headhunter23

But hows that make sense, if you had the boat firmly in the water, the air bubbles wouldn't come from no where. I'm thinking its the shaft that's letting in the air. Think of it as a water pipe, which is pretty much as it is, if the one end is all water then no air would be coming up, unless it's somehow ripping the molecules apart, but that I doubt. Unless there's another issue in which the bubbles are just from the hull as it travels over the water then are sucked in. I think we need some physics genius to properly explain whuts goin on.

Ivan[:@]

DRAGONFIRE81M

we need quick to come on he has talked about this before . i only remember parts of it and i thank one was about ripping the water apart . this is what i know i have more then anof power to run rapides . i only use half of the power . btw i just bidded on my sec nock off . so my son and i can do some racing . know i just need to work on the screaming deaman to try an keep the noise down

brushlessone

the knock off i got from ebay , had the electronics glued in with so much hot glue it was ridiculous, they had actualy slightly damaged the hull by putting it in there t hot wich made the hull ripple slightly in places under where the elecs were mounted... i damaged the botom of the hull getting the crapy servo out so i had to repair that little hole with epoxy wich was easy enough , but the amount of glue these little people put on this one was plain stupid lol
next time im going to heat up the hull to around 30 40 degrees then it should just pry off real nice and easy , i orderd two of the 370L HTX 2900 KV motors today along with 1 X 1600mah 7.4 lipoly pack and 2 X 1600mah 11.1 packs they should be here by next friday

DRAGONFIRE81M

did you have any problems with the secarty on there site. i had to bypass it to get on the site. talking about UH

i had damege to the top eage where it looked like they toached it with the glue gun


well number 2 is on its way i askes for a yellow one this time hope i get it . also ordered that same hxt 2900kv and a 30A esc from uh just need to pick up a servo . and ill get two more lipos pretty soon . man cant wait . wish i had it already its supost to be in the upper 60's towmorrow []

MKMotorsport

Jet villain is underway. I've cut and hacked the rear 2" off, and "fabricated" a wood/fiberglass flat rear transom to make way for the 29mm Graupner jet. Totally cut away the second "inner" hull. My stubby Villain is shaping up good so far. Also trimmed the stock rear hatch, er cropped it to match the hull. I'll have to get some pics of this beast up. Not sure on electronics yet.. Still need to replace the smoked bushing in my knock off jet sprint. [:-]

brushlessone

Dragon.. na man no worries with their site security , worked just fine for me , i orderd a hextronik 2900kv motor too and a few lipos to play with now all i need to do is get myself a lipo balance charger, im thinking the chinese LCD display model JB1100 has anyone had one of these ? they seem alright for the price

http://cgi.ebay.com.au/RC-Lithium-Po...QQcmdZViewItem

headhunter23

Dude check out UH and look at their hextronic model! you can charge up to 4s or do 2 2s at the same time(which the brain can do but it doesn't have balancers and you would need to make a custom cable), it's extremely quick, small and light. I have the mrc brain 989 but after picking this up for 25 bucks, been thinking of selling the brain! Yes it's not as good as the brain but who cares? They both do the same job, just the hextronix maxes at 2amps input where as the brain does... what 10-20amps?

Oh one important thing, if you have an account with uh, log in before u buy!!! I was looking without logging in at the futuba remote... 199, logged in and it was 169!!! 30 bucks savings by logging in! Nice.

Ah one note though, it doesn't come with ac, so if ya got a 12v batt layin around or those portable 12v's (these are the handiest) ya might have to plug into car.

Ivan.

brushlessone

yer ive been lookin at them all on UH ,the Hextronik one does balance them also the IMAX ones look prety supreme!! the B8 is mad az i have a swallow 2 AC/DC charger wich is prety good it can charge lipo and liion batterys but doesnt balance so i dont even want to try and charge them on it incase i ruin my Nosram lipos , all the local hoby stores have balancers for over 50$ so i think im going to buy the balancer chanrger of ebay its cheaper than all the other units and is stand alone and does nimh nicad batterys too also liek the LCD screen to read individual cell voltages

DRAGONFIRE81M

as far as ac power you can get a ac/dc transformer . thats rated at 12v @1500ma or larger if you charge higher then 1.5c got one from radio shack for $20 just put some deans on the cable so i could still use the 12v battery leads

headhunter23

I had picked up 2 balancers off of UH, i think they were 20 bucks a piece for the brain. Work fine, but it would have been nicer to have it all in one unit.

Ivan.

Quicksilver

sorry I missed the question. Well both of you guys are right. There is a maximum amount of water that can be pushed through the drive. In a nutshell the boat will go no faster no matter how much power you throw at it, this maximum is different for every drive, but it's true for all jet units. The water exiting the drive must be faster than the boat is going.

On the other side of this question, yes the drive will cavitate. This goes hand in hand with the maximum speed idea.
after that certain maximum rpm, the drive will begin to cavitate. The more power you push the drive with the worse this will become. The drive is trying to take the water in and if the impeller is asking for more and the intake says I'm giving her all shes got, but she cannot take anymore........captain" When this happens the water is drawn apart and creates an air cavity in the duct. From here, the air cavitates the impeller as if it were out of the water. Now technically this does have something to do with air getting sucked up from the intake losing prime, but at really high rpms, that doesnt even matter the drive will just rip apart the water, causing the same effect.
~~~~~~~~~~~~~
This brings me to why I am redoing the jet-villain, if I decrease intake area and place the intake further back into the hull, technically I should be able to reach a higher velocity. So this is why I cut the drive down, water now doesnt have to travel as far to the impeller, it will be more likely to keep prime at higher rpms, since a decrease in intake area in relation to the rest of the drives dimensions, will increase pressure for higher speeds. It's also best to decrease nozzle area as well, but this is harder to setup properly. Remember what I'm doing will improve MY situation and not necessarily make every drive go faster. There are tons of variables, but if you change things slowly over time, you might be able to tune your drive.

There, you made me write an essay... but then I dont get to use my brain as often as I used to.

DRAGONFIRE81M

thank you quick i needed that lesson . know if you can put a sceze on the nozzle exit you well inrease presser and up the speed . as long as you set up and the impeller can handle the presser and stress its placeing on it

Quicksilver

yeah, I tried doing that, but then the drive had trouble priming after a spin. I think the trick to either narrowing the nozzle or making a variable nozzle is to keep it short. If whatever you're using to narrow the nozzle is too long, you're going to have trouble.

In case no one has seen my variable nozzle, here it is.

DRAGONFIRE81M

hey quick do you still have that hull i know the drive is gone but you could make that in to a pretty nice jet sprint get reid of that cover replace it with a plate and add a cage bam you have a jet sprint .

Quicksilver

yeah I've still got it, I store parts in it That top disgusted me from the time I got it. Yet another project.

DRAGONFIRE81M

sweet now youll have to find a new storge place

Quicksilver

Figured since I keep saying I'm going to work on the villain, thought you guys would like some pics. Although I didn't make crazy progress, I did fix the transom and I'm sure it will work now.

Yeah and keep in mind everything is just roughed out right. The camera seems to add 10 ugly points, poor villain

brushlessone

well i decided to go with the cheapy stand alone charger balancer with LCD, i was going to go with the IMAX B8 then i read that it doesnt charge 2 cell 7.4 lipos only 3 to 8 .. wich is ridiculous... so i decided to go with the ebay one wich i think is prety damn cheap for what it does ( link below)..
So now i sit and wait for my brushless HTX motor , lipos and lipo charger to arrive then its time to have some fun on 11.1 volts !!

http://cgi.ebay.com.au/7-4V-14-8V-Li...QQcmdZViewItem

headhunter23

Hey quick, I still can't buy the prop ripping apart the water theory and creating air from water. If that were the case then essentially it would be breaking the molecules that hold h2o together, apart via a blade.
You have a website I can read further about this? Seems not to be easily found on search engines.

Ivan.

Quicksilver

Sit back, get comfy and start reading.

Really long thread, but it's more info than you'd ever want to know about jet drives and they're mostly naval architects and engineers.
http://www.boatdesign.net/forums/showthread.php?t=11254
http://scienceworld.wolfram.com/physics/Cavitation.html
http://legacy.ensight.com/news/penns...avitation.html

This issue does baffle me as well, and I wonder if they just mean that at some speed you have to suck air (I had mentioned that is an issue.) Just think of how a surface drive boats moves at 40mph. They're hardly ever in the water. But I'd swear they did say something to affect of the water basically boiling. Only clue I could give is since the impeller is creating suction and therefore pressure(negative pressure), maybe it can boil. Tough to figure out pressure on the intake. I used to know this stuff.
EDIT: I made a boo boo, the boat leaving the surface of the water would be aeration. Cavitation comes into play with high speeds and deformations of the intake duct or impeller.


Cavitation occurs when the water velocity is very high, in which case the pressure drops considerably because of the Bernoulli principle. The pressure drops even below the vapor pressure of water and the water will vaporize (or boil so to say) at ambient temperatures. Because of the low pressure the vapor bubble that is formed will grow to about 1 cm. But when the pressure rises again this bubble will experience a very strong collapse.

I like this pic

DRAGONFIRE81M

just getting started in the long ***** threads i see there talking about thrust vectoring and adjustable inlets wish a can find a pic on that to have a better idea of what theres trying to say

when i thank of adjustable intake im always reminded of a engine like the nisson sentra 2.0 ser and the ford 5.4l triton with both long and short varable intakes . the longer the beter the low end tq is and the shorter the better the high end is

headhunter23

[:@][:@]

Took knock off out today... it had been raining all night and everything is flooded. This winter blows. [:@] Anyways running the knock off in the bigger stuff wasn't the best decision! Nose kept getting caught on 7-10" standing waves. On top of that, nose got sucked under and flipped the whole thing. Bugger. It's drying now, but have decided to pull all the wood out and lighten her a bit so her nose is a little higher. Also need esc to run my other boat and test it today. If that thing skips like dragonfires did(22" abs off ebay) then she's in the garbage and the internals would be going to new villian!

Which reminds me, quicksilver your going to have to post more photos of your work, plus dimensions if that's ok.

Ivan.

DRAGONFIRE81M

forgot to michen that sorry headhunter . you cant hit the large makes on plain the nose needs to be as high out of the water that you can get it . if you can then it jump the wakes but you still have to kinda walk throw them and goose it when you get the nose up

headhunter23

Yea... I actually knew that from the 12.5" mini jet drive. Same thing was happening just on a slightly larger scale with this boat. btw can you take a few more pics of your steering in your knock off?

Thanks in advance if ya can, no worries if ya don't.

Ivan.

DRAGONFIRE81M

sure man if this doesnt help i have to make a nother one and ill take some pics of it before i install it .