Designing the Exhaust System

After puzzling over how to fasten the engine in place, the next design problem centered on the exhaust system.  The problem in brief was finding a way to position a water lift muffler sufficiently lower than the exhaust outlet on the engine so that water from the exhaust system would not flow back into the exhaust manifold when the engine was not running.  On our old engine, we dealt with this problem by fabricating an exhaust riser, or gooseneck, that routed the hot gases up a six-inch vertical rise and then back down before injecting the raw water outflow from the cooling system.  This is a common solution, and it keeps water out if the engine, but it is prone to failure from vibration and corrosion, and creates a foot or more of pipe in the engine compartment that runs at temperatures of several hundred degrees.  This must be insulated, and it ultimately radiates its heat into the engine compartment and thus into our living space.  In a cold climate that might be of little consequence, but in the tropics, where we spend most of our time, it is uncomfortable.

The problem was exacerbated by Yanmar's specification of a three inch diameter exhaust hose, which necessitates a large water lift muffler to provide sufficient volume to hold the water that accumulates in the vertical rise of the hose as it wends its way out through the transom.  Most of the mufflers designed for three-inch hose sat up too high in the space behind the engine to provide the down slope and drop of several inches required.  After extensive searching, we found a Vetus muffler that would fit down into the valley above the stuffing box, providing the required drop.  The locations of the inlet and outlet fittings were also adjustable through a broad range, as well.  This will allow us to locate the muffler in otherwise unused space, where it won't interfere with our storage bins below the cockpit, and it allows routing the exhaust hose in an out of the way manner as well.  Of course, this being a boat, there was a trade-off.  The muffler was roughly twice as expensive as more typical, competing products.  The savings from not having to fabricate a custom riser more than offset the additional cost.

The location for the new muffler, looking down from above.

  Stuffing box is below left end of tape.

The new muffler in position.  Note storage bins to right of muffler.

Rebuilding the Engine Beds

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.

Play Actor gets a new engine

It started with a thunderstorm in Grenada this summer.  We were anchored off the western side of the island, in the lee from the easterly trades, which kept us cool.  There was a big, Dutch-flagged catamaran anchored off our bow, not too close for comfort, but still fairly close.  When the storm blew through, the wind backed 180 degrees, and our sheltered anchorage became a lee shore in a squall, with the full fetch of the Caribbean of our bow.  We swung with the wind, and stretched out our 150 feet of anchor chain.  The catamaran, now off our stern, swung as well, but their anchor chain wrapped around a coral head, so they were only a few feet from us, and the people were ashore.  Before we hit them, we fired up the diesel and put it in forward gear, revving the engine.  The engine wound up with no load, and then the transmission engaged with a sharp clunk.  We throttled back, motored forward, and retrieved our anchor.  After we re-anchored, well away from the catamaran, and the storm passed, we made a careful survey of the behavior of our transmission.

We knew this symptom of old, unfortunately.  After owning and sailing the same boat for twenty odd years, her quirks are familiar.  We had experienced this problem some years ago, and realized that the forward clutch in the gearbox was slipping, the result of some eight years of daily use.  We also knew that we had a hundred hours or so of remaining use before the slippage progressed beyond the nuisance stage.  We started making plans for a transmission rebuild during our winter stay in St. Martin, a truly duty free port, where it's easy to get parts shipped in from all over the world.  This wasn't how we had planned to spend our winter, but there are certainly worse places to work on the boat, so we had a couple of months to think about it and get in the right frame of mind.

Just before we left Grenada at the end of hurricane season, we began shopping on line for the required parts, expecting that we might have a little trouble, as our Volvo engine and transmission were 32 years old.  We found a couple of Volvo dealers who listed the parts on line, and sent emails to begin the process of making sure we got exactly the right parts, in order to minimize our down time.  We had a nice sail north for a few days, and when we came to rest in an anchorage with internet service, we checked our email, to learn that parts were no longer available for our antique propulsion system.  One dealer offered a used gearbox with a 90 day warranty for $2500, but we already had a used gear box, and it would most likely last longer than 90 days.  That wasn't an attractive proposition when we knew that we could buy a brand new, modern, diesel, mated to a new transmission for around $9,000.  Given that the old engine had to come out either way, a total replacement didn't seem like a lot more work.

As we continued to make our way north, we shopped the internet for engines, downloading drawings and specifications from the manufacturers' web sites, and measuring and sketching all of the details of installation.  We found that a 40 horsepower Yanmar would fit our space nicely.  It was about 2/3 the size and half the weight of the old 36 horsepower Volvo.  The biggest issue would be mounting the engine, as the new engine would sit somewhat lower than the old one, and the mounts were enough closer together that they would not reach the beds laterally.  We had several weeks before we could actually order an engine and get started on the project, which we put to use designing all of the new peripheral systems, like exhaust, plumbing for the cooling system and fuel, and changes to the electrical system.

By the time we got to St. Martin, we had a well-developed project plan.  We spent a couple of weeks dealing with logistics and sourcing, acquiring some of the bits and pieces that we needed in order to work out remaining installation details.  Now we're ready to order the new engine.  It should arrive in mid-January, and we'll tie alongside a working dock in a boat yard and roll up our sleeves.

Meanwhile, we'll share the engineering work over the next few days.  Next up will be the question of the engine beds.

Merry Christmas from St. Martin, French West Indies

It's quiet here.  Christmas in the islands is nothing like Christmas in the States.  We like the absence of overbearing commercialism, but we do miss family. 

I spent the morning figuring out how to navigate around Blogger, and get some of our old web content posted again.  We will add more of the old material as time permits.  As I went through our old files, I realized how different cruising is now from our experience of 10 years ago -- or maybe it's our attitude ...

This blog resumes the tale of our life afloat which was begun on the defunct website, Voyage of the Play Actor, when we first left our shore based lives.  Technology has advanced considerably in the years since 2000.  The old web site died of its own weight some time ago.  It was just too unwieldy to administer, given our sporadic access to internet service.  Internet service is widely available aboard in most of the places we visit, thanks to the proliferation of wireless hotspots and other mobile access technology.  As time permits, we'll add some of the old website's material as archives, so if you want to know what life was like on a cruising boat 10 years ago, check back here every so often, or subscribe below.

Thanks for looking,
Bud and Leslie