joseywales44

My first airboat attempt has a major porpoising problem. It goes great, turns great etc until it starts to plane out, then SLAP SLAP SLAP it porpoises out of control. More throttle and it flips. I originally had the motor mounted about 3 inches back from where it is now. It porpoises the same either way. Angled motor down about 3 degrees....it makes just a tad improvement but still basically uncontrollable when it starts to plane out. It appears like the hull is fine cause it goes to plane out real easy and nice and straight.

Is the motor setup way too much for this 11 by 20 inch rigid foam boat? It's 2" thick. The motor on it is brushless with following specs: 3 cells: 18A, 190W, 850g, 49mph, 8600rpm 9 by 5 GWS prop. Try a smaller motor maybe? Smaller prop? Any ideas? Thanks!

mr. ilikehelp

maybe move the motor foward that may stop the slapping. my boat used to flip on turns do to it catching the edge of the nose so if ur boat does that then it may help control it on a plane

Mr Cajun Gator

[link=http://cajungatorairboats.com/v-web/bulletin/bb/viewtopic.php?p=4149#4149]Cajun Gator Tips[/link]

asmithnc

Sorry you are having problems. I believe it's all about weight and engine angle will not fix your problem. I would do something simple stupid to start. If you add a little drag to the transom it will smooth out the Slap Slap. Try brass tubing about 1/8" below water line to start. A little goes a long way.

joseywales44

Thanks for all you help guys!

I did try moving the motor forward as originally it wasn't stick mounted, but mounted flush to the horizontal crossbars on the engine mount. It porpoised like mad so I added the stick mount to get the motor out further towards the front. Still porpoises like a banshee, in fact it almost seems as if it's worse now with the motor further forward. Moving the battery forward doesn't seem to help much either.

With the brass tubing, do you mean run it length-wise across the transom so it's 1/8" below the water line?

Tnx, JW

asmithnc

No, run it length wise will be too much drag.
I too had serious problems with a Slap happy boat. I stated making every thing in the picture below to help correct the problem. Correcting the weight distribution is a must.

Run the tubing like the water rudder. I would try one in the middle first at 1/8" below the water line. Make it adjustable because too much drag is bad and not enough equals what you have.

joseywales44

Ah, now I got it! I'll give that a try!

Your boat makes mine look like something you'd see in Sanford and Son's back yard!

asmithnc

Thanks but it doesn't matter as long as you are having fun. If you knew how many time this boat has been trashed. Now it's sivler and black.

srw1960

My experience with porpising is that it allways comes down to weight distribution and engine thrust angle...moving that tiny engine forward wont help a bit...try moving ALL the electronics to the very nose especialy the battery as that is the heaviest part of that boat..you comented that when you changed thrust angle it seemed to help maybee you need to give it a bit more and move the weight..add the tube asmith suggested and I bet she runs smooth for ya..BTW I have the same problem with my big air boat so I am going to try the very things I have sugested to you and will try asmiths cure as well good luck

crispyspa

Listen to him. he's a very smart individual.

Mr Cajun Gator

sorry about that, I need to change the settings.
The link is going to this post.
If you place a straight edge on the bottom, is there a crack at or near the transom? Something that helps is to have a good square or sharp edge at the transom. The main thing that causes this hop is a defect on the bottom, hook near the bow, bottom raises at the transom, weight too for back.
boats that have things hanging in the water are much more forgiving than the flat bottom airboat, so the bottom is very important. Things to remember, as water passing over something or something passing over water, the water is going to do everything it can to hold on. If the water can not release smoothly it will in the case of a airboat cause lift on the front or drag. Back to when it cause lift, in most cases the thing that is causing the front of the boat to lift is not enough to hold it there, so it releases and the front falls back down, repeatedly.
You want the bottom to be straight as possible.
Sharp edges help to release the water.

asmithnc

I think the advice given here will give you or others some ideas that will help.

I have a question: 3 cells: 18A, 190W, 850g, 49mph, 8600rpm 9 by 5 GWS prop. Tell me more on the 49mph part of this.

Mr Cajun Gator

the 49mph....must be something other than miles/hour. From the numbers given I come up with a airspeed coming off of the prop at 40.7969 miles/hour.

mr. ilikehelp

mr cajun gator how can u figure out approximate airspeed?
i find that very neat how u can give a rough idea of that.

joseywales44

Awesome! Thanks for all the help!

I'll try the straight edge trick when I get home. If the brass tube and weight mods dont do the trick I may have to bust out the snader.

Note to self: Test the foam airboat first, THEN paint and seal it once the hull is tweaked!

Not sure about the 49MPH. I come from an electric plane background, so I do know quite a bit about brushless...but I'm clueless on that, sorry.

Tnx, JW

Mr Cajun Gator

Prop stated was a 9"x5" the 5" being the number of inches the air will move for 1 round
the RPM's given is 8600
8600 x 5=43,000 inches/minute
43,000 divided by 12=3,583.3333 feet/minute
3583.3333 x 60 = 215,000 feet/hour
215,000 divided by 5270 = 40.7969 miles/hour
correction 215,000 divided by 5280 = 40.7196 miles/hour
depending on the boat that it is on you can figure the max speed to be less than 1/2 of 40mph

rcairboater

make sure you don't have any hooks on the bottom of the hull. Also, balancing should be around 30% from the transom. In addition, angle your engine up.

Thanks.

mr. ilikehelp

thanks for sharing your formula for the airspeed with us mr cajun gator

crispyspa

What's a snader and how does it work?
You may want to put a layer of epoxy on the foam before testing. the sanded foam might not give a realistic "feel" to the boat.

joseywales44

"Snader" is how you spell "Sander" after 5 Hamm's Special Lights!

So last night I threw on a smaller prop, APC 8 by 4 to replace the GWS 9 by 5. Thinking maybe the boat was grossly overpowered. I placed the battery and ESC up front further also and added a piece of tubing hanging off the back to create drag. Tossed it in the waveless pond and there she was again! FLIPPER! Porpoising away the second it started to plane out. Grrrrr. That evening I took a close look at the hull and noticed under the transom I had sanded the transom bottom up on an angle about 35 degrees. You can kinda see it in the pics top of post but it's more of an angle then what they represent. According to Cajun Gator's post this is a big no-no so I busted out the RAPALA fillet knife and hacked off about 3/4" off the rear. So now the transom is 100% square at the bottom. I also mounted a lighter weight tail rudder as the one I had on previously was a tad heavy. I tweaked the motor mount to point down a couple degrees. Or should it point up? I swear I tried up previously and it only made it worse.

Weather permitting I will test it this afternoon and report back.

Tnx, JW

Mr Cajun Gator

You want the air flow towards the water/ground

stevensaero

I've experienced similar. In prototyping a simple small brushless airboat with more perpendicular sides than yours I originally sheeted the hull in balsa but did a fantastic job of getting everything slick and straight. I, however, didn't pay much attention to waterproofing the wood and skinned the boat in solarfilm. After taking some damage the balsa got wet and rippled slightly... this caused a great deal of porpoise action on subsequent runs. I ended up throwing on a small trim tab to the back of the boat to control the porpoise. In later prototypes I waterproofed more thoroughly and have had no need to use a trim tab. In fact the more perfect the hull (flat and sharp) the more stuck to the water it becomes... which can be a bit of a vice... there's a fine line between too much and too little water contact

boatnut

the porposing problem  can be stopped by adding a elevater {small} to the outboard side of the
rudders with up angle of about  10 degees.
It worked for me as I can't use any thing in the water as I have lily pads in my lake.
changing engine angle and position didn't help.
The tabs also make it dig in on the turns keeping it from flipping.