Help me understand the limitations of a jet pump in a planing hull.

shaka,from memory there were some experiments conducted some years ago ,not using the cooling water but using a separate tapping from a high pressure point and bleeding this into slots in the roof of the inlet. to address the problem of the flow "falling away from here at speed ".I do not know the results of this, perhaps somebody does.

 

Shaka, first of all; there is a variety of intake shapes combined with a variety of hull shapes, so I feel a bit uncertain of what we are actually looking at here. A few pics would help us forwards.

Then, as Joakim says, the propulsion efficiencies we see with the jets here are low, and there should be scope for improvements. The real culprit is the inlet. One big problem here is that a fixed configuration is asked to cover an extremely wide range of hull speeds, while the inlet mass flow is nearly constant (at max rpms). For low speed acceleration the lip area must be in the size of the impeller inlet area, while a fraction, maybe as low as 25 - 30 % would be optimum for high speed. I believe the only relevant and safe solution is a sliding lip section, so that the effective inlet area can be varied (could be in one or two steps, not necessarily stepless).

Now, the "blow off job" suggested earlier may save you from a crash stop due to inlet choking ("overstuffing"?), but it really spoils the inlet efficiency; you have an oversize inlet, acting as a brake, and you throw away static inlet pressure that should be used to force water at high speed through the nozzle. The lost inlet pressure has to be supplied by the pump instead!

Then anthony and speedboat; there have been a number of ideas on both boundary layer suction, bl blowing and bl turbulators (NOT diffusers, a diffuser is a flow channel with increasing area where dynamic pressure is recovered into static pressure). Unfortunately they tend to be effective either in a limited range, or the extra pumping effort is too high for the revenue.

Remember that high speed only comes with high efficiency in a dynamically supported vehicle!

 

I understand Joakim's point of view. Seems the more horsepower we throw at these boats, the more inefficient they get.

Something that you wrote about this pwc having a "fixed configuration" has got me thinking.

This jet pump is not an individual unit that can be unbolted from the hull and modified. The inlet is actually a part of the hull and is made of fiberglass. As such, I cannot change the inlet size per se without changing the hull itself. That is not within the realm of reality for me. However I may be able to install a blade/wing that may serve the same purpose.

Bear with me as I am going to try to paraphrase what you have been trying to help me understand, then I will detail my idea.

In a "perfect world" of jet pumps, the perfect sized inlet directs a fixed amount of water straight toward the center of the impeller with no interference. There is pure laminar flow to the blades traveling at the same velocity as the hull itself. The water passes through the blades. As the blades add energy, the venturi restricts/accellerates the water and you have thrust. Anything more than that creates problems.

In the real world of jet pumps, we have an inlet that is trying to accomidate to many conditions and speeds, therefore we must compromise. In this compromise, we have created some type of turbulent flow which at some point we must invest horsepower to correct. At some point even horsepower cannot get the problem solved and the system fails to process the incoming water......we have this "overstuffing" effect and the rider get ejected.

This turbulent flow from what I understand is localized closest to the roof of the inlet, and the real fix would be to reduce or reshape the roof of the inlet. This "fixed" modification for top speed would kill low speed accelleration.

Is this accurate?

Even though I cannot reshape or reconfigure the inlet, or even reduce the amount of water entering the inlet, there may be a chance that once the water does enter the that I can redirect the flow in the upper part of the inlet duct toward the center of the impeller. Would this strategy at least minimize the problem?

Clearly, you are going to need to see some pictures of what I am up against. I will try to have some posted within the next 2-3 days for your review.

Happy new years to everyone monitoring. I appreciate each of your comments.

It really is great dealing with people who actually know what they are talking about.

 

Thanks for that baekmo,I have looked through my papers to try and find where I had found this but to no avail ,I suppose something would be available commercially if the idea had been viable .In the past I decided that all, I as an amateur designer could do to maximise the efficiency of the inlet ,was to ensure that the entry is correctly sized "fair" and unobstructed and that the lip section provides the correct amount of water over the chosen speed range. Knowing what the problem is and finding a working solution are a million miles apart. But there again "a sliding lip section " now there's an idea !

 

I'm no expert but I have perused some of the raceboat sites on the web. This most likely doesn't effect your intake flow situation that you have identified as the culprit in your face plant problem but one thing the drag racing guys talk about is how different the motors have to be built for the jets. They need to move the peak torque to the right. The limiting factor is torque not horsepower, matching the impeller specs to the torque peak. You probably already know all about this to a much higher degree than myself but others just lurking may not.

 

Oh lord no! Don't start with the Torque/Vs HP thing.

There is so much mis-information about jet drives on the net, it just boggles the mind.

-jim lee

 

Enlighten us Jim,
I'm merely the messenger reporting on what the guys building the motors for the boats on the national circuits say amongst themselves.

I've neither built or raced one of these boats. Please clear up any misinformation that may have been posted either by myself or others.

Their concensus is the load on the impeller goes up with the speed of the impeller. When the load exceeds the torque available from the motor the motor quits turning any faster despite having the ability to make more horsepower. Please correct me where I am wrong, most of us are here to learn and it is sometimes hard to wrap your head around all this.

 

I was the data/computer guy on a drag boat. Tunnel hull, 1,260 HP with a Berkeley jet. We raced on the IHBA Circuit west coast USA. From Northern Calif to Texas. Also the NJBA Circuit being mostly Southern CA. It was a lot of fun. For a short wile, we set the class records and were the ones to beat.

The So Cal boys were the ones to watch, they were the ones to set up the fastest machines. So, we started having our stuff built by them.

What the So Cal boys taught us: First the Torque vs HP issue. HP is torque*RPM. So, its not one or the other. Its horsepower. The question was what's the highest RPM are you comfortable with running your engine? Build your engine to put out max HP at that RPM. Because pumps only run at one RPM, pack all your power at that one RPM. This is your starting point. From there cut your impeller down to make sure your engine gets to its peak HP RPM.

These were mixed flow pumps and the RPM was set by the outside diameter of the impeller. The axial pumps the rest of the world use will most likely be different, I don't know.

This is the first bit. Next is the placement of the pump. You want as much of the pump out the back of the boat as possible. This is because, at speed, Your touching the spoon, the intake of the pump the shoe and the ride plate off the back of the shoe. Any hull behind the intake is just dragging in the water acting as a brake.

The Shoe : Its the smooth transition of the bottom of the hull into the intake of the pump. The shape and height of this is very important. This is the first surface that begins the direction of water into your pump.

The intake : The Al casting that the pump bolts to and where your loader grate is attached. The second piece of equipment that directs water into your pump.

The loader : This is a set of fins located in the gullet/intake of your pump that direct the water, once it has entered the pump, to the correct location of the impeller. High, Low, more or less. No two pump setups need the same thing. There are many different styles of loaders.

The shoe : This bit is the trailing edge of the pump's intake hole. The sizes of shoe controlled the length, fore and aft, of the intake hole. The depth controlled how much water was loaded. The angle of the shoe controls the depth that the transom of the boat would ride at speed.

The impeller : This is the device that accelerates the water through your pump. The leading edge is pretty much responsible for your low speed acceleration. The trailing edge is mostly responsible for your high power transfer ability. Axial flow pumps don't have this trailing edge. I believe this is why they are limited in the amount of power they can transmit.

The bowl : In the bowl is your flow straightening fins. Its important to straighten the flow or you loose thrust. The longer the fins the better.

The nozzle : Contrary to what most seem to believe, smaller nozzles work better at lower speeds. Think about it, at a lower speed you can't get as much water into the pump, so you expend your power building more pressure/velocity with a smaller nozzle. The old fuel jets, no longer legal, use to have nozzle reducer inserts that they would blow out about 1/4 track or so once their speed had built up.

The ride plate : This used to be called the suction plate. This is a plate mounted under the pump from the back edge of the shoe to about the nozzle that the water that didn't make it into the pump would run across. The function of the ride plate is to keep the tail of your machine attached to the surface of the water above about 60 or so mph. Without this, your transom will lift and suddenly, you can't steer.

What we found running with these So Cal boys is that getting speed out of a jetboat is all about the pump and ride setup. If you get this wrong in the smallest way, no matter how much power you through at it, your not going to have a very fast boat.

With the setup dialed in, suddenly you have a jetboat that really works great. The sad thing is that, because its really tough to get them dialed in correctly, many performance boat "professionals" can't do it. People pay them to set up their boats, the results are poor. So they end up spending more money for stuff that also doesn't help. Things spiral. Because of this, many jetboats perform really poorly.

We used a data system that watched, among other things, the pressure at the intake, high impeller, low impeller and bowl. Using this kind of data we could get a pretty good idea what changes to make in the pump to help things. But not completly.

Pump and ride tuning is really a black art. Like the stock market, as soon as anyone thinks they had a system that worked for pump tuning, something would come along that disproved it.

There you go, that's what we found. Hope it helps.

-jim lee

 

Thanks Jim,
Quite informative!

 

It is pretty much a black art with boats of the smaller variety as well. From my perspective, it is more a matter of eliminating those things which are causing problems and allowing the hull, pump and engine to do their job. Unfortunately the problems encountered recently in the never ending quest for speed has been getting people hurt.

Thank you on that point about rpm and HP. I will discuss this with my engne builder. I believe that the exhaust pipe on this 2 stroke has max torque at 7000rpm, yet most guys tune their impellers for 7200 rpm.

A couple of points:

1. How did you adjust your shoe to get the proper angle and at the same time not change the amount of water coming into the inlet? Unless you have different shoe configurations, I cannot figure out how your crew could manipulate this.

2. If I am to believe that "inlet choking or overstuffing" is occuring in the inlet, and specificly because of what may be happening in the roof of the inlet as suggested by Baeckmo, and that I should measure the pressure changes in this area when the boat is increasing in speed, as suggested by Speedboats, tell me in your opinon what you would expect for me to see?

Would I just see a linear pressure increase as speed increases? Is their anything that you have seen in your work that everytime you encountered this data, that you knew a problem was on the horizon?

What would I be looking for that would tell me that I am headed for trouble?

Perhaps a flat graph in a particular speed range implies efficiency, or a spike in pressure suggest changing impeller pitch..... this sort of information.

Jim what I am asking here is how can I apply your unique brand of knowlege on this subject to help us figure out how to identify and correct this problem?

When I do this pressure survey, I will be using low tech equipment. Are there any inexpensive electronics out there that I could buy to help me record the data?

Thanks

 

Alright..

What I picked up was for a mixed flow pump on a tunnel hull. Pretty much I think this would work for a shallow V as well. You are dealing with a different animal. Isn't yours a PWC with an axial flow pump? I've no idea if any of this would transfer across.

Secondly, I wasn't the brains behind the team. I was just the guy that understood the computer & data system. I could get a great batch of data, but I was not by any means the guy that interpreted it. The brains were Jeff Bennett, who I believe designed our boat, The Eliminator Daytona 19, as well as the actual originator of the loader grate. And Jack McClure who is -the- guy when it comes to mixed flow pumps and how to set them up.

Jack is the guy you want to talk to

http://home.pacbell.net/jmcclure/

He's a great guy! And really knows his stuff. Jeff would be a wonderful resource as well, but I've lost touch with Jeff.

Anyway, Good luck!

P.S. Dynos give two curves Torque and HP. The So Cal boys taught us to ignore the torque curve and build your pump to run at the peak of your engine's HP curve. Building for the peak torque sounds great in theory to the non engineer types, but produces a far slower machine in practice.

-jim lee

 

Well that was´nt sufficient!

You did not listen to the one knowing what he is talking! You repeated drivel!


Let Baeckmo tell you what is up with pumps! The rest is assumptions.

 

Jim,
Your post was quite informative and you certainly have the time in on the circuit. The more I think about your answer though the more I think we are talking about the same phenomenon.

"The question was what's the highest RPM are you comfortable with running your engine? Build your engine to put out max HP at that RPM. Because pumps only run at one RPM, pack all your power at that one RPM. This is your starting point. From there cut your impeller down to make sure your engine gets to its peak HP RPM."

So you are in essence tuning the impeller to a lower torque requirement to allow the motor to spin to your maximum designed rpm. Which doesn't really seem so far away from what I had written.

"The limiting factor is torque not horsepower, matching the impeller specs to the torque peak. "

Where I seem to be mistaken is instead of matching the impeller specs to the torque peak they were lowering the impeller specs to the amount of torque available at the top rpm desired. I can see the difference but I'm not sure I would charactorize it as mind boggling misinformation.

 

Naw, I replied poorly. The mind boggling thing is all the different opinions you hear at the track, on the lakes and in the performance boat forums. Everyone seems to have a different idea how to make a jetboat perform.

What prompted my original comment :

8 to 10 years ago, on the performance boat forums this "Torque vs HP" debate was endless and heated. People in the different camps would type out all sorts of nasty thing about each other. Kinda' like the "Mac26 is it really a sailboat?" or the "What's wrong with Ferro boats?" Threads you see today in the sailing forums.

This is exactly correct. We tuned the impeller to the max HP, NOT the max Torque. The guys that setup their boats like this won races. The ones that followed the max torque curves, didn't.

The shoe.. Here's a pic from Jack's site..



Here' a shoe. If the top and bottom of the plate are parallel, the angle is zero. If the back is thinner than the front. This is a back cut shoe this will let your transom ride lower. If the back is thicker than the front it will let your transom ride higher.

To set the amount of water entering your pump, you insert or remove shoe shaped shims from between the shoe and the intake casting that it mounts to. Thicker shims allowed more water. A shim change was usually on the order of 1/16".

As for your second question, what changes to make for what problems you see? I'm not the guy. Talk to Jack.

-jim lee

 

I would venture to guess that what works on a mixed flow style pump isnt going to transfer very well to an axial pump as they are completely different animals. The pressures involved are on such a totally different scale not to mention impeller design and stators etc