DIY RC Boat with Parallel Impeller

[CarlPace]

TL;DR Summary

Need tips for a weird water propulsion system.

I'm not sure if you would call it a "Parallel Impeller," but the pics below should tell the story. I'm designing/3D printing my next RC boat from scratch, and I wanted to try something different from tried-and-true propeller and shaft system. Reasons?

1. Fun.
2. Using a "wet" brushless motor means I won't have to punch a hole through the hull to accommodate a rotating propeller shaft.
3. Gyroscopic stabilizing? Maybe?
4. Boat being "pulled" from the front intake to help maintain heading?

There are a ton of variables to consider in the design process, and I'd love any physics advice you have to offer regarding the specs highlighted below. I'm definitely not asking for something like the "best" anything--more just wondering if there are any rules of thumb to help guide the specs. Like "xyz is better for low speeds" or "abc will be more efficient at the cost of size."

Boat hull shape is strictly placeholder. Snarky criticism is encouraged.

(Forgot to ask about number of blades/space between them in the above pic)

Thanks!

 

[BillTre]

Looks like an internalized paddle wheel thing to me.

 

[berkeman]

Welcome to PF.

I'd love any physics advice you have to offer regarding the specs highlighted below.

Why are you showing curved blades? As you can see from the existing design posted by @BillTre traditional ones use flat blades. It seems like your curved blades would pick up and lift excess exhaust water (costing drive energy) and toss it forward (costing momentum).

 

[Lnewqban]

Welcome!

My humble opinion:
In theory, that is supposed to be working as an in-line pump.
When the casing gets full of water via centrifugal effect and high rpm's, the pumping effect will be close to zero.

Please, see examples of proved pump types:
https://en.wikipedia.org/wiki/Pump

https://en.wikipedia.org/wiki/Axial-flow_pump

Why not to use the proven system of ducted fans in RC airplanes?
That would work as an in-line axial pump instead.

 

[Baluncore]

If the device takes water from in front, accelerates it into a tube, to release it at the rear where energy is recovered, then the vessel will make no bow wave.
The saving in bow wave creation will be traded for wetted surface in the tube.
Then why have a pointed front on the boat ?

 

[jrmichler]

There are a ton of variables to consider in the design process, and I'd love any physics advice you have to offer regarding the specs highlighted below. I'm definitely not asking for something like the "best" anything--more just wondering if there are any rules of thumb to help guide the specs.

I tried a search using terms high speed paddle boat and found several good hits.

This one, at https://www.boatdesign.net/threads/fast-human-powered-paddlewheel.46084/, has a large human powered paddlewheel in "full hamster" mode:

Not as fast as you are looking for, but I could not resist posting it. A more applicable research paper discusses design variables for paddlewheel driven boats with speeds up to 32.8 knots, with a discussion of the potential for even higher speeds: https://www.marinepropulsors.com/proceedings/2011/FA3-1_Harte.pdf. An image from that paper:

The paper also discusses propulsive efficiency vs some design variables.

Then I searched high speed paddlewheel boat, and found US Patent 6743064, titled High-speed Paddle Wheel Catamaran. This patent puts the paddlewheel in a tunnel as alluded to in Post #5 by @Baluncore. And many more good hits. I almost sucked myself down a rabbit hole following them, but fortunately was able to control myself.

About 25 years ago, I built a pedal powered paddle boat. In undocumented, very informal testing, it was about 2/3 the speed of a canoe. It was good in shallow water, and very good in thick weeds.

 

[BillTre]

Cats seem to be popular here.
This seems somewhat like the situation @Baluncore mentioned (water going in a tube rather than between two hulls.
Would the bow wave effect be the similar?
Someone's gotta know it for catamarans.

 

[Baluncore]

Would the bow wave effect be the similar?

A fast displacement hull, must climb its own bow wave. The limiting hull speed for a displacement hull is proportional to the square root of the waterline length.
https://en.wikipedia.org/wiki/Hull_speed
That is why, for the same stability, the two longer hulls of a cat can be faster than a monohull. There comes a point where the bow waves of the twin hulls interact, so things become more complex.

 

[CarlPace]

Loving the feedback. Thanks all.

Looks like an internalized paddle wheel thing to me.

Yes and no. My thought was for the chamber to be flooded, so that instead of lifting water, it would technically be maintaining a spin cycle that jets out the back. I'm catching vibes from the others that the flooded-cyclone-chamber-thing might not pan out, though.

Why are you showing curved blades?

The design was inspired by taking apart one of these. The inside looks a lot like my curved blades. Granted, it's the reverse function, but even though that one is intended to "catch" energy, my thinking was that it might still be better to "scoop" water from the intake. I was also envisioning the chamber flooded, and creating a sort of cyclone would be a good thing.

...and in my head that all worked perfectly and made outstanding sense. Sounds like no?

Why not to use the proven system of ducted fans in RC airplanes?

I imagined those, but didn't know what they were called! Very helpful to have keywords to search. Thanks. I'll dig in on those.

My concern with that was having a motor in the center of the (call it a tunnel?), blocking the flow of water. I spent a while looking into creating my own in-runner motor, where the stator formed a ring around the walls of the tunnel, so all that touched the water was a magnet-ringed propeller.

Perhaps the losses incurred by blocking some flow with a motor would still be better than dealing with this paddle contraption I'm thinking about?

Then why have a pointed front on the boat ?

Fascinating. The hull I showed was only a placeholder, but the final was probably still going to have a pointed bow. Now, perhaps not...

I almost sucked myself down a rabbit hole following them, but fortunately was able to control myself.

My friend, I feel your pain/pleasure with rabbit holes. This boat business has been a deep one for me. Thanks for doing all that digging. The hamster wheel alone is an inspiration.

Do you think a flooded, spin-cycle chamber would be better than paddle blades dipping in and out of the water line? I can't figure out if giving the water circular momentum would be a good thing or bad.

My end goal is learning enough to design something autonomous and ocean-faring that can survive under those conditions, which is why I'm experimenting with internalizing as many components as possible--save from seaweed, crushing storm waves, etc.

This next one will only be, like, iteration 3 of 10 toward that goal. 1 and 2 are below, "Slippy" and "Peppy".

Do you have pics of your old pedal powered boat?

 

[Baluncore]

I can't figure out if giving the water circular momentum would be a good thing or bad.

Twisting water is a waste of energy. But it could be good, if you can take the twist out of it later, by converting that twist into more forward propulsion.

 

[CarlPace]

Twisting water is a waste of energy. But it could be good, if you can take the twist out of it later, by converting that twist into more forward propulsion.

The untwisting intrigues me. Would that be a matter of having the exhaust pipe at a certain angle?

 

[Baluncore]

The untwisting intrigues me.

Stators in the flow, or a turbine.

 

[BillTre]

A flooded impeller chamber would be like a centrifugal pump. They suck the water in the center and throw out to their periphery by centripetal force. The water is collected by the housing and directed to a pipe.

An axial pump uses a propeller in a pipe:

Magnetic drive pumps are common.
They separate the motor and impeller more cleanly (no seal around a drive shaft):

The ideal situation might be a mag drive directly driving a propeller from outside the pipe so you could get a completely linear flow.

 

[CarlPace]

A flooded impeller chamber would be like a centrifugal pump. They suck the water in the center and throw out to their periphery by centripetal force. The water is collected by the housing and directed to a pipe.
View attachment 339746

An axial pump uses a propeller in a a pipe:
View attachment 339747

Magnetic drive pumps are common.
They separate the motor and impeller more cleanly (no seal around a drive shaft):
View attachment 339748

The ideal situation might be a mag drive directly driving a propeller from outside the pipe so you could get a completely linear flow.

These are excellent, thank you. I'm liking that magnetic drive a lot.

 

[jrmichler]

Do you think a flooded, spin-cycle chamber would be better than paddle blades dipping in and out of the water line?

Your idea is similar to a type of air blower known as a cross flow blower. These blowers are used in applications where low profile and low cost are both important. Those motel HVAC units under the window with the annoying noisy whine use these blowers. They are notable for their low cost, loud annoying noise, low efficiency, and low cost. Especially low efficiency. Since air and water are both fluids, I would expect that a low efficiency air blower would also be a low efficiency water mover. A typical cross flow blower is shown below:

Centrifugal water pumps come in different designs, where the optimal design depends on the ratio of discharge head to total flow. This is expressed by the parameter specific speed. Specific speed is not dimensionless, so it is necessary to know the units. Even though it is not dimensionless, it is useful. The following chart shows how optimal pump impeller designs are a function of the specific speed.

This particular chart is from Marks' Standard Handbook for Mechanical Engineers, 9th Edition. A similar chart in Centrifugal and Axial Flow Pumps by Stepanoff has a more detailed discussion. A boat drive system moves a large amount of water at low discharge head, and the specific speed chart shows that propellers are the optimal choice for that application.

Fortunately, efficiency is not the only criteria that you can use to design a boat. A 30 knot paddlewheel boat would be a lot of fun, and a real attention getter.

 

[pbuk]

A 30 knot paddlewheel boat would be a lot of fun

until a paddle hits a submerged object

and a real attention getter.

particularly when a paddle hits a submerged object

 

[pbuk]

Boats that propel themselves by squirting water out of the back have been around for years, ranging from this to this.

I'd start by looking at some of those for inspiration rather than a shower pump.

 

[Nik_2213]

Agreed, pbuk: Looks like a 'jet-ski' or 'jet-boat'.

Else, a 'pedalo'...

 

[CarlPace]

until a paddle hits a submerged object

particularly when a paddle hits a submerged object

At which point, it becomes a temporary artificial applause generator.

 

[sophiecentaur]

TL;DR Summary: Need tips for a weird water propulsion system.

2. Using a "wet" brushless motor means I won't have to punch a hole through the hull to accommodate a rotating propeller shaft.

That's a very attractive idea. Fridge motors go on for ever by not needing a driveshaft seal and so do ornamental pond pumps.

 

[Nik_2213]

Two thoughts...

Some-where, I've seen a 'mud-skimmer' that could scoot across soft shallows due to shallow draft plus load-bearing 'paddle wheel'. IIRC, small hover-craft proved better for estuarine rescue work...

Second, in addition to trad 'stern-wheeler' and nimble 'side-wheelers', there are also 'quarter-wheelers'.
For you, latter may offer some advantage at speed by allowing front half of hull to lift from 'displacing'...

https://en.wikipedia.org/wiki/Paddle_steamer
https://www.sarikhobbies.com/product/paddle-tug-lulonga-plan-hull-set/