Yea I don't think the intention was to have it infinitely scaleable to absolutely any size, but to a range of impeller/shaft/bearing sizes as you stated.

My only "wish" in that category would be to make the upper limit 65mm, as I'd love to make an electric jet surfboard one day. (MHZ states that their Jet 64 is perfect for that application.)

Yup, don't really care on the min and max, just want to pick a few sizes in specific.

I think the point is to be different than what's already out there, incorporating the good aspects of all of them, not to copy one design.

Originally Posted by JUSS10B

Just commenting more or less to subscribe. That said I think all the goals make sense. The only thing that sticks out is the idea of scaling. I like that idea but it could be tricky for it to literally be scalable. With shafts and bearings coming in specific sizes, if think it best to chose 4 or 5 common sizes. That way the bearings and shafts can be spec'd to each size. I would like to see a range from 16 to 50mm. (Maybe 16 is too small but I would love to make a "pocket" jet boat").I can't voice much to the performance related items but I can give advice on printing and materials if wanted. Justin

Juss, I had this same thought a few days ago when you mentioned it wouldn't be (really) feasable. Had to think on that, but it's true. I'd like to keep it to round numbers, but maybe we should make it 15mm, 25mm, etc up to 65mm. We'll get to that once we have all the suggestions for features that we can muster. And I think we're getting close...

What would be a good size on which to base our prototype? I am thinking 35 or 45mm with the latter my favorite. But it doesn't really matter to me, I'd like to see what the popular consensus is. Please shout out your votes.

[QUOTE=Youngsta;36796955]Sounding very good
this vid is a bit old now but still the best to copy
if your shooting for a "perfect jet drive",,


Your video claims an abec-7 grade ceramic thrust bearing, I see a hybrid ceramic radial ball bearing. In the recent past you have also claimed a max rpm of 70k. An abec-7 grade ball bearing in any avilable size with a 4mm inner dia. has a max theoretical unloaded rpm limit of 45k. I question the validity...

A jet drive on acceleration reaches an efficiency limit where the amount of power required to go faster jumps up exponentially. At this point, the motor sounds like it is going faster (screams) but the boat doesn't go any faster (point of no-returns). This changes from small to larger impellers. For 19mm impellers it occurs around 25K, for larger impellers like 50mm it happens at a lower rpm (16K) and linearly in between. Of course this can be overcome with a more powerful motor requiring a more powerful speedo to take the amps (think pressure washer). But 70K is high for even SAW prop drive boats, so is a wee bit out of line. You need a speed control such as a Swordfish plus which will tell you the top RPM's of that last run to know precisely what is going on. But WTH, that is just tech talk and all most people want to know if it's fast. And Youngsta boats are supah fast off the line, there's no denying that!

A 15mm would be nice for smaller boats like the FT007. The NQD drive was a tight squeeze, and required some shaving down of the top of the pump housing and underside of the rearmost deck.

My MB rigger project is on the shelf - something is locked up. I had replaced the stock bushings with stainless bearings in an effort to attain the highest speed possible. Mickie warned us bearings wouldn't hold up long, and is why it comes with bushings. I'll tear it down soon and see what happened.

I had that same issue with said little jet. I could not rotate the outrunner outer can at all.

I discovered it was the bearings in the 250-size outrunner. Some light oil and a little working in eventually broke 'em free again, seemingly with no noticeable damage.

A jet drive on acceleration reaches an efficiency limit where the amount of power required to go faster jumps up exponentially. At this point, the motor sounds like it is going faster (screams) but the boat doesn't go any faster (point of no-returns).

Yes indeed. Drag increases by the square of velocity. To go twice as fast (lab conditions) you need to quadruple the power. Due to power losses through various means, even more power than that is needed in the real world.

That point where the power needed to push more water through is the choking point of that particular size flow path. With very high pressure you could push water through faster, but the impeller is trying to PULL water through, and thus cannot accelerate it beyond that point.

I did a bath tub test with the original Graupner mini-jet (19mm) and a 3650 3500kV inrunner, Esc 60A on 2S (6Ah, 50C).

I wish I had telemetry to tell the exact RPM and amp's, but it all worked flawless up till approx. 60% trottle. Everything beyond that point made the motor a lot more noisy and within seconds the Esc went into safety mode (limit the maximum amps). Indeed somewhere around 25K RPM.



Last time I put a 2800kV in on 2S, a perfect fit.



But still this drive has a number of design flaws:



- the "cheese" makes a hard transition inside the inflow duct

- two blade prop style impeller (no overlap, axial flow)

- bad gasket between drive body and reducer

- repriming on high speeds



Personally I would like to see something like the MKB drive. 30 to 35mm impeller for 4-6S "1/8 4x4 buggy style motors".



I think that size could give the best efficiency to size ratio, which can be equipped with a reasonably affordable drive system.



I am an (3DCAD) engineer myself, and waiting for my Prusa i3MkII

Yes, I think we've decided to go with the 5's - 35, 45 etc. instead of 28, 52 and so on. We haven't had many votes on which one we'll prototype but I've taken notice (35). a 25 would go well with a 28mm dia. motor as would a 36mm motor fit a 35mm drive and a 40mm motor a 45mm pump. This all works out fairly naturally. 56mm motor(or a TP Power 40)/ to the 55 drive and a 20mm motor on the 15mm drive. We should chart this (Airdogge?). 65mm would work for both 56mm brushless and those noisy nasty gas engines.

Of course inrunners and outrunners would have different results. One produces more torque and the other higher RPMs. You might need more than one chart.

I run a 24mm outrunner in my JT007 and 2850 inrunners in the other boats, all spinning NQD impellers. A 3650 is much too much for that puppy. Small wonder it cried out in distress when more power was added.

Originally Posted by AirDOGGe

Of course inrunners and outrunners would have different results. One produces more torque and the other higher RPMs. You might need more than one chart.

I was about to bring that on

For example the KMB 28mm drive works well with 35mm "helicopter" outrunners.



It would be fun to design an impeller for a specific RPM for an outrunner, and an inrunner. Basically the pitch of the blades can be made less for an inrunner (more RPM, less torque, but equal water column per second).



With a prop driven boat you can find the most optimal prop, with a jet you are left with only one option (exception: Gravitix)

Originally Posted by unusual_rc

With a prop driven boat you can find the most optimal prop, with a jet you are left with only one option (exception: Gravitix)

A few drives (or maybe one) offered different impellers. That's all it takes. Make a few in different pitches and let the owners/experimenters find the one most suitable for their chosen motors.

Originally Posted by AirDOGGe

Of course inrunners and outrunners would have different results. One produces more torque and the other higher RPMs. You might need more than one chart.

No need to concern ourselves with power differences, just motor mount screw spacing. Which seems to work out nicely.

Originally Posted by unusual_rc

It would be fun to design an impeller for a specific RPM for an outrunner, and an inrunner. Basically the pitch of the blades can be made less for an inrunner (more RPM, less torque, but equal water column per second).

Originally Posted by AirDOGGe

A few drives (or maybe one) offered different impellers. That's all it takes. Make a few in different pitches and let the owners/experimenters find the one most suitable for their chosen motors.

Yes, we'll get on soon with impeller design. We'll want the most efficient (performing) design with the highest top end obtainable at that level that performs best in the widest range of applications. And I believe that has been worked out and looks like the samples below. Overlapping blades with small, rounded, sharpened leading edges on a conical (mixed flow) core. I would like to try a two blade design as well for smallest drive(s). And pitch is of course part of that equation.

All sounding good so far. Someone needs to sketch out how they want the intake to look with 3-views for Justin to follow.