Turbofan

No, it isn't the be all and end all. If you are trying to traverse shallows or swamp, then a fan or other air based system might make sense. But propulsive efficiency is the cornerstone of the propulsion system design, and it explains why water jets, propellors and turbofans are all relatively inefficient at low speeds.

Think of it this way, propulsive efficiency sets the maximum efficiency that you can get from your system, and if that is acceptabe, then you can look at that system in more detail. But if the best you are going to get is a 20% propulsive efficiency, then you need to be thinking about some other way to do what you want to do or you won't have a viable system.

 

Focusing on propulsive efficiency alone is myopic. Gross thrust, power to weight ratio, durability, maintainability, and cruising speed are all factors that when taken together can override propulsive efficiency.

 

http://www.youtube.com/results?sear....2.2.0.62.109.2.2.0...0.0...1ac.1.RApXAK5WB8I

Amphibious/all-terrain capability with a single propulsion system can override simple propulsive efficiency.

 

Yes, but the result is a very specialized and grossly inefficient system. There are far better ways to skin that cat... And OBTW, it is a single propulsion system, efficient in both land and sea modes...

http://www.youtube.com/watch?v=cWKbS99fBH4

Just saying....

 

Since you can't get beyond your myopic focus on efficiency at the expense of everything else (including weight and simplicity), the simple fact is a properly designed airscrew is 85% efficient MINIMUM.

 

It is not a general rule. It will depend on disc loading and airspeed.

 

85% efficiency with airscrews is common and easily attainable, particularly in the speed regime in which marine craft normally operate.

 

http://photos.imageevent.com/qdf_files/aerodynamicsandengines/Max propeller efficiency article.pdf

Airscrews become MORE efficient with INCREASING speed.

However, they produce the HIGHEST thrust where they are the LEAST efficient.

 

You seem to think that my outlook on propulsion is myopic, but you are being blind to the simple facts.

Look at the page you referenced. Down the page they have a graph of propellr effiiciency versus speed. They don't get to 85% efficiency until they are doing 100 kts. I don't know many marine applications that go that fast, and if they do it for sure isn't for very long. Down at 50 kts the propulsive efficiency sucks, it's only 55%, and 50 kts is fast for a boat. Down in the real world of 35-40 kts, where most boats travel, the propulsive efficiency is 45% and sinking fast. A waterjet can easily to that at 25 kts, and waterjets aren't known for efficiency.

I'm not being a slave to propulsive efficiency but, simply put, a propeller in the air trying to drive a boat is dumb idea. Not only is it noisy, but it is inefficient, and is also prone to things llike hands or feet getting into the prop and doing nasty things to said appendages. If you have a specific application where you can't use a prop or a waterjet in the water it's a way to do the job, but propellers working in air are best saved for airplanes.

And while airscrews do produce their highest thrust at zero speed, the thrust is relative low compared to other forms of propulsion, so the acceleration from zero speed is not very good. If you've ever flown a light plane you would understand what I'm talking about. I had a Cessna with a 235 hp engine, and the airplane had an empty weight of 1600 pounds. Compared to a car with 235 hp and a curb weight of 2000 lbs the acceleration off the line of the plane was pretty tame. Runways are long for a good reason, that being that airplanes don't accelerate very well primarily because of poor low speed propulsive efficiency. I wouldn't have wanted to race a car with half the power to weight ratio in a competition of zero to sixity. Once you got over 50 or 60 kts things got a good bit more lively, but props in air are a very poor way to push something on the water.

 

<insult directed towards other member removed>. You're mistaking the effects of PITCH for efficiency and show no understanding whatsoever of fixed pitch versus variable pitch propellers. If that's insulting to your obviously large ego I don't give a crap.

Propeller thrust gets HIGHER the SLOWER you go, BUT you have to allow for the effects of pitch.

Typical light aircraft use a FIXED pitch propeller set up for CRUISE conditions. Got that? This trades SPEED for ACCELERATION. You want BOTH speed AND acceleration? GET A VARIABLE PITCH PROP.

 

http://www.youtube.com/watch?v=JCgavxnV5Lc

Airboat hill climbing

 

http://www.youtube.com/watch?v=OpYHj7bHZwo

Airboat versus rally car. Go to the 1:00 mark for a demonstration of propeller acceleration.

 

No, sir <insult directed towards other member removed>. The aircraft I owned had a variable pitch prop, along with retractable gear and I have an instrument rating as well as a high performance aircraft sign off, so I believe I'm much more aware of the capabilities of light aircraft. Before you ASSume something you should know something about the subject before you make a fool out of yourself.

I am not confusing pitch efficiency with propeller efficiency, the plot you refrenced didn't make any mention of pitch effects, because that's not the subject. Obviously at low speeds the power of the engine driving a fixed pitch prop is lower, so of course acceleration is even worse for a fixed pitch prop. With my aircraft I could make full engine power (full rpm), but the propulsive efficiency and the acceleation off of a standstill is mediochre at best.

And the assinine race you referenced compared an 800hp airboat with a 450 hp car that had more than twice the weight of the airboat. The power to weight ratio of the airboat in that example is roughly four times that of the car. And even in that silly example, the only way the airboat ever out accelerated the car was to jump the gun by at least two seconds. sixty foot times for a high performance car on pavement are less than two seconds. If they had started at the same time the car would have been 20 or 30 feet ahead of the airboat. Why did he have to jump the gun like that??? Well it's because the propulsive efficiency at low speed sucks.

And the shots of more normal airboats struggling to get up a small rise are a joke, the static thrust is so low that that even on a surface of tall grass, they are making like it's a big deal to get up that gentle rise. Proof enough that it's a miserabe low speed propulsion system.

Do yourself a favor. Get a static thrust calcuation for an aircraft and see what the real static thrust is for a given engine size. Then compare a similar engine peak torqe multiplied by 8 or 9 (a typical first gear ratio multiplied by a final drive ratio) and look at the difference. When it comes to low speed pulling power airscrews are a miserable way to generate thrust.

<insult directed towards other member removed>. I'm done

 

THE IDEA THE AIRBOAT IS "TWICE THE WEIGHT" OF THE RALLY CAR IS STUPID NONSENSE.

The formula for calculating propeller thrust for a given speed is:

T = 550 np (HP)/V

where

T = Thrust in lbs

np = Propeller efficiency as a percentage (decimal)

HP = Horsepower

V = Velocity in feet per second

NOW LET'S SEE IF THAT HUGE EGO HAS THE BRAINS TO TAKE THIS EQUATION, PLUG IN THE EFFICIENCY NUMBERS FROM THE GRAPH PROVIDED, THEN PLUG THE RESULTING THRUST FIGURES INTO NEWTON'S F=MA EQUATION AND CALCULATE SOME ACCELERATION FIGURES.

 

Very loudmouthed, I have to say. Looks like you want to appear as the one who knows a lots about boats, planes and props.
Ok, then here comes a simple exercise which will immediately show the facts:

This is a thread in which Dskira has asked about the possibility of powering a large SWATH with air props. So, consider this case (a SWATH yacht by Abeking & Rasmussen shipyard): http://www.yachtsilvercloud.com/SSC/specifications.htm

• Resuming the main characteristics:
• Length: 41 m
• Beam: 17.80 m
• Draft: 4.1 m
• Displacement: 600 t
• Maximum Speed: 14 knots
• Engine power: 2x1100 HP (1640 kW)

Now, you said that a properly designed airscrew (trans.: propeller) is at least 85% efficient. Very well. The goal of this exercise is to find out which one is the most suitable mean of propulsion for this boat - having in mind the intended use, efficiency, "gross thrust, power to weight ratio, durability, maintainability, and cruising speed" (your words) - among the following:

1) one or more air props of suitable diameter
2) one or more water props of suitable diameter.​

Which one of the two is:

• more efficient?
• simpler?
• more reliable?
• less expensive to install, run and maintain?

I have already done that exercise and know the answer. I won't tell you what is the answer, but I can tell you that it is not the n.1.

Now it's your turn. Please let us know what is the right source of propulsive thrust for this vessel (between the above two options) and why.