Eeyore

Looks like time to drain the gearbox fluid. Use a clean container to catch it, then leave overnight for any nasties to settle out. If you prop one end of the container up any nasties will all settle at once end. Brownish will probably be (normal) clutch wear. Sparkling is bye bye gearbox.

In addition to @Kingdom Isambard Brunel list: Check the fastenings that hold the bell housing to the engine, a small number of engines came from the factory with the wrong length fixings. A remote possibility.

regenerative braking, now there’s a useful feature on a boat 🙄

Do you know if that’s the original air filter? The “standard” engine would have had a paper element in a metal casing. The usual problem with that filter would be a massive flat spot in the throttle response due to it running lean, although this may not be as obvious in a marine application.

Craftsman are listed by inline filters, but only the fuel filter, which at about 125mm in length seems longer than the original. It probably needs the filter mounting bolts loosening to fit it. You can also search by dimension for M16 x 1.5 threaded fuel filters. The oil filter will be a Baldwin B1400 for engines up to the CM4.42, with the option of the longer Baldwin B1402 on the four cylinder. These numbers can be confirmed by searching based on engine type: Mitsubishi L2E, L3E, and S4L2SD (S4L2 alone doesn't seem to listed - anyones guess!) There will be lots of opinions on choice of manufacturer when it comes to filters 😎. https://inlinefilters.co.uk/search/filter-kit/?vehic_man=craftsman&modelex=CM3.27

As do off shore wind farms. Bit of thread drift going on here, or maybe use a wind turbine to help with the startup current 😂

Typicaly 440,000v at the grid connection next to the power station.

Just curious, you believe that its 230v ac at the power station and that there is no volt drop between the power station and your home?

The biggest unknown will be how close you are to the tipping point between reliable/unreliable. So the larger CSA will be one (albeit small) addition to the reliable side of the equation.

Would the CSA of the shoreline have any effect. I'm thinking that in my (distant) youth a 3kw extention lead would be 2.5 sq mm, but now is typically only 1.5 sq mm. So the voltage drop during startup might be an issue using a lower CSA?

Its all good. @Salopgal now has some useful info to help debunk any bull that comes their way when getting quotes.

So the highest rated unit you could reliably start on a 16amp supply using a soft start would be?

Shall I tell Webasto, or do you want to do it 😇

I guess your unit has a "soft start" system fitted to allow it to be used on a lower rated supply?

Webasto S16 unit was used as a basis for my figure, have I misread the startup current requirement? https://www.webasto.com/content/dam/global-brand/business-fields/cooling/marine-air-conditioning/documents/Datasheet_S-Series_230V_EN.pdf.coredownload.inline.pdf