TwinJetMax

That new intake is really nice.
I am just getting some more information and then I will start to build my new hull.
It is coming up to mock exam time so I will not have much time, I also have tournament in a week. But I think that I will have a boat by christmas at least. Then I can build a kick ass track on the lawn and use it for a straight 6 weeks.

Just need to build a track dow here. I think that I will try and build one where I was going to but then didn't, looking back I think that it should be fine.

Make sure that whatever steering nozzle you put on has a large degree of turning. You will want it when you are trying to get round a track.

I found that you will want the boat to turn in its own length while only applying half throttle. If you have to burst the throttle you will run into issues.

I want to see more pics of the boat of the inside, transom and underside

grael

Thanks Max !
what's the plan for your new hull ?

I tried to make this intake really short, but it's still quite rough inside. I moulded silicon inside silicon, and I wasn't sure quite what would work for a release agent, it came out sticky and with ripples[:-] I can always dremel it out smoother. Here's some more pictures for you. My son doesn't like the paint job[8D], but he's seen the power, and is very impressed with that !

I have plans to mould the intakes with a piece of aluminium angle in the mould, and the resin curing around it, for heat dissapation from the motor directly to the underside of the hull.

Also, being so small, it's really hard to mount the motor

This motor is supposed to be good for around 19 amps
( http://www.hobbyking.com/hobbycity/s...idproduct=5246 )
I don't know what I'm pulling out of it, but the original battery wires get hot[:@]

I know the plan was to have a lawn jet sprint... but seeing the power this thing has, I think that it is far more than a scale performance, I'm not sure that an 8 m x 4 m course is big enough !

According to my calculations, the jet should be exiting the outlet at around 41km/hr, supposing the boat does 31 km/hr, that's pretty fast for such a small boat !

TwinJetMax

8 by 4m is big when you get down to it.
Even though the stats say that it will take just over 1s to get end to end, you will be rarely hitting top speed. I think if the track is kept small then the boats can have really good racing, with a lot of dependance on boat design, and driver skill, while the better components will still have a slight edge.
From my quick experience last holidays, I think that the track size will be just fine. It will take practice though. There will be a lot of skill involved and the boat can't have any glitches or priming issues.
One of the hardest things I actually found was trying to turn. When you let the power off to slow down for a corner, the nozzle velocity becomes slower than the boat and hence there is no control. The trick was to slow down before the corner and then accelerate through it.


Better tidy that jet up before you make one for me.

As for my hull I want to get a really nice copy of a jet sprint hull.
The new 'NITRO' from Kingfisher boats is what I am hoping to model.

I want to get the boat really low slung and looking fast (and being fast).
I will take my time a lot more in the building process and the mold making process this time.
Then stick my current motor and esc in with a new Lipo.

Max

marshr32

Thats gonna be a pretty small track for such quick boats, you guys are gonna need some really good skills and quick reactions!

Youre right max, power is everything for turning these boats. In fact if you watch the full size jetsprint boats you'll see the fast guys keep the revs very constant for the whole run, and they dont slow down for any turns, just chuck it in. Thats good hull design!

Grael how did you measure the water velocity? You probably realise this, but jets work on velocity AND mass of water to propel the boat, so the hull would have to be ridiculously light to even get close to the velocity of the water going out the back, plus there is drag. Even if you get 20kph out of your setup, it'll still be a handful on your little track!
They are a LOT faster than scale. Mine does around 400kph scale speed

areseaer

I feel the jet should hold it down.

Sundogz, do you know the weight of your Batboat. I put everything I have for my boat in it today so I could see how it sits in the water (tub) and see if I had any leaks.
It's weight was 7.1lbs with everything placed about where I'm going to put it. Their is more left to do so the weight will go up some more.
I'm waiting on parts to do the steering linkages and trying to deside if I should order a Feigao 580l motor or not.

sundogz

The 1000kv motor on 6S was somewhat slower that this 2300kv on 3S. I don't really know how

many amps it was pulling on 6S - but I guesstimate 160 give or take. It took out the caps on

the esc as well. But I think the 180A Seaking should have handled it (360 surge), so I

emailed the seller (rcmodelland.com - or Compassho on Ebay) about returning it for a version

2 replacement (since this one has been recalled) or a refund, and here's their reply:

So I've appealed to them that it is advertised to take up to a 6S battery, and that should

include freshly charged! We'll see what happens there. I may have to repair it myself.

That is the same motor I put in the Tear Into. It comes apart if you tug on it and I had to

put washers (shims) on the shaft to keep it from pulling the impeller shaft inward - it's

meant for a heli. Watch the motor when you rev it up in water and you'll see what I mean.

The bell will pull forward (if yours is like mine).

I'll weigh it and get back to you. I know the batts and motor weigh more than the (rest of)

the boat! I've decided to go to a more efficient motor so ordered this 2800kv 540XL from

Hobbycity:http://www.hobbycity.com/hobbycity/s...677&Product_Na

me=KD36-74-06XL_2800kv_Brushless_Inrunner
I couldn't locate a Feigao 580l to save my butt, and this was less than half the price. It

is rated at 110A and should be much easier on the batteries, esc and connectors. I have the

3200kv short version in the 21" BBY and it's a freakin' powerhouse.

grael

As marshr32 says, many of the guys on the big boats don't throttle down for the corners. However, I don't think their power to weight ratios are as good as ours ?

If the course is short, then I think Max is right, the boat won't have time to reach top speed, and so you might get up to 18km/hr, in 2 meters, and then be spinning, heading in a new direction. I've noticed that my new pump only primes well when the motor's slow, as soon as I rev it up, if I drop it back in, it won't pump until I slow it down again. So... Max could be right there, about that being a big part of winning races. I made my hull stepped for more top speed, but the traditional V jetboats aren't stepped at all, they do run a lot of strakes though. If you can keep the boat flat, then a big part of cornering is simply keeping the boat pointed in the direction you want to move, but with some forward planning. A bit like an air boat.

All those of us who've run jet boats have experienced the requirement to keep power on in the turns, not knowing that is one of the causes for accidents in a jet boater's early driving, it's one of the first things they tell new drivers of the Shotover jet boats.

marshr32:
I'm calculating the jet speed using the distance for one turn of the prop (((27mm) x 3200(rpm))/16667) x 90% motor efficiency = 46.65 km/hr whoops... faster now !
And, assuming significantly less hull speed than jet speed. However, how much less, I'm not sure. Judging by what I've seen in the tank though, the boat will be going a lot faster than 20km/hr if I give it a chance

Sundogz, sorry to hear about your ESC ! You do have a case, we'll be interested to see how it progresses. Let us know.
Yes, I had a very difficult time getting the bell off the shaft. Wat I did, was to use a brass tube with a piano wire insert, long enough to hold the prop directly. Then, I dropped some urethane resin in the motor mounting joint areas so that it would solidify in perfect alignment. I have washers between the impeller and the shaft tube from the intake, so it can't move too far forwards.

I have plans for the next one though, for an aluminium cooling strip to run under the hull, and to countersunk screw fix the motor mount to that from under the hull. It's such a tiny hull that I have diffilculty fitting my fingers into it, let alone my hands !

Update:
After writing the first part of this, I remembered a couple of places that might have night lights.
My local salt water lagoon does, and this time they were on !

I took a couple of 1000mA/Hr 30C 11.1 volt LiPos with me.
Most of the time, I didn't get above 1/3 throttle[X(]
At about 1/3 throttle, I'm pretty sure the boat is doing above 20km/hr[X(]. When I gun it all the way, the boat really takes off, and moments later, it's airborne and here's where I'm uncertain what happens, it flips around it's long axis, but I think what happens is that it does a mini launch, and then the faster acceleration of the bell on the heli outrunner torques the boat mid air ?

So... not sure how to deal with this. The boat was taking on water, but out of the 2 times it flipped, it flipped all the way back deck side up again[X(] Most of the time, I just ran it up to around 1/3 throttle, and gave it little bursts. I didn't have a proper steering nozzle, just a rudder mounted mid flow, and this may even have been the cause of my problem, because I noticed later on that the steering was different at high speed... however, I think the salt water got to my servo in the end, so I've taken it home, washed all in fresh water, and then sprayed with CRC.

In the end, I probably got 6 minutes testing, without the batteries showing any sign of weakening, and I didn't use the second battery.

It's hard to analyse faulty behaviour in near dark conditions, and with weed in the water as well [&o]

There are a couple of places in the city where I may try the boat tomorrow, but they are much smaller, depends if my gear works again after a night in the hot water cupboard.

By the way.. I bluetacked one end of my ESC closed, and then filled the heatshrink up with 2-part urethane resin before trying any of this

areseaer

Sundogz, The only place I found the Feigao 580l was OffshoreElectrics. I'm not sure that 540xl you're getting will do.
My batts weight is 17 oz, they are for my plane, but it is all I got.

sundogz

You Kiwi's building your own boats - and even jets just kill me [&:] What will you do now, whittle out a better motor?! Grael, it will come down to laying the motor mount/esc mount/cooling keel in the mold and forming the hull around it. Then use 2 small lipos saddle style in the 'wings' at just the right c.o.g. to keep the nose down. Methinks the wings are why it is flying at high speeds. This Bat boat (V24) was designed that way to keep her 'floating' and stable in the big waves. But too much power and it's an airplane!

Areseaer: Offshore had one rpm range left - the 1051kv. That's not fast enough for me as I've decided to stay with one 3s battery to keep things lighter and keep from frying my electronics. The boat with one battery weighs 5-1/2 lbs. Take off aprox. 1 lb for battery and 1-1/2 lbs for motor and unladen the hull/servos weigh just 3 lbs. Since your boat is already weighing in at 7-1/2 lbs you might do better with a 580l if you can find a suitable KV range somewhere. Or use the 2300kv KB45-08L with a water jacket and 3s-5s lipos to get the performance you want.

The motor size is quite adequate as proven by Diegoboy with his Fastech jet conversion on the offshore site:[link]http://forums.offshoreelectrics.com/showthread.php?t=5468&highlight=jet+drive[/link] And he is using the long (not XL) version with those results (post 150 of that thread links to a short video)

I'm hoping this cheaper copy will perform like the Feigao. I'ts a little faster at 2800kv. If its' shorter sibling that I have in the OMC21 is any indicator, all will be good.

Compassho2005 (rcmodelland.com) wrote back (with a different attitude) Areseaer: Didn't you say your esc was the 'version 2"? I'm getting the feeling if I send it back I'll lose my money and my esc. I should probably go to Radio Shack and get some suitable capacitors to repair it myself.

areseaer

The 1051kv will be good for me because the batts I have will only handle 76amps as a 4s. Version 2 is what it said on ebay but I don't see anything marked anywhere on the esc to know for sure.

TwinJetMax

Yep Grael, I agree with sundogz there. when the traditional hull is on the plane and tries to flip, the freeboard/chine catches in the water and the boat centers. I think that when your boat is on the plane it tries to flip, the wings don't touch the water until its too late.

As for the jetsprinters, I went on the agrojet in Rotorua, which is a 450hp jetsprint. The did slow before the corners but feather the throttle on quite high revs through the corners.

aquajet

Wow Grael

Sonds like you have weight to power, not power to weight.
Rotation in midair can easilly be the motor torque.....when the motor unloads in mid air it accelerates and there is no oposite force when in the air ... rally cars used to rotate against the crankshaft rotation many years ago while airborn.If it is motor related then the roation will always be the same way , unless you close the throttle when it lifts.
The take off can be too much frontal wind areaand possibly jet angle.
At least you are still landing the right way.

grael

I have considered combining the motor into the hub of a mixed flow jet unit
That's been my thought too. If I have the motor mounted on some 4mm thick aluminium plate, then I can screw thread up into it from under the hull, so I don't need to worry about how to get my hands inside the boat to tighten allen screw/philips etc.

The wings are meant to be part of the V, and if you take overall beam (140mm) by overall length (250), then it's not too wacky a combination. However, I agree, I've already been thinking of putting the BESC and servo on one side, and the LiPo transverse mounted with an offset to get a net even balance, but so it's a lot more resistant to spin.
The other thing that would help here, is to use an inner runner motor, because the inside magnets should give a lot less torque effect in the air. Hmm.. acceleration it probably more rapid though... so the effect might not be as good as I'd hoped.

No, there might be other things wrong with it too. Better take them up on their offer. Besides, those will be special low ESR capacitors, designed to have huge current flow. Most capacitors aren't.

TwinJetMax:
I agree. I took it out in some freshwater today by the Wellington city council buildings (and the state opera house), and tried it again. The water was too rough for it[:'(]
It pretty much confirmed the inertial torquing effect of accelerating the motor bell on such a light weight boat though. I never had this problem with my bigger blue boat, and I'm thinking that I overdid the narrowness of the stern
I really love the handling of that bigger boat, so I guess I'm going to have to make another new small hull again. I think I'll stick a lot closer to my original dimensions this time, and just pretty up some minor things I wanted to redo anyway.

aquajet:
I assure you, lots of power to very little weight !
I'm planning to work on weight distribution to effect a partial cure.
Also, I need to put an external inlet scoop on, it's having difficulties repriming after air launching[:-]

TwinJetMax

I think that I will use my original jet and mod it out like marshr's. His boat is faaaasssst.
On top of that I will stick my new 5 blade prop in it and reduce the outlet diameter so that even though there will not be any more speed, there will be better suction for re-priming.
Also a better steering system will be in order.
On top of that I would possibly like to add some sort of quick release reverse bucket and servo, just for practicing so that it is not necessary to constantly go and retrieve it.

I don't think that my motor has the torque to run a 5 blade, so I might get a heli outrunner.

It certainly doesn't sound like your jet is quite right yet Graham. So I will hold off for my first boat.

grael

TwinJetMax,
Is this for your big boat, or your small one ?
smaller outlet is easier to water-lock, and it actually increases outlet speed.
I'm not so sure 5 blades will help with repriming. I think lower pitch helps a bit, and also having a short pump. The problem I'm having with my heli outrunner is the weight of the bell, concentrated at a wide radius. Huge torque to the impeller, and huge torque to the boat when airborne. Something I hadn't thought of since I was a much younger man.[] I don't want to use an inner runner though, because it will be harder to cool, and I don't think they are as efficient ?

TwinJetMax

I am hoping to get a better 'vacuum' in the intake with the closer packed blades. The feel they would also pick up water better in small quantities.
I just had a look at hobby king and I think that I will stick with my inrunner. I think that they are easier to cool because you stator is on the outside. It also has no torque effects. You can hold it on start up and nothing happens. No jolt. I don't really know about the efficiency.
What is water lock???

grael

That's interesting. I won't agree about "no torque effects". but I was certainly expecting less. I'll have to give my ones a try again.
I may try this one I already have:
http://www.hobbyking.com/hobbycity/s...idproduct=3850
I think it's powerful enough, only thing is, I'll have to cool it properly.
someone rated the HK 3200kv heli motor as only 3000kv, the slow inrunner I have is rated at 4200kv
It's either that, or admit defeat (for now), and use my 2500kv outrunner wich has a much lower mass bell (only about 1/3 of the length), and accept lower speed, but phenominal run times.[8D]

Like the S-bend on a dunny.
The steeenky air is trapped by water at both ends.

If you have a smaller outlet, then it's lower, so the water lock is more powerful.

TwinJetMax

Yes. I know that there is not 'no' torque effects, but there are very little.
I may just getting a bit excited, but I might thy and build a two stage jet. Just for fun really but it would be interesting to see how it goes.
I would really like to have two stators so that I can interchange them to see results

grael

Stators are the easiest thing to mould, even the combination stator-outlets like I do. If you get some pinkysil (cures in about 1/2 hour, faster in heat) then you can do a stator mould very quickly. Models are easy, all you need is a tube and some metal vanes soldered or brazed around a hub to suit, give it a hot wax dip, clean the sharp faces, and mould !

marshr32

Well Im glad you asked!
Personally Im cutting my own impellers from plastic bar stock cause I cant find what I want to buy. Once I get it right Im looking to move to Ali.

Made up a rig to hold my dremel, and went from there. I can take a coupla pics if anyone is interested. The way I have it set up I could do 2, 3, 4 or 6 blade impellers (assuming 6 blades would fit with the cutting tool I have) in pretty much any pitch and any size out to about 50mm dia.

Here is a few shots of my first impeller I did today. Its a bit rough and didnt get the overlap I was hoping for, but it looks a lot like a 3 bladed version of the stock NQD prop. I might just try it anyway.
Hopefully I will have time tomorrow to do a 4 blader with a little less pitch and more overlap.

Max, have you got any pics of the 5 blade prop you got?

Grael I have often thought about the jets getting an airlock in them. Maybe a good idea to put the cooling outlet on the top so the air can be bled away? Or just make it like a rooster tail like on PWCs.

sundogz

Grael, yep, sounds like you should go inrunner with the small boat. That may solve a couple of problems. They have a lot less centrifugal torque, and they are easier to cool as well - usually. I have one like your 2040 in 3600?kv with the external cooling 'ribs'. A water cooled mount is all that is required there. You really don't want her too fast in a circuit - but maneuverability is paramount. With that short length it has good potential. A well designed steering nozzle can fix many ailments too.
Your idea of a modular motor/jet - say, in several sizes makes a lot of sense. It would be easy to class races boats, and there would be more focus on hull innovation, etc. Commercially, it would allow more novices to get into the jet boat hobby as well. Hmmmm someone should look into that.[sm=greedy.gif]
You're probably right about returning the esc. There could be other problems. I just don't relish the long turnaround! But good things come to those that wait...

Marshr: Whittling impellers [sm=omg_smile.gif] Egads, you'd think there weren't any stores over there! That is just too cool, hope they turn out well.

I wonder how that Taiwan company selling aluminum NQD replacement impellers is doing? What with the knockpoff jet sprint and crackerbox getting so hard to find, that should concentrate on 3mm shafts for the Graupner mini's.

aquajet

Marsh
Would like to see what you have done there making the impeller. Ihave been trying to work out how to get a drill press to cut a prop (modify the shape to get both blades even)



Grail
It seems you dont reduce the size of the nozzle from the impeller
I drropped from 40 to 30 , maybe why I am battling to get the jet up to a better performance level

Quicksilver

They originally went down there with one paper clip and a piece of duct tape.

You guys are ridiculous, in a good way. I must be half Aussie some how, since my computer is made from 3 computers, my xbox is water cooled with a pond pump, I have a power ventilated cat litter box, I dont even bother buying parts for my wife's Ford(I just make them), I now have 7 tool boxes, 3 filled with various bits of metal, screws and the like, and then there's the boats.......Which are made from useless parts off the Ford.

grael

Marshr32, great machining on those impellers ! could you post some pictures of your jig setup please ?

And re a bleed on the airlock, I tried that once, but couldn't get it to work.

Aquajet: I do reduce the outlet slightly, but mostly I make a basic small reduction to compensate for loss of the hub C.S.A. as the flow passes the impeller.
I figure that it's really hard to get good tolerances in a small jet unit, so you need to keep the flow moving. If you block off the flow too much, then the water won't move cleanly, it will be eddying like with a stalled airplane.
Keep in mind also, that a reduction in diameter like you suggest of 40mm to 30mm, is a proportional speed increase of ((40/2)^2/(30/2)^2 = 1.777777777 , that's giving a big back loading on the impeller, so it's easy for water to squirt round places you don't want it to.
Sundogz: I'm going to substitute my 4200kv motor, and put an intake scoop on to extend the water gathering area. My intake front surface hasn't got much curvature, it's sanded smooth, but it's quite abrupt compared with my normal pumps.I'll try removing a bit more material there too. The torgue reaction as the motor winds up to full speed is pretty insignificant compared to my 3200kv heli outrunner. The outrunner is using a chromed iron bell to to hold the magnets, where as my 2500kv outrunner uses aluminium and more tightly packed magnets.
I'm hoping the boat will do it's thing then.
When I took it for it's first run in the near dark, it would reach a much higher speed before loosing control and flipping, that was pretty flat water. Yesterday, I had a smaller pond to test it in, and the quickest full trigger/full release would send the boat shooting out of the water. There was quite a chop too, and I noticed that the moment it jumped intake clear out of the water, it would loose control, confirming the heli motor torque effect.

TwinJetMax

I had a play on solid works tonight at my Robotics thing and it is amazing.
The functionality is amazing.

Marshr, I want to see some more pics of that process. As many as you can upload.
I am going to copy your jet mod except better.
I have got it all sorted.
A pair of 5 blade impellers, and two 15mm long stators,
Then use a new cone with more restriction.

I thought for a while about the shaft, but I think that the answer is to tap both ends of each impeller and have 3 shafts inline.
One from motor to stage one, one between stages and one in the nozzle.

Pics of my 5 blader: