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[darrencuk]

Ok so I have a new build idea I'm really thinking of making a air boat but I need to learn about some stuff because I understand with a boat it's direct drive and obviously you have drag but on a air boat where the prop isn't in the water u have no drag so im guessing if I ran a 400 motor on a air boat and one in a boat the air boat should technically be faster?? The thing I don't understand tho is the props I will need this is going to be the hard bit anybody have any idea how small u can go with the props sorry for bad spelling and grammar Spell check won't work for me

[MikeA]

theres a new guy on here called jostro65 hes an airboat builder maybe he can help he started thread ic to electic conversion

[Big Ada]

A water Prop pushes against water and an air prop pushes against air,two different substances.
There are several posts on here about Airboats.


Len.

[MikeA]

the point is the guy is airboat builder mainly and was asking advice on normal water props. wheres as you are looking for advice on air props and maybe he could help you. :|

[MikeA]

here is a prop chart for 400 size motors the amps and volts required also:
http://www.gwsus.com/english/product/powersystem/edp400.htm

if you track the code number of the props to here :

http://www.gwsus.com/english/product/propeller/propeller.htm

it then give you the size of the prop in mm

[Martin (Admin)]

I also believe an airboat will go much faster than a 'normal' boat with the same hull, motor and batter.... but I can't tell you why!   

[MikeA]

the point is the guy is airboat builder mainly and was asking advice on normal water props. wheres as you are looking for advice on air props and maybe he could help you. :|

I think i mightve got a bit confused bout whos posted what.

[darrencuk]

Ok thanks guys I'm thinking of running a brushless outtrunner motor don't no what kv to go for tho I'm thinking of building it out of balsa wood to make it light weight but covering the inside with resin just to make it stronger :)

[Welsh_Druid]

I also believe an airboat will go much faster than a 'normal' boat with the same hull, motor and batter.... but I can't tell you why!   

Could it be something to do with the maximum efficiency of  a propellor in water ?  A water jet powered boat will go faster than the same hull fitted with a conventional prop drive .

Don B

[MikeA]

why is that then a jet drive has 70 percent efficiency of a conventional prop

[nhp651]

because a water jet drive has two motions to it's being what it it.

Then the water is thrust out in a much greater form from it's "Venturi" nozzle ( a form of reduction of pipe diameters so that the water exiting is at a much greater pressure to that which enters )and therefore forms a much greater concentration of thrust  than a propeller could ever do on a "wall of water" where it disperses quickly to the sides.


Just the same principle as a sprinkler hose in a garden will do......if you put the hose nozzle on jet it comes out at a far greater pressure than if it's on sprinkle.

[nhp651]

why is that then a jet drive has 70 percent efficiency of a conventional prop

A water jet drive is 70% MORE efficient than a prop, not less.

[MikeA]

http://www.caleymarina.com/42/Engines/Yamaha/Outboard-Jet-Drive

if a conventional prop is changed to a jet drive there is a 30% horsepower drop. 50 hp jet boat is equivalent to a 35hp conventional prop boat.

http://forums.iboats.com/showthread.php?t=193591

a bit more

http://www.nava.ca/surface.htm

[Patrick Henry]

Mine is an Alligator Tours electric airboat, with a stock 380 motor running from an 11.1v 800a/h lipo pack(ex Twister Skylift heli) an Mtroniks Viper esc and converted to use a Planet 2.4gHz radio. I really need a proper esc, one that doesn't take the lipo pack down to critical voltage levels(lost one battery pack already)but for the moment I just guess the run time.

With the lipo pack installed it goes like the proverbial off a shovel....

[philk]

A water jet drive is 70% MORE efficient than a prop, not less.

so from that your saying that everybody who races boats be they multi racers offshore racers and full size boats are all mental and should be using waterjets because then they will win easy. no racing class except those loonies that race on very shallow water use waterjets because they are less efficient


phil

[LarryW]

 HI , THE CLUE IS IN THE TITLE,    AIR BOAT THRUST DRIVEN PROPELLER    AS USED IN EVERGLADES
   
     THIS ONE IS FOR SALE  .......LARRY.....   

[nhp651]

so from that your saying that everybody who races boats be they multi racers offshore racers and full size boats are all mental and should be using waterjets because then they will win easy. no racing class except those loonies that race on very shallow water use waterjets because they are less efficient


phil

sorry to say but i ain't rising to your bait, phil..
i don't think at any time in my explanation did i mention ANYTHING ABOUT any form of racing what so ever or speed.
but pound for pound....put a screw on a 585 motor and connect the same motor to a relative water jet propulsion unit and the water jet will have more efficiency.......torque, pulling power and grunt...............don't think i actually mentioned speed.....i mentioned efficiency.....itso facto, qed.

that's why [i do believe i am right in saying ] that the new Shannon class lifeboat,which has twin water jet drive units and powered by smaller diesel engines and nearly 8 feet shorter than the Tamar lifeboat [which has conventional twin screws and larger diesels] actually has more bollard pulling power for towing than the Tamar.smaller engines and greater pulling power to me denotes greater efficiency

[MikeA]

well according to sources a jet drive is only inefficient a low speeds (upto 30 knots) beyond 30 knots they are more efficient upto 90 knots.beyond that propellers are back in the game. So that kind of makes my theory of them bein inefficient alltogeather controversial. I think if you talk about a jet drives effectivity to move water over a proppeller then yes a jet is more efficient. but the disadvantage is it needs more power to operate and so there fore on a hole less efficient. Got to remeber that a jet drive has to be going quite fast before its creating enough thrust to move a boat. wheres a propeller doesnt need to be as fast because its treading the water itself and can start forward momentum at low speeds.