Prop design for low wattage boat

[koios]

I am building a small autonomous boat with electric power, probably in the 20w range. Power will be solar but with battery backup. I am currently trying to work out an efficient prop design.

Here are my current design specs for the boat:
LOA: 6ft
Beam: 1ft
Weight: ~30lb
Power: 20W
RPM: 3000
Vicprop gives a hull speed of a bit over 3kt and a speed for the boat of 2.2kt, 2kt would be fine for my purposes. Free!ship gives similar results.
The design is a monohull, sort of Wigleyish.

I would prefer not to have to put a reduction gear in my drive train, but the Vicprop numbers suggest a very small diameter prop (~2in). I don't have a huge draft but I should be able to have a 3 or maybe even a 4in prop - should I consider gearing down?

Should I consider a ducted prop? Posts on this forum suggest this would be good for my limited speed. If so, what changes would that make in the specs of the propeller? Also how long should the duct be?

Other people have posted about using RC airplane props. If I don't gear down, my motor specs would be roughly comparable to the under square specs of the airplane props. Can I use the Vicprop specs to choose an airplane prop?

My first boat build, my first forum post, so all help is welcome!

 

[Zilver]

Hi Koios,

I'm no expert but I have fitted my small trolling motor with a rc prop like you mentioned (I use a 10x8 prop on a trolling motor to power a small 16ft trimaran - works good). The APC propellers meant for fuel engines are very sturdy, cheap and practical to experiment with. I think they are a good way to quickly test what works and what not.
3000 RPM seems very high to me, and a 2" prop very small (related?) but maybe someone more aknowledgable can chime in on that.

Good luck, Hans

 

[koios]

Hans,

Thanks for the advice. I will try a prop like you used and experiment with it - you have way more power than me but if I keep cutting it down I should be able to roughly optimize it. I have been printing more boat-style props in PETG but there is no way my prints would be strong enough for an airplane style prop.

I presume your prop is two bladed?

As for the rpm, I really fear I will have to gear it down. Looking at anglegrinder bevel gears right now.

Laura.

 

[BlueBell]

3000 RPM is ~10x too high, 300 would be better.
You may want to consider an adjustable pitch R/C airplane prop form Germany.

 

[koios]

BlueBell,

I feared you would say that! Feeding 300rpm into Vicprop (am I expecting too much of Vicprop, since my sort of boat is probably not what they were thinking!) I get a 10in prop which seems a lot.

If I put the prop in a duct, would that change the optimal size/rpm?

Laura

 

[Blueknarr]

If I put the prop in a duct, would that change the optimal size/rpm?

IMHO the risk of floating debris snagging on the duct and stopping the people's rotation far outweighs any possible gain.

 

[BlueBell]

BlueBell,

I feared you would say that! Feeding 300rpm into Vicprop (am I expecting too much of Vicprop, since my sort of boat is probably not what they were thinking!) I get a 10in prop which seems a lot.

If I put the prop in a duct, would that change the optimal size/rpm?

Laura

Hi Laura,

I'm not familiar with Vicprop but I highly doubt it is appropriate to what you are doing.

Generally, a larger, slower prop is more efficient and when you only have 20W, you need to get all you can from it.

I don't believe you would gain from a duct as they lend themselves to high thrust, low speed.

What pitch does Vicprop say for a 10in diameter prop?

Much more information is needed however to make any further recommendations.

Pictures, Statement of Requirements (SOR), conditions, mission, etc.

 

[koios]

BlueBell,

Vicprop gives 9.9X18.6 for 2 blade and 8.8x18.1 for a 4 blade. It's an online calculator and easy to use, but their business is selling props for leisure and commercial vessels, massively beyond what I am looking at. So I have no reason to believe that what the calculator puts out is realistic for my "edge case".

So what's the mission? I suppose this is really a toy. I am really interested in autonomous robots, I love the sea and I think solar power is cool. It would be nice if this could patrol our local lake, scaring geese and measuring ecological markers but I would also like to take it kayaking in Boston Harbor.

For requirements, it would be good to run off say 40-50W nominal of solar power, be manageable - hence the 6ft/30lb, maybe slightly longer or heavier. I want this to be self righting so cats are out. 2kts is fine, I don't want this scooting away.

My biggest concern right now is trying to spec the motor and that quickly gets into specing the prop. I had thought I could drive the prop on a shaft from the motor, but it's hard finding low revving motors at least at reasonable voltages. From what you say, especially "...and when you only have 20W, you need to get all you can from it." (so true!) I will look into gear motors to get down to those rpms. I don't like gear motors because it's one more thing to fail but that may be my best bet. Mostly right now, I want to start doing experiments with different motors and props to find what works with the panels. Your suggestions are super helpful in getting a place to start.

From your comments and those Blueknarr, I'll scratch the duct off the list!

Laura

 

[philSweet]

I think a cordless drill would make more sense as a motor-gearbox hack. They actually have cordless drill boat races in England once a year.

 

[BlueBell]

I suppose this is really a toy.

Oh good, just for fun.
So screw around with what you've got and learn from that.
Lots of old threads on here to review using the search engine.

A self-righting, dome-top with a flexible solar-panel powering a little hand drill
mounted on a deep keel would get you started.

Personal Message me your email and I'll send you some drawings.

 

[Dejay]

Cool project!

I've played around a bit with JavaProp that you can download and design a propeller using more complex methods, but I haven't been able to generate an efficient propeller (>65%) yet. I plan to order the "propeller handbook" later to learn more.

Gearing cost only a little bit of efficiency and it might be much less likely to fail first than other parts of your boat. I would go with an old cordless drill too.

Theoretically you could design a catamaran to be self righting too. But probably not worth the bother.

Maybe you'd more practical advice in a community for building RC boats.

 

[koios]

Thanks philSweet, BlueBell & Dejay! I'll look at cordless drills - I have seen videos of the races and it's a good idea. I like the idea of an 18V motor because it should get the most out of the panels without having to put a buck converter in. Most drills seem to be way above my power budget - typically well over 100W. I was hoping to find a way to use the design of the prop to properly load a motor in the 1-2A range and so for now I am going to pursue some gears.

I'll look at JavaProp. Your 65% efficiency - that's measured, presumably? I have been hunting around on the other parts of this forum but I should look at RC forums. So much of that market is driven by LiPo specs, super high currents and low voltages, the opposite of my problem.

Anyway, thanks all! I will let you know how I get on with gears and some bigger props.

 

[Dejay]

No JavaProp only makes predictions, if you google the webpage you can find descriptions of the algorithms used.

 

[koios]

Dejay,

JavaProp is interesting, lots for me to learn. My best prop is 39% so far!

 

[Dejay]

39% with 3000 rpm?

I guess for a low watt application you could just 3D print a prop. There are some threads here talking about javaprop. But it's another complex topic to learn