Air propulsion

[nikm]

Could i get some help with these kind of propulsion.I am building boat with air propulsion but I do not have any experiance with it.I need to calculate for 2 m large propeller to achiwe speed of about 15 kn,how powerfull my engine has to be and at witch revolutiones.

[FAST FRED]

You will need to do the work to decide how much thrust is needed for your boat and desired speed.

As a rule of thumb each HP on a submerged boat prop can create 20 to 25 (rare) lbs of thrust.

Unfortuniatly pushing on air is very inefficient , so all you get is 5lbs of thrust for each hp.

That means at the same speeds 4 to 5 times as much power and fuel is required to push a boat with an air prop.

Better choice might be a Hovercraft.

FF

[Rick Willoughby]

As Fred said you need to know the boat drag at the design speed to design the prop.

Air props will be more efficient than a water prop if the boat is easily driven. You could expect efficiency around 90% if you have the right set up.

If you know the design speed and boat drag at that speed I will give you the best prop design.

If you do not know the drag then you need to give some detail on the boat you intend to push.

Rick W.

[FAST FRED]

Air props will be more efficient than a water prop if the boat is easily driven. You could expect efficiency around 90% if you have the right set up.

There is a HUGE difference between the thrust Output on water or air , regardless of the props efficiency in the medium chosen..

The best I have heard of on air propulsion was 7lbs thrust per hp, about 1/3 or 1/4 of a simple water prop.

7lbsIS more efficient than 5lbs , but still expensive to produce.



FF

[Rick Willoughby]

Originally Posted by FAST FRED

Air props will be more efficient than a water prop if the boat is easily driven. You could expect efficiency around 90% if you have the right set up.

There is a HUGE difference between the thrust Output on water or air , regardless of the props efficiency in the medium chosen..

The best I have heard of on air propulsion was 7lbs thrust per hp, about 1/3 or 1/4 of a simple water prop.

7lbsIS more efficient than 5lbs , but still expensive to produce.



FF

Fred

You would not consider an air prop for speed under about 20kts for a powered boat. Even then you need an easily driven hull to be better than a water prop.

Above 40kts the advantage definitely swings in favour of air prop from a performance perspective.

This does not conflict with what you are saying. All it means is an air driven boat will not make a good tug. But if you want to go fast at maximum efficiency then an air prop will be the better option. Far superior to any water prop even the surface piercing variety. The constraint of course is that there is room to swing it.

Attached example shows the sort of air prop required to get 90% efficiency with 30kW. Thrust is a moderate 900N (say 200lbf). Of course would need a very easily driven hull to have 200lbf of drag at 60kts (30m/s) - maybe something on foils.

It was not dumb luck that Mark Drella chose an air prop for decavitator:
http://www.youtube.com/watch?v=l2UOH65QOI4
This is only doing around 20kts but is very low drag. Power is about 600W so thrust is something like 12lbf (calculates as 15lbf per HP)

Rick W.

[Rick Willoughby]

An example of an application suited to air prop. Would not be super efficient because diameter is on the low side but not too bad:
http://www.youtube.com/watch?v=HHAlU46qt2E

Rick W.