Jeremy_H

I see your thinking. I'm looking at pivoting fins to aid that differential

Jeremy_H

This is a trial fin to get me going on seeing if angles work. I've fitted a means of repeating a set position should the fin be taken off for speed runs:

arcdude

Are you trying the fin on both sides or just the left? I'd be curious if curved fins would work on both sides simultaneously, they'd likely have to be absolutely zero degrees or lift and diving may occur. Interesting?

Jeremy_H

On this design the outer edge of the sponson edges in turn to provide bite, I can imagine that any thing that lifts it in a turn will be bad. It's a lot of speculation, trials will be best. Just one side at the moment, if my pivoting fin works then two.

Jeremy_H

A poor video of some turn fin tests

http://youtu.be/6A8UYzH6W-k

arcdude

The slim angled fin seemed to hold the curve much better at least what I can see on the vid, were you getting greater speed with it? Too bad the pond seems small can you actually get a full speed run? The boat does run nice!

Jeremy_H

Yes the slim fin is best at the moment, it's faster with it for sure, you can hear the engine unloading more.

The curved fin bites much harder on a tight turn, but it's too far forward so the front stays locked whilst the stern digs in, resulting in this if you put too much rudder in :

http://www.youtube.com/watch?v=xrPzN3XX-QM&feature=share&list=UUw8b4mq7g44FwvGq97q6aKw&index=1 Watch that on YouTube itself as this forum crops them

I think the reason the slimmer fin is better is simply because it's not biting so hard, the rooster tail is less fine on the curved one, but far more volume of water in it, pretty much matching the right sponson's. So the next stage is to slim down the curved one and see what can be done to get it closer to the center of balance. It would actually be OK to go with the slim fin as it is, but I'd really like to get to a place where can go hard over at full speed without drama and get a tighter turn than the slim one achieves, then I'll look at the pivoting action.

One thing though is that the setup of the curved fin was easy, I started with straight in line, and tweaked it to suck the inner sponson down a tiny bit. I think some careful setting up with straight edges held against the fin helped with that, dead fore and aft and parallel to the deck on the curved bit.

Jeremy_H

Oh and no, the boat is still accelerating when I have to turn, in fact the boat is turning pretty much the whole way around, so not seen it at full chat yet. I might pay a visit to Big Pond this weekend and see what prevails.

arcdude

Now was that a barrel roll on the video? Can you get enough of a straight run to see if there is any interference due to the fin, sucking down and so on? Can you strap a GPS on it to get a speed value, not sure if the motor is broken in or not?

Jeremy_H

Hmm, more of an axial roll really hehe.

Weather is perfect again today so may head out to Big Pond if I have time, perhaps tomorrow after the club meet, there's about a 200m run available there, it's just a hack to get to with all the gear, the guys including me are rigging golf trolleys to carry our gear the 1/4 mile over boggy moorland to get to it :-) . GPS's are notoriously inaccurate for speed over short distances, which for the benefit of anyone reading this I'll explain. Generally a GPS will tell you how accurate it is from any given position, my Garmin Etrex usually tells me it's about 15 -17 feet. If the boat then travels, say 50 feet at full speed, if the GPS updates every second it could see that as only two points, if we take it as 15ft accuracy for ease each point could be plus or minus that amount, so as much as 80 feet travelled [2 points x maximum '+' error for each therefore distance travelled+(15x2)] or as little as twenty feet [2 points x maximum '-' error for each therefore distance travelled-(15x2)]. So, if the GPS error sees 80 feet it's going to give a max speed reading which would probably make one smile a great deal but would be as useful as breasts on a fish. There are so many YouTube videos with folk proudly showing the speeds obtained but I'm sorry, if the distance travelled is short it's not reliable and you're kidding yourself. Hence why Radar is the only way speed readings can be used to declare formal records etc. One of my other hobbies is Parakarting, using foil 'powerkites' to propel buggies, and there's loads of debate about this so it's well known to me.

However, my view is that GPS readings give something of an indication, and it's fun seeing what can be gained and I'm happy to join the party on their use, it just won't stand up in court as it were. So yes, I could strap my Garmin to the boat and see what I get. I estimate that at the moment I'm doing about 35 mph around the pond, I see it as being 40-45 in open water which was the target for the build, and I reckon the curved fin is robbing it of maybe 5-7 mph, the thin fin perhaps 3-5.

I set the fin to neutral to start with, then after some runs tweaked it to give a little bit of pull down to aid keeping that inner sponson held down. Tiny amounts though, too early to draw finite conclusions.

On the motor front I have a direct comparison, my son's boat has the same engine with lots of run time and his is audibly revving far more than mine, but he has a Master Airscrew 12x6 on his, mine has a APC 12x8, so that's all very well but it's thrust that matters and it would be wise to measure that rather than RPM. My gut feeling is that there's more to come on mine. I add ZX1 to my fuels as I'm a complete convert on the stuff, but it does mean running in takes much much longer.

By the way I've been thinking about the rooster tail differences. We must take into account that the finer tail coming off the thin fin could be the result of greater speed, it's not necessarily indicative of the holding power of the fin.

Unfortunately one of the niggles with all of this is that getting reliable video is tricky, I have the gear, but I need someone to operate it. The vids above are simply off a phone rigged to the transmitter, and the wide angle lens on phones crucify the perception of speed due to perspective issues. What's really needed is a long focus lens, a stable mount, and a smart operator to follow it. I'm seriously seeing if the 'Soloshot' system can be used, this rig automatically follows a transponder on the subject, I don't know if it's fast enough though, I'll see. I like making videos (check Jes437 powerkite vids on YouTube), and to date there is not a quality video of one of these boats, there's plenty of fun vids like my current ones which are great, but it would be good to have a properly produced version some time.

Jeremy_H

Much improved running on a slimmed down turn fin, and a mod to the transom:

http://youtu.be/ewegPZtoeCA

Jeremy_H

This is another short vid where I setup the camera in a way that would show the boat at speed taken at Big Pond, but it proved too tricky to follow by my daughter who had a good go at it but couldn't resist laughing and eventually gave up , trying to sight that on an LCD screen through a 400mm lens is a nightmare, so other ways are under consideration

https://www.youtube.com/watch?v=0Hqs2Ykmxoc

arcdude

Well done Jeremy and hats off to your daughter for the fantastic video skills, she's having fun which is the end result to all this. In regards to the GPS I somewhat agree to it's inaccuracies caused by time delay especially at a high rate of speed, but as you state it gives somewhat of a ballpark figure as to what one is achieving speed wise and I for one am not about to invest in a radar gun to acquire a more accurate result. Out of curiousity what is ZX1 that you add to your fuels, I've never heard of it? I would agree the slimmed down fin seems to have less drag at least by what can be observed in the video, it's sure scooting along nicely, looks like it handles well. what was the mod to the transom that you mention? The power kites look like a blast at least the buggy style ones, when I was a teen I fashioned a sail on an old ten speed bicycle, it did work but was too hard to control the bike and the sail at the same time, everyone else around spectating must have thought I was nuts! What is it with this fascination with wind power, fun stuff.

Jeremy_H

ZX1 is an oil additive that removes most of the friction from bearing surfaces. It is not a coating, it modifies the surface molecules of the metals, and the treatment works by it replenishing itself from the dose given. Now, I could go on forever about this stuff, but in short I was involved with it professionally and as much as it sounds too good to be true, you'll have to take my word for it, it's good. Probably best if you have the time to check out their site fully, I have voluminous personal evidence of it's validity, and there's only one way anyone ever really believes it, and that's through using it: http://www.team-zx1.com/

The oil industry hates its presence.

On the rear of my tub you'll see an overhanging tub bottom. I did that because I thought if it ever sustained damage it would be easier to fix a ply edge than the bottom corner of the transom itself. I know this is a common practice on riggers, but I had no information on the length of the overhang so made my own up. I like to use photos and video to analyse a boat's operation, and to aid that I tweak the images to draw out whatever element I'm looking at. I had developed a suspicion that my overhang was causing problems, this stemmed from a comment from a club member, that the boat drifts. Now that's easy to see and no surprise, but it occurred to me that my rigger-like overhang is used on boats that don't drift, water prop riggers turn corners freakishly fast, no drift. As such I could imagine that as airboat riggers do drift (a consequence of there being no lock down from a water prop) it was possible that some turning issues I've had might be caused by pressure getting on top of the overhang and making it dig in. What I was finding was that intermediate speeds the boat resisted rudder input. I corrected this originally by adjusting the end point of the servo to give me more throw, but then it was way to much at higher speeds, hence why I flipped it in the first session video after I adjusted the rudder.

To explore this I referred to this photograph. It's a still from the video above, simply adjusted from brightness and contrast to isolate the water flow in the rooster tails, doing this drops out the finer spray. It shows the boat during a slight right turn, very slight. The ejection off the sponson is the line at the top of the tail, the turn fin fitted here is the much heavier streak in the middle, but right at the bottom at the stern of the boat, normally less obvious because of spray from the sponson, there is a comparatively massive pressure zone bearing on the side of the tub which bursts into mist as it leaves the tub side. I could easily imagine that pressure getting on top of the overhang could cause it to dig in, rail, hence resist the rudder input. Simple when you think about it.


This photo was taken on a 1/3200th sec exposure, it's easy to see why the spray zone might hide this effect on a boat travelling past at maybe 35-40 mph, where those individual drops coalesce into a blur. Incidentally check the mass of water ejecting from the curved turn fin:



Anyway, to fix that for trials I rigged this, couple of CF cheeks to block off access to the top of the overhang, at this time I believe it has fixed the problem, I must make it permanent and drag free to finish:



The curved fin has been cut down to this:

arcdude

What you've created is essentially a trim tab on the transom, I have a cracker box I purchased some time ago and it has them in the kit. What I've been able to find out on the web is people have been adding a side piece similar to what you have done with the exception they bend it on a 45 degree angle. What if the tab were shortened to accomodate the angle, it would then be behind the side of the hull, but then you'd have refinishing to do.ZX1 seems to be an interesting product and I see it's only available in the UK, I don't disbelieve the claims they now have come out with a product that literally sheds water off anything including your shirt and if I'm not mistaken Nissan with be offering it on upcoming vehicle's, for a price of course. Hope that wake from the turn fin is in a turn? do you have it set at 0 degree's?

Jeremy_H

Yes it's a turn. Zero degrees vertically, about two degrees of downforce.

Jeremy_H

There's global ZX1 outlets in the contacts page on their site

Some above deck aerial tubey thingies:

Jeremy_H

Now that you mentioned it, just for a bit of fun I've dug this out of the footage I have of that day, to show the wake when straight, this is with the straight turn fin. Don't blink .

http://youtu.be/-qHkixoJhOU

arcdude

That's hilarious and there wasn't enough time to blink The height of the wake is quite visible surprisingly, why would there be such a wake from it if it's straight? Would the curved fin have a negative effect at lower speeds because the angle of attack would be somewhat at an incline in relation to the water? Figure this all out so I'll know exactly what to do when I get to that point,

Jeremy_H

To be honest I don't see anything clear on fin output, I see two trails off a three point boat, I've been trying to figure out which two points it is haha

Jeremy_H

In essence yes, at work we call them 'neutral fixed tabs', i.e. a tab that is at the same angle as the hull bottom.

I'm completely in the dark about the 45 degree thing you mention, any pics?

arcdude

I either have to up my medication or get new glass's, possibly delirium. To me on the 1/3 right side of the screen it appears to have more wake disturbance on the left than right. I'm getting dizzy watching it blip past Lol.

arcdude

Okay did some digging through my pics and here is what I found and from the descriptions I have read it helps prevent the hull (transom) from digging in on a turn.

Jeremy_H

Hehe, it's almost addictive

Yeah I know what you mean, there is a deeper trough on the far side, but I think I know what's occurring here. Well, it's a theory

First off the fin, remember this one was angled at 45 degrees, and it's shaped into a simple airfoil, it's carbon, and just 2mm thick. In a straight line it's going to cut in, the spray off these is quite fine. I've not used turn fins before, but I'm used to making my own rudders and have played with all manner of shapes, so I've seen the effect on water before. It's that I'm not seeing here.

What I see is the result of slaps, the boat is going over it's own wake, you can see the return run at the far side of the lake still expanding.

I've mentioned elsewhere that this boat's rear end is often out of the water, there's plenty of pics of SI3's doing that, and I'm pretty sure, certainly on the original full HD version, that I can see the path of the rudder dead in the middle of those two main lines running in clear water.

So, my theory is that it's running on the sponsons alone for most of this pass, that either circumstantial, or perhaps torque forces are making the left sponson hit the ripples harder, but the last 1/3rd sees the tub touching down.

I'm going to lie down.

Jeremy_H

Right got you. Something to put in the bank I think, often referred to as 'Drag Tabs' that design will suck down like a goodun, but some sharp corners would improve things.

Transom Trinkets