Hi Steve d.

I like your logic in making the decision.
Insignificant cost, they do no harm and have no contraindications, why not buy them? At most they will be useless, patience.
Good boy.

Having said that, since this is a forum and, in addition to planning, drawing and building, we also like to talk about these topics, I make some considerations.

Meanwhile, congratulations to GregHiltz who not only gave you his opinion, but also indicated the type and capacity in microFarads to choose the capacitor to buy. I like helpful and pragmatic comments.
He rightly pointed out to get a ceramic capacitor.
Without knowing it, a person could have chosen the electrolytic ones which are not good because they are polarized and therefore the polarity must be respected.
I would have needed to do some calculations to establish the capacity of the capacitor to use, so I greatly admire those who (perhaps also based on their own experience) immediately give an answer.
Experience and practicality are very important; I have always held these two aspects in the utmost consideration.

Colin confirmed the possibility of using capacitors in parallel and/or inductors in series in another topic and this reassures me a lot.

The capacitors on the motor will filter out interference caused by the motor.

Disturbances on the radio link between transmitter and receiver can be caused by other sources.
From direct experience I can tell you that some interference could be caused by the external environment, coming from sources that are not always identifiable. In that case there isn't much you can do except change seats.

From indirect experience I'll tell you some causes could also be switching power supplies, such as non-linear but switching BECs (Not appropriately shielded).

Doing (serious) tests is not so simple and not decisive.
If I remember correctly, in another post (which I have not yet been able to dig up) Doug (RNinMunich) showed the screen of an oscilloscope, writing a very interesting post.
Again if I remember correctly, but I wouldn't swear by it, the display showed the classic duty cycle of a PWM controlled power output. However, the variable rectangular wave of the duty cycle was contaminated by some clearly visible disturbances.
However, I have to look it up better and reread it. I could probably understand it better today.
On purpose! If Doug intervened on this topic he could certainly give us a lot of useful information and advice.
However, the oscilloscope can detect what passes in the circuits, and therefore in the wiring. To observe whether the disturbances actually propagate in electromagnetic waves (more precisely in radio frequencies) a spectrum analyzer would be needed.
Some time ago I observed fixed radio frequencies caused by PC power supplies (which we certainly do not consider RF transmitters) on a Rohde & Swartz FSH8.
Both the oscilloscope and the spectrum analyzer are very expensive but in these areas, in my opinion, superfluous.

Furthermore, I also want to make another consideration, the loss of the signal for a very few fractions of a second can be harmful for the RC model of a car or an airplane but is much more unlikely to cause serious problems for naval RC models, unless it is of a very significant and persistent disorder.
RC ship modelers most likely do not even notice temporary signal losses possibly caused by noise.
It is no coincidence that disturbances introduced by equipment placed close to the receiver are a widely discussed topic among dynamic modelers of racing cars and airplanes, but much less discussed among RC naval modelers.

If you are very unlucky it could happen, at least in theory, that one of the cables is tuned (of the length corresponding to the wavelength or fractions of it) for an external disturbing frequency and acts as an antenna.

Finally there would be another cause of disturbance caused by the arrangement of the wiring which however I want to leave out because it is very difficult for me to explain in English.

I confirm (as I have already written in another post) that with transmissions on the 2.4 Ghz band you hear much less talk and discussion about disturbances and interference unlike transmissions on the 27 and 40 MHz.

Right so capacitors on order. Only a few pence each so why not.

Little correction needed here, brushed motors don't affect transmitters, that should be receivers. I don't know how far away an unprotected motor casts radio interference.

I only have one boat with a 27mhz radio which runs 99% of the time perfectly. Who knows what causes problems (intermittent) that 1% of the time.

Might try a test running one of my unprotected boats next to the 27mhz boat. That can be done on the table.

Lew
Florida ⛱️, USA

I agree with Toradog if someone is using 27 Mhz and your boat gets close to the persons transmitter
, maybe testing your boat poolside this may also cause an interference problem. I use 0.1 mag capacitors between each terminal and the case and between each terminal

Absolutely no problems for me. Have not have had any problems with brushed motors (no capacitors) and 2.4ghz radios.

Lew
Florida, USA

Ion, if you are still using the older (AM or FM Crystal) type R/C sets, which always were prone to wayward RF glitches, then yes, using 2× .047mf ceramic capacitors do help if you are getting glitches in the servo action caused by the brushed motors.
I havevfound with the newer technology of the 2.4 mhz systems, they bind and lock the Tx and Rx together very well, and with all the systems I have geared up, find no glitches at all.

I'd stay on the safe side. It would do no harm to install them. Not installing them may cause some harm. And cost and space are negligible. So, the decision should be clear.

If you are using a 2.4ghz radio, probably not. HOWEVER, some people that you run with might still be using crystal radios and your motors could interfere with their boats if you were to get close to theirs.
I winded way of saying that installing them is being safe rather than sorry.😀

Do I need them?
Or are they a throw-back to glitchy R/C gear of old?