Video from yesterday:
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Good evening jer.. ya know the boat is so like a river barge just ment to go forward with the flat sides and upturned bow.
Im in agreement that the sides need a straight drop from the deck then a nice angle down to the bottom all the digging in would b gone. Just my 2cents
No fear!!!!!!!!! I think thats the ticket now i bet gyro rudders will need to be deployed....
Well that was interesting fun! :-)
High winds made it a totally unfair test, but I found some more limits lol!
My 2 cts; your air rudders swing to much in each direction and makes airboat act like a regular mono. Second, the weight of boat is centered and not distributed across the CG line to make airboat glide. Third ,the angle you cut the sides was to much and depth of airboat is too deep which let air catch to much air. The bottom edges of airboat should be rounded so edges don't hook in water but glide over water and rounded and transom be square and straight horizontal and vertical. Where I picked this up from Cajun of Liberal, La. who builds airboat for a living and did have a airboat site . I've had great success from his info of which I just have typed.
Holly wind burn batman, that was fun...... i think to my eye the rig ran better, it did not seem to dig in on turns as badly...
Lease jeremy dont kill your project to satisfy my thoughts. I feel the design is workingnow if you dbl
The speed who knows how it will handle.
I do agree the cg inline my b causing an issue some cross weight might not b a bad idea
As far as the angle being to sharp i cant agree on that one..
But with the wind during testing ALL is really mute right now.. cant wait to see it in this config on a calm day
Well doing this has certainly opened a can of worms . Good! Previously the boat felt cumbersome but it's certainly lively now.
Before I continue though I must talk about that wind. Weather sites were showing it as 16 - 31 mph. I know a bit about wind through another hobby, and in this case the geography around the pond was contributing to some gnarly conditions. Immediately to the left of the camera is an open flat and wide valley, the wind was coming along that at a slightly oblique angle, rising up a slope to the bank of the pond. So, if I'd measured in on the ground I would have expected to see that stated wind speed about realised with no problem, add the turbulence the bank adds and it was a bit of a snake pit on the water. It's a pity that the video doesn't really show the winds effects much, the only plants waving around are native reeds and Gorse bushes, as plants that wave around a lot don't grow on Dartmoor. One seasoned expert, the proprietor of the UK's leading RC Powerboat outlet http://www.modelmarinesupplies.co.uk/ simply stated it was too windy for an airboat, and I think he was right hehe. Other boaters were airborne with well trimmed CMB90 engined Aeromarine Sprint Cat's. So cumulative speed of air passing under the hull could have been 40 mph. So worth keeping that in perspective.
However, for any wind to be able to lift a boat it has to get under it in the first place, and I agree that the sides are allowing that. I think the angle is right, it's an optimum for perpendicular force distribution in any number of areas, but I think what you mean is that it's cut too deep, so the angled side is too large a surface. I have to see that as being a possibility, it's a big lifting surface on the outside of the turn, and a reduced buoyancy surface on the inside, plus the inside lift from the water is being released far closer to the centerline of the boat, all of which contributes to the roll. But, at this time I do not believe that the boat will lift under it's own speed in sensible conditions, it was in danger of being blown over when stood on shore let alone under way on the water so I'm yet to be convinced that it's actually a problem in itself. This boat is not fast so any risk of it turning over under it's own apparent wind generation is just way out there.
The distribution of mass has not been mentioned before. I know exactly what you mean, and it's right that weight distributed to the periphery of the boat would resist rotational forces far more than mass accumulated along the centreline. That's good to know as the final boats are more in keeping with the more desirable distribution.
The corner between the bottom and the sides is in fact radiussed, whether it's enough is worth discussing. Again I'm familiar with the different properties between a sharp edge and a round one with water running off them, it's classic surfboard rail design considerations for the same reasons, where I need some more input would be to clear what that actually means for a boat like this. A hard edge on the outside of a turn would promote lift, but the inside edge would be too, resulting in break out of the inside edge and so into a drift. A radiussed outer edge will still lift but less so, and the inner edge would suck in resisting setting the hull into a skim. So in my head the radius is contributing to roll as a consequence, the inner edge is not lifting enough anymore and allowing the hull to carve. That is quite contrary to the report from Cajun of Liberal, I'd need to be sure that you got that one the right way around. Some more info may re-direct me on that one, but there's another consideration that I'll come back to. On previous airboats I've enhance that area of the boat by fitting spray rails, sometimes square section, sometimes triangular so they work differently depending on angle of attack to the water flow. With spray rails a hull's slip can be controlled by design.
The transom is square edged and dead vertical.
It may help some to know that I work for a company that makes fast luxury motor yachts, in fact a global leader in the field, and up till now I've not talked about hull design much because it's not that important on this test hull. But, I will at least be approaching that aspect on the final boat with that basic foundation of knowing a bit about boats. I mentioned that I'm in mechanical engineering, but that has always been in the marine field. Maybe I should have mentioned that.
Using full throttle was not a happening thing, I occasionally did so heading down wind, but had to chicken out on the turn out each time as that air lift was scary one way and the blown spin out the same the other way, in the vid you can see it going light a couple of times. It actually flipped again later off cam but landed right way up, engines had been killed though. Getting back to the roll I'm not surprised to see a hull acting like this at low speeds, On the few occasions when I got it to fully plane it showed signs of a possible flatness to the turn.
The book on air rudder design has opened right back up. In this last test this is the area that actually changed the most, or at least new properties were witnessed. The weathervaning effect the boat has is massive. What may not be apparent from the video is that the amount of countersteering I was having to do to keep a straight line was huge on occasions, and often simply not enough to rudder what there to overcome that with the reduced throttle I was having to run, quite scary at times. On occasions as the boat turned down wind the back end would be kicked around by the wind into unplanned spin outs. Fitting the original smaller rudders back on improved this problem a lot, and it maintained enough steerage. So maybe the shape and sizes of the rudders could be reconsidered, but not the angle of operation. Again going back to 45 degrees for the same reasons as the hull side this is optimum redirection of force, so this permits maximum scope of steering range for a given rudder design, it's the way that scope of movement is used that's the issue. Here, I tried applying dual rate as I have been doing all along, previously this would give me the usual finer control over steering input at the higher speeds as is predictable, but when trying it here suddenly I didn't have enough scope to capture the unpredictable gusting turning force of the wind. So the resulting effect coming across on the video is manic steering. Some of turns were not in fact me at all, think about that, in fact it's me trying to keep it as smooth as I could. Later I powered up the gyro and a fair deal of my input was handed to that, it worked a treat resulting in slightly better tracking than manual with far less input from me, but that's another story.
For me then this has been a worthwhile exercise, I think I did OK in the conditions and as the idea was to expand on air rudders and that's certainly been opened up. Plenty of smiles too, so looking forward to seeing what comes out when it's able to run in sensible conditions. Does the hull now have enough straight line lift to plane fully? Will it lie flat in high speed turns? If it doesn't, what can be done about that? will it simply act mono like that that's the way it is? I think the front end is suspect on the hull now too, but need to get it out of the water more to know for sure.
Here's my prediction: It's going to struggle in high speed turns because of a conflict between the drag on the inside edge, and the lift of the sides. This will make the outer side lift till the resultant drag on the inner edge is able to overcome that, it'll then slam back down and repeat the process over and over again, so all through the turn the outer edge will be bouncing. A possible solution is to widen the boat (no!), another could be an inward facing rail to increase lift on the inner edge to lift with the outer so they come up flat, or maybe from what i've learnt about air rudders mine should be parallel, or even turned upside down.
So plenty to look forward to, a fascinating exercise, I'm so pleased I'm here right now, really enjoying this. So, moving on.........
Engines for the final boat have arrived. Budget SC's, not had these before but they seem to fayre well if looked after and great value for money. They appear to have been crafted well with some nice touches. I'll start a process of running them in whilst I progress with the rest.
I've started working on the engine setup for the target boat, looking at rubber mounting to help with noise, throttle will be the test boat arrangement with maybe some other mods to improve adjustment. I'm diverting a little sideways to look at exhaust systems. This boat is far too loud and the next will be worse, so I need to look at quietening it, at the moment additional silencers are under consideration, with possible water injection which does loads to take the crack out. Not a simple subject as we all know that 2 strokes rely on timed back pressure pulses to breath properly, so have to work within that. Tuned pipes would have to be convoluted to fit, so that's another option. Plenty of fiddling to do
Last edited by Jeremy_H; 09-28-2013 at 08:43 AM. Reason: typos
I went thru the video many times and when you turned sharp, it lifted up a lot. If you dialed in less amount of turns like 20 degrees would max would lessen the lift. I agree on the wind lifting the boat up. I have a Anarchy which I put car wheel weights on the extreme inside edges at CG line which helped in gliding and tho it is 16" long and a different design than your airboat. Bam, it just hit me, your top heavy. Lower the weight toward the bottom will solve the blow over and when turning, the lift will be solved + weight across the CG line. Now just like ballast in a regular rc boat. Now a low balance helps boat turn sharp better than a high balance point and less blow over. This is my last posting on the subject, so I won't tick you off, ok! I'm retired and been thru the testing period and that is what I've come up with. Cajun told me 20 degrees is plenty on air rudder. This was many , many years ago. I have a BBY 29" mono and a 31" Villian EX which I've muddied and run to fast for me, but what FUN. Electric.
Mikeup i doubt u will tic him off he seems the type with an open mind...he has tried many a sugestion wth no regrett just to allow us a part in this build.
As far as spray rails go id try them. And i wonder would some hull stepping be of any use?????
I thank you jeremy for the time you have shared along with spreading your smarts about.
Do to my growing inability to walk more then 30 yards without extream hip and leg pain( went to a uc football game and damn i about did not make.the walk to my seat) i have yet to get my rig on the water so my mind is in build mode for another foamy...
Very interesting to watch about you, but we believe that the two engines arranged in this way, you do not pass the exam and did not meet your expectations. A similar case had the hull.
Hull the other had you better, but it was a flop.
Therefore, a new body to AirBoat 26ccm will be made as in the first picture with minor modifications top model. Of course, we will post construction AirBoat lost photos.
Greetings from Poland
Anyway, my apologies for your gaining a negative feel from my response, maybe a product of the written word where the tone of delivery is not always clear, you're welcome to post any view you see fit, this is a hobby and all of this is with that in mind.
Last edited by Jeremy_H; 09-28-2013 at 09:51 AM. Reason: typos
The final hull has a step, it's shown on the drawing at the top of this thread, I'll definitely not be doing that to the test one though. I think already there's a serious conflict between such hulls and air rudders which may be too much of a compromise.
Last edited by Jeremy_H; 09-28-2013 at 10:18 AM. Reason: typos
We are sorry for our English, but we use translator that doesn't reflect what we're writing.
There are photos of old good shape of hull and new bad shape.
On our case the new design didn't work.
This post we've written without translator.
Greeting from Poland
Last edited by St.Marek; 09-29-2013 at 01:12 AM.
Hah! Brilliant. Yes, got it now .
Interesting, hard chine better than curve for you. So much of this can be different from one boat to another, to achieve the same aim. I think mine would be better with hard chine.
I enjoyed the session with the test boat this morning. Having turned to the mid sized rudders upside down, and applied a bit of down force from my winglets, it aided the turning but predictably it was hopping in turns. As Mikeup pointed out the CoG has become critical on this boat through the loss of side bouyancy, just too far the other side of what it was. I left the spray rails at home like a goon so concentrated on working with what I had. I'm happier after today as the rudder decision is becoming easier.
There's some shaky vid to come of some better running, but for now some pics taken by a clubmate:
I'm going to break president. I would consider you have reached max on this airboat and don't change a thing!!!!!!! The airboat did everything a boater wants it to do and extra!
I think you're right. Apart from all the known's we've discussed this boat is pretty close now to what could be achieved with it.
I'm moving on with the project and this boat will stay together as things develop, I'll probably try the rails to be honest at the least, my curiosity is aroused now. But getting back to the main idea here I do feel that the twin engine thing is viable, actually worthwhile. Putting the turning thing aside, it's easy to forget that this heavy and large hull is being moved along at a respectable pace by little engines, just a .20 and .25, so I'm very happy with the performance the desire to have twin engines is giving me so far.
What I've learnt specific to the project:
In - Line twins can develop more power than the same two engines side by side. It's possible that two engines the same might not do that, but this use of pressure recovery by using a fast engine on the front, and an over-pitched more powerful engine on the rear is giving me more power than the sum of the two running separately. I've no idea where Marek get's his conclusions to that from.
The rear engine is utterly dependent on the front one for cooling. It will survive for a time on say third throttle but above that and it's suffering, so keep an eye on fuel consumption and ensure the front engine will have more fuel to run for longer.
Whilst keeping the fuel tanks off the pylon has saved the height of the CoG the height they still need to be is a challenge to deal with unless I go back to the thought of pumps. Right now it works fine, but my cobbled together pylon and mounts is an indicator that that area is going to have to be kept at light as reasonable. All effort must go into getting the CoG down, and spreading the load wide. I knew that, the original drawing of the final boat shows props below deck level, and the position of the center hull in relation to the sponsons, the hull itself has the spread of weight.
I'm still hearing the case for air rudders. They are great fun and not to be underestimated in design needs. Right now I'm thinking about how the final boat might have to change at the rear of the hull to accommodate them. I think the test boat will help towards that to a degree, possibly more than I anticipated.
Sound is a real issue, but the rigid aluminium engine mount assembly is sounding like a biscuit tin and rubber mounts are going to be a must for these twin engines. Vibration is very must present and it must be isolated from the rest of the boat to cut noise down. The final boat is a column pylon, not this bridge affair on the test boat, so it's going to be lighter already, but that too might benefit from being resiliently mounted, dunno. The exhausts need something special too, I'm working on simple additional cans with water injection at the moment. The powerboat boys use these to great effect to take the crack off the exhaust note. I'll probably try them on the test boat.
Finally, as much as it's too loud at the moment, I do like the sound of a twin under way .
Ok, so next I'll sort my test mounts for running engines in at start that process with the SC's.
I will fit the water rudder I put together when I have calm weather to be able to judge the differing hull needs.
Start making the next test hull which will be all about what's underneath. I want to know more about steps, in particular venting them, as I have a hankering to try something I've not seen, but probably does exist. I'll look into that some more this week to see what's known about it before I take it any further.
Last edited by Jeremy_H; 09-30-2013 at 08:56 AM. Reason: I've just noticed, it you look at the last few seconds of the last vid you can see how a breeze spins the boat around
Bravo i agree with mikeup it seems to b running very well... now more power lol faster faster...