I am a retired engineer with proven, practical info on shrouding/ducting props. If done right you might double thrust, make less noise use less gas. use lower hp, less costly and lighter engines. You guys have a lot of skill. If there is sufficient interest I will post some practical proven info. I am interested in any of your comments or questions and will respond.

I would like to hear your thoughts.
Ron

I'd be interested to read what you have. Do you have any real life results in an airboat situation? I know that you just can't throw any prop in a shroud and make it work, the prop has to be designed to work in a shroud. I've seen experiments with shrouds in the past but no real world use of the setup. I'd think that a person would need to have a prop manufacturer on board to see real results.

I'll move this thread to General Tech Talk

Following

Yes, I'm interested in learning what you've discovered

A lot of current R&D is aimed at military drones or getting a Phd and seem heavier on computerized fluid dynamics than clarity. Here are a few info links for starters culled what real world work shows are the key points.

- THE DUCTED PROPELLER FOR STOL AIRPLANES, 1964 - 8 pgs – Raspet- Miss State
- MARVELETTE – tests- quote from conclusion, “AN INVESTIGATION OF THE SHROUDED PROPELLER PROPULSIVE SYSTEM ON THE MARVELETTE AIRCRAFT” 1964- 80 pgs – Raspet
- MARVEL “XV-11A - DESCRIPTION AND PRELIMINARY FLIGHT TEST”
- 1967, 105 pgs. Results from 49 test flights of an experimental military STOL aircraft from the above project. There is a useful hart on about page 41-42 from the Marvel STOL work
- https://theses.lib.vt.edu/theses/available/etd.../WGraf_Thesis_2005.pdf,
pgs 14-15, 32,33
- ntrs.nasa.gov/search.jsp?R=19930084866 a NASA pdf file - by Robert Taylor of NACA , pgs 11, 12, 13, 15.
- The NACA Kruger report ON WIND TUNNEL TESTS AND COMPUTATIONS CONCERNING THE PROBLEM OF THE SHROUDED PROPELLER
Pgs 16-18 and 62, easiest found via google.

A blade manufacturer has recently stated: "Also, we achieved 8.3 pounds of static thrust per HP using a 33 HP Generac engine driving a 4-blade 72” propeller using a reduction drive. No duct used. This was for a powered parachute flying machine." (a basic chart of actual thrust data shows typical thrust for this size prop at this loading would be 9 lbs T/hp. If he delivered 33 hp to the prop this should yield about 240 lbs thrust so that is OK for a non-military cost but still expensive, airboat prop.

Key ducted prop points, keeping in mind that you are sane and don’t want to go more than double 50 mph are:
- Inlet configuration – should be like a fat upside down airplane wing airfoil shape.
- Prop tip clearance which should be no more than 1% of diameter, preferably less.
- Propeller pitch picked for cruise speed. For less noise a few wide blades with tip speed under about 450 ft/sec.
- A reasonable duct or shroud or whatever you want call it could be from ¼ to ½ the prop dia.
- For higher power 5 fixed stator blades rigidly fixed behind prop gain thrust and could provide something to hang rudders on. For less noise the number should be less than the number of prop blades. Say 2 blades 3 stators. 4 blades 5 stators.

At mini (or maxi) sane airdrive speeds you don’t need to go overboard with streamlining. It gets important as you go above 50 mph. but use some common sense about drag.

There are other refinements worth discussing but these are the main ones that follow the KISS principles. 8-10 lbs thrust/hp seems a reasonable target.

As I suggested in my take on air drive for small boats in, (Mini Tech - Air drives - Small boats.) I do believe this is worth looking into as I for one always try to get the most out of the least. 2429

Would like to know what you guys feel are good engine choices for a moderate 25 to 50 hp range project, preferred hull size and type. Also, what diameter prop seems about right and not too big. and how much thrust is essential.