[khaos]

Forgive my boating ignorance please. What makes a V drive all that? From what I have gathered its a motor a gearbox and a prop. The drive shafts connect the motor to the gearbox and the gearbox to the prop?

To the uninformed (me) it seems like an engine/tranny/prop would be the same. I have no agenda here I just don't get it. I am rebuilding a boat and thought that Jets might be an awesome way to go. Of course I dont want to give up speed. I want my cake and all... The boat I'm building is a 36' offshore boat.

I was at the Key West Worlds and I didn't see any jets on those boats that were my size. They all looked like the motor/tranny/outdrive type. Even the turbine powered boats.

Is this a size or weight thing or drag racing only thing?

Again, I have no agenda other than to learn more about this sport I love.

 

[khaos]

Nobody has any input at all. Did I cross some line here?

 

[Beer:30]

Did I read right that you want to run jet drives in a THIRTY SIX FOOT boat? About the biggest boat for jet propulsion is 26 feet. Even that is way inefficient.

The reason V-drive or outdrive or outboard power is better than jet is positive displacement - a prop actually grabbing water and propelling the vessel. A jet is pushing water to push the vessel.

A 26' boat with 450hp and jet would probably top out at 45-48 mph.

The same 26' boat with 450hp and outdrive (or pobably even V-drive) would be a 60-68 mph boat.

Think of it as the props actually applying the HP to the water. The jet is applying the HP to the impeller, which is only pushing water through an orifice. That water (pressure) pushing against more water just slips and never actually "grabs" to propel the boat.

 

[velomax]

No line crossed, just to much info to be put into 1 reply, all of your question have been answered in one way or another, if you search the threads you'll find answers.)THumbsUp P.S some of the cats on this forum are good at posting links to those threads, maybe they'll chime in.

 

[BAJA JA]

If jets are so inefficient ,why do some very large ships use them.

 

[Chop Shop]

If jets are so inefficient ,why do some very large ships use them.

Very LARGE boat moving SLOWLY. Two key words. Large and slow.

 

[gn7]

If jets are so inefficient ,why do some very large ships use them.

large ships don't use jets for propulsion. Steering and manuvering, yes. But not for propulsion.



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[gn7]

Did I read right that you want to run jet drives in a THIRTY SIX FOOT boat? About the biggest boat for jet propulsion is 26 feet. Even that is way inefficient.

The reason V-drive or outdrive or outboard power is better than jet is positive displacement - a prop actually grabbing water and propelling the vessel. A jet is pushing water to push the vessel.

A 26' boat with 450hp and jet would probably top out at 45-48 mph.

The same 26' boat with 450hp and outdrive (or pobably even V-drive) would be a 60-68 mph boat.

Think of it as the props actually applying the HP to the water. The jet is applying the HP to the impeller, which is only pushing water through an orifice. That water (pressure) pushing against more water just slips and never actually "grabs" to propel the boat.

that would depend on the size of the jet wouldn't it.

BTW, worst discription of a jet I have ever read. HP in the water VS HP in the impleller. Really? That's the cause of the jets limitations?



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[SNiC]

...That water (pressure) pushing against more water just slips and never actually "grabs" to propel the boat.

In this thread's scenario, No argument, props would be more efficient than jet drives plain and simple )THumbsUp

But with all due respect, I believe your "explanation" is misleading.... water pushing against water is not the physics behind aquatic jet drive propulsion. If this were true, a jet boat with the place diverter full up would have zero forward motion as all of the water is pushed into the air. Aqua jet drives develop propulsion via Newton's 3rd law: "for every action there is an equal and opposite reaction"

At low speeds, the jet drive is more efficient if the nozzle is exiting at or below the water line as the "lake water" offers only minimal restriction to the low mass and velocity of the water exiting the nozzle and the "lake water" actually aids as a "push board" to the opposing forces. But at high speeds, the "lake water" acts as a restriction to the water exiting the jet thus reducing the mass and velocity inside the jet drive itself. Therefore, at high speeds the jet drive is more efficient when the exit nozzle is raised above the water level.

 

[gn7]

If this were true, a jet boat with the place diverter full up would have zero forward motion as all of the water is pushed into the air.

Really? Directly into the air from the nozzle? Just water shot into the air moves the boat along? Like shooting a fire hose from the top of the transom.
I think you tried to convince me of that once before.
Just water shot directly into the air. :bangmyhead:



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[SNiC]

If this were true, a jet boat with the place diverter full up would have zero forward motion as all of the water is pushed into the air.

Really? Directly into the air from the nozzle? Just water shot into the air moves the boat along? Like shooting a fire hose from the top of the transom.
I think you tried to convince me of that once before.
Just water shot directly into the air. :bangmyhead:

I am saying NO! The water shooting into the air or the water is NOT what is "moving the boat forward", nor is it the the "water shooting against water" is what is moving the boat forward........that is what Beer30 is saying.

However, if you attached a fire hose to the transom facing backwards, and pushed water through that hose at high volumes, YES, the boat would be moved forward....

 

[gn7]

If this were true, a jet boat with the place diverter full up would have zero forward motion as all of the water is pushed into the air.



I am saying NO! The water shooting into the air or the water is NOT what is "moving the boat forward", nor is it the the "water shooting against water" is what is moving the boat forward........that is what Beer30 is saying.

I agree with you! Maybe you should read my post again......

Really? Then what is.



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[SNiC]

Really? Then what is.

Are you thinking of thrust as in a Jet Aircraft turbine?

 

[gn7]

Are you thinking of thrust as in a Jet Aircraft turbine?

It has to push off against something. Its not magic.



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[SNiC]

It has to push off against something. Its not magic.

No its not magic...its physics and where the "pushing" actually takes place.
How does a jet turbine engine propel a rocket in a 100% vacuum environment? Nothing to push against there!

Jet boat:
The water enters through the intake......the engine turns the jet impeller and compresses the water thus pushing the water away from the impeller. As the compressed water pushes away from the impeller it is pushing back against the entire impeller assembly trying to exit and decompress, this pushing force is in the opposite direction of the flow, thus propelling the vessel forward. Newton's 3rd Law!

As I alluded earlier, at low speeds (RPMS) the exiting water thrust is a factor as the volume of water exiting the nozzle is a greater force than the opposing force applied against the impeller assembly, thus a reverse gate which does direct the thrust of the exiting water flow and is effective and works well.

A jet turbine (rocket) works in a similar fashion with the exception the jet turbine ignites fuel gases which expands and compresses and exits the jet nozzle at high volumes and force. It is the ignited compressed gases exiting and pushing against the turbine in an opposite direction as it exits is the actual force propelling the rocket in the opposite direction. Rockets work better in a vacuum - in fact, according to NASA, they develop about 10% more thrust in a vacuum than in the atmosphere. So how can this be? One of the great fallacies about rocket propulsion gasses is that it needs to have something to "push" against to make it go. In fact, atmospheric air partially chokes off or slows down the gasses coming out of the rocket nozzle which, in turn, reduces thrust.. Same physics applies to a jet boat except it utilizes water, not air and gasses...this is why a jet boat performs best with the nozzle aimed slightly above or out of the water.
Rockets and Jet Boats are perfect demonstrations of Newton's Third Law.

 

[aussiesmokinjoe]

that would depend on the size of the jet wouldn't it.

BTW, worst discription of a jet I have ever read. HP in the water VS HP in the impleller. Really? That's the cause of the jets limitations?

Sorry off the topic guys. But I have seen these jets when I picked up my Hamilton pump. I have climbed in the nozzle lol. They are huge!!!

 

[SNiC]

Sorry off the topic guys. But I have seen these jets when I picked up my Hamilton pump. I have climbed in the nozzle lol. They are huge!!!

Those are unreal! What do those monsters go on!

 

[gn7]

Those are unreal! What do those monsters go on!

They go on boats. But those type move the boat by pushing against the water in the lake, not like the kind you run.

No its not magic...its physics and where the "pushing" actually takes place.
How does a jet turbine engine propel a rocket in a 100% vacuum environment? Nothing to push against there!

Jet boat:
The water enters through the intake......the engine turns the jet impeller and compresses the water thus pushing the water away from the impeller. As the compressed water pushes away from the impeller it is pushing back against the entire impeller assembly trying to exit and decompress
.

Your the one compressing and decompressing water and I'm the one that doesn't understand physics:happy:



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[aussiesmokinjoe]

Those are unreal! What do those monsters go on!

I have always seen them set up as twins on very large vessels "obviously" lol. There are some clips on you tube too will big ships punching out wide twin rooster tails with these jets.
Also the people mover jet cats on Sydney harbour are running large twin axial single stage pumps.

 

[aussiesmokinjoe]

https://www.youtube.com/watch?v=dKs3jR-Rk1E&feature=youtube_gdata_player