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The Unofficial all things Jet drive thread

Old 06-26-2011, 06:42 PM
  #4701  
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Default RE: The Unofficial all things Jet drive thread

In order to get a more complete picture of just what is going on, I installed a switch in the EDF circuit so I could isolate the decoy jet. What I found right away was that the water jet would cavitate on higher speeds. That is why you don't see a jet trail, as it quits pumping. But even if it stops working, my thinking is that the EDF jets should propel it onwards regardless, albeit without steering. I even tried putting a large rock in the nose to try and help it stay primed, but it didn't help much if at all. I did change the angle of the jets slightly by installing 2 layers of velcro under the back which tilts them downward slightly at the front (perhaps lifting the nose). It could be that the 'runaway' water jet motor is drawing all the amps at that point (but the ripples in the water say the EDF's are working). I'm still perplexed by all of this. Here is a better video of the phenomena. Pay no attention to my clumsiness when connecting the battery! http://www.youtube.com/watch?v=CnGKqLHjmgE
Old 06-26-2011, 06:52 PM
  #4702  
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Default RE: The Unofficial all things Jet drive thread

One thing seems evident, and that is that this boat (hull) with the deeper bow section (stepped V hull) is designed so the aft section rides on a cushion of air when at speed. That's not good for the water jet. But it never really gets to that point, due to premature cavitation.
Old 06-26-2011, 07:11 PM
  #4703  
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Default RE: The Unofficial all things Jet drive thread

hmm... In that video it does seem to get a bit farther in its planing process than in the first one. Try shutting the jet off and running on pure EDFs (tape something like a credit card to the nozzle of the jet so you still have steering)

I think the main problem is as you stated in your previous post - the hull type. The EDFs with their high speed/low volume flow cannot get the boat on plane (with the Jet drawing amperage) and the jet cannot get the boat on plane due to cavitation.

And BTW, that is a WEIRD hull... I wouldv'e thrown it in the trash just by its looks and made something out of styrofoam... but that's just me
Old 06-26-2011, 09:00 PM
  #4704  
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Default RE: The Unofficial all things Jet drive thread

Hi Sundogz,

I had to reread to see that you have a jet drive in it as well as 2 x EDFs.

Firstly, the jet cavitating. put a constriction in the outlet nozel, just taper it in slightly. Easy to do with friendly plastic aka polymorph plastic, else car filler etc etc. That will load the output of the pump slightly, and keep the jet unit filled around the impellor.

Secondly, I'm thinking that the aerodynamics of that design are giving a lot of lift to the rear of the hull at higher fan speeds, because like in a wing, you've got more velocity over the top, than over the bottom. Because the EDFs are at the back of the hull, all that lift effect is on the back of the hull. It's not so bad when the jet isn't cavitating, but as soon as air leaks in from the outlet of the jet(due to lack of back pressure), the impellor starts spinning air, and the intake looses suction, and that's when you see the hull bunny hopping, as there's nothing to hold the stern of the boat down. Every bunny hop ends in a nose plant, and so you loose speed. You can only stop that by angling the back of the EDFs up slightly.

Have you taken static thrust readings of EDF and jet separately ?
If there's no steps in that hull (and it looks like there aren't), then I don't think it will be able to handle the higher centre of thrust of the EDFs, a boat like that is designed for outboard engine prop thrust, under the hull. As soon as the jet looses prime, the centre of thrust becomes unballanced, and it really does try to drive the nose down.

Have a look at float plane float designs, they all seem to have steps up front, so that the high thrust centre still allows aeration under the hull.
Old 06-27-2011, 04:58 AM
  #4705  
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Default RE: The Unofficial all things Jet drive thread

ISIRC10, yes that is what I will do next - disable the jet drive and see what happens. Thx for the rudder idea. Weird? Throw it in the trash? Ha! Beauty is in the eye of the beerholder, I suppose.

Grael, this is a 'stepped' hull of some sorts (see pic). And if the jets are lifting the aft section, you wouldn't know it by the way the bow comes down when the power is shut off. The only thing that makes sense to me is if the cavitating jetdrive motor is drawing all the amperage so the EDF's are starved at that point.

I'll keep youse posted. But it's looking like I will end up removing the waterjet and going to two esc's and a 'V' tail mixer for throttle and speed control, like what came with it - but a bit more powerful
Old 06-27-2011, 05:40 AM
  #4706  
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Default RE: The Unofficial all things Jet drive thread

Now that you've shown that the jet drive is cavitating that means it's not due to too much amperage draw from the drive motor since when there is no load (no water or little water being pumped) the amperage on the drive motor would go down thus allowing the edf's motor to be almost fully supplied. I have a feeling that without the jet drive working it causes too much drag for the edf's to overcome. That and the extra weight it's lugging around. I was hopefull this would work, but not sure if a solution can be found becvause of the hull design. As far as the shape of the hull goes isirc10 it grows on you and I kinda like mine now.
Old 06-27-2011, 06:04 AM
  #4707  
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Default RE: The Unofficial all things Jet drive thread


ORIGINAL: damfurst
I have a feeling that without the jet drive working it causes too much drag for the edf's to overcome.
That is what I thought initially - that the waterjet was sucking the aft of the boat down and keeping the EDF's from gettting to maximum velocity. But a cavitating pump doesn't suck things down, so that didn't make sense. And like you said, a runaway motor doing no work should use LESS amperage, not more. Damn Gremlins!
Old 06-27-2011, 12:49 PM
  #4708  
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Default RE: The Unofficial all things Jet drive thread


ORIGINAL: damfurst
As far as the shape of the hull goes isirc10 it grows on you and I kinda like mine now.
To each their own...

sundogz - Yeah, the Vtail route is the simplest for you to go now. That boat could make for a nice retrieval boat and still have the power/speed to play with when it isn't being used as a rescue.

Old 06-28-2011, 06:57 PM
  #4709  
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Default RE: The Unofficial all things Jet drive thread

Hi ! Here my new work boat ! All made whit aluminium sheets, the jet drive is made whit 1 1/4 copper drainage pipe,
45mm 3 balde prop reduce to 35mm, 3/16 shaft and graupner u-joints, motor is hpi 27T saturne motor, draw 190 watts on 7.2v and 1.5lbs trust

[youtube]http://www.youtube.com/watch?v=H08cio5qiXA[/youtube]
Old 06-28-2011, 07:32 PM
  #4710  
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Default RE: The Unofficial all things Jet drive thread

Nice looking boat! I especially like the way the steering nozzle allows it to turn on its own axis.
Old 06-29-2011, 08:36 AM
  #4711  
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Default RE: The Unofficial all things Jet drive thread

Here's the built up !
[youtube]http://www.youtube.com/watch?v=HROUtpIgYp8&feature=email&email=comment_received[/youtube]
Old 06-30-2011, 05:14 AM
  #4712  
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Default RE: The Unofficial all things Jet drive thread

Hi all, nice to see this thread still going strong!! it was very helpful when i was modifing my nqd jetboats. i have bought a kmb jetsprint boat now grp with a 28mm jet. I got in contact with thomas kehrer from kmb to ask what type of "brushless" setup i could try and he recommended a 1600kv outrunner 540/600 size motor 1500w 600a and a 50 to 60 amp esc and run it on 4s 14.8lipos or 5s 18.5 cells, which i did, but with low kv such as this even running 4 or 5 cells it still goes basically does not give me eough revs and is the same as the 700 neodym brushed motor i took out! So it seems i have been given so strange advise. wheather kmb do not want people over stressing there jet units with too much power i don't know?? On checking with you guys on here it seems the most popular motor your all using for the 28mm jet is 3000 to 3600kv motors 120a plus esc's running only 2 and 3s cells. Also does anyone know what the best brushless rpm for the 28mm jet? If would anyone be kind enough to recommend a good motor and esc to use for the size jet and boat im using i would be very grateful guys,many thanks regards to all.
Old 06-30-2011, 02:33 PM
  #4713  
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Default RE: The Unofficial all things Jet drive thread

That sounds like good advice to me! I am running a 25mm jet with a 3900kv motor on 2S (28,860 rpm's), and a 40mm jet with a 2200kv Neu on 4S (32,560 rpm's) in a 28" boat and get respectable speed from both. A 1600kv on 5S (18.5v) has a potential of 29,600 rpm's. Do you suppose there is something else going on? Low power motor not reaching that rpm range maybe? What are the temps of the esc, motor and lipo's after a long run? What motor, esc and lipo's are you using?
Old 06-30-2011, 11:49 PM
  #4714  
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Default RE: The Unofficial all things Jet drive thread

Hi, thanks for replying The temps of both esc, motor and cells are all running very cool after a full batterys use, the motor i bought from china, but there is no makers name on it, its the usual purple colour,all its says on the box is xyh A4160 kv1600. the esc im using is a hobbywing seaking 90A, thats good for 2 to 6 cells and a 6v bec. The 5 cells are 4000MAH, 15C made by "win force " another purchase from china. As i said they are reading 19v full charged. But they do seem on the heavy side. At first i thought is was the overall weight of the boat with cells, it weighs just over 2kgs. Its a fully built kmb boat with the scale bucket seats and dummy engine and rather heavy grp lid. But i have seen other boats on youtube identical to mine with a single jet and they are going fine with bl setups. Could it be the cheap cells im using? i would expect a little more voltage reading than 19v for that size battery or is that about right? I have also just tried 2 x 11.1 3s cells connected up in series on the bench and there reading some 25.4v and the motor rips so i have a feeling it could be those cheap 5s's, and a low kv motor, what do you think? many thanks for your help regards nick
Old 07-01-2011, 04:43 AM
  #4715  
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Default RE: The Unofficial all things Jet drive thread

15C! Yes, I suspect that is your problem. I'm surprised they haven't puffed up yet. I would go to the highest 'C' rating you can afford. I run 35C and 40C Rhino, Flightmax and Turnigy and have had good results. If I were to need more batteries I would go with the Turnigy 'Nanotech' series. Great reviews and good prices.

Now onto your other suspicion - weight. You can cut down on weight by going to a higher KV motor and a battery with less cells. On a boat this size that is not a problem, but larger boats generally require the low KV motor - higher # cells to keep things running cool. So IMO try different lipos first, that may be all that is needed. And if you are still not happy, try a higher kv motor. But get a good one - Neu, Leopard, etc. A cheap motor will run inefficiently and put the load back on the batteries and esc.
Old 07-01-2011, 07:24 AM
  #4716  
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Default RE: The Unofficial all things Jet drive thread

Hi, oddly enough the cells are running very cool, I may try a higher kv motor, my local hobbyshop said to try leopard motors so I my try one of them and back down on the cells and run it on with higher c's. Do you think the 90a esc can handle a higher kv motor or shall I upgrade that as well? Thanks nick
Old 07-01-2011, 08:59 AM
  #4717  
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Default RE: The Unofficial all things Jet drive thread

It has been my experience that running higher kv's and lower volts really draws a lot of amps, and I have always had to use a much higher amp rating than anticipated. My personal rule of thumb is to get twice the esc that I think I'll need. I know this is not very helpful, but I have burned up many esc's in the past, and this is the conclusion/solution I have adopted.
Old 07-01-2011, 09:15 AM
  #4718  
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Default RE: The Unofficial all things Jet drive thread

Yes good point, when you use 2 and 3 cell lipos do you use two in parallel or just singles and higher mah and c's?
Old 07-01-2011, 09:33 AM
  #4719  
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Default RE: The Unofficial all things Jet drive thread


ORIGINAL: orangewheels01

Hi all, nice to see this thread still going strong!! it was very helpful when i was modifing my nqd jetboats. i have bought a kmb jetsprint boat now grp with a 28mm jet. I got in contact with thomas kehrer from kmb to ask what type of ''brushless'' setup i could try and he recommended a 1600kv outrunner 540/600 size motor 1500w 600a and a 50 to 60 amp esc and run it on 4s 14.8lipos or 5s 18.5 cells, which i did, but with low kv such as this even running 4 or 5 cells it still goes basically does not give me eough revs and is the same as the 700 neodym brushed motor i took out! So it seems i have been given so strange advise. wheather kmb do not want people over stressing there jet units with too much power i don't know?? On checking with you guys on here it seems the most popular motor your all using for the 28mm jet is 3000 to 3600kv motors 120a plus esc's running only 2 and 3s cells. Also does anyone know what the best brushless rpm for the 28mm jet? If would anyone be kind enough to recommend a good motor and esc to use for the size jet and boat im using i would be very grateful guys,many thanks regards to all.
its been a while seince i was last on here . do u have a auqajet with a grumpner 28mm ? if not i did that hull with a grupner drive and 36mm 3600kv 100a esc watercooled and in running 5cells 18.5v . it runs great but youll need every bit of 100amp crated lipos or yell fry the batterys
Old 07-01-2011, 05:13 PM
  #4720  
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Default RE: The Unofficial all things Jet drive thread

Single 5000ma 30+C lipo's.

Dragon, fancy seeing you here! Long time no type. What have you been doing lately?
Old 07-01-2011, 06:36 PM
  #4721  
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Default RE: The Unofficial all things Jet drive thread

Not to much . new hobby is 2 seater 250cc dune buggy .
Old 07-01-2011, 07:17 PM
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Default RE: The Unofficial all things Jet drive thread

Hi, no I don't have an aquajet, there's a hull on eBay at the mo with a Graupner drive. I'm gonna try a 3290kv leopard motor with 2 or 3 cells and see how that goes?
Old 07-01-2011, 07:22 PM
  #4723  
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Default RE: The Unofficial all things Jet drive thread


ORIGINAL: DRAGONFIRE81M

Not to much . new hobby is 2 seater 250cc dune buggy .
Not to much !?! Are you kidding? That's freakin awesome - post a pic or two (put a jet boat on the thing to keep it relevant)

Old 07-02-2011, 02:13 AM
  #4724  
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Default RE: The Unofficial all things Jet drive thread

heck you can go off topic every once in a while who know could spear new ideas heres some pics

i remember now you made the cool nozzole garde for the jet sprint . you can go higher in rpm on that drive i have spone the grumpner over 60k .
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Old 07-02-2011, 05:46 AM
  #4725  
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Default RE: The Unofficial all things Jet drive thread

Yup, looks like fun there! I see it's aimed at the mud hole, but there's no mud on it (yet). Nice hearing from you.

Orange: That Leopard should do the trick on 3S. Be sure to get a good battery

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