Hi everyone

Some of you may recall that I'm designing my own 600mm hull [1] and that project is in the back burner at present while I learn some things to make it go better! In the mean time I am practicing with a farbricated construction method that I originated on my first boat [2] where I build a frame and then bond a skin onto it to form the hull. Of course, I haven't invented this by any means, but I am just learning the method and need to practice it before using it on my scratch designed hull.

So I have found a plan for a 1000mm jet [3] on this forum and scaled it to 700mm. It has a slightly deeper V than I woud normaly go for but this will hopefully add to the learning experience. The BermyLizard hull looks really nice in all the videos I have seen of it so I have high hopes that this will behave nicely when complete and allow me to test some of the features that I may wish to include on my 600mm hull.

I am very lucky to have a 45mm pump [4] from Sundogz (which is a beautifully designed piece of equipment) and I will be testing that at the same time. It is paired with a 1600Kv 4465 motor (may switch to a 1000Kv unit) and a 125A esc which will be powered by two 3S packs in series (22.2V)

The 45mm pump is not a perfect fot for this hull in a 700mm scale but instead of making the hull larger I have elected to modify the hull drawing to suit this pump. I have chosen to do this because
1) I would need to go up a sheet size in material which will efectively double the cost of the materials
2) The other hulls this pump is being tested in are of this scale so will give a better comparison
3) The practicality of a really large jet boat (for RC) would be so poor that it would be a pain to put in my car/ walk to the river with etc.

These hull modifications include the transom, transom doubler and delta. Minor modifications have been made to the longitudinal formers and the mid later former. Later in the project I will have to modify the skins that make the bottom of the hull because of the changes around the delta which is now 25% wider to accomodate the bigger pump. I have included a screen shot of the modifications I have made to the hull frame in this post.

I am going to have the hull laser cut from birch ply. I will assemble it as per the original 1000mm hull [3] and skin the vital areas on the bottom with composite to keep it nice and tough.

The files are already at the laser cutters so I should get that back in a week or so and be able to start assembling.

While I am waiting for these parts to arrive I will have a look at designing some of the secondary control systems that I want to include such as remotely operated trim tabs, adjustible CoG etc.

Some of you might think this is all a bit familiar, I was originally doing this in plastic but I have had real trouble getting the plastic to bond to itself so have decided to refocus my efforts and go with the method I have described. I'll post somepictured from my phone in a subsequent post so you can see what is to come.

[1] https://www.rcgroups.com/forums/show...sign-and-build
[2] https://www.rcgroups.com/forums/show...ats-here/page5
[3] https://www.rcgroups.com/forums/show...n-%281000mm%29
[4] https://www.rcgroups.com/forums/show...-Drive-project

Here's some pictures of my attempt in plastic. You can see where it's come apart at the bow.
If I do use this it will be to make a mould so I can have the hull in fibreglass.

I have always liked that Bermy Lizard hull design, it is the quintessential jet sprint. Looking forward to watching your build.

Subscription added to this thread!
Happy to see you are going to try to use the 45mm pump

If you can tell me your used CAD software, I might be able to send you all the pump parts in a native format. I guess this is the triangulated stl file?

Please send me a PM!

1600kV sounds as if it is too much.
However I found out that many commercial available scale drives are revving in the area (tip speed exceeding 35 to 45meters/second) where the real scale drives tend to suffer from cavitation. My guess is the cavitation limit for model jet drives lies far higher.

In other words, with the correct pitched impeller and outlet diameter I would dare to make the gamble!
I can help you out with that too.

One of the exciting parts of this jet drive is with 4 impeller pitches X 3 stator sizes we get = 12 options. Now we can fit the jet drive to the motor size and KV to get the power and RPM for our individual piloting needs. This gives us a lot more motor possibilities with this jet drive. Thank you Sundogz and Unusual-rc.

You are welcome Kayaker!

That is what the real life jet drive manufacturers do: one size drive with multiple differently pitched impeller avialable, to match a certain motor.

At this moment I am busy with a newer version of this drive in size 35mm. I will try different impellers and exit nozzles. I am also designing a scale hull for it.

Originally Posted by unusual_rc

That is what the real life jet drive manufacturers do: one size drive with multiple differently pitched impeller avialable, to match a certain motor.

That makes perfect sense. I'm going to need a few different impellers I think to properly work out how it should be set up for this hull.

Its going to be a fun experiment!

There seem to be 3 materials at Shapeways that might work for impellers. I had them make all 4 impellers in regular Nylon. The biggest impeller was $12 USD.

They have a new 3D printer by HP and charge 2X the price of their regular Nylon prints. It is supposed to be stronger. They say; “HP Nylon Plastic is a Nylon 12 material 3D printed by fusing nylon powder together using black agents and heating lamps.” About $26.

Next is what they call stainless steel. It costs about $56.

Originally Posted by Kayaker

There seem to be 3 materials at Shapeways that might work for impellers. I had them make all 4 impellers in regular Nylon. The biggest impeller was $12 USD.

They have a new 3D printer by HP and charge 2X the price of their regular Nylon prints. It is supposed to be stronger. They say; “HP Nylon Plastic is a Nylon 12 material 3D printed by fusing nylon powder together using black agents and heating lamps.” About $26.

Next is what they call stainless steel. It costs about $56.

I had kind of discounted shapeways because of the assumed postage costs but I just checked and it's only $5 to the UK. I'll probably just go for it at that!

I'll just go for the cheapy cheap material though. When I'm decided on the best impeller maybeI'll get a nylon 12 one made.

I still think a 1000kv motor might be required so I'll maybe order one of those too in due course.

Presumably my higher kV motor will prefer a lower pitch impeller?

Do we know what pitch mine is please?

Originally Posted by dunk911

Presumably my higher kV motor will prefer a lower pitch impeller?

Exactly!
Basically a 1000kV needs twice the pitch compared to a motor of 2000kV on the same battery pack, if they are compatible in output power and torque!

Originally Posted by dunk911

Do we know what pitch mine is please?

That is the question!

Could you please provide me with a link of your motor? I need to know what the maximum amperage draw is, so I can make an estimation of the power on 6S.

By the way, does a 125A Esc not sound a bit on the low side for such a large motor?
My 4092 inrunner needs a 160A for example. This will be suitable for the 35mm version of the free printable jet drive.

I can make "in between" pitches and exit nozzle diameters if necessary.

Sundogz sent a 60mm (22 degree pitch) impeller to me and Dunk at the same time. I got the black painted one and and Dunk got the white one.

I now have two 22 degree pitch impellers made by Shapeways from Nylon, a black and a white one. After comparing them I think the black painted one was put in a vacuum paint bucket. If this is true then the paint would fill in most of the air voids making the painted ones stronger. This may be worth the extra $1.

Like Dunk, I will try them in white Nylon to see what works with my motor and ESC and if I brake one I'll try the HP Nylon. I installed the impellers on shafts with set screws. This may not be strong enough with the Nylon impellers. Roll pins like Sundogz's did on the black impeller he sent to me seems a stronger way to do this. Dunk's has a roll pin also. I only have hand tools so will have a machine shop drill the shafts for pins if set screws fail.

For testing this jet drive I got a Swordfish 240 amp Pro+ and a Leopard 40mm x 92mm, 930 Kv/rpm per volt 3Y. I'll try this motor on 6s and 9s. I expect this motor to be on the small side for this 45mm jet drive. I think it would be happier with a 56mm motor. This jet drive may work well at higher RPM's like Youngsta sees with his jet drive so I may end up with a lot of motors with different KV's for testing.

Originally Posted by unusual_rc

Exactly!
Basically a 1000kV needs twice the pitch compared to a motor of 2000kV on the same battery pack, if they are compatible in output power and torque!



That is the question!

Could you please provide me with a link of your motor? I need to know what the maximum amperage draw is, so I can make an estimation of the power on 6S.

By the way, does a 125A Esc not sound a bit on the low side for such a large motor?
My 4092 inrunner needs a 160A for example. This will be suitable for the 35mm version of the free printable jet drive.

I can make "in between" pitches and exit nozzle diameters if necessary.

It does seem low but I'm expecting to only use this on 6s so the current will be lower. All my connectors are rated around this too. Low current happy boat is my philosophy.

https://hobbyking.com/en_us/kb-44-65...er-1650kv.html

I believe it is this kiddie. There is a slightly longer, 800kv available that I might get if needs be.

Thanks for working this out for me :-)

Originally Posted by Kayaker

Sundogz sent a 60mm (22 degree pitch) impeller to me and Dunk at the same time. I got the black painted one and and Dunk got the white one.

I now have two 22 degree pitch impellers made by Shapeways from Nylon, a black and a white one. After comparing them I think the black painted one was put in a vacuum paint bucket. If this is true then the paint would fill in most of the air voids making the painted ones stronger. This may be worth the extra $1.

Like Dunk, I will try them in white Nylon to see what works with my motor and ESC and if I brake one I'll try the HP Nylon. I installed the impellers on shafts with set screws. This may not be strong enough with the Nylon impellers. Roll pins like Sundogz's did on the black impeller he sent to me seems a stronger way to do this. Dunk's has a roll pin also. I only have hand tools so will have a machine shop drill the shafts for pins if set screws fail.

For testing this jet drive I got a Swordfish 240 amp Pro+ and a Leopard 40mm x 92mm, 930 Kv/rpm per volt 3Y. I'll try this motor on 6s and 9s. I expect this motor to be on the small side for this 45mm jet drive. I think it would be happier with a 56mm motor. This jet drive may work well at higher RPM's like Youngsta sees with his jet drive so I may end up with a lot of motors with different KV's for testing.

Didn't mean to ignore you there matey.

Yes, I think I will go for the epoxy paint for an extra dollar! good tip, thanks!

So 22* is quite a shallow pitch in my (limited) experience, the NQDs that I have been playing with up to now are more like 45*. These pumps are very different though so hopefully it will work out just right. Plus there is only one of these pumps actually out there and running at present so I know we can never be sure what pitch will work best. I mean, even if this is the perfect pitch, I'll not know untill I've tried the wrong one too!

Anyway, the aprts are paid for and should be with me in a week. Very excited to be starting gluing! I'm really looking forward to getting this done. It's going to be a really nicely done hull.

Final piece of info: I have been working on a slide so I can have a mobile CoG to help trim the boat on the fly. Ive come up with this as a proof of concept. Just got to scale it up and re configure it so it has a double sided servo head with one fixed end so the servo moves to increase its throw to double. Lovely!

Linear servo slide (0 min 9 sec)

Wow, I didn't know this conversation was happening, sorry about that. Duncan, Kayaker is right, you have a 22 degree pitch impeller - what Michael refers to as 60mm pitch as it moves forward (ideally) 60mm for every rotation. This may not seem like much - compared to the NQD, but consider the blades overlap each other and there is little 'blowby' or slippage.

What I do know is this pitch is like being geared to the road and put a strain on my (cheap) 200A esc! I will be trying a lesser kv (1000kv) motor next time and will have lesser pitched impellers ready to try.

If you haven't already done it, I would get the least pitched impeller (11 degree/30mm pitch) made by Shapeways in Black Strong & Flexible Plastic (sintered nylon) to try on your present motor. That would be my gut feeling anyways. I've already worked out the ideal roll pin location for that impeller.

I like your 3D printed 'linear servo'! And if you can work out the logistics I believe it will be much more effective than a trimable nozzle. Looking forward to watching its' developmemt.


Kayaker, if set screws work - even in the new hardened nylon I will be surprised. But ya never know. Looking forward to your tests.

Thanks matey. I'll procure an impeller in due course. I need to build the boat still so no immediate rush as yet! I'm not sure how to sort out ordering from shapeways but I'm sure between the 3 of you there will be an answer! :-D

I'm really looking forward to having the adjustable ballast thing to try. I'm not sure how easy it will be to gauge though. I think it would work better on a hull with more frontal displacement than the bermy lizard one I have modified (to be even steeper v slightly). Maybe it would work better on kayakers boat. Either way, it's going to be fun!