Max engine RPMs for jet drive

All marine inboard engines seem to produce max power and torque between
4,500 to 5,000 RPMs. I understand that this is because props lose efficiency
at anything above this. Does this also hold true for Jet Drives?
An impeller is a form of a propeller?
Thanks much, Stan

 

Jet drives are usually rated to the impeller. This is typically;

Impeller rate (kW)/ (RPM/1000)^3

You just make sure the impeller is below the above value of your engine at the given rpm.

 

The limiting factor for jets in general is impeller inlet cavitation at low to medium boat speeds. It boils down to a critical inlet peripheral speed (in fact, relative inlet velocity at impeller inlet tip). Depending on the shape/losses of the inlet, the duct and the impeller design, you will find the maximum tip speeds in the range from about 35 m/s to some 45 m/s. So, with a known inlet diameter you can get a reasonable idea on the rev range from these figures. The engine "evolution", resulting in higher rpms at max power is problematic for jets, and many of the new marine engines require a gearbox in order to get along with a jet, which is a complicating pity.

Regarding propeller rpms, you very seldom find a 4500 rpm direct drive; you always use a stepdown gearbox. Just as with jets, propellers are not supposed to operate with cavitation (unless specifically designed for it), and there is a limiting tip speed for props as well. A propeller for direct drive would, in most cases have a diameter that is far too small to give reasonable efficiency (=too high area loading).

One of the few areas (apart from pure racing), where you still find direct drives from 5000 rpm engines has been in ski-boats. Here, the acceleration is more important than fuel economy, and a small propeller will allow higher engine revs during the start. The thrust developed during acceleration is a balance between losses in the prop and available shaft power, and in this case a propeller that is "suboptimal" in diameter may give a higher, and more controllable acceleration.

 

Marine engines I've seen develop max torque at somewhat lower rpms and max HP at something higher than max torque rpm. But I think that rpm at max HP is the real issue here.

 

Ad Hoc, baeckmo, and jonr, I want to thank the 3 of you for taking the time to respond
to my question. I really appreciate the effort. It is now time for me to settle on a brand of jet and a model. I'm looking serious at American Turbine. Again, thanks much for the assistance. Stan

 

Rasorinc,
American Turbine has lots of great info on their web site. AT's can turns more RPM's than some other pumps.
Here is a link to the the SD309 impeller selection chart: http://www.americanturbine.com/information/impeller_chart_sd309.htm

 

Unfortunately, some of the jet salesmen are not aware of the cavitation limits of the equipment they market. I have been called in by a number of clients to solve "bad performance problems" caused by lack of understanding lately.

You are welcome to discuss your application here before the brown substance hits the fan....... It's far easier to avoid jet problems than to correct them.

 

baeckmo, thank you. The engine I plan on using is a Subaru 6 cylinder boxer motor (opposing) that came out of my 1992 Subaru SVX rated at 230 hp. It has 120,000 miles on it and I plan on doing some overhaul and speed work on it. This is a very strong engine and I could easily run it up to 500 hp. It is fuel injected but no turbo.

Mating this to a jet drive is a little scary for me but I do want a jet for safety reasons
my 3 young grandchildren. If I install ceramic coated exhaust, I will pick up 50 hp. If I put a blower on it I can pick up about a 100 more hp.
\My boat is a planing hull and measures 8'-6" max beam x 30'-6" long. I anticipate at full load it will weigh 5,000 pounds. The V starts out at 9.5* at the transom and the next frame also 9.5* then goes to 14* at frame 3. The next 4 frames are 18.5* with the 4 frames leading to the bow starting out at 26.5* then 30.25*,33.75, and the last frame before the bow is 37*. The bow angle is 40* and the transom tilts back 14*
Frames are spaced 31" oc to 36" oc. Engine will sit just fore of the pump with a very short shaft and weighs about 350 pounds. Most bottom frames are 52" each side. I will probably shoot for 400 hp at a max of 5,000 rpm. SO the question is what pump to mate it to? I like American Turbine as they are US, can ship parts out asap and are priced out reasonable, have always responded to all my questions, and seem like nice people. I need to proceed as I'm reaching a point where I have to design for the pump and install those designs. Thanks for any help or suggestions. Stan

This is the boat (hull) I'm building extended 6'. https://www.boatdesigns.com/products.asp?dept=735

 

Just got a call from American Turbine in answer to a question I sent by e-mail. All the pumps on their website are Mixed flow pumps not Axial. After providing some more info he said their SD-203 is what he would recommend and it is axial flow ( not listed on their website) and he is e-mailing me all the info on it. I will post it when I get it. Here is a link to another jet that I've been following for 4 years. Do not know if they are selling yet to the public. http://intellijetmarine.com/

 

There are very few "hard facts" about performance of the AT jets in the public domaine. You should request a thrust/speed diagramme before anything else.

But I found a flow test comparing AT with Berkeley on a webside, listing flow capacity at varying powers for AT's AA impeller, which gives a hint of what to expect. With the AT impeller selection chart, the corresponding rpms can be found. In this configuration, you would have, for instance, 300 hp at 4050 rpm and a flow of 3722 gpm (=0.235 m3/s). This is less than half of an axial pump flow for the same power.

From this info, we get the pump specific speed (see diagrams in the thread "Axial vs diagonal jets"), and from there the max possible pump efficiency. Now we have the info necessary for a rough calculation of the possible thrust at, say 30 knots (which may be a reasonable speed for your hull with this power).

The result shows that this pump is not suitable for your purpose; the max thrust would be about 5700 N, giving a propulsive efficiency of some 38 %. A suitable jet would have an efficiency of about 52 % (thrust 7600 N) at this speed. The reason for this is that the AT diagonal pump is a high-pressure / low flow unit. The nozzle velocity is close to 40 m/s at 300 hp; the ratio jet velocity/boat velocity is about 2.6, which results in huge losses in the jet wake.

This type of jet is suited for light boats (high power/weight ratio) attaining speeds of about 50 knots, where the high jet velocity is right. At medium speeds, like the example, a correct jet would require 220 hp instead of 300 for the same thrust!

The limiting factor is inlet area, I'd say you should have an inlet diameter of minimum 230 mm in order to get the flow capacity needed for good efficiency. Now with the limits for peripheral speed, you will probably need a gearbox to match impeller rpms with your engine. Example: inlet dia 240 mm, max tip speed 45 m/s gives max rpm =3580 rpm.

It's a good sign of honesty that the AT guy has suggested another pump for your project, I'm interested in what he has to offer in this case. For their diagonal pumps, they can supply an inducer, which increases the cavitation performance. If they have inducers for their axial pump as well, you may get by without down-gearing, but you should prepare your engine to give its top power at the lowest possible speed.

 

baeckmo, I sent you an e-mail with some info on the axial pump American Turbin offers.
Could not figure out a way to post it here. Stan

 

With the hp you have available and size / weight of the vessel you may wish to consider the Scottjet, possibly the Flo-pro as well, but at 2.5 tons it is alot to move. As Am Turbine suggested the SD309 would be way to small, and as Baeckmo pointed out wouldn't offer enough thrust for and significant performance. Most of the guys running '309's in the Pacific NW in boats that size run a dual rig. Am Turbine's SD312 may be okay (10.5" mixed flow impeller), and the 4,500 - 5,000 rpm is on the operating limits for Hamilton Jet's HJ212.

 

I'm taking a look at these jets. British company with US outlet and I will call them tomorrow. They have inlets over 500mm and are axial. But again, this is not an area I have knowledge on. http://www.ultradynamics.com/products/ultrajet-model-range/ Any comments really appreciated.

 

That looks to be a derivitave / in the same class as the HJ212. The HJ212 has a 210mm diameter impeller and Inlet hole of 525mm x 260mm. I would check with Ultrajet what the RPM constraints are for their pump. Swinging a 9.8" axial flow impeller will require a significant amount of torque, of course they may have a fine pitch to enable it. The HJ212 also has a mechanically operated reverse bucket, negating the need for the hydraulic pump, hoses and cooler seen on the UltraJet.

Be interested to hear UltraJet's stand on the RPM and your selected motor.

 

You should definitely look at the Hamilton hj212 or even the hj241 for a 31' boat. Lots of hamiltons sold here. They have several distributors in the US and are well known and used in the pacific NW as Speedboats mentioned. AT does even make a larger 231 axial flow pump, but now your getting in the price range of the hamiltons.