Custom jet

I am working a a project that would include a custom jet drive system. I know how a jet drive works, I just need a few details to bring it all together.

I i plan to use a 1250gpm pump fired off by a 250hp diesel. Would that be big enough to sufficiently move a ~2000lb boat down the water. And about what nozzle size would I need? I was the thinking 3.5" to 4". Pretty much all aspects of the drive system will be custom fabricated.

Any help is appreciated. thanks

Fred

 

Buy a jet, there are many good ones around.

 

:/ that wasn't the answer I was looking for. I know its not going to be the easiest job I've done. I'm a welder/fabricator and mechanic, I don't have a problem building what needs to be built. I'm just looking for some guidelines to go by as far as displacement/power to weight requirements.

 

Can be done, but you will probably not even get close to the performance of a produced jet.

However, I have seen some custom builds for smaller craft. They used the shaft, impeller, stator and nozzle from a PWC. All they did was custom build the intake.

I would suggest studying the specification of some of the water jet manufacturers like Hamilton. Here is a link to the brochure for the HJ212 (a very popular jet for planning boats) I think it lists the nozzle at 4.3"
http://hamiltonjet.com/links/HJ212Brochure.pdf

 

Hmm, you mean water in here and out there? It's a bit more complicated than that. This idea of using an existing pump is popping up regularly here, but it does not work. Let's leave your unrealistic weight estimate uncommented right now and focus on the pumping issues. You may gain some insight by reading the thread "Axial vs mixed flow pumps."

First you have to understand that the hydraulic power produced by the pump is the product of pressure times flow. A pump for 250 hp (=184 kW) and a flow of 1250 usgpm (=0.079 m3/sec) will be a radial pump with a low specific speed (ns=0.3). It will have a maximum pump efficiency of 70 %.

It will give a pressure of: p=184000*0.70/0.079 (=1630380 N/m2, or 16.6 bar). Neglecting piping losses from inlet to pump and from pump to nozzle, this will give a jet velocity through the nozzle at zero boat speed ("bollard pull") of 56.5 m/s.

The resulting thrust at bollard pull is now jet velocity times mass flow; 56.5*0.079*1000 (=4464 N).

Now in order to get an idea of the thrust versus speed we need to check available thrust at, say 10 m/s boat speed (roughly 20 knots). This time there is a pressure recovery from the advance velocity if your inlet is reasonably shaped. The total pressure is increased to about 1660380 Pa (16.9 bar), giving a jet velocity of 57.0 m/s. The thrust is now the difference between jet velocity and inlet velocity (57-9) times mass flow. The inlet velocity is here taken as 0.9 times boat speed because the pump is inhaling the boundary layer. Resulting thrust is 3796 N at 20 knots.

The nozzle has to let 0.079 m3/s through with a velocity of 57 m/s, leading to a diameter of 42 mm, provided it has a smooth, rounded inlet.

So, now you plot the two thrust values in a diagram with thrust over speed, giving roughly a straight line, falling from 4466 N at zero speed, to 3796 N at 20 knots. Next problem is to find the resistance for your hull, complete with engine, pump, water, fuel, crew and, and, and....... The point where the two curves cross is your operating point.

And here comes the doom: This performance is absolutely crap!!!! A decent, high flow, low pressure jet pump for this power (250 hp) would typically produce a flow of 0.55 m3/s, giving a thrust of about 8800 N at 20 knots and about 11000 N at bollard pull. You may put it this way: with a suitable pump it would take less that 100 hp to get the performance you get from your 250 hp setup! With the lower weight of the smaller engine, the hull resistance will be lower and the available thrust will give even higher speed.

The lesson: It's all about flow; radial pumps for a given power produce low flow. Axial pumps (including propellers!) produce higher flow and smaller losses; that's what is needed! FORGET RADIAL PUMPS AS PROPULSORS!!!

 

Thanks for the backhanded help with my project baeckmo. All I asked for was a little direction. Next time someone reaches out for a little help, just remember, not all of us are masters of hydrodynamics.