The Shipyard ( Dry Dock ): Builds & Questions > Working Vessels
Gasoline powered Tugboat
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:
--- Quote from: Colin Bishop on December 13, 2023, 08:35:41 pm ---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
--- End quote ---
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:
--- Quote from: tonyH on December 14, 2023, 09:57:59 am ---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. %)
--- End quote ---
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:
--- Quote --- 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.
--- End quote ---
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
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