Turbofan

I Hate math , and so prefer easy rules of thumb, ballpark figures.

For light aircraft that fly 50-150 mph a simple rule is 1 hp will produce about 5 lbs of thrust , 7 is theoretically possible , but seldom seen.

On a conventional boat 0 to 30-40 mph or so one HP will make 20 lbs of thrust, higher is again theoretically possible , tho seldom seen.

Sorta makes the argument as to why boats are better pushed with a shaft and wet prop easy to understand.

 

Those numbers are pretty odd.
How did you arrive to them?

 

So is there a rule of thumb on resistance to movement for boats and planes?

 

as an aircraft mechanic I can tell you one good reason not to put a turbine on a slow effect ship. Turbines burn about double the fuel a piston engine does. Almost any aircraft designed with a piston engine that later has a turbine engine added either started with really big fuel tanks, or (usually) they figure extra spaces to add more fuel tanks. On an aircraft they will put up with this for extra altitude ability or even an extra speed advantage, along with the longer life cycle of a turbine (thousands of hours, vs. hundreds for a piston).
For a large slow moving SWATH it doesn't help much of anything to burn almost double the fuel for the same speed. For the amphibious type landing craft the nearly double airflow through the engine would be much harder to filter out all the dust, salt, bugs and junk, remember they don't just cross the water.

It really all comes down to the amount of airflow through the engine, combustion takes place in a small range of fuel to air ratio, a turbine uses more air, and as a result burns more fuel.

 

American tanks use turbines, a lot of military vessel use generators with turbine, chopper use turbine, hovercraft use turbine. I agree no turbo fan, but the point that low speed vehicles can't be propelled with a turbine seams not a good argument.
I agree with the consumption of fuel, but even that can be offset with other advantages.

 

Yes, it is typically a cubic relationship, that is drag (thrust, power) vs velocity (rate of movement).

 

yes, the M1 tank is turbine powered, it is an amazing powerful machine. it's biggest downside is that is a gas hog. Those Turbine engines suck the fuel down at a crazy rate, even sitting still at idle. causing our military to spend a ton of money and man power on getting fuel to the front lines (a big issue in the Iraq war when the front line was flying forward)
You will notice that while Israel buys the M1 they put a diesel in it, thanks to some tricks to make the tank lighter (reactive armor for one) they get a tank that is nearly as fast, with a much longer time between refueling.
Helicopters use turbines specifically for weight savings, the best example of a piston helicopter that changed to a turbine being the Robinson 44. they now manufacture a R-66 with a turbine engine. The R66 holds one more passenger, has a higher useful load, a bigger baggage area (the turbine is about half the size and weight of the piston one). But it also has double the fuel tanks thanks to the extra fuel burn.
However Helicopters are not slow, they are slower than fixed wing aircraft, but still do over 100 kts in the slowest. They also need a ton of extra power to do the special operations, like flogging the air into submission to allow the helicopter to defy gravity. (hovering)
My dad worked for GE in their power generation division. They focused on making aircraft turbine engines into portable generators. a gas turbine cannot compete with a piston engine on the small end of the scale, the turbine does become the more useful option when you get to the point you would need massive amounts of engine displacement, or more than one common sized engine.
The other place a turbine is better is for extremely long duty cycles. A piston engine generator or pipeline pump will need more frequent shut downs for oil changes, there are gas turbines on pipelines that are shut down yearly for maintenance, otherwise they run non stop.
getting back to the original idea, on a swath ship, once you get beyond the power two large diesels could put out a turbine makes some sense.

 

Thank you Rowerwet. You know quite well the turbine world.

 

Two comments come to mind here.

Our local 36 knot cat ferry changed out their turbines for diesels.

Our remote, unmanned Coast Guard stations run small, two cylinder diesels for three years straight.

They have 25 gallon dry sumps.

 

US/Canada border on the 49th parallel?

 

Got ya thinking 'eh OP...