Gearbox out for service : Thamesway 12000

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    Czarina Blue

    I apologise in advance for reiteration, but thought to make my own topic in case it got more of a viewing:

    I have recently suffered intermittent loss of forwards propulsion simultaneous with the Perkins 4108 hunting up 100 revs. This cycle occurs about once a minute for 15 secs, and I see my hull speed drop off during the higher revs. The prop shaft is still turning during that time but I imagine that is from the force of flow over the prop blades?

    Spoke to Kevin from THamesway Marine this morning and he says the bearing will be worn and the Gearbix a Thamesway 12000, probably a Mark II, will need to come out and be overhauled. This will cost around £1200 inc VAT and parts, with the courier charges on top, and it’s a heavy item.

    Unfortunately I am in Sicily, in Siracusa, and not expecting this (!) I am now looking for a mechanic / engineer to help with this. I understand from the forum that this may be possible with the boat in the water, as Voltair and Trevor have managed it in the past.

    Just wondering if this box is worth overhauling bearing in mind its age and the difficulty of getting parts apart from via the UK, or there is any advantage in considering a more modern adaptation? From the forum it sounds as if people have stuck with the old box, and just renovated it.

    Any further advice on its removal and or replacement with more modern versions, I would much appreciate.




    Duncan –
    Rainshadow, hull #126 has a pressure switch on the gearbox. If the oil pressure is not high enough, the gearbox will not engage. Are you sure you have enough oil in the gearbox sump? Or could the pressure gauge be failing?

    We have never experienced the behavior you are seeing, but I just thought I would the simpler questions before you consider a gearbox rebuild – just in case.

    As you say, the prop will freely spin, so the shaft rotation does not mean the gearbox is engage.


    Czarina Blue

    Thanks Marilyn, that’s an interesting point, and a mechanic was fiddling with the gearbox pressure gauge recently as part of diagnosing a faulty alternator when he was trying to understand what the light on the dashboard did ( he expected it to go out once the alternator started charging, though I explained to him it was a gearbox oil pressure light. I hadn’t however realised it was a switch that controlled clutch engagement. I believe that I’ve kept the oil level high enough in the main gearbox, and I’m surprised Kevin at TMP didn’t mention it this morning I will mention this to the mechanic and to Thamesway tomorrow to see what they say. Thanks again for your advice. Duncan

    Czarina Blue

    FYI Kevin says that pressure gauge is only a warning light of the pressure status and has no power to disengage or control the clutch.

    I’m sure there was sufficient oil in the gearbox dump so I’m going to have to bite the bullet I think.

    Regards, Duncan

    Czarina Blue

    Having understood that the main gearbox level should be checked from cold or with the engine off for a significant amount of time, despite what both the Perkins manual and the Thameswaysmusl say (!) I Havd managed to remedy what we thought was a worn clutch on our Thamesway 12000 Mkt 2. We are now motoring again without a hitch so I think we’ve escaped the dreaded ‘gearbox out’ for now.

    The pressure gauge which on my boat gives a read light on the dashboard by the wheel only on starting or if there is low pressure in the gearbox due to lack of oil. The gauge is not, as I understand it from Thamesway,a switch. It doesn’t need to be a switch because the way the gearbox works is that if the pressure gets low enough the clutch can’t grip the shaft and turn it, due to la k of pressure-induced friction, so the outcome is the same.

    From the Type 12000 Mk2 Manual: page six
    “An oil pressure gauge is provided as an extra to indicate the oil pressure in the hydraulic system…(pressure figures)…the pressures read by the gauge are not critical but, should the oil pressures fall appreciably below these figures, insufficient pressure may be exerted by the Operating Piston to prevent the cone clutches from slipping. This will not only result in loss of power at the propeller shaft but will also accelerate clutch lining wear.”

    So if you have a slipping clutch for goodness sake check your main gearbox oil level which should be not higher than the mark on the dipstick ( though Kevin at TMP said it could be as much as 1 cm above the mark but really not more). If this is definitely at the correct level then you may indeed have excessive clutch wear.’ A full service with TMP is currently approx £700 plus vat, not including couriers week turn around at their end.

    Cheers, with much relief!

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