We found a number of design challenges as we worked out the details of replacing the engine. The first of these was how to modify the engine beds. Play Actor's engine beds were hollow fiberglass stringers with the top reinforced to hold the bolts securing the engine mounts. Viewed from the side, they had a saw tooth profile, with a separate inclined plane under each mount. It was clear that they had been configured specifically to support the Volvo MD 17 C, which had the front engine mounts significantly lower than the rear ones, relative to the axis of the crankshaft. All of the modern engines that we considered had the four mounts at the same height, and the plates of the mounts were much lower relative to the crankshaft and output shaft than on the Volvo. This meant that our existing engine beds were too high, as well as being about 2 1/2 inches too far apart.
A common solution to this problem involves fabricating new brackets to attach the engine mounts to the engine block, and using after-market mounts, which come in many different configurations. This is relatively easy, quick, and inexpensive, but there is a downside to this approach. The geometry and the elasticity of the flexible mounts relative to the output shaft of the transmission is carefully developed by the engine manufacturer to ensure that, as the engine vibrates in operation, it oscillates about the transmission output shaft in such a fashion as to minimize movement of the output flange. Engine manufacturers spend a great deal of engineering effort in this area to avoid dynamic alignment problems, and to make sure that the resonant frequency of the system is outside of the normal operating RPM range. This keeps the engine from vibrating excessively in normal operation. Raising or lowering the elastic mounts relative to the engine's center of mass, or moving them in or out laterally, can radically alter the way the engine moves as it vibrates under load, significantly increasing vibration, in some cases causing the propeller shaft to strike the stern tube. To avoid undoing all of Yanmar's careful engineering, we decided to rebuild the beds.
The old beds incorporated a 15-degree incline, to match the engine to the angle of the propeller shaft. Newer engines are not designed to accommodate that much incline. The configuration of the crankcase sump is such that a 15-degree incline can lead to lubrication problems. Because this is a common problem, Yanmar and other manufacturers offer transmissions with an angled output shaft to reduce the inclination of the engine. Yanmar in particular offers a 7-degree down angle transmission, which means that our new beds will need to be built to provide an 8-degree incline. The beds must be structurally sound. They support the weight of the engine, and must be able to hold it in place during a rollover. Further, they must absorb the thrust of propulsion and transfer it to the hull -- that's what moves the boat. Some quick calculations reveal that the thrust to move Play Actor at hull speed is on the order of 1,000 pounds. Another way to look at this is to consider that besides supporting the engine in all possible orientations, the beds must resist a force of half a ton trying to slide them forward or aft along the inner surface of the hull.
One way to tackle the engine bed issue is to cut the old beds out and start from scratch. We considered this, but it would involve removing a lot of cabinetry around the engine compartment to allow tabbing in the new beds. Aside from the expense, we will be living on the boat while we are doing this job, so we want to avoid the disruption. Given that the old beds are well secured, we decided to cut the tops off, leaving a hollow, open box of fiberglass with walls that are 3/8 to 1/2 inch thick. We will cut the top edge to an 8-degree incline fore and aft, and the height will be about 1/2" below the desired finished bed height. We will laminate a teak plug to fill the open box, and bed it into the box and to the hull, along the bottom, with thickened epoxy, so that the teak becomes the core of the structure. To accommodate the narrower footprint of the engine, we will laminate a piece of 1-inch to 1 1/4-inch thick teak along the inboard side of the beds. Once this is in place, we will cover the whole bed with fiberglass cloth and resin. The final step will be to bed a length of 4 inch by 4 inch by 3/8 angle iron to the inner edge of each bed. The angle iron will be attached to the bed with lag bolts through the vertical flange, and the horizontal flange will be drilled and tapped for the engine mounts, as shown in the sketch.