At present it's just forward planning, my motor looks like it us a Coreless brushed, i realise now that it's totally useless for the job, j will have to revert to a gearboxed motor that will give me 30 rpm. I will just have to redesign the mounting when I get to that part, but thanks for all your help.👍

Hi NickD

Oh no, you're right, if it doesn't have space you can't put gears on it.

I have never activated rotating radar antennas, because the ships I built are older.

I'm thinking about how you could resolve this but I don't have your view.
For example, couldn't you hide the engine in the hull and reach up to the radar antenna with a long, thin rotating axis?
In the plastic tube instead of the motor you could pass the axle.
In this way you could opt for an engine with lower rpm and equipped with reduction gears.
I'm guessing because I don't have any images to look at.

Since I have no experience in this field, I make general reasoning, think of them as thinking out loud.

I hope a modeler who has already installed rotating radar antennas can help you.

On a brushed motor everything is easier, because you can reduce the rpm by acting on the voltage. In this case a resistance (adequate in ohms and watts) can create a voltage drop useful for your purpose.
However, within certain limits, because under a certain voltage the engine no longer turns without having managed to reach the desired number of revolutions.
However, brushless motors (I know their working principle but I never use them) are different. If I'm not mistaken the number of revolutions is determined by the electronic controller. Without the electronic control it wouldn't even turn, unlike the brushed motor which just needs to be connected to a DC voltage.
Do you have a photo of this engine? Does it have two or three cables?
I've never tried with brushless motors, because I don't have them.

Start from a very high value of 48,000 rpm, which seems like a lot to me.
There are brushed motors on the market, even smaller and cheaper ones with lower rpm than yours.
Are you tied to this engine for a particular reason or could you change your choice?

For the Radar I have got the 4mm brushless motor to be hidden in some plastic tube, I can't really incorporate a motor with a gearbox to make it look authentic, is it not possible to connect 100 ohmn resistor or one off suitable size to the power line to reduce the motor speed from 48,000 rpm down to say around 30 rpm.
I am not brilliant with mathematical calculations. Otherwise can I not just connect in a potentiometer to control the speed of the Antenna rotations.?🤔

Hi Wolfang, but it's a radar engine, I don't think you connect it to a stick, hahahaha.
In brushed motors I believe the acceleration still affects the tension. I don't want to be wrong.

😉😉Or you accelerate less.

Hi Nick, sorry I completely forgot the second question, but Rodc answered you very well.
Generally, to reduce the rpm of a brushed electric motor, the voltage can be reduced or mechanical gears can be applied.
To reduce this much I don't think it's enough to reduce the tension but you have to resort to gears.
It's not very easy to make them yourself, so I think Rodc's advice is the best.

Hi Nick.

To know exactly how much your ship can weigh at most, you would need more information.

What type of ship is it? It is important to know the relationship between length and width.
Do you have a project or a drawing or at least some photos to show us?

If you have a real reference ship, look for the displacement and write it here, so we can do some simple calculations and considerations.

The overall weight of the ship corresponds to the displacement.
The overall weight of the ship is the weight of fresh water that would be contained in a volume equal to the submerged volume of your ship.
For these reasons you can arrive at the data you need experimentally and without calculations with a simple test in water.
In general, I advise you to always carry out static tests in the bathtub before launching in a pond.

If you want to do the practical test, draw your waterline, put the ship model in the tank, fill it with other weights (removable but stable) until the water surface coincides with your waterline.
Make sure that the position is correct, that is, that it is not tilted anywhere.
Then weigh the whole ship including the added weights, well that will be the weight to be respected.

However, approximately, I can tell you that a 120 cm vessel certainly exceeds 5 kg but you should stay within 7/8 kg..
But it is a very rough estimate, because I compared it with mine which is 115 cm long (overall) and very slender. Her displacement is just under 5 kg. A widther ship easily surpasses them.

I hope I have been of some help to you.

For the radar antenna arrays you want abt 20rpm. I suggest you buy a geared-down DC motor on AMAZON. THEY ARE PURPOSE-BILT FOR THAT SERVICE, inexpensive too, under $20CAN as I recall.