aquajet

Twinjet
may not be a good idea to have 3 shafts in line and screwed together . The junction will almost certainly not be stong or straight. Threads have a poor clearance tolerance and dont align very well


Grael

Do you run a bush or bearing after the impeller. Iam thinking of doing away with the bush at the tail end , but concerned about vibration problems

I think I will try a new tail housing with only 2or 3 mm reduction in size
I reduced the size based on what Ihad seen in full size jets, and what I could make out from the swash drive pictures. I see full size jetsruna lot of reduction in some applications

marshr32

2nd that. Sound like a lot of trouble. A 2 stage jet is a good idea tho.
I dont think you'll need a bushing thru the first stator, so I'd just thread as much as you need for both impellers and use a lock nut on the rear one. Then turn down the threads on the end to the right size for whatever bush is in the final stator.

That prop looks like it will do the trick well, 2 in series should be really good. Thanks for the pics.

Aquajet, everything I have read about jets says a larger venturi gives acceleration and load carrying ability, whereas a smaller venturi is the go for high speed. Of course you need to have the power to drive it, as Grael says it will create backpressure.
If you want more speed, what if you turned a reduction nozzle that fits inside your current 30mm outlet to bring it to say 20mm and see what happens? I guess the other way to do it is with more impeller pitch.
I ran my knockoff without the rear bush once and had bad vibrations, never again!

Will take pics tomorrow of my setup, I totally ran out of time today. I was thinking about it tho, and I reckon I could cut stators on it too with the right guide.

grael

I do normally run a bush after the impeller. Otherwise, you will need to extend the shaft tube quite close to the impeller.
the jet skis are the pumps I've seen with the greatest nozzle restriction. They use a lot of stator blades though, and also a very low pitch impeller with a lot of area.
The Pros are: less entrapped water due to more effective use of water contained. Greater relative thrust of jet to boat at high speeds compared to a non constricting outlet.

I don't go more than 5 blades for my stator at the 35mm size and below, I think it's asking too much to put more surface area in the way of such a low volume flow.
Efficiency will be partly proportional to flow volume per unit of time relative to surface area of jet pump housing, stator, impeller, steering nozzle and outlet.

TwinJetMax

Okay, Thanks for the advice here guys.
I think that I would like to run a bushing in the first stator to stop any vibrations. But I will try and run a single shaft with a shim on the shaft at the second stator.
So here is a very crude SketchUp Graphic of what I am wanting to do.
It has 3 blade impellers because they are easier to make.

I don't really know if this will increase performance, but it will be really cool.
What I am hoping for is thai if one impeller loses prime then the other will pull water through. Essentially allowing me to not worry about losing prime.

DRAGONFIRE81M

sorry but i havent looked it up yet . but isnt most dual impeller set ups for increaseing the presser. first starting out with a larger impller to drow in a large amount of water the a smaller impeller in the sec stage to compress the it to a higher presser. am i close

marshr32

Yeah pretty close. Normally the second impeller is higher pitched because it receives water at high speed from the first impeller. Its a way to both increase pressure and reduce cavitation, especially in whitewater.

So max, are you not gonna have a bush in the last stator, and leave the second prop with a blind hole? That would prob be easier actually.

marshr32

Pics of my impeller cutting setup. Its pretty basic and needs refinement but seems to work ok.

First pic is an overall view.

Pic 2 is the drive end. Just a prop adaptor that came with one of my motors that I grub screwed to the shaft so its in the same place every time.

Pic 3 shows the spanner mounted. Using a 12 point ring spanner means you can get 2, 3, 4 or 6 bladed props.

Pic 4 shows it set up and you can see that I just use a bit of pipe cut on an angle to guide the cutting path.

Pic 5 is obviously just a close up.

Note that the angle on the pipe is NOT the angle that ends up on the blades. The twist effect on the blades means the actual pitch is steeper. Basically you set it up so it moves however far the length of the prop is in whatever angle of rotation you want each blade to be. I have now modified the pipe to work a little better and give over 180 deg of blade sweep.

Also in pic 1 you can see shims, this is because the cutter needs to be at different heights relative to the prop shaft line to cut the front and back of the blades parallel. I do all the fronts then all the backs then take out whatever is left in the middle.
Aquajet, if you wanted to use a drillpress you'd need to move the shaft setup laterally to cut each side of the blades.

I'll try to get a video of it in action sometime if that would help.

sundogz

Yep, that'll do for a 'crude' graphic!. Those ideas are sound. Here's another few: Ist impeller larger, lower pitch, 5 blades, unthreaded with drive dog and (unthreaded) bushing in stator. Short, abrupt intake ramp. 2nd impeller smaller, higher pitch, 3 blades, threaded for shaft end. Single shaft runs through all. Should eliminate priming problems and increase nozzle pressure (thus speed). If you design the steering nozzle to have trim adjustment, you'll be able to control hull-design discrepancies as well. Sdg

I've decided to return the 180A Seaking for a replacement, but have reservations about it all - and so am ordering a 150A Etti Navy Race (w/bec) esc from Offshore today so I can get back in the water sooner. It's rated for 24v (they recommend 5s) and should be fine for the Hobby City generic 540xl rated at 110A (underrated, I'm sure) with a water jacket. We'll see.

sundogz

Marshr, that is definitely a cool rig (or jig). All you need now is a means to balance the finished product. Tres cooleros (?]>

CRAPPY COMPUTER=Edited entry
My computer sometimes cuts out when I am typing in this forum and so I have tried using a word processor and pasting it in, but with iffy results. So what I do now is type my ideas quickly (usually needs cleaning up) and then I go back and edit it - sometimes several times before I get it right. So why I'm telling you this is that sometimes others are online at the time - and so if this is you, you might outta go back and re-read my entry - say 30 minutes later, because it may have changed significantly. Sorry, but I am too broke to buy another computer, because of my high dollar hobby [X(] Mo' parts, mo parts', mo parts! If I were smarter, I'd start making my parts like them Kiwi boys [sm=wink_smile.gif]

damfurst

Pretty cool marsh. It's amazing what some guys can do with bits & pieces & brain power. Depending on how you cut you plastic pipe guide you can also vary the pitch? It'll be interesting to see how your impellers work in the drives.

I've still been working on the nqd crackerbox. I've got the 22mm drive mounted, but not much further than that. Been very busy with work & getting daughter off to college. Also, kind of working toward making my first hull for an airboat. Tired of sucking up moss this time of year when the wind doesn't blow. Anyway I did put a brushless setup in one of my abc hobby jetskis. I used a sv27 brushless motor & esc with 11.1v lipo(yes I finally own 3 of them). I think it will do fine. I didn't check the speed yet with this setup, but is close to 14mph that I had with my 2700kv purple motor on 9.9v. I don't think this one will overheat though like my purple motor did earlier this summer.

aquajet

Marshr
that has to be a perfect example of the KISS logic
Playing with the shape of the plastic guide can change the shape of the blade as well.
Will try making up something similar when Iget my jet back up and running.
Iwould like to make a impeller with a 150% blade area, like the jetskis and swash drive unit have, and I need to make up your idea a bit more ridged to try cut a piece of ally
Will first try Graels way in wax to get a proflie and see what it looks like.
Do you think a strong enough blade could be cast in plastic after being made in wax Grael.
Pleasekeep us posted as to the performance of the bades Marshr
Looks great though, and cant believe how simple it can be.

TwinJetMax

Wow marshr, that is smart stuff. I think that I might have build one of those also.
I think that it entirely possible to build some stators as well.
I wonder if you can get a sanding bit with the same dimensions as the cutting tool. That way you could actually get it a lot smother on the machine after the initial cut.
Where do you get the plastic stock from?

I am hopefully going to get some rubber mounts this afternoon.

marshr32

Yep, the shape of the guide directly affects blade shape. I modified mine yesterday to give more overlap like youre talking about (1st pic).
It definitely will need to be stronger to cut ali, but it shouldnt be too hard to modify really. It will be pretty slow cutting in ali tho.

Max, I just got the plastic from work as a piece of scrap and got them to lathe a hole in the centre for the shaft. I only have 2 bits left now tho, after a couple of experiments gone wrong, so im not sure where I'll get the next bit from. Hopefully I can get it right with these next 2 and go to ali.
A sanding bit would be an idea if you can find one small enough, mine is about 10mm dia!

marshr32

Yeah they do need some finishing when the come off the machine, but I reckon if Im careful they should be reasonably close to balanced.

What word processor do you use? Have you tried using the simple old windoze notepad?

sundogz

Office Word. And it gains something when saving it as regular .txt that you don't see until pasting. I'm using Notepad now, and if I don't have to edit, everything went OK. Sdg

TwinJetMax

I might try Mulford plastics. I know that they are selling bar stock now.
The other option of corse is just to go down the hill to Ulrich Aluminum and use alu.
I think that it would be nice to construct a graph so that you can easily calculate what you need to cut into the pipe. It would just take a bit of maths.

aquajet

twinjet
What you really want to do is work out the pitch and angular section you want the blade to be in solidworks, then expand the curve to the guide plastic diameter in solidworks and print it out as a flat patern.
The flat patern can be stuck on the outside of plastic guide pipe and cut to a exact shape.
If you know how to do that once all other profiles will be a walk in the park. I tried on a friends machine, but cant even make a prop. He says when he gets time he will look into it for me , but that can be months.

marshr32

Yeah that would be doable but a bit of a mission. Where would you measure the pitch? cause it changes as you go out from the hub. So even if you said measure it at the outside edge it would be different for each size impeller when using the same guide (not a prob if you always wanna do 20mm ones for the knockoff jets)
Probably the easiest way to classify them would be to say each blade sweeps 180deg in 15mm length therefore its a 30mm pitch for a whole turn. Or 120 deg in 15mm so its a 45mm pitch. Whatever combination of blade sweep and impeller length you have could work out at the same pitch (like 120 deg in 10mm gives 30mm again) so you can try 2, 3 and 4 blades at the same pitch and compare.
Personally Im just gonna say I know how many degrees I want each blade to sweep, and I know the length I want it to move in that arc so I cut the guide to that.

grael

Marshr32, your rig looks very promising !
Aquajet, yes, the procast and easycast products have about the right rigidity/flexibility balance, I'm sure you could make an impeller with them. I'd do it a bit thicker than the injection moulded black props though, and you would want to vacuume the resin before pouring.

TwinJetMax

It is not that hard.

>>> the pitch is inversely proportional to the radius that it is measured at.
I will take pitch to be rise over run. // this is not conventional but is still useful in this application.
For the example I will use 100% blade area.
eg. The radius of the prop at the centre is 2.5mm and at the outside it is 10mm.
The length of the prop is 20mm
>>> Therefore, the pitch at the outside is (20/20pi) x b // I left it in this format so that it is comparable.
and the pitch at the inside is (20/5pi) x b // b is the number of blades.

>>> The pitch of the pipe to cut is (20/2r x pi)+ x b // where r is the radius.
>>> If you want 150% blade area increase the run 50%.

>>>>>>> To put that all into a formula to get the cut for the pipe: ((length of prop)/2(blade area constant)(radius of pipe)pi) x (number of blades)

To get the pitch in mm you can just use your method eg 20mm in 90 degrees is 80mm.

I think that the above formula give a better idea of what is going to be cut. It includes all of the variables, especially blade overlap.

I just love doing maths on a saturday morning.
I hope it's right.

Edit: I keep seeing things wrong with my maths.[:@]

areseaer

Twinjetmax, Did you get your ASP 91M mounted? I decided to get one for my Shockwave, was cheaper than batterys and should give longer run time.

TwinJetMax

Nope not yet. I can't find a local source for rubber mounts. I will get some off the internet this week.

grael

Have just had another run of the small boat with the 4200rpm/v in-runner.
I'm having trouble with my lathe getting a proper centre for making couplings, the one I made would be OK for lower RPM, but it's very slightly out of true.[&o]

Anyway, the in-runner cured my flipping woes, but the water is too rough again today. I need flat calm water to see how fast it can go. Also, a proper nozzle ! I wasn't getting the most precise control, and was trying to give it very small bursts on full throttle in the small pond, but ended up crashing it into the bank.. 30-40km/hr+ ?, and broke the deck off the hull. It does handle ok in the rough water for it's size, but it's simply too light and beamy for good performance in very rough water. I ran an inlet under an aluminium angle bracket I'd mounted the motor on, and this worked great for cooling. However, in the crash, the motor armature outer assembly came off the end that I mounted to the bracket. I may end up just having to use a small out runner anyway for reliability, and to match it to the boat better.

TwinJetMax

That is not good Grael.
What do you think of my maths above?

areseaer

Did you try an auto parts store?