Episode 34 Diesel Engine Part 5 - Construction of the Floor Plate

The construction of the floor plate has to be the most geometrically challenging project I have ever undertaken. It has consumed countless hours over the past two months, and I have been working on it more than the engine. If I were doing this at work, I might use 3D laser-scanning to create digital twins that I could play with, tossing them around weightlessly, cutting holes and filling gaps with ease on the computer screen. But, I am doing this at home, which means using the analogue prototypes of hull and engine, and slowly figuring out how to join the one to the other.

The floor plate’s first function is to provide a strong platform, glued securely to the hull with epoxy resin, onto which I can bolt the engine beds. The hull is curved, and the engine beds will be made of wedges, cut to the right angle between the axis of the engine and the hull. The second function of the floor plate is less obvious, but just as important, which is to provide the reference surfaces and planes from which I can take the measurements for the engine beds.

The construction of the floor plate used 20 pieces of spotted gum 19mm floor planking, assembled in four layers, joined with birch dowels and epoxy. I also made up several jigs to measure things and to act as guides to the cutting operations.

I chose spotted gum because it of its density, strength and durability, or resistance to rot, according to the New South Wales Technical Publication Series 5 Timbers in Boat Building. Another reason is that it is available as machined flooring planks. Given that I would be assembling the floor plate in my own garage, I thought it would be better to start with something sawn to machine-grade tolerances. The floor planks also meant I could build up the layers with the grain criss-crossed, like plywood, increasing the strength of the composite construction in all planes.

I used Norglass Epoxy with Norcells as a filler or extender. The slow hardener gave me more time to get things positioned, and allowed the epoxy to run into all the cracks and voids it could find, reducing the opportunities for water to get into the timber. I had also tested International Epiglass, but it cured faster and gave off considerable heat, which might have stressed the composite construction. Before gluing I wiped down the surfaces with Methylated Spirits and then MEK paint thinner. Fun fact; MEK dissolves some types of plastic gloves.

Another aspect of this approach was that it built up my experience of working with epoxy resin before the Big One, which would be pouring the epoxy into the join between the floor plate and hull; a daunting point of no return and something that had to go right the first time.

Typically, I mixed the hardener, epoxy and Norcells in the ratio 1:3:4 by volume. This yielded a white paste with the consistency of runny custard. Liquid epoxy has no surface tension, unlike water, and if it finds the tiniest crack, it will find a way through. It does not run fast, but it does run far. I found a friend in clear packing tape, which would stick to the timber, but deny any adhesion to the cured epoxy. I became adept at constructing packing-tape dams around my assemblies to stop the liquid epoxy dribbling down the sides. 

My experience with the runny-ness of liquid epoxy portended a potential problem with the gluing of the floor plate to the hull. If the hull were horizontal, life would be easy, but it is not. It is at an angle of about 6 degrees to the horizontal, meaning that any epoxy I pour into the join would simply run out of the downhill side, forming an immovable puddle of useless gunk in the bilge, forever fixing whatever it touched permanently in place, except, of course, the floor plate. I built a dam around the outside of the floor plate with extruded aluminium angle, sealing the remaining gaps with Parfix gap filler. The addition of packing tape should keep the liquid epoxy in place until it has set. I will report back on this operation, when it is done.

The assembly of the spotted gum planks took several weeks, including an afternoon with my mate, Naman, and his tools to get the sides trimmed and squared up. He is a furniture-builder, and went to town on getting the angles right. It was interesting watching him work. Like other craftsmen, he would take perhaps 15 to 30 minutes setting up a cut, the cut itself being quite brief. No wonder Leo has spent the last four years restoring and rebuilding the classic sailing yacht Tally Ho (I watch his YouTube episodes religiously).  The last thing I asked Naman to do was to route a scoreline along the centre for reference. I told him not to fuss too much about the bottom surface, because it was about to get hacked. When I got the plate home, I shaped the curved base with the wanton use of a belt sander, checking it with a jig to mimic the curved hull.

Having got the base of the floor plate to mate with the hull, I measured the dimensions for the top surface. This is where I needed my measuring-jigs. There are no published dimensions for the engine or its mounts, so it was a case of measuring what was there and transferring the dimensions to the floor plate and my measuring jigs and guides. 

However, one cannot simply measure the dimensions between the bearings on one side to the other. The engine is in the way. Also, it is impossible to measure the height of the engine sump from the floor, because the underside of the engine has an irregular shape, and the whole thing is at an angle. My most recent jigs include a positive template of the underside of the engine cut into a piece of pine (the negative came from a breakfast cereal box), and sawing guides, cut at the right angle (hopefully) to the floor plate. With these two jigs with the shaft flange as a reference, I estimated the clearances under the engine, and hence the angle and dimensions of the slot cut into the top of the floor plate. 

I might have overthought it a little, but in anticipation of this slot, I glued the lower layers of the spotted gum sideways, to reduce the exposure of end-grain to a potential source of water from the shaft seal. These sideways-on pieces provided extra strength to the longways-on pieces, as I cut through the upper layers. The last dry-fitting on the boat showed me that I had got the angle wrong, and I needed to cut the slot deeper to ensure sufficient clearance to the underside of the engine, allowing for its biggest vibrations. I don’t know exactly what clearance is sufficient, but I reckon that if I can get the tips of my fingers into the gap (about 10mm), it should be enough, even allowing for the inaccuracies in my measurements. Deepening the slot with my circular saw left a couple of accidental gouge marks that might need to be filled. The filling is mainly cosmetic, but I worry about the opportunities that the water might take to get into the grain of the timber.

The photos show the completed floor plate. The handles are temporary, allowing me to lift the 16.5kg thing and jiggle it into place on my work bench and in the bowels of my boat. What remains are some final checks, some coats of epoxy or varnish to seal the surface, and a thorough clean of the hull. Finally there is the scary job of gluing it in place with only one chance to get it right. Once it is in there, it is never coming out.

 

Underside of floor plate. The jig at the back mimics the curvature of the hull.

Topside of floor plate following its squaring up by Naman.

Perimeter dam around hole in floor, made from aluminium angle, to stop the liquid epoxy running  away.

Negative of underside of engine, made from a breakfast cereal box.

Transferring the negative onto a positive template of the underside of the engine.

Saw guides, used to cut the slot into the top of the floor plate.

Chiselling out the slot. The chisel set originally belonged to my great-grandfather, who was a merchant mariner, probably used for his apprenticeship.

Dry-fitting the floor plate with the engine template.  This shows that there is insufficient clearance, meaning that the slot needs to be made bigger.

Using new cutting guides and the engine template to test the larger slot for  clearances. This time, it looks OK.

The finished floor plate, looking at the rear. When the floor plate is in the boat, we would be looking forward. The slot accommodates the gearbox and flange, so that the flange can be fitted to the prop shaft. The handles are temporary, but are jolly useful in moving this thing around.