sundogz

Wow, you gotta check this out. Alien Boat is selling an aluminum impeller for the NQD Tear Into (knockoff) boat. cgi.ebay.com/1-Prop-Aluminum-For-RC-JET-Boat_W0QQitemZ270427316399QQcmdZViewItemQQptZRadio _Control_Parts_Accessories Curious thing, it says "3mm", but that is the shaft size of the mini Graupner - the NQD shaft is 2.3mm. Looks like the Graupner screw. It has MadMax's video and credit as well. Toocool! Sdg

TwinJetMax

the 3mm is referring to the grub screw. I think.

grael

>
I have a metal lathe, but I make them in wax, and then take a silicon rubber mould from the wax.

We are trying to set up a class small enough to be run in a back yard jetsprint course.

(ISIRC10): Epoxy resin has better bond strength, and I don't think it messes with foam. The recommended way, is to paint your foam with PVA, and then glass over. Spray with adhesive after, and the glass will stick around the corners.

aquajet

Ok so they are hand made but you turn up the nozzles and jets then insert the wax vanes and cast it as one unit.
Ican turn the bits up, but havnt worked in wax yet. will have to go try find some. Is it something fancy I must ask for when Igo the glass shop.
What material do you use to cast the part after you have a silicone mould. The plastic I have seems a bit brittle.

When you say "spray withadehesive after"
Do youglue the glass to the mould in problem areasthen work the resin in or is this a fast set adhesive to get the glass to go around outside corners better.

grael

No, actually I make it in one piece. I machine the wax into the outer shape, and then mark it out into the number of vanes and the correct angles. I use prime numbers for the vanes. Then, I place the stator outlet on a bent axis, and bore through where I've marked the divisions. After I've bored out all water passages evenly, I remove it and tidy up the vanes with a craft chisel. It's very fiddly work, and easy to destroy several hour's work with a mis-stroke of the chisel. You can also heat the chisel and melt drops of wax against the model to repair minor defects. I use regular candle wax, but there is an Engineer's wax available, and for pressure moulding, you want bee's wax. (it's a bit more like plasticine.) There's a shop in the south island of NZ that sells beeswax at very reasonable prices. Topmark sell calibrated sheets of wax, but that's extremely expensive.

I'm using polyurethane two part plastic easycast, and/or procast. Look up www.topmark.co.nz , they supply to modelcraftandhobbies.

It's a long tack time glue, gives you time to position and wrap the glass around exactly like you want. you probably don't need much if any away from the corners.. I hadn't thought about that.

aquajet

Thanks a mil for the info . Icalled the resin shop locally and they had the calibrated sheets, but I decided I would pass on that for now.
Never gave candles a thought though. Will go get a few and melt them down and try my hand at it. Ihave tried glueing plates in , but cant get it to my satisfaction.
Will rethink the way Imake the jet next time as well, to try make manufacture easier. At the moment Iwant to try turn faster and just waiting for the belts &pulley to arrive, as well as make a better stator to improve the flow. The way I have the stator at the moment it has to be a stand alone unit. Next unit willbe a vast impovement ( I hope)
Any reason for prime numbers. Ihave been trying to get even numbers up to now. Is it because the prop is even numbers and better flow can be achieved from even to prime?
Will also speak to the guys about a stronger two part plastic.
My current jet has roller bearings and a tail bush. Starting to think it is a bit of a overkill. was even fitting a seal to the new unitswhen Iam happy the thrust is adequate.

grael

Mostly, the prime numbers are to reduce parasitic vibrations. It's done in jet turbines, but it's not so critical in water jets.

The "shore hardness" figure shouldn't be too high (brittle), or too low (too flexible)

Easy cast urethane resin is about right for an impeller. It also sands really well.

http://www.topmark.co.nz/articlelive...Full-View.html
I see they've added a clear easycast, that would be great for moulding jet units with !

sundogz

Yes Twinjet, you are right. Iwrote them about it and here is their reply:


hi..(sundogz). yes this prop shaft is ~2.3-2.5mm. dimeter ~ 2cm. if you want shaft ~3mm. i can made. thanks.

bayshun

http://www.youtube.com/watch?v=JoRSMh9DtiA&feature=fvw this vid i found is interesting there is a sub with what looks to be graupner mini jets.

sundogz

I don't see any airfoils or control surfaces - do you supposethe steering nozzle of one is oriented vertically while the other is horizontal?

bayshun

i think your right because at one point it looks like one is going down and the other is gong left. its quite the conundrum.








Ben

mineralvann

dont subs use ballast tanks? After getting the mini rio working reasonably well again im having roblems with the repriming, considering either a screen to reduse intake are or a aluminium plate to see if that helps. The only thing im worried about with the aluminium plate is that the intake would be flat on the inside like this __/ instead of going straigt up as it is now and it may create some unwanted turbulence in the waterflow. And im still having problems with leaking getting probably 2 dl of water on every run, ive sealed up everything with epoksy so the only place i can see it leaking from now is the bearing. Instead of ordering the bearing that was recommended to me i took two bearings and put one inside the other, dont think any of them was waterproof but didnt think it would be a problem, will try placing the original rubber seal in behind the bearing and see if that does anything.

edit: after wathing the video again im thinking the skin is flexible and it steers by manipulating the angles on the fins. But is the intake at front? its driving upside down too

edit2: after using the internet i might have solved the mystery, and the solution is hidden in cheap nqd boats

TwinJetMax

When you are running the boat, water should not come through the bearings. There is actually a negative pressure inside the intake caused by the impeller.
In saying that it may leak at rest through the bearings. A cause of not re priming well is that it could just be sucking air through the bearings.
I don't think a ride plate will help your re priming though.
Give us some pics to work with.

Max

TwinJetMax

Hey.
It is my birthday so that means a new project!

I have a 0.91 nitro engine and a 1m tunnel hull.

That is about it for now. I am not going to have any time to work on it this weekend but I can't wait to get started.
Here are some pics...

ISIRC10

Well then, happy birthday! does this mean that the little hull is on the back burner for now?

I cancelled my order from hobby lobby, placed one at funrcboats, they have the drive in stock. Hopefully mine gets here before the 31st (I am leaving for vacation)
My boat is getting there, I sheeted the plug with plywood and then broke up the plug once the CA had a chance to set. still need to weigh it, but right now it is very light. I may want to keep this boat all natural with the finish... I like the look. May post some picts if time allows.

grael

Happy birthday Max !
That hull looks good with the .91[8D]

My mini jet hull isn't demoulding very well, so it's lots more work yet. I do have one that demoulded, but it's over weight, with a top on, it comes to 100 grams, but I should be able to achieve around 60 grams hull + deck weight, maybe even down to 50 grams with a really good vacuume.

I may convert my big fibreglass hull (stepped, shallow V) to a rescue boat for another project, although it's a bit on the heavy side, I fibreglassed it thick enough to take the original pocket bike engine.

ISIRC10

grael, sorry to hear that. I was looking forward to seeing how yours turned out!

My hull weighs 30 grams without the top or any reinforcement. I predict its final weight to be somewhere in the 55-65 gram range when I am finished with the fiberglass and top. I hope 65 grams isn't too heavy. What do you think?

Projected date of jet drive arrival: Wednesday, the 29th... won't have much time to test the boat before I leave, but hopefully I should have a working boat by then!

grael

ISIRC10:
That's certainly light for a ply boat.
Don't forget the weight of paint or varnish though !
I find that C.A. adhesive gives a pretty tough finish, but it's brittle.

mineralvann

I think the repriming problem startet after i grinded up the intake because it was damaged from some rocks on one side, and i also dropped the intake screen rods at the same time(not sure this is the cause though because i redid the whole boat). Maybe the rubberbushing will stop any air getting past the bearings and help with repriming too. Do you think some water can get through the bearing when it looses prime? kickback of water or something? I tried filling the boat with water to find any leaks and the only place water is coming out is the jet drive, its not a steady flow just dripping but still, i cant understand where else its leaking from. I tried running it for about a minute while holding on to it but i couldn't see any water getting in. And below is a paint sketch of what i was tried to explain in my previous post. Fig 1 is where i though i would place the plate and fig 2 is how it looks from the side now and fig 3 is how it would look like with the aluminum plate. The red arrow points to where i think turbulence will occur. But now im thinking it would just be easier and better to just screen the intake(then i will get some protection too)

sundogz

Ihad the same problem (repriming). It will help if you flare that part out (that you are talking about). That is called a shoe, Ibelieve- it is sort of a scoop. Ihad my doubts about its effectiveness, but it helped to reprime my boat quicker. Sdg.

TwinJetMax

Sorry guys, I am totally over RC.
I have found a new way to control my boats.

aquajet

Looks great.

few minor problems you may want to look into before you test it

Cats and water are not friends

say goodby to your boat...... all things come to those who wait - except a cat ..... it may come back if it feels so inclined but after being dumped in the water the cat may not share the same afection for you or the boat

But hey ...... you wont know how good it is until you try
Should make for a exciting day .

sundogz

Happy belated birthday Twinjet.Good looking pilot, but I'm having difficulty picturing it in water. Somethinglooks amiss, but Ijust can't put my finger on it Begood, Sdg.

marshr32

I finally got around to doing thrust testing.
With 11.1V lipo the boat weighs just under 700g.
In the bath pushing against the gf's digital scales it reads 13XXg (the numbers were all over the show) so its around a 2:1 thrust to weight
I couldnt be bothered putting the boat back to stock for a before and after reading but I dont think it would be that much lower when stock.
The new stator works brilliantly. when the steering is straight the jet out the back doesnt even touch the steering nozzle now. It also turns the boat a lot better, not sure if its the extra leverage from the extra length of the stator section or what but this mod is definitely staying! It also looks more like a proper jetboat now, that always annoyed me a little.

Twinjet, that big tunnel hull is gonna be awesome. what jet unit were you using again? You could make your own mixed flow unit (aka american turbine) for a real scale look marathon boat.

Grael, any reason youre using chopped strand mat for your boat? Wouldnt some light cloth be stronger and lighter?
Looking at your pics, do you get a big lump of cured resin where the vacuum tube comes out?

grael

That was only the first mould, so I wasn't bothered about excess resin there. I actually had issues because I didn't prepare enough resin, and from not having the paint fully dry on my model.[X(] (didn't release properly.)

Chopped strand has tear resistance from any direction, cloth is more directional. Mainly, I used chopped strand there because it flows around corners much better than woven mat.

Anyway, I've redone my hull from the first one taken from my mould (which is pretty stuffed now), and I've boxed and set concrete around it. I also made some major alterations to the deck, and I'm waiting for the mould for that to set, also in concrete.

I used silica sand mixed with portland cement, so hopefully I'll get good fine detail. Max may go OK with his plaster of paris mould, but in my experience it can crack pretty easily, hence my use of concrete. Also, it's really hard to sand away too much , but you can wax and polish it.

The pictures: in the first frame, you can see my first attempts at decks in the bottom two, the last bottom one is the deck mould. At the top left, you can see my new deck mould, I decided to have a curved profile all around, to add strength and make it look better. I also added the two little vent like protuberences near the back. Top middle, is my new plug, which is sitting in concrete now. It lacks some finesse.. hopefully I'll be feeling motivated to put that into the concrete when I demould it. Top right, is my first mould, taken by resin infusion vacuume bag casting, but after a difficult demould, and the beginning of some repairs.

The second frame: side on pictures of my deck (top), boxed, waxed, parmin10 release agent applied, and ready for concrete.

Last frame: the top, to give you a bit of an idea what the finished boat should look like.
[8D]