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Nov 11, 2016, 09:27 AM
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Help!

Help with rc airboat in water, thrust angle?


Hello, my scratch built airboat does not want to drive straight at all in water, as soon as I throttle up at all it tips, digs the bow in and spins around. My uncle suggested it's the hull design, I think it's rotational thrust, but I'm not sure what to do here and any help would be appreciated. We did make a video showing the issue, we even tried putting heavy rocks in the back to adjust the cg further back, but it didn't help at all really. Here's a video of what's happening though;

Rc airboat test NEED HELP!! (0 min 37 sec)


Also, if anyone has any water tested and proven plans for a foamboard airboat that would be great, I'm at the point where starting over sounds like a good idea

Edit; http://www.flitetest.com/articles/fo...at-on-the-snow Here's the original plans, the guy who designed it did it for snow, so that's why it's not water ready. Do you think theres a problem with the hull design , specifically the front corners causing it to dig in on turns?
Last edited by Siezed; Nov 11, 2016 at 09:53 AM.
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Nov 11, 2016, 09:38 AM
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You can't have a flat bottom . You need something to keep the boat straight on water. Look at the bottom of boats and you will see what you need.
Nov 11, 2016, 09:48 AM
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Quote:
Originally Posted by txt killer
You can't have a flat bottom . You need something to keep the boat straight on water. Look at the bottom of boats and you will see what you need.
A flat bottom is part of the purpose of an airboat, it allows it to run on land and water, plenty of flat bottomed airboats do fine on water. I know a v hull would be better but the whole purpose of this is to be able to run on any surface. If there's plans for non-flat bottomed boats that do as good on snow and grass I'd be interested though.
Nov 11, 2016, 10:04 AM
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You need strakes on the bottom to keep the boat going straight. All boats have them.
https://tse1.mm.bing.net/th?&id=OIP....9&rs=0&p=0&r=0
Nov 11, 2016, 03:22 PM
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How come i don't see them on any airboats i look at?
I really don't think strakes would have any effect on this, all airboats don't have strakes and the problem is the front is grabbing and plowing water, then making it turn, if you look at airboats they a lot of times dont want to just go straight, like on grass, this would be impossible with strakes the thing would just dig up the lawn or not turn at all- https://www.youtube.com/watch?time_c...&v=GRvBPJ8Z6m0
Last edited by Siezed; Nov 11, 2016 at 03:49 PM.
Nov 11, 2016, 04:15 PM
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Lay a straight edge on the bottom of the boat, running fore and aft. Look at it from one side. If there is a gap between the straight edge and the boat, with the straight edge touching only at the ends, you have a "hook" in the hull. This may account for this boat's behavior.

What about the thrust angle from the prop?

And do adjust your ballast to the rear a bit so the boat is slightly nose up.
Nov 11, 2016, 04:31 PM
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Quote:
Originally Posted by Siezed
How come i don't see them on any airboats i look at?
I really don't think strakes would have any effect on this, all airboats don't have strakes and the problem is the front is grabbing and plowing water, then making it turn, if you look at airboats they a lot of times dont want to just go straight, like on grass, this would be impossible with strakes the thing would just dig up the lawn or not turn at all- https://www.youtube.com/watch?time_c...&v=GRvBPJ8Z6m0
It's built into the shape of the hull. The boat can't be a complete flat rectangle bottom and correct ballast makes the boat go in one direction.
Nov 11, 2016, 05:08 PM
Just Plane Nutts
AirDOGGe's Avatar
A simple strake set-up like this one with some strips of spruce or whatever you prefer will help with steering stability, and still provide all the clearance needed: http://www.aquacraftmodels.com/boats...03-strakes.jpg


Shifting weight won't help with plowing here much, as I can see the bow plow down deeper and deeper the higher the prop/motor speed. It's probably too late to move the motor farther back any significant amount, or mount it lower and use a smaller 3-bladed air prop.

Angle the motor/prop up 5 degrees to move the thrust angle closer to the center-of-mass of the boat and try testing again. Try 8 deg if 5 proves not enough, or 2-3 if apparently too much....this is "dial-it-in" time... The strakes and new thrust angle should get you going.
Nov 11, 2016, 07:04 PM
Steps? What steps?
zozer's Avatar
Go here and ask your question.

http://www.rcuniverse.com/forum/rc-airboats-246/



.
Nov 12, 2016, 06:28 AM
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Haha, ok you guys have sold me on strakes, they will be something I try after I fix this bow plowing issue, I think they may help especially if I try "lifting" strakes that have a taper to them, not exactly sure how to go about it on a packing tape bottom but i'll figure something out.

As for the design, I really think it's thrust angle and rotational force, I know from simple physics this design will drive the bow down, regardless of cog. What I did was cut the entire mast off and mount it further back to give myself a bigger lever to get kinda close to the cog with the motor angle. Now I just gotta get to the lake to test it out.

Thanks for the ideas and solutions, and also I'm still looking for a better foam plan for an airboat, I might just make my own but in all honesty this was just supposed to be a rescue boat for my newly built sea plane, not such a long r&d endeavor, I thought of just leaving it the way it was because it taxi's ok, and that's all I need to push a plane in, but it feels unfinished and bothered the heck out of me
Nov 12, 2016, 06:40 AM
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Quote:
A simple strake set-up like this one with some strips of spruce or whatever you prefer will help with steering stability, and still provide all the clearance needed: http://www.aquacraftmodels.com/boats...03-strakes.jpg
Do you think if I just glued some BBQ skewers in this way it would work? I also noticed that's a v hull, I've yet to see a flat bottomed boat with strakes which is why I'm a little hesitant, if they don't work i've ruined my bottom and will probably need to trash the whole boat.
Nov 12, 2016, 09:54 AM
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This is not a V hull?????? http://www.aquacraftmodels.com/boats...03-strakes.jpg
Nov 12, 2016, 10:41 AM
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Quote:
Originally Posted by txt killer
Well it has a v shape, look at the back, not sure if it's technically a v hull cause the front isn't a v but it has an angled hull instead of flat. I'm telling you all the flat bottom airboats I have seen have pure flat bottoms, so your comment about all boats having strakes is just wrong, plenty of pure flat bottom boats plane and turn fine, strakes are an optimization, like an airfoil, planes fly with absolutely no airfoil, just not as good, same with flat bottom boats.
Nov 12, 2016, 10:44 AM
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Anyways here's what I did, not pretty but so far on grass it's handling way better, can full throttle turn no problem where before it would just flip. I'll update after I try it on water;
Nov 12, 2016, 10:59 AM
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Quote:
Originally Posted by CGAux26
Lay a straight edge on the bottom of the boat, running fore and aft. Look at it from one side. If there is a gap between the straight edge and the boat, with the straight edge touching only at the ends, you have a "hook" in the hull. This may account for this boat's behavior.

What about the thrust angle from the prop?

And do adjust your ballast to the rear a bit so the boat is slightly nose up.
Thanks I didn't even think of this, checked it and if anything it's bowed out, as for ballast we tried heavy rocks in the back and it's really the same, as soon as substantial throttle is given it digs the bow in and grabs the side.


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