Way to combine turbojet and prop drive

[RatliffFranklin]

A paddlewheel or paddlewheels placed in the exhaust stream of a turbojet engine could drive propellers through spur gears. The amount of horsepower delivered to the props would depend on the size of the paddlewheels. The question is would there be anything to be gained by this? And if so, would it be enough to warrant the effort?

[ted655]

A jet drive is less effeciant than a prop. To use it to drive a prop is a waste of equipment, space, money. Plus, you lose most if not all of the advantages of the jet & end up with a poor prop design. Add in the loss's in performance due to drag, friction, power transfer and you end up with nothing more than the attention such a wildabeast contraption would bring at the dock.
Can it be done? yes. Should it be done? Hey, everybody likes to own "unique".
Send me a picture.

[RatliffFranklin]

Originally Posted by ted655

A jet drive is less effeciant than a prop. To use it to drive a prop is a waste of equipment, space, money. Plus, you lose most if not all of the advantages of the jet & end up with a poor prop design. Add in the loss's in performance due to drag, friction, power transfer and you end up with nothing more than the attention such a wildabeast contraption would bring at the dock.
Can it be done? yes. Should it be done? Hey, everybody likes to own "unique".
Send me a picture.

I didn't mean WATER jet or jet drive.

I said TURBOjet.

Although Top Fuel drag boats have gone much faster, the official record for prop-driven boats is 223 mph set by the gas turbine powered Miss Budweiser.

A gas turbine is simply a jet engine in which 100% of the hot gas stream is converted into shaft horsepower.

The question I posed is could there anything gained in a system where only a portion of the hot gas stream is converted into shaft horsepower and the rest left to generate direct thrust?

[RatliffFranklin]

Originally Posted by ted655

A jet drive is less effeciant than a prop. To use it to drive a prop is a waste of equipment, space, money. Plus, you lose most if not all of the advantages of the jet & end up with a poor prop design. Add in the loss's in performance due to drag, friction, power transfer and you end up with nothing more than the attention such a wildabeast contraption would bring at the dock.
Can it be done? yes. Should it be done? Hey, everybody likes to own "unique".
Send me a picture.

However, I have sometimes wondered if there is any theoretical upper limit to the speeds attainable with conventional boat jet drives.

Let's say for example this hypothetical system of paddlewheels placed in the exhaust stream of a turbojet were in the exhaust of a 3,000 lbs weight 10,000 lbs thrust General Electric J-79. You could potentially be looking at half the shaft horsepower of a small destroyer.

What kind of performance might be possible if this powerplant were in a jet drive boat weighing, for the sake of discussion, only about 7,000 or 8,000 lbs?

[timgoz]

The J79 alone weighs approx. 3,850 lbs. With the other assorted mechanics & marine equipment, it would be hard to get it all in a proper boat at 8,000 lbs.

Take care.

TGoz

[Quicksilver]

Callan racing uses them, fastest offshore boat in the world, search youtube for videos also, theyre out there.

"In 2002 Callan Marine of had purchased the 43' MPYD-designed Tencara and retrofitted twin T55-L7 gas turbines making more than 6,000 horsepower. Running in the Super Boat Unlimited class the Callan Marine proved to be extremely fast when in 2002 it became the first offshore power boat to break 200 mph, although unofficially.

In 2003, Callan Marine took top gun at the Lake of the Ozarks Shootout with a thundering 203 mph run. In 2005 it repeated the feat at over 201mph. "

[ted655]

Opps. sorry. Of course, TURBOjet. Well that's different because I don;t know a thing about jets.
One question?? Would not the thrust at WOT not literly drag the prop through the water like a bad skeg? Except at slow speeds I don;t see how the prop drive could work in conjuntion and be of any help. What would the tip speed of the prop (s)have to be at full throttle?

[RatliffFranklin]

Originally Posted by ted655

Opps. sorry. Of course, TURBOjet. Well that's different because I don;t know a thing about jets.
One question?? Would not the thrust at WOT not literly drag the prop through the water like a bad skeg? Except at slow speeds I don;t see how the prop drive could work in conjuntion and be of any help. What would the tip speed of the prop (s)have to be at full throttle?

Top Fuel hydros accelerate faster than the WSR holding jet boats, so you tell me.

[RatliffFranklin]

Originally Posted by timgoz

The J79 alone weighs approx. 3,850 lbs. With the other assorted mechanics & marine equipment, it would be hard to get it all in a proper boat at 8,000 lbs.

Take care.

TGoz

It's not that hard.

http://www.americanjetcars.com/arfons/art/art61.htm

The J-47 first generation engine in this boat weighed about as much as a J-79.

[granite]

If you put something in the way of the exaust of a turbojet and used it to drive something through a gearbox then that would be a turboshaft engine. as commanly used in helecopters, and turboprop aircraft.

I think you have weight issues with the reduction gearbox in boats when using this sort of system

[ted655]

Originally Posted by RatliffFranklin

Top Fuel hydros accelerate faster than the WSR holding jet boats, so you tell me.

Do they do this by being driven by paddle wheels or directly off a high HP engine?
If I understand, there are 2 sets of fans in a jet. The first turned by the fuel being burned, the second driven by the exhaust of the 1st.. A paddle wheel would be yet a 3rd fan. At some point there has to be a demonishing value.

[RatliffFranklin]

Originally Posted by granite

If you put something in the way of the exaust of a turbojet and used it to drive something through a gearbox then that would be a turboshaft engine. as commanly used in helecopters, and turboprop aircraft.

I think you have weight issues with the reduction gearbox in boats when using this sort of system

Weight can be an issue, but looking at gearbox weight in isolation isn't going to tell you whether or not it's a problem.

I've seen some giant (4,000+ hp) helicopter gas turbines that were made by mounting a power turbine to the back of a Pratt & Whitney J-60 jet (their competitor to the General Electric J-85).

[RatliffFranklin]

Originally Posted by ted655

Do they do this by being driven by paddle wheels or directly off a high HP engine?
If I understand, there are 2 sets of fans in a jet. The first turned by the fuel being burned, the second driven by the exhaust of the 1st.. A paddle wheel would be yet a 3rd fan. At some point there has to be a demonishing value.

A turbojet is an engine in which the first set of fan blades (the compressor) is powered by a second set of fan blades driven by mixing fuel with the compressed air and burning it.

This same arrangement is used in turboshaft engines. In some turboshaft engines, such as the General Electric T-58, there is a fluid rather than direct mechanical connection so it is possible to convert these turboshaft engines into turbojets by taking off the power turbine.

A turbojet serves as the core engine of a turbofan. In a turbofan a third set of fan blades driven by the core engine augments thrust by accelerating a stream of cold air. In high bypass turbofans this stream of cold air accounts for most of the thrust generated.

[granite]

[quote=RatliffFranklin;123978]Weight can be an issue, but looking at gearbox weight in isolation isn't going to tell you whether or not it's a problem.

[quote]

Agree totaly you need to look at the HP at the prop versis the overall package weight for both a turboshaft and a std internal combustion engines. to make your decision.

Typically you can get a large ammount of power from a turboshaft engine for its weight, however it is low torque and high speed which needs converting to be able to do anything usefull.

Another disadvantage of a turboshaft engine is the throttle responce, it takes much longer for the engine to spool up to full power and chop back than a typical internal combustion engine. they need complex fuel control systems to stop flame out and surging.

Salt water is also not good for the compressor blades and requires regular washing.

[RatliffFranklin]

[quote=granite;124126][quote=RatliffFranklin;123978]Weight can be an issue, but looking at gearbox weight in isolation isn't going to tell you whether or not it's a problem.

Agree totaly you need to look at the HP at the prop versis the overall package weight for both a turboshaft and a std internal combustion engines. to make your decision.

Typically you can get a large ammount of power from a turboshaft engine for its weight, however it is low torque and high speed which needs converting to be able to do anything usefull.

Another disadvantage of a turboshaft engine is the throttle responce, it takes much longer for the engine to spool up to full power and chop back than a typical internal combustion engine. they need complex fuel control systems to stop flame out and surging.

Salt water is also not good for the compressor blades and requires regular washing.

When you have a fluid connection between the hot gas generator and the turbine, such as in a General Electric T-58 or a jet engine driving a paddlewheel, you can run the engine at a constant throttle setting and use flaps or waste gates that divert the hot gas stream to vary the power output.

You are correct that gas turbines don't like salt air.