Nozzles vs. Open Wheels

Hello everyone,

I am sure that there is a very logical answer for my question, and I hope I can find it here. After having worked on towing vessels with both open wheels and kort (not actually sure of the type) nozzles, I don't understand why more new builds don't take advantage of the increase in bollard pull and efficiency they would get with nozzles.

My initial thoughts were that companies avoided nozzles because of the increased cost to build coupled with an increased likelihood of propeller fouling. The advantages gained would of course be the increased fuel economy and bollard pull.

Because I don't understand the costs of going with nozzles vs open wheels, I don't know if a simple cost vs benefit analysis logically leads to the choice of open wheels, or if there is an irrational fear of change that is holding companies back. For the purposes of this question assume that the vessel are Canal Sized tugs between 1200 and 2000 BHP.

Thank you,
Phil

 

Welcome to the forum Phil.

What do the guys in the industry that you worked with think is the reason?

I am a bit surprised and can think cost both in manufacture and maintenance could be reasons. The only other thought I have is do they spend an equal amount of time in transit with no tow where there is little to no benefit gained.

-Tom

 

The most frequently cited reason is propeller fouling risk and severity increases, but I think that is more anecdote than sound research.

Even when traveling light boat wouldn't there be an efficiency increase with nozzles? They also charter the boats at a higher rate after installing nozzles so include that in your assessment.

 

phil brady-

Nozzles do not increase efficiency in the usual sense. One way to look at this is that a nozzle can have about the same mechanical efficiency as an open prop the same diameter as the maximum diameter of the outside of the nozzle. Thats for a perfectly crafted nozzle and does not take skin drag into effect. Often, when a nozzle is selected for a tug, it is because the nozzle is cheaper than a pod; and a straight shaft arrangement is considered a no-go for side thust or some other reason.

Also, nozzles don't seem to scale in a straight forward manner. For canals with fixed speed limits, they might be more attractive on bigger boats and not so much on smaller ones.

 

Speed limit may be just the answer, but you'd think the guys he works with would know this...

-Tom

 

Nozzles are real headbangers. You can nondimensionalize a particular prop and scale it pretty reliably over a huge range of powers. But choosing the type of setup is a different story. The crossover points between various applications just don't track the same nondimensional quantities.

It's a common trap to forget this and scale one application across a crossover point.
It happens in all sorts of industries.

 

I was thinking fuel efficiency and not mechanical efficiency. I was thinking that a less horsepower engine with lower fuel use could be used since the nozzles increase the thrust 20-30%.

I think pods are not considered because of the risk of damage when operating in shallow water, but why would side thrust be a major consideration for a vessel operator?

What do you mean when you say that nozzles don't scale in a straightforward manner? Don't put too much emphasis on my use of the phrase "canal class" as it was more a reference to the size of the vessel than the service. Although, when operating in the ICW, operating speed is frequently less than maximum.

Please be patient with me. I am obviously not a naval architect and have only a basic understanding of propellers and nozzles.

 

They only work (are of benefit) in a small speed range. Most boats operate at a higher speed. The speed of 8-12 knots is typical, which is good for towing but not for general purpose boating. On most boats it is just best to get the biggest prop you can.

 

I am not an expert in this, but I think that the nozzle creates a lot of drag, and at "higher speeds", the drag of the nozzle overcomes the added thrust.

 

I am concerned with towing vessels operating between 4-6 knots most of the time with a maximum speed of 9 knots.

 

Then they make sense for you. There is a couple of companies that make them for towboat companies. They sell for about $800+. They are not cheap, and it is important that they fit properly and the prop is designed to work with it.

 

Here's a pic of a rice nozzle set up(incomplete in pic) http://www.boatdesign.net/gallery/showphoto.php/photo/11472/ppuser/12438 that we put into a motorized barge, some needs to be met were- lower draft - smaller gap to hull bottom- increased bollard pull for light towage- "better" close quarter maneuvering with fwd / rev x 2 engines. The main requirement was the draft as this vessel is for delivering excavators & concrete trucks etc to waterfront sites along with associated delivery support of waterfront construction/ships stores/line handling etc etc etc, the owner wants the biggest "bang for buck" to enter these markets with a versatile platform that incorporates positioning winches, drill tube, spud shutes, knuckle boom crane, bow ramps, push knees. Jeff.

 

The nozzle is only effective (when gain in thrust is more than loss from additional garbage in the water) at relatively slow speeds and for highly loaded props, i.e. for props that have to develop large amounts of thrust per unit of disc area.
That is because they are mostly used for for tugs (high thrust is obvious priority), river boats (drastic draught limit mean only tiny propellers are possible).

 

Worth remembering that nozzles are like vacuum cleaners and draw in any bit of potential fouling material available. I've only experience of a very small river boat nozzle, but it was hopeless, as it continually got fouled with weed and plastic bags etc. An open prop tends to throw a lot of this stuff outward and doesn't have the same high suction effect on the inlet side.

May not be a big issue for big ships, but it was a show stopper for me. After just one test trip I decided it was a hopeless option, despite the shallow draught advantage the partially tunneled nozzle gave.

 

That looks interesting. Do you have more pictures of the vessel to satisfy my curiosity unrelated to the op?

On topic how did the vessel do when it began operations? Were the owners happy with the nozzles? Did they experience fouling in excess of their gains in power?