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  1. #1

    Airboat Outrigger build (foam core)

    Hi guys I have been in contact with a few of you guys and am pleased to find like minded individuals who are willing to invest of themselves the time and effort in a project. My name is Al by the way and my project is to construct an outrigger with a foam core and lite ply exterior and fiberglass to top it off. I have searched top to bottom and have been unable to find a lot of information out there to have a firm footing as to what will work and what won't. So I'll blunder my way through this and hopefully have a nice model in the end that will provide many hours of enjoyment. I'm new to this whole posting thing but am willing to share what knowledge I have with everyone and hopefully benefit from others experience in the process.

    I will be taking tons of pics along the way so please feel free to ask for further information or a closeup snapshot along the way and if you have any input I will welcome it with open arms. I have built other rc boats and full size boats in the past and spent several years building airplanes and rebuilding airplanes, anyone who has flown knows what I'm saying.

    So the basics, as for size it will be around 40" with approx 18"x2" sponsons and will be powered by a lonely 90 ST looking for a home. The tub will be 4" wide and tapering to about 2.5" at the transom and approx 38" long. I have always thought a foam core would be a different way to construct a boat so that is the process I will be using for this. My expectation is to have a rtr weight of 4.5 - 5 lbs. As some of you may have experienced it may be somewhat intimidating for the first time builder to construct a project of this design so if you're all willing I may be somewhat simplistic in some of the construction methods in the event it may spur a first timer to give it a try. (Not trying to dumb it down or bore anyone) I say simplistic but basically demonstrating a model like this can be built with the most basic of hand tools and a tight budget.

    I will be drawing something up and posting a few pics in the next day or two, til then!

  2. #2
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    For the engine being used, I think you're RTR weight and size might be a tad small/light, but the nice thing with foam is you can always add to it. I'm curious to see how this works out for you.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  3. #3
    Yeah sounds great, keep us posted.

  4. #4
    I'm in the same boat as to how this will work out, someone has to give it a try. This is basically a guessing game based on what information is available and what knowledge a builder has from previous experience. Essentially the 90 ST is a 60 size engine so that may play in my favor weight wise.

  5. #5
    So did a little doodling and have come up with the plan/templates req'd for this one off masterpiece. Gotta be positive....
    The tub is 38" long and 4" wide at the front tapering to 2 5/8" at the back. The side profile is 2 1/4" at the deepest point around 1/3 length of the hull from the bow and is 1 1/4" high at the transom. The sponsons are 18 3/4" long and 3 1/8" at the deepest location and will be 2" thick straight through the length, the bottom has a two plane profile with 4" flat plane at 5 degrees at the rear and the sharp edge taken out between the two planes (Just like curves). The ride plane of the sponson ends at 4 3/4" from the rear of the sponson so about 1/4 length of the sponson. In my plan I have tried to achieve a neutral lift attitude by a slight curvature on the top of the tub and sponson's, the thought process is to control lift with the motor alone and to set the C of G slightly behind the ride plane of the sponson. So all I have done is make the templates out of parchment paper for baking, for the hull I just folded it in half lengthwise and used a fiberglass garden stake to bend to the curve I was satisfied with, cut it out and opened it up to what is in the pic. The side profile is basically flat throughout the bottom with a slight upward curve at the front 1/3 and a similar curve from the front to the transom on top. I have included a couple of pics of the templates so please feel free to comment good or bad, better to rectify a possible mistake before it happens, Cheers!

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  6. #6
    Had trouble uploading pics, my wife's camera create's a 7 MB file so I had to use a different camera to get a small enough pic to upload (no wonder a woman's purse is so big?) sorry about this.
    Last edited by arcdude; 04-09-2014 at 10:32 PM.

  7. #7
    Your wife's cam should have a setting to reduce the quality/file size.

    Ok, thoughts...Quick ground rules. This is a forum, and the written word carries risk when the delivery is not heard, please understand that what follows from me throughout your build is me enjoying being part of the discussion of your process, not anything negative or intended as rebuke.

    There's a lot of dialogue about the right shape for the front of sponsons. Some say curved, some say straight angles. To me it's about what's doing what, so where it's wet under full power it needs to be flat, what's in front of that dictates how it reacts to differing depths. A straight path increases in lift proportionally as more surface area enters water, as does the drag. a curved surface's lift increases exponentially, so does the drag. All of that stuff applied to boats having to penetrate water by design, hydroplanes only do this to get going, so to me flat angled sponsons are easier to make, but any suggested advantage of one over the other is academic because if that area is in the water enough to care about such efficiencies you've got a bigger problem that having the right shape to deal with it! So in essence you've done what I do, curved entries followed by a lift plane which is flat in the wetted area, in my case because I think it looks better and is stronger.

    A tapered tub I'm troubled by. This means a lot of the hull pressure will be exiting the sides of the tub, this will be fine if there's enough thrust to simply overcome the loss and the CoB allows it, but I'd need to know your CoB before harping too much on that. Whatever, I'd be looking at parallel personally.

    I've noted on a lot of airboat riggers that there's a little too much adherence to established water prop designs in some areas. This works, but two things come to mind which I'm considering for my own rigger project (this is a gift engine I've been given, a small Pylon racing engine, so not in the league of yours). The sponson supports are the usual two parallel shafts, and I think these need to be as high as possible as keeping them clear seems to be something of a challenge even on water propped boats, in my case I'm thinking of arches: Click image for larger version. 

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ID:	1985766. The other thing I've not got to yet is prop protection, One of the quiet benefits of Chris Selph's design is the massive deck which keeps spray off the tub and sponson away from the prop. From my experiences with the little red hydro limited protection means intolerant to rippled water. So I'm coming at this aspect again and methinks those arches are going to grow a bit if I want to be able to run it in other than millpond conditions.

  8. #8
    Quote Originally Posted by Jeremy_H View Post
    Your wife's cam should have a setting to reduce the quality/file size.

    Ok, thoughts...Quick ground rules. This is a forum, and the written word carries risk when the delivery is not heard, please understand that what follows from me throughout your build is me enjoying being part of the discussion of your process, not anything negative or intended as rebuke.

    There's a lot of dialogue about the right shape for the front of sponsons. Some say curved, some say straight angles. To me it's about what's doing what, so where it's wet under full power it needs to be flat, what's in front of that dictates how it reacts to differing depths. A straight path increases in lift proportionally as more surface area enters water, as does the drag. a curved surface's lift increases exponentially, so does the drag. All of that stuff applied to boats having to penetrate water by design, hydroplanes only do this to get going, so to me flat angled sponsons are easier to make, but any suggested advantage of one over the other is academic because if that area is in the water enough to care about such efficiencies you've got a bigger problem that having the right shape to deal with it! So in essence you've done what I do, curved entries followed by a lift plane which is flat in the wetted area, in my case because I think it looks better and is stronger.

    A tapered tub I'm troubled by. This means a lot of the hull pressure will be exiting the sides of the tub, this will be fine if there's enough thrust to simply overcome the loss and the CoB allows it, but I'd need to know your CoB before harping too much on that. Whatever, I'd be looking at parallel personally.

    I've noted on a lot of airboat riggers that there's a little too much adherence to established water prop designs in some areas. This works, but two things come to mind which I'm considering for my own rigger project (this is a gift engine I've been given, a small Pylon racing engine, so not in the league of yours). The sponson supports are the usual two parallel shafts, and I think these need to be as high as possible as keeping them clear seems to be something of a challenge even on water propped boats, in my case I'm thinking of arches: Click image for larger version. 

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ID:	1985766. The other thing I've not got to yet is prop protection, One of the quiet benefits of Chris Selph's design is the massive deck which keeps spray off the tub and sponson away from the prop. From my experiences with the little red hydro limited protection means intolerant to rippled water. So I'm coming at this aspect again and methinks those arches are going to grow a bit if I want to be able to run it in other than millpond conditions.

    Thanks Jeremy, appreciate the input. I do respect another's opinion and am not offended in the least, I don't think any of us in this forum wish's to see another fail. I know what your saying about the written word but, when is it enough, that's what questions are for I guess.

    As for the sponson design I'm only following my previous experience and the latest developments that are made public. Previously I had built a hydro 4' and I noted it took til about half throttle to get up on plane due to the water adhesion and my best guess is it may have been attributed to the curved ride pads. Please observe the enclosed pic of the sponson with the straight edges aligning the bottom in the following posting, as I mentioned the bottom is essentially two angles and I have just removed enough material to appear curved so the impact on transitioning from one plane to the other should be minimal, I hope. Also I have drawn a horizontal line on the sponson template to indicate the bottom of the hull so I will have 1 1/8th" clearance under the hull and the boom tubes are an additional 1 1/8th above that bringing the total tube to water clearance to 2 1/4" above the water line.

    The tub itself I mulled over for quite some time and here is the reasoning behind that madness. I recall when holding an airplane by the midsection at full throttle the prop has a definite control of the direction in which it is directed (can't explain it any better than that) and as you move the tail up and down it follows the direction of the pull of the prop so in my thinking the only lift I want is controlled lift thus the standard I guess front end and to taper the aft part of the hull to expel the lift rather than holding it. To my understanding of the dynamics of the whole thing a high percentage of blow overs are caused by uncontrolled air pressure lift. Another point to this is the Japanese air boat basically has no lift surfaces at all except for a rear stabilizer and is one of the worlds fastest to the best of my knowledge.

    I have given the thought of spray a little consideration and if the boat runs as expected there shouldn't be any issue with the sponson's set out at about 24"s so if that becomes an issue when this beasty goes for a run I'll have to address it at that time. I have come up with a tentative design for an air inlet scoop because of your similar issue, whether it'll work or not?

    Is the above drawing a rendering of what you propose to build? pretty sharp looking, I like the arched sponson connection. The water I'll be running is a major river with lots of current so that is how I came to the clearance between the tub and water surface.

  9. #9
    Okay got some wood cut out today, all material is 1/8th inch lite ply with the exception of the outer sponson sides which are 1/16 aircraft ply just because I had it handy. All I did was trace the outline onto the wood and cut them out with a razor blade knife and clamp the opposing sides together and gave a final sanding to get both sides identical. I placed the sponson onto the side to demonstrate the side profile of the finished boat and placed straight edges on the bottom of the sponson to show the two angles which are basically flat. I know there may be some reservations on the use of lite ply so I did some experiments a while back and fiberglassed a piece of wood with 1/2 oz cloth and was surprised to find how strong it became. Before the glassing I could easily leave a finger nail imprint in the wood and after glassing it was very difficult to leave a dent. If I was building a race boat I would likely be a little more concerned about scuffs and the occasional altercation with another boat but for a straight line runner and a few donuts there shouldn't be an issue. In the least I think the side profile looks pretty cool, at least enough to smile!

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  10. #10
    Here is the latest update, I glued 3 layers of 1"thick x 4" wide and 38" long high density styrofoam together with 3 hr epoxy glue and placed paint cans on it to weigh it down. I marked the basic outline of the wood templates onto the foam so that I will avoid most of the cured glue while cutting the foam out as it can be a little difficult to shape. Next I squared and clamped the side wood templates onto the foam and chose to cut the foam with a hacksaw with the blade turned sideways and if carefully done there isn't much sanding to do on the foam to match the templates. Once I cut and sanded the two ends I moved the clamps to the inside and finished cutting and sanding the full length of the hull and presto the bottom to top shape is done. you'll have to excuse my sidekick aka quality control inspector Maggie she has to be involved in everything I do. And no I don't work on my kitchen table wifey wouldn't be too happy, it just provides the best picture taking spot at the moment.

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  11. #11
    The humpback look of the side profile is one I like.

    Great start!

  12. #12
    Thanks Jeremy.

    For the next step to complete the basic hull I drew a center line at the bow and stern on the templates and the foam to make sure everything was nicely lined up to cut the sides. The templates were clamped on as before and cut with the hacksaw and lightly sanded to mate all the surfaces, caution had to be exercised to keep it all perfectly lined up so I didn't end up with a skewed foam to glue the templates to. All the surfaces are temporarily taped around the foam to get a proper perspective into how the finished shape will turn out. Coming along nicely!

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    Last edited by arcdude; 04-13-2014 at 04:26 PM.

  13. #13
    For the next step I decided to use the blue styrofoam as I had a chunk of it in the two inch thick needed for the sponsons. The outline of the wood sides were traced out onto the foam and inspected by my trusty sidekick and I decided to cut these out with the scroll saw this time for the sake of speed but, can be cut out in the same fashion as described earlier for the tub.

    All side parts for the tub and sponsons were clamped together and drilled out at the same time using the horizontal line on the template (which is the bottom of the hull) to get the proper alignment and to ensure the boom tubes will line up exactly when complete. Next I added doublers on the inside of all drilled holes to strengthen them, on the outer side of the sponsons I added 1/4" because that will be the only support for the end of the tubes and 1/8th" on the remaining holes as can be seen in the pics. Another little trick I use to toughen up holes of this nature is to add a touch of thin CA in around the hole and it strengthens the surfaces substantially to prevent wearing of the hole. The side wood components were lined up on the styrofoam cores and marked in order to do recess cuts into the foam to match the doublers that were added which were easily done using a razor blade knife and coping saw.

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  14. #14
    Do you plan to fix the sponsons, or have them removable?

    Your cat is useful to judge scale

  15. #15
    Yes I suppose the cat is good for scale, she also provides temporary weight when needed, wouldn't be the same without her.

    As for the sponsons yes the intent is to have them removable to allow for different options and because I'm going with the lightest material I can find for the build so the possibility of breakage may occur. My current predicament which has boggled the mind for quite a while is how to attach them. The best option I can come up with so far is to drill down from the top and place an approx 1/8th" pin down through with less than an 1/8th" sticking out the top, next take a nylon washer and drill a hole to match the protruding pin and screw the washer down to hold it in place. I haven't proceeded with the idea because there has got to be other options available so if anyone has a suggestion I'd be most appreciative. For the boom tubes I've chosen to use 9/32" carbon tubes, an insert can be added if they prove not to be strong enough. I had earlier purchased aluminum rod and 1/2" tubes and collars but am dissatisfied with the weight so opted to go with carbon. I took a couple of pics of the tube in the sponson to give an idea how things are going together.

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  16. #16
    After much deliberation I decided to attach the stubs to the sponsons by taking the stub which is 9/32" and inserting a 1/4" wood dowel into it and once it is placed into the outer sponson a hole will be drilled through in the center of the end of the dowel to place a screw with washer as seen in the pic. The dowel is epoxied into the carbon tube which had to be sanded lightly until a snug fit was achieved and the ends of the dowel will be epoxied as well to help prevent moisture from entering the wood. The outer sponson tubes are 11/32" outer diameter, both shafts are for archery which I already had, the inner is for a bow and arrow set and the outer is a bolt for a crossbow and are both easily obtainable at most hunting outlets. As far as strength goes there shouldn't be an issue, I took one of the shafts and tried to bend it with my knee (hardly scientific) and there was a considerable bend in the tube without breaking and the same amount of force would have bent a half inch aluminum tube. IMO it is better to have a little flexibility than too much rigidity, guess it all boils down to where you want it to break, in the body or the easily replaceable tubes.

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  17. #17
    I'm struggling to picture what you mean. Is the screw going through the sponson side?

  18. #18
    Quote Originally Posted by Jeremy_H View Post
    I'm struggling to picture what you mean. Is the screw going through the sponson side?
    I have temporarily installed one to show how it will go, the wood dowel will be epoxied inside the carbon tube and a pilot hole will be drilled into the end of the tube once it is installed into the sponson and a screw placed from the outside into the end of the tube to secure it into place. Hope that helps, here is the pic of it installed.

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    Last edited by arcdude; 04-21-2014 at 02:23 PM.

  19. #19
    Got it. I guess you're happy with the screw going into end grain? I figure it'll grip by spreading the wood into the bore of the CF if the screw is large enough.

  20. #20
    Quote Originally Posted by Jeremy_H View Post
    Got it. I guess you're happy with the screw going into end grain? I figure it'll grip by spreading the wood into the bore of the CF if the screw is large enough.
    Well I have to say that I'm less than comfortable to be honest. This is one item of a few that has dogged me from the mere idea of building this boat and can't seem to get much input from other sources unfortunately. As you say with limited expansion the idea was that it would provide adequate strength inside the CF. It is maple so little grain to worry about. I haven't stopped thinking about this because as mentioned earlier there has got to be other options available, so without a viable alternative this is where I'm at.

  21. #21
    Off the wall maybe, but what about elastic bands?

    Decent rubber band used for catapults etc could pass through the centre of tube from one sponson through the tub to the other, like a tent pole, and hold the whole thing in compression, you just need some form of cup to allow a knot in the elastic to site flush on the sides of the sponsons, and the shafts arranged so they socket together to a shoulder. It would be no problem to stretch the band out before knotting to get a firm setup.

    I'll get my coat

  22. #22
    Quote Originally Posted by Jeremy_H View Post
    Off the wall maybe, but what about elastic bands?

    Decent rubber band used for catapults etc could pass through the centre of tube from one sponson through the tub to the other, like a tent pole, and hold the whole thing in compression, you just need some form of cup to allow a knot in the elastic to site flush on the sides of the sponsons, and the shafts arranged so they socket together to a shoulder. It would be no problem to stretch the band out before knotting to get a firm setup.

    I'll get my coat

    Creative minds at work here, i did consider elastic bands and a spring go figure, only not through the tube however since you mentioned it I tinkered with the idea a bit. I also took another light weight 1/4" aluminum tube and installed an rc racing car clip into the end and considered that as well, no go, don't like the idea of metal to metal rattling around. It occurred to me the original idea is sound but inadequate to hold the screw in place without the worry of it coming loose or the wood dowel absorbing moisture. so I took a piece of carbon tube and a 1/4" nylon bolt and scuffed up the inside of the carbon with coarse sand paper and cleaned all parts with acetone especially on the nylon bolt in case there was a release agent on it. I decided on Gorilla glue as it expands and will fill in the threads on the screw and adhere to the scuffed up surface inside the carbon tube, after it set I cleaned the excess glue off and cut the head of the screw off and drilled and tapped the nylon in the center and am now able to install a 4-40 socket head cap screw. I banged bashed and pulled on that little screw and the nylon will not move out of it's new home in the carbon fiber tube, voila I created a threaded insert that should do the trick. In the pic I show a socket head screw in place and the type of nylon screw used to make the insert. Thank you Jeremy for your suggestion, appreciated.

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  23. #23
    Cool, much better idea

  24. #24
    I finally found a solution to removable sponson stubs as pictured below, I glued the nylon bolts into the ends of the carbon fiber stubs and filed the end square and while I was at it I glued a nylon bolt into the opposite end so when I bolt through the outer tube and the inner tube to bolt them together I don't have to worry about compression of the tubes. I bolted the stubs onto the outer sponson side to show how everything fits together and the finished stubs as shown. Alright now that this challenge is finally dealt with I can actually get onto building the sponsons.

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  25. #25
    That'll Work.

    Are you using full length rods from one sponson to the other, or half rods from tub to sponson on each side. I had a chat with one of the competitive electric rigger guys from my club this evening and he was using half rods, simply because it costs less to replace them in a ding. Racing riggers must be fraught with risk of damage so it sounds like a sensible idea for that side of the hobby anyway.


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