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Author Topic: Simple airboat with predictable steering  (Read 338 times)

SailorGreg

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Simple airboat with predictable steering
« on: September 07, 2019, 12:39:35 PM »

I guess quite a lot of Mayhemers are like me, using a patch of water currently clogged with weed.  The only real answer (apart from some major aquatic gardening) is an airboat.  The Hobbyking Swamp Dawg is a favourite at my club, but is prone to some fairly random manoeuvres, as are quite a lot of airboats of the classic shape.  I have built an airboat which actually steers much more like a conventional boat, which is something of a boon on restricted or busy waters.  You might say this takes away half the fun, as skidding sideways and pirouetting suddenly is part of the airboat scene, but if you are after a slightly less frantic session, read on.

The main difference in my boat is that the sides are sloped in at 45 degrees.  This leads to heeling as you go into a turn and a more predictable course.  The boat looks like this –



There are no real plans, other than the doodles I did before I built it, so any measurements that follow are just taken from the boat as built and none are critical.  The hull is 18” long, 8” wide and 2” deep.  The construction is a simple egg box with three identical frames (one being the transom) and two longitudinal members. One frame is 6.5” forward of the transom, the other 12.5” forward.  Here is a cross section of the hull.



And here is a picture of the hull after the bottom and side panels had been added from 1/16” ply.  I probably overdid the thickness of the ply for the skeleton, but that was what I had to hand at the time.



You can see I have added material to the tops of the side panels to give the deck something to seat on.  The pylon for the motor is a piece of ˝” balsa, shaped to something like an aerofoil over most of its length, and with some pieces added to build it up where the motor attaches.  There is also a plywood disc glued to the front to give the motor mount screws something to bite on.  If you build this, you may have to adapt this to fit the motor you choose.  Here is a closer view of the bottom of the pylon – I was keen to make sure it was braced firmly as all the thrust is transmitted through this join.  You can also see the piece of brass tube at the transom which is the pivot for the rudder, cut down flush once the deck had been fitted.



I coated the inside of the hull with epoxy, which also helped reinforce all the joins. Here’s a couple more pictures –





Once completed, I added the propulsion and steering bits. The motor is one of these, and the propeller one of these. They seem to do the job just fine, using a 3s LiPo.  Here is a view of the moving bits –



And here is the internal view.  The 1300 mAh LiPo sits nicely next to the pylon in the stern compartment.  The fact that it is off centre doesn’t seem to make any difference.



The cover for the electronics can be any shape you want, but mine is a streamlined shape, hinged at the front with some tape and held down at the back with a small magnet.



The shroud for the prop is the container from some Frogtape masking tape with the bottom cut out of it, which coincidentally was a perfect fit for this prop.  For different sizes of props you might want to look at plastic flower pot rims.



And finally, I planed the edge of the hull down to a small flat to let me stick some of this around the edge – slightly less vulnerable than the sharp wood edge that comes with joining two sheets at 45 degrees.
Well, that’s it.  A fairly simple build, and as you can see from the pictures, mine has had some pretty hard use.  But it’s great fun, and when winter comes, it works great on ice!

Greg

GG

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Re: Simple airboat with predictable steering
« Reply #1 on: September 07, 2019, 01:52:57 PM »

Greg,
       Let me be the first to say that it looks like a nice design and you have described + illustrated it well.


Airboat models don't need a complex design and the information you provided ought to be more than enough for any competent person to build from.


Hope you have encouraged a few more to try building their own airboats.


Glynn Guest   
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SailorGreg

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Re: Simple airboat with predictable steering
« Reply #2 on: September 07, 2019, 01:59:25 PM »

Thanks Glynn.   :-)

SailorGreg

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Re: Simple airboat with predictable steering
« Reply #3 on: September 07, 2019, 03:29:37 PM »

I should perhaps have mentioned after my throwaway comment about it being fun on ice, that of course it then steers exactly like any other airboat, and if you forget that you might end up like this -






Whoops!  I had to rebuild the rear quarter after that little faux pas.  :D :D   Take care!


Greg

martno1fan

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Re: Simple airboat with predictable steering
« Reply #4 on: September 08, 2019, 08:55:50 AM »

Make one from PU  foam board cover with thin glass cloth and epoxy then you wont have any issues regards damage also its quicker and easier to do ,hollow out areas for electrics etc and make a cover and your good to go, its easy . Also if you want better steering extend the air rudder a little at the bottom so it sits in the water works a treat  :-)) :-)) .
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SailorGreg

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Re: Simple airboat with predictable steering
« Reply #5 on: September 09, 2019, 05:29:53 PM »

Yes, others have suggested extending the rudder down so you have both air and water rudders, and I guess that would work pretty well.  But if you are running in weed-ridden waters, the projection down will quickly snag a clump of weed.  The object of this was a design that could run over weed (and ice) but would steer more like a conventional boat.

That said, once our lake is clear again I might knock up a combination rudder to see how it works.

Greg

roycv

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Re: Simple airboat with predictable steering
« Reply #6 on: September 09, 2019, 06:01:20 PM »

Hello I have a similar airboat, sort of inherited, and when turning it drifts sideways and I would think a small well angled dagger board  to avoid picking up weed, centrally placed for the boat to turn around would work well.  Not done yet. 

When turning you have to kill the forward motion of the boat before it starts to turn in a new direction and you have to think ahead, once you know how the boat behaves on the water.  Best sailed away from scale boats, probably best with other airboats.

You probably do not need a water rudder as the air rudder is very effective.  It turns to face the new direction but carries on travelling in previous one.  My one has a 380 motor running on 6 cells with a 3 bladed shrouded prop.  It goes very fast and just skims on the surface of the water.  Lots of fun!
regards
 Roy
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GG

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Re: Simple airboat with predictable steering
« Reply #7 on: September 09, 2019, 08:01:37 PM »

Must admit that my two airboats (Skimmer 400 and 5/600 - MB Sept 07 and Oct 14) plus the similarly hulled waterjet powered Scudder (MB Winter Special 2013) all managed quite well without having a rudder in the water.  Yes, high speed turns involved a degree of sideways skidding but rarely did the hull "dig in".


I did try adding longitudinal rails to the bottom of the hull to see if it improved things but the only effect appeared to be a loss in speed whilst turning.


Having seen a few commercial airboat models running, I get the impression that they may be rather heavy with "soft" (i.e. radiused as opposed to sharp) corners on the hull. Both features would hinder model airboat performance.


One feature that is essential is some form protection around the airscrew for safety.  RC and free-running (which some nominally RC models can become) are usually frowned upon on public waters.


Glynn Guest


 
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RST

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Re: Simple airboat with predictable steering
« Reply #8 on: September 09, 2019, 09:25:51 PM »

Maybe try and focus on the big fin at the back. No point in tapering it like a feather for aesthetics. It needs to capture full flow and deflect in way of the fan. Should work better if sanded to a rough aerofoil i.e. radiused at front and thinner at the back.  Sticking a rudder in the water is a bit counter, productive. Agree though, sharp edges, chines, same for any higher speed application. It's been a while since I did this myself. Agree also on prop vowels but you have this sorted. Back in my "youf" plant pot rims were a useful source, and we usually had one prop or another which fitted a thick enough one.
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