Engine Mount Design

Deployed — Stowed — Raised for maintenance

Quidnon is rather indifferent to choice of engine: just about any outboard over 25hp will do. But the leading contender right now is Yamaha T50 or T60 (the number is the horsepower rating and the T stands for high-thrust, which is bad for high-speed planing and good for pushing a heavy barge). The engine is installed in an engine well, making it an inboard outboard, if you will.

This arrangement presents a few design challenges:

• Normally outboard engines are hung directly on the transom. When not in use they tilt until they are out of the water. But in an engine well there is no room for the engine to tilt; instead, it has to slide up and down inside the well.

• Normally, control cables for the throttle and the shifter are led forward from the engine, but in an engine well the engine is up against a bulkhead, so control cables have to execute a tight 180º turn as they exit the front of the engine, which is something that cables can’t do.

• Also, engines are usually bolted into place (they come equipped with clamps, but these are far less secure than bolts). But bolting the engine into place when it is in the engine well would require reaching down, and perhaps hanging down, into the well—a very awkward working position—and this would have to be done every time the engine has to come out for maintenance.

• Lastly, what happens when the engine hits a submerged rock or some other obstruction? Does the engine’s lower unit get destroyed, or does it just get banged up a bit while it is the engine mount that fails. And when the engine mount fails, how does the owner repair it on the spot using provided spare parts and without having to haul out the boat?

Quidnon’s engine mount design solves all of these problems.

Backing up a bit, some contraption—be it a shop crane, a hoist or an improvised A-frame—is used to lift it into place, usually with the boat ashore. In exceptional circumstances two or three big, strong men can simply lift it into place with the boat backed up to a floating dock, but this is a risky operation. On Quidnon, which is designed to never need a haul-out, it has to be possible to install and remove the engine with the boat in the water.

This is done using a swinging hoist that is part of the aft deck arch. The hoist is used to lift the engine off a floating dock or a dinghy, lift it up to the deck (a rise of 10 feet/3m or so) swing it toward the engine well, and install it onto the engine mount. This would have to be done repeatedly because outboard engines don’t last that long, with 1200 hours being the typical point at which people give up on trying to make them run reliably and buy a new one.

Someone who motors all the way down the Intracoastal Waterway and back every year, putting around 300 hours on the engine each time, usually needs to replace the engine every four years. A lot depends on the amount of preventive maintenance, and with enough care that number can be doubled to eight years. A complete overhaul can stretch it out to ten. But Quidnon’s expected useful life is at least 30 years, so it will need to be repowered at least three times, and perhaps as many as ten. Luckily, outboard engines don’t cost that much. A Yamaha t50 with 0 hours costs around $10k. Now, $100k is quite a bit of money for most people, even over 30 years.

The amount of preventive maintenance an engine receives is generally a function of how easy it is to get at. Impossibly awkward and cramped engine spaces that are common on sailing yachts with inboard diesels will quite predictably receive less than the optimum amount of attention from their owners; but if the engine can be pulled out and placed on a stand without so much as breaking a sweat, working on it and tinkering with it will be a pleasure and it will receive all the care it needs. Quidnon’s dedicated engine hoist incorporated into the aft deck arch and the design of the engine mount are intended to make working on the engine easy and pleasant.

The engine mount consists of just seven major components:

• Four brackets that are attached to the forward bulkhead in the engine well, two at the top of the well and two at the bottom

• Two rods that run vertically between the two sets of brackets

Upper bracket

Lower bracket

• A slider car which slides up and down on the two rods and incorporates a large, solid piece of hardwood to which the engine is bolted

Car

The brackets and the car are welded up out of A500 steel and subjected to the same galvanization process used for anchors and anchor chains. They consist of tubes, square and rectangular pieces of channel and plates.

The two rods are of stainless steel because they have to be smooth to keep the slider car from binding up and can’t have a surface finish because it would wear through over time. The rods have caps welded to their top ends to keep them from falling through the brackets.

The brackets and the rods are not in direct contact. Instead, neoprene rubber inserts are used to isolate the galvanized steel from the stainless to keep them from galling together through galvanic action, to dampen the vibrations in the rods (which would otherwise ring like a bell at certain engine speeds) and to prevent engine vibrations from being transmitted to the brackets, the bulkhead and from there to the rest of the hull.

The car which slides up and down on the rods and on which the engine hangs uses similar inserts but of different material: Delrin plastic instead of neoprene rubber. This material is slippery and allows the car to slide easily. The inserts are forced into the pipes at each end of the car and slide freely on the rods.

The order of assembly is as follows.

• Hoist the engine aboard
• Bolt the engine onto the car with it hanging above deck
• Lower the engine into the well and skewer it into place by dropping in the two rods

There are a few more minor details:

• When the bottom of the engine hits a rock, the bottom two brackets are the designated points of failure. What fails on them is not the brackets themselves but the nuts that hold them against the bulkhead. The nuts are stripped off the pieces of threaded rod onto which they are threaded. In turn, the pieces of threaded rod is screwed into a socket that is installed from the opposite side of the bulkhead.

Designated point of failure marked in red

The repair procedure then involves lifting out the engine (it is left hanging on a lanyard, which is the usual precaution to losing it), removing the stripped pieces of threaded rod (they have a slot at one end, to accept a screwdriver), screw in new pieces of threaded rod (provided as spares) and reattach the brackets using fresh lock washers and nuts. Then the engine (with the damaged bottom unit replaced or repaired as needed) can be put back into place and into service.

There are a few more elements to the design that are small but critical:

• A small custom 3D-printed part to make it possible for the control cables to face aft and up instead of forward.

• Lanyards for each bracket, so that they don’t go swimming if they are torn off the bulkhead in an underwater collision.

• Clips to keep the rods from falling out in a capsize.

One last element of this design will most likely need to be determined experimentally: how much and of what kind of sound insulation to install inside the engine well to keep the noise level in the aft cabins low enough so that people can sleep soundly with the boat moving under engine.

The engine mount was the last important conceptual part of the design that needed to be completed. Now that it is, the work of putting together detailed construction drawings can begin. It’s taken a long time, but we have finally arrived at that stage. Thank you for your patience!