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  1. #1

    Calling all experienced outrigger builders, need advice.....lots of advice!

    Have built a few boats in the distant past....long long ago and am now considering an outrigger. What I have for an engine is a ST G-90, so looking for some speed on this one....gotta challenge ourselves once in a while. I have briefly spoken to a few fellows on RCU to date and they have been great in offering what knowledge they have and it has been greatly appreciated. What I need to know is basic dimensions length, width and whatever other tips may be helpful. I am a light builder and am aiming for approx 4.5 lbs or so with some luck and a tail wind.

    Thanks guys

  2. #2
    1QwkSport2.5r's Avatar
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    Quote Originally Posted by arcdude View Post
    Have built a few boats in the distant past....long long ago and am now considering an outrigger. What I have for an engine is a ST G-90, so looking for some speed on this one....gotta challenge ourselves once in a while. I have briefly spoken to a few fellows on RCU to date and they have been great in offering what knowledge they have and it has been greatly appreciated. What I need to know is basic dimensions length, width and whatever other tips may be helpful. I am a light builder and am aiming for approx 4.5 lbs or so with some luck and a tail wind.

    Thanks guys
    The G90 is timed to turn a little slower on a stock muffler so don't expect it to rev to 16,000rpm happily. A tuned exhaust is needed to get some extra power from it. I have the older version which is the S90K. I put a Jett muffler on mine and run a 11x8 or 12x6 APC at 13,500-14,000rpm and low nitro fuel (5%). I have it on a 3-point SI3 and really like it. But the SI3 is about 7.5-8lbs RTR. 4.5 pounds RTR weight is gonna be hard to do. You will need to build it pretty strong to handle he vibration the big .90's produce. Building strong usually involves extra weight. Figure 42-48" long for the center tub and an overall width of 20-24" depending on if you want straight line speed or if you want it to turn worth a darn. Wider for turning narrower for speed. Wet rudders are a must and a turn fin might be needed. The turn fin goes on the sponson on the side you want to be turning. Generally it's the left side I think. Check out the hydro airboat forum at RC Airboat World here: http://www.rcairboats.net/phpBB/view...0cc6398bc65b01

    Theres several nitro riggers and some electrics also. Might give you some ideas.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  3. #3
    Thanks 1QwkSport2.5r You say you have a G90 on your SI3 what kind of speeds are you getting from her? the tub itself will be approx 40"x3.5" and about 1.5"-2" in depth and lite ply over foam, don't think I'll get any lighter than that. As for width I was contemplating 24" outside dimension so at about a total 48" in length it should handle the motor which will be inverted. I've also read the G90 will really pop to life on a tune pipe, so that'll be a consideration after the build. For the onset I'll be trying higher pitch props for top end speed rather that quick response at lower rpm's, at least that's the thought for now. Thank you for the site info I'll definitely check it out. Should a turn fin be necessary I would think one of the turn fins that are bent inward half way down might be the way to go for oval maneuvers that way it'll help hold the inside down in the turns and hopefully counteract the high CG with the air engine.

  4. #4
    1QwkSport2.5r's Avatar
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    I have an S90, not a G90. They are a little different due to some different internal parts (crankshaft and piston for sure possibly the rod.) and I think the S90 is timed a little bit faster. I haven't ran it much because my little SI3 with the Jett .35 has been my goto for a run at the park. I suspect to see 50-55mph on a good day from the big SI3 maybe more. I'm using a tuned muffler not a tuned pipe. I don't think the .90's really benefit much from a tuned pipe because they're not timed real high. You will get a boost in power but not as much as say a S61 ABC. They can't breathe well enough due to having a .90 piston/liner in a .60 case. The gas passages are much smaller in the .90's.

    Liteply over foam isn't likely to be strong enough for the .90 especially on a pipe IMO. If you incorporate some aluminum for strength and fiberglass the tub then maybe. I wouldn't be running anything bigger than a .60 on a wood over foam build personally. I build with balsa frames, and various thickness birch plywoods. I can stand on top of one of my riggers crap sponsons with one foot and it won't crush, crack, or break. My little SI3 .20 series with the Jett has run 49.8 with a 3" rudder and wrong CG, and that boat is 5lbs RTR. I expect to see 60+mph this year. I think 4.5-5lbs is unrealistic for a .90 sized build since the engine weighs about a pound with muffler or a little less as it is. So if the engine is a pound, that gives you 3.5-4lbs for the hull, control rigging, electronics, paint/finish, etc. I wanted my SI3 .20 to come in at 3.5-4lbs and I'm glad it's heavier. It'd be airborne by 40 otherwise.

    This is all my opinion, so don't be offended that I'm not totally on-par with your ideas.. I love the idea and can't wait to see what you build, but I have to look at it from a realistic point of view. Weight can be of a benefit to a point on fast boats.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  5. #5
    1QwkSport2.5.r Trust me no offence taken, it would take a lot more than that and I respect your opinion derived from your own experiences. The G90 is rated at Practical RPM Range: 2,500-16,000 according to Supertigre's website with a 12x8-13x6 prop so that's not too bad. So you haven't tested the big SI3 yet? As far as strength goes I have considered inlaying square carbon tubes through the length of the hull but you know what foam and lite ply for the size is pretty cheap and worth the experiment. My objective as with your own ideas is taken from airplane dynamics so the stress's will be somewhat diminished and if not the carnage should be spectacular if caught on video. The 4.5 stated is what I'm shooting for and if it comes out a little heavier oh well, just a benchmark I'm setting for myself, maybe unrealistic but it's sure going to be fun. Should the hull run perfectly horizontal as I'm expecting it to it may just work out fine, the beauty of the design is it wouldn't be too hard to downgrade to a 46 or so for a more practical fun boat. I would like to ask you a question about the SI3 front sponson angle sideways are they perfectly horizontal at the rear of the running plane? Thanks again
    Last edited by arcdude; 01-28-2014 at 10:24 PM.

  6. #6
    1QwkSport2.5r's Avatar
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    Quote Originally Posted by arcdude View Post
    1QwkSport2.5.r Trust me no offence taken, it would take a lot more than that and I respect your opinion derived from your own experiences. The G90 is rated at Practical RPM Range: 2,500-16,000 according to Supertigre's website with a 12x8-13x6 prop so that's not too bad. So you haven't tested the big SI3 yet? As far as strength goes I have considered inlaying square carbon tubes through the length of the hull but you know what foam and lite ply for the size is pretty cheap and worth the experiment. My objective as with your own ideas is taken from airplane dynamics so the stress's will be somewhat diminished and if not the carnage should be spectacular if caught on video. The 4.5 stated is what I'm shooting for and if it comes out a little heavier oh well, just a benchmark I'm setting for myself, maybe unrealistic but it's sure going to be fun. Should the hull run perfectly horizontal as I'm expecting it to it may just work out fine, the beauty of the design is it wouldn't be too hard to downgrade to a 46 or so for a more practical fun boat. I would like to ask you a question about the SI3 front sponson angle sideways are they perfectly horizontal at the rear of the running plane? Thanks again
    The realistic rpm of these engines is right around 12,000 in an aircraft on the stock muffler. Airboats are like cars - can run higher rpm for short periods of time. I was getting 13,800rpm on a 11x8 APC prop and Jett muffler. It would take a 10x9 or 10x10 to turn 16k but at the huge sacrifice of thrust. If you can get 13k on a 12x6 APC you're doing pretty good.

    I have ran the big SI3 several times, just haven't checked speed. Now that my GPS is fried, it's gonna be awhile before I can replace it. I have videos of my big SI3 on YouTube.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  7. #7
    Thanks 1QwkSport2.5r for the input, I've read a little about the older style ST 90 apparently it has a two piece crank. As far as how the G90 will perform I have no idea yet i yanked it out of my mustang and I was only starting to break it in so time will tell. There is several SI3's on youtube so I probably have seen your run, they all performed great and very predicable at least from the keyboard side.

  8. #8
    1QwkSport2.5r's Avatar
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    The crank is one piece, the front housing comes off. They can be run in reverse by rotating that front housing. Either engine are good engines, just keep the nitro low and give it lots of time to run in. They throttle rich in the midrange until they have time on them.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  9. #9
    1qwksport2.5r Thanks for the tips I'll keep them in mind if the 3+ feet of snow in the yard ever disappears!

  10. #10
    Question for all if anyone has an opinion and at least point me in the right direction, What would be an acceptable weight for a sixty size outrigger (complete hull with sponsons attached and no gear installed)? I will probably be looking at about 4 feet in length with a main tub of 4" wide. There must be a ball park figure, or is it just set at the lighter you build it the faster you go. I have built numerous rc airplanes and a few life size boats and a couple of rc boats but nothing along the lines of a rigger so some idea of what I should be shooting for would be an asset, Thanks


    Also is there any interest in starting a new build thread for this boat, I'm sure you guys have seen everything before. I don't want to waste anyone's time or my own?
    Last edited by arcdude; 02-06-2014 at 10:06 PM.

  11. #11
    1QwkSport2.5r's Avatar
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    My .46 rigger is about 6lbs RTR as it was with the old sponsons. So a .60 size rigger should probably be 8 lbs or less I would think... That's totally a ballpark figure for the way I build.

    And yeah, I'd be making a build thread. Pics are great to look at IMO. You never know what you might learn from the next guy.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  12. #12

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    What would be an acceptable weight for a sixty size outrigger (complete hull with sponsons attached and no gear installed)? I will probably be looking at about 4 feet in length with a main tub of 4" wide. There must be a ball park figure, or is it just set at the lighter you build it the faster you go....
    Weight affects acceleration, not top speed. Well, it does some but the important thing is to balance weight with lift. We often have to add weight to our water prop boats to keep them on the water if we run them too fast. If you boat is heavy and doesn't perform well, increase the amount of lift by adding angle of attack to the sponsons. It is all a balancing act.



    .

  13. #13
    Got RPM Thanks for the input, appreciate it.

    Question in designing the hull and sponsons do you try to get as much angle on the upward surface as on the bottom in order to prevent it from blow over?
    Also in the inboard models what is the water clearance to the bottom of the tub (ball park)?

  14. #14

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    Sponson tops are generally neutral to very slightly downward. You want a net increase in lift from the sponson bottoms, this comes from both hydrodynamic (water) and aerodynamic lift (air packing under the sponsons). Ground clearance for SAW-type boat is usually around 25mm.


    .

  15. #15
    Got RPM Thanks for the info. At what point (height) does the hydrodynamic pressure decrease or lose adhesion? As I have things plotted out at the moment my ground clearance would be about 1 1/4 inch's ground clearance, she gonna fly?


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