
I define pitch in this case as the angle between the propeller blade and the streaming water.
The water pressure in its turn is depending on how much power you have available on one hand and the hull speed at a particular instant on the other hand.
Isnt this exactly the good thing about Autoprop when motorsailing?
A fixed propeller on the other hand could be considered as quite stupid.
Hopefully you will bear with me when I give a little background to my original question.
I have used methods and diagrams in David Gerr's wonderful and intriguing book "The Nature of Boats". If you want to dive even further into the subject I recommend "Propeller Handbook" from the same author. I dont know if I can share these diagrams with you since they of course are copyrighted by the author.
Anyway here is how it goes:
Basic data about my boat: lwl 33 feet, displacement
18000 lbs, future engine power 87 hp at 3000 rpm.
In the Displacement
Speed Chart (Page 246) we will see that for such a boat 45 hp is needed to reach the maximum hull speed of 8 knots.
Now, how much power is necessary to reach 5 knots?
From the diagram we can see that 10 hp is enough.
My future Lombardini of 87 hp at 3000 rpm will deliver 10 hp at 800 rpm as I can see from the power vs. rpm spec. This is about idling speed.
Now back to the Autoprop issue.
If the Autoprop does what the advertisement promises it will automatically assume the optimum pitch due to its self pitching characteristics.
With optimum pitch in this case I mean a pitch that with the available engine power of 10 hp will drive the hull as fast as possible.
Will it be 5 knots??? That would be disturbing, I want to be able to go slower.
All the above are of course calculations although well founded.
Is there any support for my findings in real life?