Thursday, January 6, 2022

Marine diesel engine rebuild, part 1: removing propeller and shaft

WHIMSY came with a Beta Marine 28 diesel engine built in Sept 2000. This is a marinized version of the Kubota BD 1005, meaning among other things that it is cooled by a seawater heat exchanger rather than with a radiator and fan like in a car. They've also added a convenient handpump to make oil changes easier, but mine has been dangling in a wet bilge apparently and has rusted bits that will need replacing. The engine paint is chipped off, there are bits of corrosion and rust, etc.  However, despite cosmetics, diesel engines in general are tough, and will run forever as long as they get regular TLC in the form of clean fuel, regular oil changes and plenty of ventilation. 

Though I discovered the anode zinc pencil on the heat exchanger had been eaten away to a nub, suggesting poor maintenance and additional possible damage to the heat exchanger, the engine ran fine when I had the first test sail back when I first bought the boat. It has been sitting around since then, gathering dust. I can turn the prop shaft in both directions easily so I don't expect it is siezed. I needed to pull it for a rebuild, including head gasket change, and to make improvements (larger capacity alternator, for example, to handle charging a large lithium LiFePO battery bank) 

I wanted to get access to the bilge area and the area under the cockpit too. I plan to generally clean, fill-in the areas of the bilge that are inaccessible by hand (under the prop shaft log in particular) and paint the entire area with white gel coat followed by anti mildew paint on the upper areas (as in the anchor locker, which also tends to be a dark humid environment.) While the engine is out, I plan to reroute bilge drain hoses, install the autopilot, etc. The galley sink drain standing pipe is an open question, I may just glass-in and brace the drain standing pipe more securely, or seal it up and reroute the galley sink drain through a sump pump arrangement overboard above the waterline? I am tending towards the latter for safety though it will be more inconvenient to clean out a sump pump of bits of food and grease.

I also need to repack the prop shaft stuffing box. I had considered a watertight shaft seal but I think this many-hundred year old solution to having a rotating shaft through a hole underwater without leaks, is a pretty good one as it is, even though there will be a constant drip from the stuffing box. Maybe I'll find some way to direct the drip into a the sink sump I was thinking of building...

So anyway, the first thing to do is to remove the propeller, to be able to further disconnect the prop shaft from the engine transmission box. After removing the prop nuts, my prop just popped off easily using a gear puller, no drama no fuss. I just had to push the rudder hard to starboard to get the prop entirely off the prop shaft. The prop shaft zinc anode is new, and I had the cutlass bearing changed already.

Checking the prop, it has some minor nicks but seems fine. I noticed it had a nylon bushing, so it was probably designed for a 1" shaft, while WHIMSY has a 7/8" prop shaft. The bushing may be responsible for the wear on the prop shaft key, so I guess I am buying a new propeller and saving this one as a backup. This is a three bladed 12" diameter bronze prop, so we're talking about $500 new. (Have to figure out the old prop pitch.)

The prop shaft coupling unscrewed easily, though it looks very rusted. It includes an R&D flexible coupler to take-up torsion forces. Ideally I'd like to install a thrust bearing and cv joint (but I don't think there is enough room left over along the drive train that would allow me to also service the stuffing box.) As it is, the forward force of the engine is transfered to the hull via the engine mounts, which is not ideal; a thrust bearing would remove that force off the engine mounts, and the cv joint would resolve alignment issues. (When installing marine diesel engines,  you're supposed to make sure the the prop shaft coupling is aligned to the transmission correctly, checking with feeler gauges, but there is now way to check alignment once the engine is operating. CV joints actually require a little misalignment. )

Once the coupling was unbolted, it was easy to just pull the prop shaft from inside the engine compartment. I had made sure there were no bolts or keys on the other end of the shaft before just sliding it out of the stuffing box. 

The prop shaft seems fine, there is no corrosion, no groves, etc.and no indications that it was rubbing inside the shaft log due to misalignment Prop shafts often are subject to electrolytic corrosion due to stray current, or crevice corrosion esp. in areas along the stuffing box, where the packing deprives the steel of oxygen. The problem is that what may seem like surface discoloration can be evidence of very deep and so invisible corrosion on a prop shaft. Mine thankfully had no issues though I was surprised how short my prop shaft was, about 3 feet. 

Anyway, because I had pulled the prop shaft from inside the boat, there was no need for me to remove the coupling itself off the shaft. This was a relief because with the very rusted set screws on the coupling, I could tell this was a job for a professional machine shop (that will probably end up cutting off the coupling: if you do this, make sure to cut the coupling over the shaft key, to protect the prop shaft itself in case you cut too deeply, then use metal wedges to loosen the coupling off the shaft. If you do manage to get the coupling off easily by just undoing the set screws and using a coupling removal tool (or maybe you have a split shaft coupling, which consists of two parts bolted together over the prop shaft which can be removed much more easily) note it is generally a good idea to not reuse old couplings since they may be warped. Installing a new coupling on a prop shaft should also be done at a professional machine shop to make sure it is mounted square to the shaft. If the coupling itself is warped or misaligned, then the prop shaft and engine will never be aligned right, resulting in excessive transmission and prop shaft wear, maybe even a permanently damaged prop shaft or engine. A new one-piece coupling is about $70.