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[1967Brutus]

Since most likely in this section there is the most experience with propulsion systems on tugboats, I'd like to ask you guys:

For a 105 mm diameter fourbladed prop in a tugboat, what would be the expected thrust at a given amount of power, and what kind of RPM would I be looking at? Does anybody know if there is this kind of info availlable in for example tables or online propcalculators or such?

[Akira]

You have listed the diameter of the prop, but other factors need consideration. 1) the shape of the prop, 2) the pitch of the prop, the operating rpm of the prop. I'm not a naval engineer so there are probably more factors involved.

[1967Brutus]

That's the thing: I actually AM a naval engineer, and I do know what factors are in play, but...


I cannot give a pitch, because it is a controllable pitch propeller (4 blades), and I do not know an RPM, I only know that I most likely have 80 Watt availlable at somewhere between 1000 and 1500 RPM.
So I need "general propeller data" to be able to make an estimate on the eventual reduction ratio, IF needed...

If anything else fails, I would be interested in for example, people that use a 4 bladed 105-ish mm prop and ever have measured, to hear their ree running and moored shaft RPM and Watts (Volt x Amps)

[tonyH]

I think you could be better off by reversing the matter and using old propellor design formulae which seem to be simpler.
You'll have to choose a series of discrete pitches covering the range you're working in and you should get a graph.
Good luck anyway

[1967Brutus]

I think you could be better off by reversing the matter and using old propellor design formulae which seem to be simpler.
You'll have to choose a series of discrete pitches covering the range you're working in and you should get a graph.
Good luck anyway

That is exactly what I intend to do, because I am aware that a controllable pitch propeller does not have a fixed set of parameters... IF ONLY... If only I could find propellerdata, and I seem not to be able to find that.
All I see is propcalculators that are laid out to find a prop for a boat and installed power, but I want it the other way around: I want to find a power and RPM for a given prop. The imput fields of those prop calculators however do not allow the search the way I want it. So I am a bit depending on those of you that have measurements done on their props, so I can collect data and construct a graph.

I have contacted the original manufacturer, Raboesch, which still exists and even is located in my home country, but I have not yet heard back (sent my question Sunday evening).

I am at sea right now, but if I haven't heard from them by the time I get home, I'll either call them or even visit them (they are about 100 km away from me).

[Colin Bishop]

Selecting scale model boat props has always been a black art depending more on empirical comparisons than theoretical data.

In your case not many conventional model boats employ props of that diameter and the number of those using controllable pitch must be negligible so you are extremely unlikely to find any convenient tabular data.

You might be better off setting up a test rig in a tank and measuring the various inputs and outputs as best you can at varying pitch settings on the prop. Power consumption can be readily measured using a wattmeter but output thrust would be more difficult and might need to be assessed by eye.

Colin

[1967Brutus]

Selecting scale model boat props has always been a black art depending more on empirical comparisons than theoretical data.

In your case not many conventional model boats employ props of that diameter and the number of those using controllable pitch must be negligible so you are extremely unlikely to find any convenient tabular data.

You might be better off setting up a test rig in a tank and measuring the various inputs and outputs as best you can at varying pitch settings on the prop. Power consumption can be readily measured using a wattmeter but output thrust would be more difficult and might need to be assessed by eye.

Colin

That would require me to build a fairly large test tank, and buy a large electric motor and measuring equipment. I know large boats are relatively rare, but I was hoping that, since people building large boats tend to be a bit more finicky, might have measured their amps and RPMs.

I am an engineer, and I have some empirical data on a 50 mm controllable prop, but from 50 to 100 mm is a bit too large of an extrapolation to be really reliable.

Oh well, it was worth a shot...

[tonyH]

What I was thinking of when I suggested reversing the situation was using a simple flat-bladed prop as an example, with a 1:1 pitch, calculating the possible thrust, allowing for a guestimate of the slip, from the 80 watts you think. If you repeat, changing the pitch in increments, you should get a graph which would give an indication. Also, a decent prop should be more efficient so the graph should understate.
Better than doing a Computational Fluid Dynamics degree anyhow.

[1967Brutus]

What I was thinking of when I suggested reversing the situation was using a simple flat-bladed prop as an example, with a 1:1 pitch, calculating the possible thrust, allowing for a guestimate of the slip, from the 80 watts you think. If you repeat, changing the pitch in increments, you should get a graph which would give an indication. Also, a decent prop should be more efficient so the graph should understate.
Better than doing a Computational Fluid Dynamics degree anyhow.

I know what you mean.
The thing is: For aero propellers, calculators that return a thrust and absorbed power, for an imput of propsize and RPM, are in abundance.
I cannot find that for ships propellers.

Mind you, I may not be expressing myself properly, English is not my first language.

If you have such a calculator that calculates the thrust and RPM for a given imput and propsize, I would be much obliged.
I can reverse that by messing with the numbers to create the graph. But the calculators I find, do not work very convenient for that purpose.

[Colin Bishop]

I know large boats are relatively rare, but I was hoping that, since people building large boats tend to be a bit more finicky, might have measured their amps and RPMs.

Many large model boats are warships which, like their full size counterparts, use multiple screws. A 100mm prop is huge by comparison; there are very few single screw boats with this size and most are tugs as you know. (of these steam is often the favoured power plant as it offers lower rpm combined with high torque.)

Matching motors to props at model sizes is a question that often comes up but there is not much in the way of published data and in most cases the empirical route is taken whereby you try to identify a similar model which works satisfactorily and extrapolate from that. Back in 2021 I contributed an article in Model Boats magazine which was intended to help modellers in selecting motors and props by publishing the details and characteristics of a number of typical scale model boats which could be used as rough benchmarks for comparison purposes. It was practical rather than scientific as I am no engineer. If you PM me your email address I can send you a copy but the largest propellor size mentioned is 70mm.

There are large plastic plant troughs that could be used as a test tank and don't cost much. You mention a large electric motor but surely with a prop of that size you will be using some form of gearing to bridge the gap between motor RPM and prop rpm to maximise efficiency and reduce current drain?

Tony's suggestion could give you a rough estimate of the thrust.

It would be certainly interesting to hear from any forum members who run large single screw boats.


Colin

[1967Brutus]

Many large model boats are warships which, like their full size counterparts, use multiple screws. A 100mm prop is huge by comparison; there are very few single screw boats with this size and most are tugs as you know. (of these steam is often the favoured power plant as it offers lower rpm combined with high torque.)

Matching motors to props at model sizes is a question that often comes up but there is not much in the way of published data and in most cases the empirical route is taken whereby you try to identify a similar model which works satisfactorily and extrapolate from that. Back in 2021 I contributed an article in Model Boats magazine which was intended to help modellers in selecting motors and props by publishing the details and characteristics of a number of typical scale model boats which could be used as rough benchmarks for comparison purposes. It was practical rather than scientific as I am no engineer. If you PM me your email address I can send you a copy but the largest propellor size mentioned is 70mm.

There are large plastic plant troughs that could be used as a test tank and don't cost much. You mention a large electric motor but surely with a prop of that size you will be using some form of gearing to bridge the gap between motor RPM and prop rpm to maximise efficiency and reduce current drain?

Tony's suggestion could give you a rough estimate of the thrust.

It would be certainly interesting to hear from any forum members who run large single screw boats.


Colin

First bolded: that is what I am trying to collect data for, by asking the question in first post.
second bolded: Will do, and as long as there is enough range in the propellers in that article, I might be able to determine a mathematical relation and extrapolate. I surely appreciate the offer for practical help.
third bolded: There are several ways around that, Direct drive with variable voltage should be not too inefficient and lead to decent calculations, and a geared motor sure also is an option, because in the end, Watts are Watts, and it is rather irrelevant which method is chosen (I am a naval engineer and fairly familiar with all kinds of methods to determine power and efficiency, both electric and IC). I am fairly confident that I can work something out and correct for scale etc etc.

As it is, the drive is going to be a slow-speed single cylinder fourstroke of which I have no data yet, only estimates and goals.
Way I see it, I expect about 80~100 Watt at 1100~1500 RPM, but RPM still has to be selected (sound is the determining factor here) and the actual power to be had, depends on me modifying carb and camshaft (valve timing, these engines are timed for 10K and I need to change that).

I have experience with this kind of drive (thread in the pleasure boat section about Krick Alexandra). I have a 5 cc fourstroke tuned for 1500 RPM. 7 years ago I measured that engine to about 17 Watt and that was enough to drive a Marx 50 mm controllable pitch prop, but just about...
Back then, that was with original valve timing. I recently altered valve timing and by feel, torque (and thus power) doubled but I can't say for sure because it bottomed out my torque measurement set-up. So I would say I am at least able to get 30W from that 5 cc, extrapolating says I should be able to get 80~100 from a 15 cc, give or take.

Now, the situation is, last week I suddenly came into the poisession of this controllable pitch prop, and I have blades for 86 mm or 105 mm.
The idea to go for a 15 cc engine followed the purchase of the prop, not the other way around, so now I am tryingto figure out if the whole idea is viable in the first place.

Just to give an insight in my train of thought here.

[1967Brutus]

Colin, PM sent. Thanks for the offer.

[Colin Bishop]

Thanks for the additional information, I think we had assumed you would be using an electric motor. A low RPM 4 stroke is of course a different matter. I will send you the article but it may not be of very much use in your circumstances.

Would it be worth looking at advertisements for electric trolling motors (for fishing dinghies etc.)? Some of these have props of around 100mm  and there is often thrust and power data quoted for the various motors. You might be able to extract some relevant data.

Colin

[tonyH]

https://the-contact-patch.com/book/fluids/f1518-propeller-thrust
This looks interesting
Sadly, my son is now in Canada and he's a hydraulics engineer who did his masters on CFD. That said, I'm pretty sure that his answer would be "trial and error" simply because there are so many variables, not the least of which is the prop being in very shallow water.
Another factor in play relates to why the need to know. The prop size and the revs are inter-related anyway so it could be worth looking at some of the steam threads on here where models are used to tow one or two "passengers" along in dinghys. OK, the acceleration is not great but are you planning on towing or just sailing solo?
Also, see if you can find a book by Sidney Barnaby who wrote one of the first guide to prop designs in the 1920's. Worth reading! The copy I had may now be in Nick (raflaunches) reference library.

[1967Brutus]

Would it be worth looking at advertisements for electric trolling motors (for fishing dinghies etc.)? Some of these have props of around 100mm  and there is often thrust and power data quoted for the various motors. You might be able to extract some relevant data.

Colin

Now THAT is a possibly very useful suggestion!

[1967Brutus]

OK, the acceleration is not great but are you planning on towing or just sailing solo?

I am not really planning anything, I just want to build "a little engine that CAN"...

I don't expect to be towing a lot. It is going to be a large and awkward to handle boat, not something you bring along every sunday, but I just want to be able to "drain the pond and wash all the fish ashore" when I jam the lever to deck.

[tonyH]

 

[1967Brutus]

Thanks for the additional information, I think we had assumed you would be using an electric motor. A low RPM 4 stroke is of course a different matter. I will send you the article but it may not be of very much use in your circumstances.

Would it be worth looking at advertisements for electric trolling motors (for fishing dinghies etc.)? Some of these have props of around 100mm  and there is often thrust and power data quoted for the various motors. You might be able to extract some relevant data.

Colin

E-mail received, many thanks again, Colin!
Going to read it tonight, and let the info sink in.
From other sources, I could more or less destill out that per 1W of power (as measured by current, brushed motor, Bühler to be exact but I do not know the type) an approximate of 0,4N of thrust can be generated. Person that shared that with me, said his boat did 10N at 24Watt with normal props in Kort-nozzles, and with props designed for use in tunnels he reached 15N at 36 Watt.
Both measured with tied up boat.

I do not know the diameter, but it is a start that I can theoretically achieve 0,4N per Watt.
That does not yet give me any info on the RPM or the diameter, or the relation between those two.

Some simple idiot-mathematics however would suggest, that maybe tip velocity  is a factor, and currently my 50 mm prop runs 1500 RPM, which seems quite OK, therefore for a 100-ish mm prop 750 RPM would be a nice value to start with.

Just going by disk-surface (I would not be surprised if disksurface is a factor) it is for starters not unreasonable to assume, that for same tip velocity, but disk surface quadrupling, absorbed power could be 4 times as high.
Maybe the blade surface/disk surface ratio plays a role in it as well, I need to think about that.

Mind you, this is just some redneck engineering right now, I still need to read those articles you sent me.

But at least I think I start to see some light at the end of the tunnel

Will keep you guys updated.

To the moderators: If this thread would be more appropriate in the R&D section, feel free to move it.

EDIT: Oh, and Colin.... I don't do "electric"... :p :p :p :p

[Colin Bishop]

EDIT: Oh, and Colin.... I don't do "electric"... :p :p :p :p

 

[Shipmate60]

I would tend to think 1500 rpm is about right.
If it was me I would use something like a 550 motor geared down to that speed on 6 volts.
If more speed required up the voltage, if too fast trim the ESC.
Also a Marine Engineer with a few years experience not greatly into indicated power ratings as the wind and waves are not to scale.


Bob

[1967Brutus]

I would tend to think 1500 rpm is about right.
If it was me I would use something like a 550 motor geared down to that speed on 6 volts.
If more speed required up the voltage, if too fast trim the ESC.
Also a Marine Engineer with a few years experience not greatly into indicated power ratings as the wind and waves are not to scale.


Bob

It is a bit hard for me to copy you guys all in on what I find on other forums, but right now, picking up things left and right, I am inclined to think that this 105 mm prop will absorb somewhere in the neighbourhood of 50~60 Watt at full pitch and 750 RPM

That is in part based on that my current prop (the old Marx 50 mm variable prop) absorbs in the order of magnitude of 12~15W at 1500 RPM and full pitch. Both props appear to have a max blade angle of 45 degrees if I am not mistaken.
Since 750 RPM for the large prop results in by and large the same tip speed as 1500 RPM on the 50 mm prop, and blade angle is the same, but disk surface is 4 times as high, I think power absorption is also 4 times as high.

I think, 1500 RPM is too high, that would make power absorption increase by four, to about 240 Watt.
Mind you, I throw out numbers like it's nothing, but these are ONLY estimations and deductions, I could be wayyy off here. but it is supported by the following: I more or less measured my current prop at roughly 15 Watt at 1500RPM, and according to Marx, the manufacturer, that prop was rated for 500W and 10K RPM.
Now if I go by "double the speed=four times the power" that would be 60W at 3000, 240W at 6000 and if I want to double that, that would be 6000 times root of 2, which is about 8500 RPM. It's not quite there, but close enough to regard "double speed is 4 timesthe power" as to be a reasonable assumption.
 

am thinking: 1100 RPM should be possible with a direct drive, and the beauty of a variable pitch prop, is that I do not HAVE to have full deflection, now do I? IF, big if, power absorption at 750 RPM would indeed be about 60W, then 1100 RPM (which is pretty close to 750 times root of 2) would be about 120 Watt. 1100 RPM still is low enough to generate a cool tugboat sound...

Since electronics can do amazing things, and I have aquaintances with amazing programming skills, maybe I can have a feedback loop on the engine throttle lever, cutting the pitch back if the throttle passes a pre-set position, preventing the engine to overload. That would result in a control loop that would limit the pitch to full engine load in a bollard pull or towing situation, but release the pitch for a free running boat, and anything inbetween.
Heck, possibly I can even arrange something like that via telemetry feedback and programming in my FrSky Taranis. Who knows?

[Subculture]

I have a small MAP publication titled 'Model boat propellers', which has very good practical advice for matching speeds, calculating thrust etc. Sadly long out of print AFAIK, it was first published in 1972, and revised in 1980.

Books on fullsize propeller design tend to be a little too involved I think.

[Colin Bishop]

Thrust is one factor. Another is the shape and displacement of the hull you are pushing. A lot of variables to get to the end result.

Colin

[1967Brutus]

Books on fullsize propeller design tend to be a little too involved I think.

Correct... Believe it or not, nautical college does not spend much time on the subject, given that a ship has a propeller and that's that. No one is going to change one iota about that thing, they are in general too expensive.

Several ships that I worked on had fairly extensive data on the installed propeller and I have out of interest tried to wrestle myself through those books, but man, that is tough stuff.
The basics I can understand about the relation to pitch and speed, speed and power, and such, but only on a level as more or less expressed in my previous post, which still basically is "redneck engineering".

But I am slowly getting at least in the ballpark, and things seem to settle in the direction of about 1100 RPM. That should allow the engine to develop decent power, and the prop to absorb that power, give or take. Which is a relief, because it saves me the construction of a reduction gear.

[1967Brutus]

Thrust is one factor. Another is the shape and displacement of the hull you are pushing. A lot of variables to get to the end result.

Colin

Yup.. and a lot of THAT info, I do not yet even have a clue, as I do not yet have a boat.