crapshooter

Hi Guys, i am building a rescue airboat for my r/c powerboats, the design is a pontoon style with the motor on the rear , its a tractor type puller no a pusher prop . my question is; i have twin air rudders as used on most airboats and was wondering if i need to glue on 2 small fins on the under side of pontoons so the steering will be more accurate with no sliding when it comes to lining up a dead boat in the water for rescue. was also thinking on having 1 x rudder added to the bottom of air rudder to go under water abit for more controll. what are your thoughts guys ? thanks.

1QwkSport2.5r

If you're not running in weedy water, run a wet rudder only; no air rudders. This will give the ultimate control and provide much better steering accuracy. Strakes on the bottom aren't really needed if the hull design is sound.

One of my flat bottom airboats (18x32"; 8lbs AUW). First video with 20cc 4-stroke, second with 20cc 4-stroke. Wet rudder only.

cardinalflyer

My Feedback: (11)

I would recommend a water rudder in addition to an air rudder. It will provide more positive control for slow maneuvering.

1QwkSport2.5r

If you use a wet rudder, there is absolutely no need for the extra weight of air rudders. Watch my videos in my last post. The video of the boat with 10cc engine shows just how tight you can turn at slow speeds. Trust me. I've built 10 airboats; all but one use wet rudders only.

crapshooter

thanks guys, i have already built the platform with twin air rudders attached so it would be alot easier if i just add an under water rudder to the bottom of one of the air rudders. this air boat is not built to go fast its just for retrieving a dead boat so speed is not needed .

crapshooter

gave this boat a test and it definetly needs a water rudder plus i think its too light as the wind made things very hard for me to bring the model back to shore , would some extra weight added help ? the model is very light at present.

1QwkSport2.5r

What are the dimensions of the boat, AUW, and prop size/rpm used?

crapshooter

IMG_0235.JPG size of this craft is 21 inches long by 15 inches wide, power is a 2408- 21t-3a outrunner with a 9050 slow fly prop and 18 amp esc running on a 1300 mah / 2s lipo. can,t measure weight but being made of foam and balsa you can imagine it being quite light.

Got RPM

I like your airboat! A light boat is usually good, but in your case the air rudders are acting like a sail. They are probably over-powering you prop causing your steering issue. Airboats can be a challenge to maneuver in windy conditions.

What is the speed of the boat in calm conditions? What is the Kv of the motor? I suspect that you don't have enough thrust.


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1QwkSport2.5r

I'd dump the air rudders and use a wet rudder only - for starters.

crapshooter

not sure on the motors kv but it is designed for a 3d profile plane and the recommended prop is around the 9 inch dia x 50 pitch. if i hold the model in front of me and hit full power it does have some pull and plenty of air pushing back. as you guys mentioned i will remove the air rudders and make up a water rudder then see how she runs, its a shame i have to remove the air rudders as this hull if you call it that goes over the water lillies with no issues plus its like a hovercraft on land. oh well for now back to the drawing board. i will let yous know results soon. thanks guys.

1QwkSport2.5r

It takes a lot of trial and error to get air rudders to work well. Steering accuracy can be very good, but it takes a lot of trial and error. It also requires the right amount of thrust from the motor/prop to work well. Air rudders only work well when you're on the prop (throttled up). So steering at low speeds isn't nearly as effective. The caveat to wet rudders is they don't work well in weedy environments. You can circumvent this by incorporating a "break-away" in the rudder to allow the rudder to pop up over obstacles and drop back down once past the obstacle. What this entails is basically spring loading the rudder to it will return to the down position. The trick is having enough spring tension to keep the rudder down when underway under power but low enough tension to lift over obstacles.

crapshooter

good tip there with the spring loaded rudder. as this model is mainly designed for a rescue boat i do need it to have good steering at low speeds when lining up a dead boat for collection , i am happy with the way this model sits in the water plus how it planes nicely just got to get the steering good and maybe even some extra weight is required so its not like a feather on the water which i found out last week when it was a bit windy on the lake for the first test.

expresscraft

It might be too light to really push a dead boat. Maybe find a way to have it take out a fishing pole line, then just reel the dead boat in. Some gas boaters use a tennis ball on a fishing pole and try to cast over there dead boat. your airboat could have a tennis ball with a raised fishing line and then loop it around or something the dead boat. have something like a bow hook on your model boats.

Got RPM

The weight of the retrieval craft isn't that important, it is the thrust that matters. My latest recovery craft is much lighter than the large gas and FE boats it is designed to recover, but it has high thrust levels. A steerable air prop genernates over six pounds of thrust and it moves the target much better than heavy tug-like boats do. But as long as you can recover the target, that's what matters.


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expresscraft

If all you have is RPM and no torque, air or water prop will cavitate not really push. Less pitch on a airplane prop turns more RPM and is faster, until the weight is to much, no pitch no load capacity.. does not really matter id it recovers your boat efficiently or not., That is where you make the choice if recovering your boat is the reason you went to the lake or running it was.

Got RPM

I am not sure what you are trying to say. Perhaps you should try an air prop-powered boat. The recovery craft in question produces a measured six pounds of static thrust, it isn't just fanning the air. It is not "torque" or "rpm" which moves a vehicle, it is the thrust produced that does the work.

Tell us us about all your successful R/C recovery boats. Tell us that you never have to recover a race boat, or a sport boat that stops in the middle of the pond. Tell us that you never plan to recover your boat, just run it. Sheesh. Good luck dude.



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expresscraft

LOL take a look For all your r/c boating needs - Dock I only served in the Marines in a raid company were we attacked in zodiacs and Rigid raiders, to learn just a little about real boats, like off set navigation with just a compass to hit the shore in the right spot from 50 miles out. then After that I opened a full line hobby shop in Cocoa Fla, you know the Air boat capital of the world. And have built model boats for a living since 1989. Including Air boats, Even though the hobby shop was based on Planes a bigger market. I have flown Helicopters for years as well as BVM jets ect. and even J powered model rockets. Do you really want to pull my string?....Dude. My wife has build from raw Materials more boats then most hobbyist have assembled from already made parts. Even in a Air plane a airplane prop needs to have the right amount of PUSH to move the Air frame.. I did not say what he is doing will not work, just giving him another option to recover his dead boats. OH and 2 of the boats we make are Airboat hulls, one is a deck over chain saw powered boat the other will take a .60 2 stroke, but runs much better on a .80 Four stroke. 2 stroke would build a faster airplane and does, but does not push the boat over the dry ground like the four stroke does.....WHY? because of torque. to swing a bigger higher pitched prop to move weight under more load..In real life if you have a airboat that will not run dry or what they call the hill....It is garbage.

crapshooter

i have tried a 9 x 7 prop and it pushes alot better than the original 9 x 5 prop, need to complete the water rudder and maybe add some lead weight [ fishing sinkers ] so its not so light on the water.

1QwkSport2.5r

A 9x5 will have more thrust than a 9x7 will. Those higher pitches props tend to slip more until your model gets going faster where they "hook-up"' better - think of it like a 3-5" pitch is like a 4:56:1 gear ratio in a big 1-ton truck compared and a 2.73:1 luxury touring car as a 6-9" pitch prop.

More pitch means more speed at the cost of acceleration and more prop slippage. Less pitch means less top speed but more thrust/pulling power.

crapshooter

maybe i could need to try a 10 inch prop with a lower pitch as for some reason the 9 x 7 is better on this boat than the 9 x 5 . might try a 10 x 5 and see how it goes. as mentioned i don,t want speed just pushing power.

1QwkSport2.5r

If your power plant will handle it, an 11x3 might do even better. Diameter will give better results than pitch.

crapshooter

yes this motor is a slow fly type thats designed for the use of larger slow fly props sso i will try a larger diameter lower pitch prop , these are very cheap to buy from hobby king so i don,t mind grabbing a few different sizes for testing. i might even buy another larger low kv motor as aslo there very cheap. thanks for your help. i will let you know how things go.

1QwkSport2.5r

If you want the best possible performance, use APC props. They are far more efficient than the ultra cheap HK stuff. Also, do NOT use wood props on airboats. Wood props will shatter if they hit the water at rpm. Nylon props like Master Airscrew are okay, but APC flex far less and are much truer pitched than MA props are too.

crapshooter

Thanks. i don,t think i will have any issues with prop flexing using this slow fly low kv motor.