Jet steering nozzle design

Hello all,

I'm wondering if I could get some help, or a push in the right direction on a steering nozzle project I'm starting on.

So background information - the steering nozzle I want to make is for the early Kawasaki stand up jet skis. They have 40hp motors and a tiny little jet pump, but are a crazy amount of fun. Back in the 80's and early 90's a company called Ocean Pro made a cast aluminum steering nozzle to replace the stock plastic nozzle. The difference in control that this nozzle offers is unbelievable. However, people know this and even at 20 something years old they still cost about $200 usd. I have a bit of engineering knowledge (4 year degree in mechanical engineering), a 3d printer, some computer software and too much time on my hands. I figure I'll make my own steering nozzle out of 3d printed ABS. I'd like to comment here, that I'm not interested in directly copying the other steering nozzle, just learning from it. I'd also like to add that in this point in time I'm not worried about my material choice and how it will hold up hanging off a jet pump.


Now for the questions....

Attached to this post is a picture looking down into a stock steering nozzle (bottom) and the aftermarket steering nozzle (top). For now I'll ignore the change in the length of the steering arm - it's an easily understood change, and is also related to other aftermarket parts in the steering system. Snooping around the jet ski forums, all I could find is that the aftermarket nozzle is more of an oval shape, has a divider and has a smaller exit area than stock.

- How do you size the exit area of the steering nozzle versus the exit area of the reduction nozzle?

- Does the divider do anything beside look cool?

- Does the short and wide shape of the exit nozzle help increase steering force?


again, any answers would be greatly appreciated, as well as any reference books or papers for me to learn from!

thanks a bunch!

 

I'm sure beackmo (member) will chime in with his usual extremely in-depth and excellent replies too.

In the mean time, i would suggest you start your background reading here:

"Waterjet Propulsion" RINA Conference 1994, UK London.
"Waterjet Propulsion - Latest developments", RINA Conf, 1998

These two conferences shall have papers with answers that you seek. Just lots of reading...

 

The smaller nozzle with the same mass flow rate could provide a higher thrust as the exit velocity will be higher than with the larger nozzle. There is a bit of a fine balancing act for any given impeller to get the maximum thrust at the boats operating speed with the amount of horsepower available. On some pumps you can actually change the size of nozzle to get small changes in performance.




As the thrust generated is from the change in momentum of between the water coming into the impellor to the water exiting the pump, the two variables are mass flow rate and the change in velocity

So for a given impeller, if you reduce the nozzle size, the velocity goes up but for a given horsepower the amount of mass will go down.

A change in nozzle size in your craft , if the speed of the craft is higher with the new nozzle steering pump, the pump has been further optimized if higher speed is what you are trying to achieve

A limiting factor for changing the nozzle size is the conditions imposed on the impellor due to the nozzle size. Normally the bowl pressure will increase with a lower nozzle size which can then cause the impellor to cavitate which will reduce thrust.

Ie you cannot take a 12 inch impeller designed for say 300 horsepower and reduce the nozzle to a 2 inch diameter and expect the pump to produce more thrust. The impeller would cavitate and you would lose thrust.



Recently on another thread, a contributor pointed out the Intellijet, variable nozzle, variable pitch impeller. At prototype stage, this appears to be a viable, though I suspect, expensive pump for the small recreational jet boat market but for commercial applications it might be a winner. Check out the entire video selection.

I cannot tell from the picture but I assume the center divider has some width going back to front on the nozzle, and will provide another area meaning a larger area for the water coming out of the nozzle to impinge upon to provide a further turning component.

Similar to twin rudders as compared to one

 

Ad Hoc, thank you for the reply! I hate to ask to be spoon fed information, but I wasn't able to find those papers on the RINA website or floating around anywhere on the web. Any advice on where I could track them down? Thank you!

Barry, thank you also for your reply! I'm a little confused though. It was my understanding the the steering nozzle had little effect on thrust. Or possibly I'm not familiar enough with up jet pump systems. On the jet skis, the water goes through a reduction nozzle and is accelerated, and then goes through the steering nozzle. I'd imagine that the steering nozzle wouldn't be able to accelerate the water further since it is open to the air on the back side.

Yes the divider is about 2 inches deep into the steering nozzle. Interesting theory about the added area for the water to be turned from. I wonder if two dividers might be better than one?

thanks again!

 

You have several possible routes:

1) Try your local library, and see if they can get an ILL..not sure about the US, but this is a free service in the UK. (ILL = inter library loan).

2) Go to your nearest University, check out their library...(if they have a Naval Arch course). If they do have a naval arch course, but not the transactions, again, request an ILL.

3) Email RINA and buy them. I have hard copies, but they most probably are pdf these days.

 

I do not refer to the steering outlet as a nozzle for the same reasons as you do as it does not help in accelerating the water. I assumed from the picture that the aftermarket pump nozzle was smaller in diameter than the original and based my comments on that.
As the water is exposed to water to atmosphere immediately after leaving the pump nozzle, it just impinges on either the side of the original nozzle, or on the side of the aftermarket nozzle and the divider.


Basically, the impeller accelerates the water and if the effective diameter of the impeller was the same as the pump nozzle you will still get a thrust. As you reduce the size of the nozzle, the pump static pressure can build with a resulting higher exit velocity.

My understanding, is that the higher static pressure in essence puts more pressure on the downstream side of the impellor to develop more thrust. But for a given impellor, there is only a small range that you can change the nozzle diameter to get an increase in thrust. If you reduce the nozzle size significantly, cavitation will increase and you can lose thrust.

If you plot the engine max hp available at say 500 rpm intervals and the impellor horsepower requirements at 500 rpm intervals, ( ignoring for now, to keep things simple, the inlet speed effects as the boat picks up speed) the engine max hp graph will be above the impellor hp requirements and they will meet when the throttle is full advanced producing less than max hp, and it reaches the impellor hp requirements

Normally, the engine never reaches its max hp, because for that rpm, the pump does not require the max rpm to turn it

By reducing the nozzle size, which increases the pump pressure and therefore increases the horsepower requirements, you are actually able to be using more of the engines available horsepower. Albeit at a higher fuel burn.

Not sure if this is as clear without actually plotting it
But some rough numbers and I do not have a hp graph in front of me

Say your engine produces 100 hp at 5000 rpm and 90 hp at 6000 rpm. Your pump requires 90 hp at 6000 rpm. Then that is where engine stops increasing rpm. Wide open.
Maximum engine horsepower output AT THAT RPM matching the pumps requirement AT THAT RPM

But , depending on your impeller design, perhaps you can nozzle the pump down so that the rpm drops to 5000 rpm, and you are now able to have 100 hp available to it.

Again, the impellors ability to take this increased downstream pressure without cavitating and actually reducing thrust becomes the limiting factor

 

Nozzle divider: Vertical steering fin (rudder) that increases the mass of water that is directed in the desired direction -- the more fins the merrier (up to a limit of 1 or 2, depending upon the nozzle size because more baffles rob more power due to friction losses).
Nozzle shape: Oval shape with its major axis horizontal to compensate for the area reduction caused by the introduction of the steering fin into the stream, in order to maintain (nearly) circular cross sections of water for efficiently directing the steering force.