Hi Eric,
Here is the link to the operating instructions.

I had similar problems and reset everything to factory settings. Then I used the boat that I need most often as my first boat.
Our club is aware of the problem, which is why everyone has a remote control for each boat. This means that someone else can ride along if you allow them to. In my case, it's my grandchildren who enjoy cruising around with Grandpa.

Hopefully, a few others will come forward with better answers.
Your Michel-C.

https://www.spektrumrc.com/ProdInfo/Files/DX6i-Manual_EN.pdf

Register on Spectrum’s web site. This will allow you to download updates and apply them to your transmitter.
This may fix your issue

I have just done an AI on your question and it has come up with this-🤔🙄

An older Spektrum DX6i that fails to retain settings (like servo reverse) and reverts to model 1 is
likely suffering from a failed internal memory capacitor, corrupted memory, or, most commonly, severely degraded battery power/contacts. The transmitter cannot save data because its internal power is too low to maintain settings when the main batteries are turned off.
Immediate Troubleshooting Steps:
Check Battery Voltage: Ensure your AA batteries are fully charged and putting out adequate voltage. Low batteries frequently cause memory loss in older DX6i units.
Clean Battery Contacts: HeliFreak users recommend cleaning the battery contacts and checking the tension of the spring clips in the battery tray to ensure a tight connection.
Check "Back" to Main Screen: When making changes, always return to the main screen (where the model name and voltage appear) before turning off the transmitter. Turning it off while in a menu can prevent saving.
Perform a Factory Reset: Model Boat Mayhem suggests holding the sticks down and outward while powering on the transmitter to trigger a factory reset. You can also go to Setup List -> Copy/Reset -> Reset.
Replace Transmitter Batteries: If you are using rechargeable AA batteries, they may be old and not holding a charge under load. Try a brand-new set of alkaline batteries.
If these steps do not fix the issue, the internal non-volatile memory or the component responsible for holding settings is likely dead, which usually requires a repair from "Horizon Hobby" or replacing the transmitter.
Hope this gives you something more to try, Steve..🙄

Rechargeable batteries hav a lower output voltage than alkaline, even right coming off the charger. A lot of electronic devices were designed to use alkaline and then the hobbyist thinking he/she will save $$$ by using rechargeable, will substitute nicads and learn the hard way. Also, rechargeable hav no where near the ampere-hour capacity of an alkaline

I have an old 6I which had problems with the digital Trims
Spektrum repairs had been transferred to Germany.
I found a repair company in Hertford called Logic RC Ltd
They were absolutely brilliant fast service and very reasonable price. Tel 01992 58226

That is good information Roger - a good UK service centre for Spectrum is just what we need - thanks for the "heads-up'.👍

Bob.

Logic are the UK appointed horizon hobby service centre, great service, very fair prices

Thanks all for the really helpful suggestions, particularly the link to Logic RC - RogerA1, could you check the telephone number you listed please? Looks like there’s a number missing..

Hi Eric
Sorry missed a 5Number is 01992 558226

Logic RC phone number correction I missed a 5 from the number should read 01992 558226

Thanks RogerA1, I rang and got the correct email address (support@logicrc.com - but had a reply from spektrum@logicrc.com) with the following reply:

Hi Eric

I would say with out seeing the radio it’s a main board issue the issue there is if it is the boards are no longer available so would be unable to fix.
You are welcome to send in the radio for investigation

Regards
John Norris
Logic RC
www.logicrc.com

That’s that then! Being sold for spare or repairs!
Thank you all..
Eric

I’ve followed your suggestion @mike stoney, thank you! One boat per radio is the way to go.. now got a problem with trying to program the ESC that came with the boat! Will open a new chat.

Hi Eric,

You may not have any fault with your Spectrum tx as the Sea King ESC is a "forwards only" type - it has no reverse function!

This may be causing you some unnecessary concern.

If you are familiar with how to "call-up" and "lock" a new or different model (or function) into your Spektrum radio, it should not loose its memory or switch to a different model.

Any function (including model selection) that is not "locked-in" will not be retained when the radio is switched off.

Check you have good batteries in your TX that are providing a strong current as this can cause you some problems if they are weak and unable to deliver enough power to support the function of your transmitter. (see Steves-s previous comments on this thread ).

I have several older types of Spektrum transmitters than yours and none of them loose their settings, but if you are not familiar with them, you may not be aware of the way they can be asked to change .....and retain ....their settings?

If you do not already have one, I would suggest that you get yourself a copy of the owners handbook for the Spectrum DX6i and read it carefully so you understand how it works and functions as they can be very confusing - especially as different manufacturers programme their transmitters in very different ways.

These manuals can be found online if you cannot find a hard copy to buy.(see the comments by Mark Stoney on this thread).

Model boats have by far the least demanding use for a very capable radio like yours as you will not have any use for most of the functions that it has to offer, but you should be able to use all of its memory functions - including using it for as many models as your DX6i can store.

Don't worry, your radio is probably doing what it should and may not have any faults at all.

If you have a local model shop or local radio control user who is familiar with the Spektrum DX6i radio (or any Spectrum or some JR radio systems) they will be able to help.

If none of the advise offered here by all the contributors helps, then try contacting Logic RC again to explain your problem and ask if they can give you a price for servicing - and possibly changing the internal memory capacitor if it has failed.

Bob.

I've just read from the beginning. I've had dxi6 for many years, well into double figs, I'm using dx8, gen 1 and gen 2, and nx8, with planes, so I know the transmitter quite well.
When a model is set in the memory, it won't get lost if the batteries get flat. I store my dx6i which is for boats without batteries, for many months, pop the batts back in, all the models are there in the memory.
The scroll roller is a problem if it starts to malfunction. It might be that, and the on off switch can start to malfunction over time. I've had both issues, the roller can cause model memory to jump. Don't disregard it until logic have looked at it. If they advise against a fix, they usually won't charge you. It's a great transmitter. Just be mindful 2.4 ghz doesn't like water so mount your receiver above the water line or you could get signal loss, and remember the hull will roll in turns.