The Boat Tinkerer: Episode 31 - Diesel Engine Part 2

Sub-projects

The overhaul of my diesel engine is, in fact, a number of sub-projects run roughly in parallel. Some of the sub-projects, such as the rewiring, cannot start in earnest until the earlier projects have been brought to some sort of conclusion, if not to completion.

Another such sub-project is the rebuilding of the engine beds, which I had delayed until I had confirmed that the old diesel engine would go back into the boat. A new engine might have required an entirely new bed configuration.

The engine beds are the two parallel beams on which the engine brackets and mounts sit, as shown in the first photo. As the engine lay not horizontally, but on the same plane as the propellor shaft, the engine needs only four brackets and mounts. If the engine were laid horizontally, it would need six brackets and mounts and a universal coupling between the gearbox and shaft.

Finding the angle of the propellor shaft proved more difficult than I had expected. If the boat were out of the water, it would be quite simple; all I would need to do was to remove the propellor shaft from its tube, shoot a string line or laser through the tube into the boat and mark where the string line or laser would hit the mast or a bulkhead (it would hit the mast unless the boat was seriously out of whack). However, removing the shaft means breaking the shaft seal, which means that any water outside the boat would rapidly find its way inside the boat, sinking it quickly. Whilst the boat was in the water, all I had to work with was the existing beds and the near-vertical face of the connecting flange, which is about 90mm in diameter with a hole in the middle.

After much measuring with inclinometers, and the usual boat-yoga needed to read the numbers, I found that the rear part or upper step of the existing beds was probably on the right plane, about 5.5 degrees from the horizontal. The absolute angle was not important; what was important was getting a reference line inside the boat for the new beds. Furthermore, after yet more boat-yoga, and several measuring jigs, I determined that the rear part or upper step of the existing beds was also the right offset from the plane of the propellor shaft for the new bearings.

The geometry is incredibly complex, and difficult to describe in words. The difficulty in measuring things was made harder by the absence of reliable reference lines or planes - almost nothing was straight, flat or with nice sharp edges to measure from. I went through half a dozen measuring jigs, just to get the fundamental dimensions.

What I found was some shoddy workmanship in the original build. Although the rear part or upper step of the beds had been set properly, the front part or lower step was set at the wrong angle and it was too short, requiring the builders to bolt on an additional 2 inches of timber just to provide enough material to screw the mounts to. The second photo below might be a little hard to read because I traced both port and starboard beds onto the card as a rough reference. The black lines show what is actually there, and the thin blue line shows what is needed. The wrong angles on the lower front steps might have contributed to the snapping of the forward starboard bracket under the started motor, which has now been re-welded.

Furthermore, as I probed into the construction of the beds themselves I found what amounted to a heap of timber off-cuts, including soggy pine, with plenty of air voids and globs of un-bonded resin. I had hoped to saw off the tops of the beds and replace the surfaces on which the engine bearings sat, but there was not enough sound timber or glass, and what was left was unusable. The third photo below shows the beds partly removed with my battery-powered demolition saw (my new favourite toy).

I could not tolerate the shoddy work and resorted to cutting out beds entirely. To give you an idea of the state of things, I removed most of the timber off-cuts in the beds by ripping them out with my bare hands after some initial attention with the demolition saw, such was the poor state of fixing and bonding under the hull. I worked with my hands as much as possible, taking care not to send the demolition saw through the boat hull, for obvious reasons. I did not even need a hammer to dislodge the timber off-cuts.

After the initial removal of the beds, I saw what I thought to be a crack in the hull, which might have beed caused by a grounding by a previous owner. I cut out more of the floor and cleaned it up to find that the crack only extended to a superficial layer of spilled resin and did not extend to the hull. To my great relief, the hull itself appeared in perfect condition with no cracking whatsoever.

Whilst investigating the crack, I emailed my insurers to ask advice. When I had established that the crack did not extend to the hull, my insurers acknowledged my diagnosis and suggested that I keep a photographic record for future reference. Incidentally, my insurers also informed me that the crack (or something similar) could only trigger a claim if it were related to an identifiable event. Shoddy build-quality was not covered. In any event, it was advisable to inform my insurers about the work I was doing.

One more job was to give the bilge a good clean, which I did with dishwashing liquid, a scrubbing brush, and a garden hose on “jet” setting, followed up with swabbing the exposed hull with methylated spirits. Given that the main driver for this project was to de-smell the boat, I used the opportunity of an exposed hull to remove as much as possible of some 40 years’ accumulated oil, grease, grit, resin spills and generally smelly muck. The resulting bilge soup filled four 10-litre buckets with filth, and went into the disposal tank at the nearby slips. It is not a job I would like to repeat. I’ll clean the hull further when I get to gluing or epoxy-ing the new floor into place. For every hour of measuring and cutting, I must have spent another hour of cleaning up.

The final photo below shows the engine bay after the removal of the beds and floor, after initial cleaning.

I will address the rebuilding of the beds in a consequent blog.

Critical dimensions

I found these by measurement, as there appear to be no record drawings in existence

Diameter of propellor shaft = 1 inch (25.4mm)
Diameter of propellor shaft tube (found by wiggling the shaft from side to side and up and down) = about 35mm, which means the shaft is central in the tube when it is 5mm from either the left-right or up-down limits of wiggle.
Amount of fore-and-aft wiggle in the shaft = 10mm. At rest, the water pressure on the outside of the propellor shaft pushes the shaft forward until the propellor rests on the P-bracket at the rear of the boat, This also extends the seal inside the boat. This means I will need to arrange the engine so that it pushes the shaft backward by 5mm, getting the propellor clear of the P-bracket, and so that the seal is not fully compressed.
Additional length of shaft due to polyflex coupling = 25mm. This means I will need to align the engine so that the flanges touch, mark it off, then move the engine forward* by 20mm to allow for the addition of the poly flex coupling and the 5mm adjustment mentioned above.
Diameter of polyflex coupling = 127mm. This means I will need to ensure that the new floor is low enough to not touch the polyflex coupling, which has a larger diameter than the original flanges.
All four engine mounts are in the same plane, unless the rear mounts are mounted upside-down, which was the case with my engine. If the rear brackets are mounted upside-down, the beds need a step of 85mm.
Offset from plane of propellor shaft to underside of mounting bracket = 19mm (maybe 20mm?)
Offset from plane of propellor shaft to underside of new mounting bearings = 66 to 90mm. This means the mid-point is about 75mm, which is the offset to the existing bed. The upper limit of 90mm allows for some of the adjustment stud to remain exposed, which is needed because the stud has flat surfaces for gripping the top end for tightening the nuts.
Minimum dimension between inside faces of beds = 300mm
Minimum width of beds = width of new poly flex bearings = about 85mm
Maximum dimension of outside faces of beds = about 465mm.
Fore-aft dimension for new floor plate = 510mm
Athwartships** dimension for new floor plate = 570mm
Versine of hull curvature over 570mm = about 20mm

* Forward means towards the front end of the boat, which can be confusing as you spend all of your time on the engine looking towards the back end of the boat. So, moving it forward generally means pulling it towards you.

**I'm using a lame excuse to use the word, which is basically the left-tight or port-starboard direction