Inlet opening with protective screen

[kach22i]

One of the safety attractions of waterjets is they will not chop up manatees and swimmers like a prop will, right?

Would adding an inlet screen to strain out floating water bottles and seaweed do more to choke off water supply to the water pump/jet drive than not?

I seem to recall on some forward facing inlets that vanes in one direction or another may be present, not a real grid (eggcrate style) though. I assume that with these types of inlets hitting a submerged log would pretty much be a disaster for the drive system.

My interest is in the flush type inlets as seen below which rely on "boundary layer" effects to draw water into the feed tube supply. Is there any kind of protective cleanable screen you can put in the inlet - flush with the hull surface which will not totally distrupt the "boundary layer" effect?

http://www.amc.edu.au/areas/cavitation/projects/

Flow problems, with flush type inlet ducts, relate to ingestion of the hull boundary layer and the complex viscous flow within the inlet streamtube and duct.

http://www.khi.co.jp/tech/ne135g06.htm

The flow field around the inlet of a water-jet propulsor is complex three-
dimensional branch flow with both internal and external flows.There are few examp-
les of computations which have investigated the hydrodynamic characteristics of
such flow fields and it is also very difficult to measure the flow fields in
detail.

[kach22i]

I just found this:

http://www.mercurymarine.com/jet_drive_faqs#1093

What is the shallowest water I can drive my jet boat in?

Because the jet pump will suck loose objects, including rocks and sticks, from the bottom, we recommend maintaining at least 3 feet clearance between the boat bottom and the bottom of the lake or river.

How do I keep the weeds out of the inlet grate?

1997 and newer models can use the Hydro Surge gate with auto weed clean-out system. For older models there is a weed rake.

I'm still looking for pictures and or drawings.

[RAWRF]

It's the "cleanable" part that is the problem, which is the only advantage that outboard jets have over inboard jets. I put stainless aerator screen inside my intake, but I don't ever run through weeds. Inboard jet assys are so difficult to take off for cleaning that it would be a serious overhaul to do so. Someone would have to redesign the whole intake system specifically with cleaning in mind. As far as sucking up rocks goes, I run mine in 6 inches of water and only suck up sand, because of the screen. Also, I didn't notice any power loss with a screen, but there would be with something like punch plate or with an intake grate like the Mokai.

[RAWRF]

Here's a pic of a typical jet assy, it's off the unit I put on my canoe (a very large canoe). The intake grates are pretty heavy duty actually, especially on regular boat models like the Berkley and Hamiltons, logs are not a problem because the intake is flush with the bottom of the boat. Without a screen, small sticks and rocks can be disastrous to a PWC impeller like this one, but full size units will chew them up with little or no damage.

[kach22i]

Thanks RAWRF, that grille seems to be what I remember on the bow inlet jet drive boat out of an old Jane's high speed craft issue.

I think there is a very good reason that the direction of the grille vanes is parallel with the direction of the craft motion. It would be interesting to find out if this has anything to do with "boundary layer" effects or something else I've not considered.

[Quicksilver]

rawf, what kind of screen do you use?

I'm building a new rc boat, that I intend on running rapids with. I made a loader grate with a pretty tight grate, like 1/8" openings. Hope it's enough

Here's video of my other jet boat, which didnt have a proper grate, during this particular video, it sucked up a rock and grinded my impeller.
http://www.youtube.com/watch?v=G7hnO1qYobs

[RAWRF]

I used stainless steel aerator screen, similar to the kind found in most faucets. You can buy it in different size mesh, I think mine is 1/16".

[RAWRF]

BTW Quicksilver, how big is that RC boat and what kind of engine does it have? Where do you get the engines and components? That seems like an interesting hobby. Did you build the hull yourself?

[RAWRF]

The vanes on the grill are to minimize water disturbance, jets need a smooth flow of water through the intake to maximize power.

[Quicksilver]

well it's 26" long, using a 29mm drive. Has a 700sc motor, running 14cells. Of course I'm not done building it yet, however I've already built a 30" deep vee, with a similar drive on the same motor with 16cells
1cell=1.2v

the jet drive can be found here http://www.jet-drive.de/

the hull is older and not really readily available but it is abs plastic.

The motor is from here.

http://www.offshoreelectrics.com/MainPage.htm

All the electronics are water cooled as well as the motor and its brushes.

Cost to build the same thing.
maybe roughly $500 if you have nothing and want the best, for a similar setup.

here's some video's of the previous boat, they go in order as the boat was upgraded.

http://www.youtube.com/Electi0neering

The new boat will have some upgrades, like carbon fiber armored hull, the loader grate, the better drive, and a variable nozzle. That last one is in development but seems very doable.

I'll try out some screen and see what does better

[kach22i]

Originally Posted by RAWRF

The vanes on the grill are to minimize water disturbance, jets need a smooth flow of water through the intake to maximize power........................I used stainless steel aerator screen, similar to the kind found in most faucets. You can buy it in different size mesh, I think mine is 1/16".

1. An aerator adds air to the water stream which is just a different kind of disturbance, right?

2. A screen will do different work than vertical vanes, what is each doing and why.

3. Where does the "boundary layer" of the inlet come into any of this?

4. I'm not trying to do what everyone else does, I'm trying to understand what they are doing. I am NOT an engineer, so keeping this in simple terms will help, thanks.

[RAWRF]

The aerator in a faucet is the whole unit, I don't think the screen itself adds air, it serves to work as a rough filter for keeping solids out of the little plastic piece. You are right tho, the screen must disturb the water a little. I was just saying that I didn't notice an appreciable difference in power by adding a screen. I think a screen flush with the hull surface would cause problems though because it would disrupt the water more than when it is inside the suction tube even slightly. I noticed this on the screen I put over the foot valve on my water pump for dredging, that if the foot valve with the screen was being moved because of fast water movement, i.e. in rapids, there was an appreciable loss in power at the suction end of the setup. This can partially be explained by the foot valve being dragged backwards with the hose possibly causing a drafting effect on the water flow.

[kach22i]

Originally Posted by RAWRF

The aerator in a faucet ............., the screen must disturb the water a little.

Sorry if I edited what you wrote to adbruptly, but I see something.

Aerator adds air.......................similar to a surface piercing propeller?

Are the goals different but the results the same (more speed)?

[Quicksilver]

A surface piercing propeller works by reducing drag by the blades(one in the water at a time) and allowing a much higher pitch prop unlike a subsurface drive. You dont really want aeration, but it reduces drag in surface drives. A jet drive runs on a different principle and requires the water to be aerated as little as possible. Water is not compressible, but air is. You add air to a jet drive and you lose thrust. Aeration at high speeds is inevitable, as the water gets ripped apart physically and forms bubbles. This is the jet drives limitation, which is where variable inlets and nozzles and variable pitch impellers come into play. All to reduce this effect.

To put it simply, a jet driven boat can only go as fast as the jet can produce more velocity than the vessel. Meaning you stop accelerating when the jet cannot translate the incoming water into a faster velocity than it is entering the drive. This is where bubbles go and screw everything up and limit jet driven boat's top speed.

Vertical vanes will produce less bubbles and less drag then a mesh screen, but it wont protect the impeller as much as a screen. It depends on what your planning on doing, running in 3ft of water or 3inches? that kind of thing.

The aerator he speaks of and correct me if I'm wrong is simply for the reason I made a grate, to protect the impeller from debris. It reduces performance, but certainly improves reliability.

Hopefully this makes sense, coincidently I'm also not an engineer or a naval Architect, but I love learning about both. I was originally a marine engineer major in college, than architectural, than went for teaching and ended up a biologist, go figure. So I know very little about everything

rawrf- that makes sense about the drafting effect, which is why I hadnt seriously considered an actual screen, by putting it in the duct the water in is still forced into the drive, decreasing the effect of drafting. Good idea

[alaskamokaiman]

I own 3 Mokai's and have used different grates some that let larger rocks in that damaged the impeller.
The slots in the grate should be small enough that what comes through is expelled out with minimal damage to the impeller.
I am no expert but I think that if the slots in the grate are cut with the beam that it would cause vortexes in the water making the water rough for the jet thus a loss of power.
I would think that a screen with 1/16 inch holes would clog with debris (plant or gravel) quickly you would want the small stuff to be blown through and not collect. As thins collect the volume of water is restricted and again loss of power.

You could have a stomp grate made for your unit that makes easy work out of cleaning an inboard jet. Silver Streak Boats of Alaska makes stomp grates you can Google their web page to see one if you do not know what it is.

Cheers