The gearbox on a friends’ yacht started playing up recently. It is a simple Hurth box with a three position gear lever – forward, astern and neutral – which is all you need for the average sailboat. From time to time it would slip out of gear when going forward and then a few seconds later it would grab again and go back into gear. This was ok for a time, but it was noted that as is always the case, these malfunctions gradually deteriorate and become worse, never better, and that at some point in the future it would need some attention. Sure enough it gradually got worse and was staying longer out of gear before going back in again, to the point where the decision was made to take some positive action.
Apart from this being potentially dangerous i.e. not being able to go astern when docking etc., or subsequent damage arising from being unable to avoid a collision when under power, it is good seamanship practice to keep right on top of these maintenance jobs when they occur – not much later when they could potentially put you, your boat, your crew and others at risk. This particular gearbox is a simple Hurth (Hurth 150) one speed box attached to a marinised Perkins 4108 (48hp) diesel engine of hazy history. The boat was built in 1988 and the engine could well have had hours on it prior to it being marinised. The Hurth would have been fitted new at that time and twenty five hundred hours have been run on the combination in the ensuing years.
Following a considerable amount of research we have located a marine engineer who can service and recondition the gearbox providing we can get it to him. This is the point we are currently at now, and facing up to diving into the bilge to uncouple the shaft from the gearbox and remove the gearbox from the engine. The date has been set and all going well we should have it out of the boat and to the engineer by the afternoon of Tuesday 14th April.
Easter has arrived and true to form we are experiencing classic Easter weather – overcast, drizzling rain, but warmish – it never fails, no matter what time Easter falls! That being the case we brought forward by one day the task of stage two of our removing the gearbox project (challenge) – never a bad idea when you are dealing with mechanical people who, with the best will in the world, very often fall short on the promised deadline.
Fortified with a strong cup of tea Patrick (the owner) and I set to work having removed all the side panels from around the engine. There is a nice bright cabin type light installed in the engine room so at least light will not be problem. First task is to drain the ATF (automatic transmission fluid) from the gearbox and this is done smoothly with the drain plug coming undone quite easily. Next, the gear cable is disconnected from the gear lever and strung up out of the way. We remove the cable bracket as well as it will also be in the way when we come to wrestle the gearbox off the bell housing of the engine. So far so good, with all nuts undoing reasonably easily.
Now, we come to what is to prove to be the most heat generating part of the operation, and that is uncoupling the shaft from the coupling plates. The shaft needs to be slid back toward and through the stern gland about eight centimetres, to allow for the coupling plates to come off and the gearbox to be worked off its studs on the bell housing. It doesn’t want to budge. Nothing generates heat more quickly than two amateur mechanics in a confined space heaving on an obstinate piece of machinery that doesn’t want give up its purchase. The shaft has been quite happily rotating whilst driven by the engine, but once de-coupled does not wish to be turned by hand and definitely not wanting to be slid sternward into and through the stern gland.
The shaft has obviously bedded itself into the stern gland packing and lodged in a comfortable position from which it objects to being changed. The next step is to dig out the gland packing and this is laboriously achieved with a sharp screwdriver and time. It has been awhile (ten years) since the gland was packed so it was stiff, hard and dry. With the gland packing removed and using the largest lever available we attempt to separate the shaft from the coupling plates. It moves about half a centimetre and stops once more. Several sweaty minutes later and a combination of levering between the plates and a large driver rotating the coupling plates from side to side the shaft head plate and shaft is worked back five to six centimetres.
This looks ok but we were to find out later it was not quite far enough. The next task is to unbolt the coupling plates. These are very cunningly bolted from both directions and neither set of bolts have enough room to fully exit their respective plate. This was where we discovered the shaft and its plate had to go a little further aft. By this time we knew what we were doing and a couple of good heaves and it was done. With much debate as to how to remove the bolts from the coupling plates, many minutes later and a lot of wriggling they came away. It was at this point we began speculating on how we were going to re-assemble them! Cross that bridge when we get to it was the consensus.
With the coupling plates removed we were rapidly closing on our target. Six nuts are all that now stand between us and the completion of this part of the job. Surprisingly, they all come undone sweetly and with a gentle lift and tug off its studs, the gearbox comes away from the bell housing and is laying on the cabin floor.
Into a suitable sized bucket, lowering it carefully into the tender and rowing ashore is accomplished without mishap. We shall have it on Barry the gearbox engineers’ workbench tomorrow morning sharpish, leaving him to do the reconditioning which is a specialists’ job.
We have set ourselves the target (reminiscent of a ‘Top Gear’ project) of having the gearbox back in our hands and re-installed on the boat this coming Saturday – watch this space for part 2.
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