Jet Boat Question

[Andy Stone 1Participant @andystone1]

HI I've just bought a 40mm water jet motor, not knowing very much about them or what size hull it would drive has anyone had any experience with them ? another hopeful question Id like to ask, What size and type of Battery would the drive unit need / The motor doesn't have a water cooled motor, would it be a good idea to fit one to keep the motor cool ? sorry about all the questions but as you can see by my questions I'm a dummy where they are concerned

Regards, Andy

[Andy Stone 1Participant @andystone1]

[ashley needhamParticipant @ashleyneedham69188]

Hi Andy. Assuming it has a brushless motor, then the voltage of the battery will be set by what the motor is designed for. If the instructions don’t say, then look on eBay or somewhere and find the same unit to compare.

In the edf world, same size units can have different kv motors for 2 3 or 4s size batteries but as far as I know this is not normally done for water jets (but stand to be corrected).

Running as designed the motor should not get hot enough for water cooling to be needed. It may get quite warm and that’s to be expected, and having in-boat ventilation is certainly required. A good flow of air through the boat stops the interior becoming an oven.

Generally these things take a lot of current so larger capacity batteries should be used. The esc is likely the one item that DOES need water cooling, and there is normally a water take off point in the jet unit for that.

Ashley

[Chris FellowsParticipant @chrisfellows72943]

Morning Andy

That is quite a big and powerful water-jet so I think you need to look at hulls over 30".

If you look in Build Blogs, I'm just about to start a build called Scimitar. I'm using a 29mm water-jet in that and at 34" I'm pretty much on the limit for the unit, but I'd already produced some drawings which I just wanted to modify.

I don't usually water-cool my motors but I am with this, partly because it came with a jacket but also because the higher kv motor will be running at high revs for an extended time. If you're going to water-cool the ESC as Ashley suggests you may as well do the motor anyway.

Need to know more about the unit but many are designed not to exceed 20k. rpm and so from this and the motor kv, probably somewhere between 2200 – 3000 as a guide, will determine the battery size.

Chris

[Dave Cooper 6Participant @davecooper6]

Hi Andy,

As Ashley alludes to, the unit is roughly comparable to an EDF (Electric Ducted Fan) in the aviation world. I am just about to do some pressure recovery calc's for a Jet Provost by-the-way….

The big difference in the boat world is that water is a much denser medium than air and that affects things greatly. Try and get some static thrust data from somewhere (Manufacturers maybe ?).

Failing that, there's a good argument for a good old bath test. I think I would look around for a second-hand hull that's big enough to take the propulsion unit and a suitable battery. Hack it about a bit, use silicon sealant or whatever until it floats and see how it performs…..

You could also have a look on YouTube and see if others have done similar experiments. Come back to us with your findings.

Happy modelling,

Dave

ps Just seen Chris's post he has far more experience than me !

Edited By Dave Cooper 6 on 12/08/2023 11:13:06

[ashley needhamParticipant @ashleyneedham69188]

Absolutely. A wattmeter is invaluable here.

My ducted jet unit experiences are from the Stingray build. It was a 25mm unit with a 4000kv motor. When built and tested in the water the performance was good but the thing was drawing well over 30A….

Given theStingray hull is not specifically designed for planing (now There’s a thing) I re engineered it for a 28mm 2200Kv outrunner via a double coupling and the performance was on a par, for about a 12A current draw…A big difference.

Ashley

[Chris FellowsParticipant @chrisfellows72943]

In the scheme of things Ashley, even the 30 amp draw was low.

For my 29mm impeller water-jet a 120amp ESC is recommended! Starts getting expensive!

Chris

[ashley needhamParticipant @ashleyneedham69188]

I used a waterjet on the Stingray purely because I had to hide a prop somewhere.

I would question whether a waterjet gives more performance than a normal prop setup, especially viz. current consumption. Obviously if you want to model something with a waterjet than you have to use one but otherwise….

Ashley

[Chris FellowsParticipant @chrisfellows72943]

They are more inefficient, so for models more current draw and for full-size more fuel guzzled and for high speeds a prop is better. As we know in certain situations water-jets have their advantages.

In my case just fancied building something different again after the stern-drive.

Chris

[Andy Stone 1Participant @andystone1]

Hi Ashley, Dave and Chris, thanks for your replies and tips, I think Ive a lot more to learn regarding Jets, water cooled ESC,s and the power that a large motor I have which is a brushed 775 motor will draw, Particularly as the workspace won't have any cooling apart from the water jackets of the ESC and motor. Im still in the design stage of my hull so its good to ask these questions before I get carried away.

Ive another question without an obvious answer, Hopefully you can answer it. Some types of servo`s are described as waterproof, Are they really as submerging an expensive one could be a really costly mistake. I get water resistant but not waterproof

Andy

[Chris FellowsParticipant @chrisfellows72943]

Andy

It would be interesting if you could post up some pictures of your water-jet and its make and model, surprised it's got a brushed motor.

What size hull are you going for and type of boat?

As for the servos I wouldn't worry too much. The only place water could get in is at the top (and Im guessing there's a seal of some type around the shaft) and you'd have to have a serious amount of water in the boat to get as high as that!

It's going to be a learning curve for me as well which is part of the fun and reason for doing it.

Chris

[ashley needhamParticipant @ashleyneedham69188]

Servos in a boat shouldnt be getting wet! However there are circumstances where ‘an amount’ of water is inevitable and petroleum jelly is a good way of keeping water at bay. A smear of silicone sealant around the servo lead entry point and a good dollop of PJ around the top should keep out most of the wet stuff, barring a proper submerging.

Chris. Yes sometimes you just feel like trying something out.

There are other sorts of water jets, or rather, pumps, as well!

Ashley

Edited By ashley needham on 13/08/2023 23:30:46

[Chris FellowsParticipant @chrisfellows72943]

Oh yeah, I forgot about the entry points for the leads!

I think my water-jet will be my one and only foray into that sphere of propulsion. I've got other Faireys to build, with props!

Must get that timber ordered for Scimitar, my stocks of thicker ply are seriously depleted.

Chris

[Dave Cooper 6Participant @davecooper6]

Just something from my EDF research, a well-designed 'nozzle' at the air (water ?) exit point can accelerate the mass-flow to good effect and give extra thrust….

The physics of this is quite complex, but, there are some YouTube videos that explain it better than I can. I suppose the end of a fireman's hose is a good example.

Could be fun just to play with different lengths and bore sizes of nozzle just to see what results could be gained.

Dave

[Chris FellowsParticipant @chrisfellows72943]

The guys who 3D print their own water-jets design their own modifications and carry out experimentation but their needs are more extreme with using boats on rapids and waves on the sea. They tend to go for big water-jets in small jet boats.

I've just bought a stock item and am just looking for typical sports cruiser type performance on a lake and its about the build as much as anything.

Chris

[ashley needhamParticipant @ashleyneedham69188]

If you are steering by twiddling the end nozzle then it is tricky to experiment with different lengths and shapes.

In the edf world a narrowing of the exit tube can be advantageous, but having a nice clean entry and exit for the air is the most important thing, and one of the issues with my Bluebird model, and why the cabin is so large.

Ashley

[Andy Stone 1Participant @andystone1]

Chris,

The Type of boat intend to build is semi-submersible, the reason I asked how really were the waterproof servos were I particularly don't want to damage or have to replace it. I would also have to alter my design which isn't set in concrete yet but would rather not

The hull will be scratch built, something like a cigarette design. The hull length is 70cm, the size of the 3 ply wood I had available. Its not a bad size car hull either. Hope to receive the jet in the post tomorrow,

Andy

[Chris FellowsParticipant @chrisfellows72943]

It doesn't sound as though you are making it easy for yourself Andy! But why should you?

When you say cigarette, do you mean the off-shore racer Cigarette or cigarette shaped? Semi-submersible?

Be interested to see what the drive looks like when it arrives.

With mine the servos are mounted on and part way along the drive. For additional water-tightness you could have them in a sealed box in front but you might be struggling for space. I use Hitec servos and if I was looking for water-proof I think I would be happy with theirs as they state they meet IP67 which is the electrical standard for stuff used outside.

Chris

Edited By Chris Fellows on 15/08/2023 10:21:16

[Chris FellowsParticipant @chrisfellows72943]

You may have already thought of this but as you are concerned about the servo(s) what about the rest of the installation?

Also you need at least one hole through the transom for steering and two if using a reversing bucket as well. These holes are usually protected by a rubber bellows but they protect from splashes rather than immersion if that is the likely case with yours.

Chris

[ashley needhamParticipant @ashleyneedham69188]

I use aircraft slinkies and they are quite effective at keeping the water out as long as the outer is 6 inches or so in length. It also means you can put the servo almost anywhere in the boat and the run needs very little support. Ashley

[Chris FellowsParticipant @chrisfellows72943]

I've used them in my stern-drive build (if slinkies and snakes are the same thing?) but have used two so that one pulls and the other one pushes and vice-versa.

Can you just use one to push and pull or is the cable too flexible? Chris

[ashley needhamParticipant @ashleyneedham69188]

All my snakes/slinkies are single. As long as there is not too much unsupported inner it’s ok.

Ashley

[Andy Stone 1Participant @andystone1]

Chris, things aren't meant to be easy, its good to challenge yourself from time to time, Your right Im not making things easy for myself. The hull is a similar shape to the cigarette offshore racer, long and lean but not too long, Water ingress is a problem I'm going to have to tackle at some stage, Most and nearly allof the electronics will be enclosed in a water type enclosure, Its just the one servo Ill just have to rack the old grey matter for an answer, things aren't meant to be easy !

Whats a slinkier Ashley ?

Andy

[ashley needhamParticipant @ashleyneedham69188]

I’m calling it a slinkie but probably the technical term is probably a ‘snake’. A thin 2mm inner plastic ‘wire’ inside an outer sheath.

Used for operating aerelons on model aircraft usually as you can gave a really long run and they are almost frictionless. As the sheath is a decent fit over the inner, Its for all intents and purposes a waterproof seal if used in a boat (not sub)(but even then if the run is over about 6 inches it will be ok)

I use those adjustable metal servo ends. Seat the inner in the socket with superglue and use wire crimp pliers to crimp the sleeve over the inner. The socket bit also screws into plastic ball joint ends.

Ashley

[Richard SimpsonParticipant @richardsimpson88330]

Have you thought of using submarine bellows? This is one that has been used to seal an external penetration for a steering servo.

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