waterjet underwater

While searching the forums I couldn't find an appropriate answer, so I had to start a thread. If there is a thread I should reference, please post the link.

I am curious of the efficiency and effectiveness of a pump which remains immersed at all speeds.

Ill lay some simple ground rules for the sake of discussion:

Let us say the pump is attached to 20-50 hp
Place the pump in a bulb, with the nozzle fitted to the trailing edge of the bulb.
The nozzle would be fixed, and shrouded so that no gaps between outside waterflow and nozzle discharge exist.
The discharge would be directed by traditional rudder.

Is this physically possible as a propulsion method?
I would like to operate in semi-displacement speeds not to exceed 25kts.
Id like to be in a craft no more than 25' LOA, with a beam in the 6' - 7' range.

Any information is greatly appreciated.

E

 

A boat that is 25ft LOA will be planing at 25kts. It would need to be very light to do this speed with 50HP.

The more wetted surfaces you put under the water the more drag you create. Even if it is nicely faired it will have significant drag at 25kts.

A high velocity jet is not as efficient as a propeller designed for the application. A good propeller will achieve efficiencies as high as 90%. A typical outboard prop is around 70%. A good jet is similar.

You will see on the following link that a major advance with jet development came in 1954 to get the nozzle above the waterline to reduce this component of drag:
http://www.hamiltonjet.co.nz/hamiltonjet_waterjet/waterjet_history

You get some idea of the physics if you look through the development of the Hamilton Jet. This started life as a pump. The important factor from efficiency perspective is to minimise the changes in velocity (most importantly speed - changes in direction are OK providing they are streamline and not tubulent). You aim to accelerate the water flow in the most streamline fashion.

Kort nozzles are similar to what you are describing only with the prop being akin to an axial flow impeller. These are typically used in tugboat or trawling applications where high thrust is required at low speed. Also good steering at low speed.

Rick W.

 

Excellent info Rick, thank you.

Ideally this will be a semi-planer, which would keep any pumping appendage underwater in the aft sections. I guess something akin to some of the box keeled or displacement glider types. A kort nozzle is a consideration, its just the function of attaching an inboard engine to it efficiently for this horsepower application that made me curious of the pump usage.

E

 

REFOLO ASD

Length hull > 6 m
Breadth at transom > 2.4 m

Engine > 1 x 25 hp
Speed > + 25 knots

At this speed the boat is in planing condition.....

 

How can I learn more about Refolo?

E

 

Estags this may or may not be of interest to you but hear we go. Rick gave you the basic outline for water jet application relative to planning and or displacement applications. Hamilton's 212 is a excellent choice for planning hulls as is Doen water jets. This hasn't been done as far as I know but the most efficient method of flow would be a straight line from the intake to the impeller. I have often thought that in a displacement hull the shallow water capabilities would be out standing having the intake just above the keel line. You could still use a conventional and tried bowl but the intake would be forward of a midship. From there the intake would funnel water channeling it down a tube {straight} to the impeller that would not necessarily be off the transom. At mid ship on the hull there would be a tunnel for the bowl. The steering could still utilize a nozzle with a reverse bucket. The hull aft of the tunnel would have channels that would direct the flow carved out in the hull fanning from port to starboard for directional control. There also could be a ruder system but also on hull bottom perhaps incorporated into the channels aft of the tunnel. The channels could be incorporated as strakes for stability. I know this sounds complicated but if you can imagine it as i can better than I can write it its a very simple design. On a jet plain the jet engines are mounted so the flow of air is directed straight to the impeller. That is the basis and most efficient method for volume to feed the impellers. This also is the most inefficient part of any water jet! The curve the water takes to the impeller from the intake. Air is introduced lessening the efficiancy. A straight line of travel to the impeller from the intake not only would make the jet more efficiant but aid in sea keeping in heavy sea conditions due to the more forward entry and exit of the propultion. The spacific area of placment would be at the candelever balance point of the hull. At that placement there is the very least amout of movement of the hull in any conditions. There for maxamizing the control and propultion of the vessel. I dont know how to put this into word very well I hope I made sence.:

 

To a point you did. I can see certain efficiencies in the idea, and it follows some of the things Id like to accomplish. So much of the effectiveness of aircraft engines is completely lost on moving water, id like to apply some of it in conceptual form. My target was actually a pair of inlets which would give essentially similar results to your straight pipe, which will receive zero surface effect, and hopefully reduce aeration to the pump intake and impeller.

If you have any drawings (or can draw them and have someone scan them as an image, itd be interesting to see.

Thanks!

E

 

Regarding the positioning of the waterjet nozzle, as mentioned in the original post.
To understand why most waterjets have the nozzle above the on-plane waterline, let's take a look at where the thrust of a waterjet comes from.
Ignoring for the moment the internal components, a waterjet is a classic example of Newton's laws at their simplest - for every force, there is an equal and opposite reaction force. Force is the product of mass times acceleration, or put another way, the rate of change of momentum.
To apply a forward force on the boat, the waterjet accelerates a large mass of water in the opposite direction. In early prototypes, with the output nozzle underwater, this posed a problem. Water is dense and incompressible, so the outlet stream quickly encountered a lot of back pressure from the surrounding water. It's very hard to get a high flow rate at a high speed with the surrounding water resisting the output stream (momentum = mass * velocity). Thus it quickly becomes very difficult to give the water leaving the jet a significantly greater momentum than the water around it.
When we move the outlet above the water, this picture changes. Air is compressible and about 600 times less dense than water, and so presents very little back pressure. Now it's not nearly so difficult to get the outlet flow to have both a high volume and a high speed, thus the change in momentum of the water through the pump is higher, thus the force is higher.
In terms of optimizing the things, let's visualize two boats of different sizes and speeds but requiring the same thrust force to cruise at their design speed. The light, fast boat will benefit from a high jet velocity, which necessitates a small nozzle and thus a reduced flow rate. The heavy, slow boat will be more efficient with a higher flow rate at a lower velocity, thus it will have a large diameter, slow turning impeller and a wide nozzle.
I've been bugging HamiltonJet periodically about a jetpump for a 10 m boat I'm working on that may or may not be built sometime early next decade. (They're an excellent source of honest, accurate numbers). One thing that their techs pointed out to me, an issue which is easily overlooked, is that when a propulsive coefficient for an inboard or sterndrive is quoted (ie the 70% efficiency claims we see from certain makers), that propulsive coefficient usually applies to the propeller alone in an ideal cavitation tunnel. It does not usually take into account the drag created by the shaft, strut, skeg, fin, gearcase, etc.; including the entire drivetrain in the propulsive coefficient puts the sterndrives and outboards in the 0.5 to 0.65 range. When a propulsive coefficient is provided for a jetdrive, it refers to the complete system, including the losses in the pump and nozzle, and the extra drag (if any) created by the unit. So when HamJet quotes a propulsive coefficient of 0.55 at 30 knots, they have already included the drive losses; when Volvo or Merc quotes 0.7, there's a very good chance they're not including the drag of the exposed driveline components which will bring the overall efficiency of the drive system down to the 0.5-0.65 range.

 

Matt, thats EXACTLY what I was trying to ascertain. Thanks so much!

Off to rethink some ideas, its all about shallow draft today.

Im trying to get 3" of draft in a 15' flat bottom dory, with the combined ability to propel myself forward. Ideally, Id like to be able to get the boat out past the shallows after delivering a rope and having the hp to assist in removing boats from the shallows.

Thanks again!

E

 

Glad I could be of help.
On the idea of a straight-through jet. Are you thinking of something like the configuration below, but in a boat, and with a single jet at the tail/stern? (That's a Dassault Rafale by the way.)
On a semi-planing / semi-displacement hull with a transom, I would expect the transom to not be submerged at cruise speed, thus a jet mounted through-transom would exit above water.
Oh, and Ranchi - You've been raving about how awesome this ASD hull of yours is, like, forever. When are we going to start seeing them at dealers and boatyards at this side of the Atlantic? I know a lot of people who would jump at the chance to buy something like the one you put up a few posts ago.

 

I actually was looking at putting the propulsion in the keel section of a tunnel-stern seabright skiff. Shallow draft, and a way to keep the pump intake out of the mud if she gets aground. That way you still have propulsive effort when the boat hits the mushy stuff and stands a chance and getting herself out of the goo.

E

 

Mat Yes like that! I thought we were talking displacement hulls but the same principle would apply for fast jetboats also. As you mentioned the thrust would just have to be directed threw a smaller exit orifice at the surface. Less hp and torque would be required to achieve 100mph than current American Turbine pumps require on say an Eagle hull because you wouldn't loose so much volume with the intake breaking water as much. Shoes,wedges,loader impellers,over sized bowls,stove pipes, all are designed to correct the angle and retain volume for that type of hull so instead of redesigning the pump to fit that type of hull. Design a hull that would accept a unit with a better path of least resistants that always stays loaded. To make it simple instead of having a droop snoot and a place deverter to trim as your angle changes with the dynamics of the hull in flight, why not have the hole unit angle to achieve this dynamic. In this case your intake portion of the pipe would be near the transom. The propulsion and the power would be connected as one moving part. If a gimbal was used i guess this wouldnt have to be but for simplicity sake we'll say it is. As speed increases and wetted surface does the angle of the unit and engine adjusts accordingly. I cant recall what this was called but Glasstron had a stern drive like that on their CVX hulls. The tunnel to the intake on the hull would be much longer than current designs and chine angles on the hull would sweep upward aft into the angle the would set in at speed. Multie chine to also have a degree of usfulness at lower speeds aswell. I may be a dreamer but I can emagine it. I have drawings and it looks sweet but need to get a hold of a scanner and post the idea I guess. Call it the Dessault Barren