Nozzles vs. Open Wheels

What was the HP of the vessel? What kind of nozzle was this?

This was honestly more of the response I was expecting to hear. How often did the boat foul with open wheels?

 

It was a very low power river boat, with electric power. The nozzle was small, 8" internal diameter, with a machined prop to fit closely inside it with a tip clearance that was around 1/8". Power was less than 1hp maximum, so not at all representative of the big vessels that I think you're interested in. I wanted to explore a high efficiency, shoal draught, system, for use in 18" or so of water on inland waterways.

Fouling with an open prop was occasional (maybe once or twice an hour in some of the more weed-infested stretches), but I could pretty much always clear it by throwing it into full astern for a second or two to chuck the weed off. This trick didn't work with the nozzle, I needed to get under the boat and clear it every time.

The nozzle was a home brew, like a Kort, but moulded into a shallow tunnel in the hull. The boats out of the water and I may be able to take a photo tomorrow.

 

Okay, there's enough stuff now that maybe I can explain the scaling issue that I see as a big part of nozzle applications.

Jeremy had an 8" prop, so maybe running 8 pitch as well. Square props are pretty good efficiency wise. But imagine a much bigger boat that was also trying to run at the same speed. The RPM would be less, but it doesn't usually drop as fast as the diameter increases, so you get flatter and flatter pitch ratio props. This means that the speed of the blade through the water is getting faster with increased size and that means proportionally more friction for the same thrust- ie lower efficiency. Nozzles come into their own when the drag penalty of the duct can be somewhat offset by the friction reduction of running a smaller AND higher pitch prop. They have to be built very heavy. Chopping the top of a crab trap is one thing. Sucking the concrete base through the duct is something else. Many ducts on workboats such as bridge building barges have serious strainers on the front and back. They aren't looking for efficiency though. They can just about plow their way to a bridge pier.

For efficient ducts, you have to get the ratio of the inlet diameter to the prop diameter correct. This determines the pitch ratio of the new prop. Efficient ducts are not much, if any, smaller than free props. A lesser overall diameter basically demands lower efficiency.

 

My prop and duct were optimised (using Javaprop) so the pitch etc was matched to get best efficiency. The thrust and efficiency was impressive for such a small prop, so the nozzle was very definitely working well. I was getting the same thrust with pretty close to the same power as I had been getting with a 12" open prop.

The downside of the nozzle working so well was that the velocity through the nozzle was high, creating a lot of suction at the intake. The biggest issue from fouling was probably that the stuff had nowhere else to go once in the mouth of the nozzle except jam around the prop and the nozzle, whereas with an open prop I'd guess that a lot of the time stuff just gets flung outwards and clear.

 

Jeremy, (or TSpeer or M. Drella),

I was wondering if you had pressure profiles or streamlines for any turbofan jets showing the different stagnation points at takeoff and cruise. The other point that has gotten short shrift here is the use of ducts to extend the cruise speed when the tow conditions have been set. It is an awkward trade off between efficiency and speed extension and would seem to involve some rather sophisticated flow analysis to shape the duct. I'm guessing nearly all the effect is accounted for by the change in the radius of the stagnation point on the duct's leading edge, but I haven't seen anything published on this.