Efficiency Theory

[El Prosecutor]

I have been reading off-site about comparison of different drive systems, and keep hearing that jet drives are "inefficient".
I would like to hear some facts on the issue from people who aren't jet haters:
What are the inherent efficiency problems with jets?
What has/can be done to improve their efficiency?

[centerhill condor]

I think they mean dollars/hour! a pump is efficient otherwise the F-16 would use a prop. I think they're also concerned with static thrust. A prop is more "efficient" at 0 mph. But when you get relatively low speed the jet catches up quickly. that's what they taught us at rocket scientist school. The new navy attack ships use a jet and most modern nuke subs use a shrouded prop (read jet). Also, for a jet to be really efficient the suction and discharge require variable geometry like the F-16. This is still in the investment capital phase but it does exist.

[u4ea32]

Gas turbine airplanes and ships make sense for many reasons: the chief reasons are (a) very simple, (b) very reliable, (c) very light, and (d) very compact. They do lose on energy efficiency however. In fact, putting a propellor on a jet engine roughly doubles the fuel efficiency.
On a boat, there are some situations where a water jet is more efficient that the alternatives, and some where it is less efficient.
If you've ever driven a jet boat, you already know when it works well: when you are moving at a moderate but clearly planing speed, like 30-40 knots, the jet boat works GREAT. Its obvious because you juice the throttle, and the boat GOES!!!!
When going slow, its obviously very inefficient: you need a LOT of power to mush along at 8 or 10 knots.
When going fast, we all know its inefficient too because the outdrive and prop boats eventually catch up and go flying by.
We also know that jet boats are much more efficient for acceleration.
When the system is efficient, its fuel efficient. When its wasting HP, its also wasting gasoline.
Fast ferries typically run 30 to 50 knots, and they ALL use water jets. They really are more fuel efficient in this range: less drag because there is no shaft, strut, or outdrive in the water.
When you start going faster, water jet pumps become less and less efficient because they work by moving water faster and faster, not just by moving more water. The pump picks up the water and has to accelerate it out the nozzle. I won't give you the physics equation, but the horsepower required to double the acceleration of the water (e.g., to double the speed of the boat) increases by 4, not just two.
On the other hand, propellors become more and more efficient the faster you go.
Props are wings, and the lift of a wing increases exponentially as the speed goes up: That's why jet planes really work -- they simply go faster, so the same wing can lift a LOT more.
Double the RPMs (double the speed of the prop) and the lift of the prop (the force pushing the boat forward) goes up by 4, not just by two. That's why you can trim the drive up, jack the outdrive, or ride the prop at speed -- you hardly need any prop in the water at speed. The same prop gets more efficient, eventually making up for all the drag of the shaft, strut, and/or outdrive. And trimming the drive or riding the prop gets rid of that parasitic drag too.

[Wicked Performance Boats]

Cold beer " gets rid of that parasitic drag too." IMHO Budlight

[lil man wayne]

This aint aero space science so zip it there!
Jets suck then they blow and aslow they go.Hell even a small block and an i/o can take out a big block jet boat with 200 less hp.
Prop is thee most effecient way to go.Just cant go in shallow water like a jet can go so a jet does have at least one advantage

[probablecause]

If you are going to own a jet boat, you should not be worried about MPG. Just hammer and enjoy the big block.

[SmokinLowriderSS]

This aint aero space science so zip it there!
Jets suck then they blow and aslow they go.Hell even a small block and an i/o can take out a big block jet boat with 200 less hp.
Prop is thee most effecient way to go.Just cant go in shallow water like a jet can go so a jet does have at least one advantage
Ya know, :idea: I keep hearing this ...... and something definitely similar about strap-on props outrunning jets, but every time one of 'em has tried in my area (last weekend was the latest, and a 16' V-drive in Okla fell over the holliday), they keep watching me go away, AHEAD of them ...... strange. :argue:
Last time a guy running a boat shop told me I could go faster with my 300 HP on a prop (i was buying a tach), and I told him that didn't explain all the 300HP V-6 wackers dad & I had never lost to, he rocked back on his heels and looked as if I had slapped him. :cry:
I won't mention the 502 Baja every weekend last July/August.
:D

[El Prosecutor]

If you are going to own a jet boat, you should not be worried about MPG. Just hammer and enjoy the big block.
For clarification, I agree totally. My original question was about HP to performance efficiency, not fuel economy.

[Aluminum Squirt]

Props don't seem to be very efficient where I boat :boxed: . They usually end up burried in a sand or gravel bar with most of the outdrive still connected to it and the boat a few yards away. The yardage is usually directly proprtional to the velocity at which previosly mentioned outdrive/outboard/v-Drive connected with the sand bar/gravel bar/rock/stump/floating tree........yielding exactly 0% efficiency when compared to a jet boat. Of course previously mentioned prop boat may be traveling down river with the current, producing some amount of MPH on the GPS but since it is not under its own power, this number is assumed to be 0 MPH when using this formula. The above formula may be used with any amount of horsepower in either boat, the results will be the same-Aluminum Squirt

[mrossum]

Props don't seem to be very efficient where I boat :boxed: . They usually end up burried in a sand or gravel bar with most of the outdrive still connected to it and the boat a few yards away. The yardage is usually directly proprtional to the velocity at which previosly mentioned outdrive/outboard/v-Drive connected with the sand bar/gravel bar/rock/stump/floating tree........yielding exactly 0% efficiency when compared to a jet boat. Of course previously mentioned prop boat may be traveling down river with the current, producing some amount of MPH on the GPS but since it is not under its own power, this number is assumed to be 0 MPH when using this formula. The above formula may be used with any amount of horsepower in either boat, the results will be the same-Aluminum Squirt
that is my kinda logic. wheres my beer?? :cool:

[El Prosecutor]

Same hull, same engine, V-Drive vs. Jet. Who wins on a deep glassy lake in:
1. 100 yds.
2. 1/4 mile.
3. Top speed
and WHY?
What size outboard on the same hull would it take to be competitive with the above?

[rrrr]

LMAO, I didn't know F-16s could be compared to jet boats.....
A gas turbine and a pump powered by a gasoline engine aren't even related.