I have a sailboat, and I have a motorcycle. Sometimes it's useful to put the motorcycle aboard the sailboat, sail to some destination, take the motorcycle ashore and ride somewhere beyond convenient walking or cycling distance. A restaurant, or shopping, or just exploring. It's easy to do that where there is a dock - most have a gangway wide enough for a wheelbarrow, or motorcycle. This is described in another Instructable here.
However, I often anchor in places where there are no docks, and go ashore in a dinghy. That's a small inflatable - I can get a bicycle on it, awkwardly, but certainly not a motorcycle. It happened that I had an old small sailing catamaran, missing some parts (stolen by metal thieves). I wondered if that could be adapted to create a "landing craft" for the motorcycle. It turned out that it could, and works quite well, which is what I will describe.
Step 1: The Catamaran
The catamaran is an Aquacat, a small sailing cat with a single sail, as shown in the picture.
It is about 10 feet long and 7 feet wide, with two fibreglass hulls. The superstructure consists of a tubular aluminium frame, with a canvas trampoline in the centre portion, and aluminium arches supporting a mast and rudders. The hulls were originally completely filled with styrofoam and polyurethane foam, making them unsinkable.
Some years ago, the canvas rotted and broke, and later the frame went missing, I presume taken by thieves for the metal. The hulls had also leaked slightly, allowing some water in so that the foam was wet and the hulls heavier than as built. I had removed most of the foam and fitted drain plugs and some buoyant bags so that the hulls were easier to carry. They are no longer "unsinkable", although there is enough emergency buoyancy that they will not actually sink to the bottom.
Step 2: Design and Stability
Small boats have a stability problem - you may hear the expression "never stand up in a boat". This is an issue when the boat is lighter than the passengers or cargo - if they move, the boat will move the other way. Also, the centre of buoyancy is in the middle of the portion of hull below the waterline, and if a passenger stands up, so that the combined center of mass is above the waterline, the system is unstable and the boat may capsize (turn upside-down). The dynamic stability (whether it will actually capsize) depends on the shape of the hull. If the centre of mass falls as the boat tips (perhaps due to a wave), the boat is unstable. This is more likely in a round hull shape like a kayak. A catamaran is a very stable design - it may in fact be more stable upside-down, but it takes a lot of force to tip it over, so in fair weather it is very unlikely to capsize. It is particularly stable against roll - side-to-side. This is an advantage if a motorcycle is parked on a kickstand facing fore-and-aft - the catamaran does not roll enough to tip the motorcycle over.
The catamaran is less stable fore-and-aft. Normally, if weight is added to a boat, the hull sinks lower in the water, and if it is V-shaped it becomes increasingly more buoyant the lower it gets. The aqua-cat hulls are a U-shape rather than a V-shape, and have little freeboard. So if weight is placed at the bow, the bow will sink low in the water. If it actually goes below the water, there is no extra buoyancy with additional weight, and the boat will start to tip over bow-first. So the cargo platform only covers the middle of the catamaran in length, although it extends almost the full width. The motorcycle is relatively stable fore-and-aft when in gear so it can't roll forward in any case.
These considerations mean that a relatively small catamaran is capable of supporting a motorcycle and rider without risk of capsize - moreso than a monohull of the same displacement.
Step 3: The Cargo Platform
Since the original frame was stolen, I had to make a new one. Originally, I used scrap material to prove the concept worked rather than buying new. Since the foam, which provided much stiffness, was removed from the hulls, I placed 2x3 inch wood members along the top of each hull, bolted to the original frame supports. Additional 2x3" lengths are fastened across these from side to side, and a 2x8 foot sheet of waterproof 3/8" plywood placed on top of those as a cargo platform. This is strong enough, but only just. The 2x3 members were later replaced with 2x4" cedar, since the originals were weakened by the holes for the frame supports. The 2x3" crossways members were reinforced with strips of 1x2" after one broke when I tried to ride the motorcycle off the cat rather than walk it. The motorcycle is normally placed centrally fore-and aft on the plywood sheet. Additional sheets of 1/4" plywood (not shown) are sometimes placed either side to provide a total 6x8 foot area for light cargo. The centre plywood sheet is bolted to the fore and aft crosswise members to provide shear resistance (to stop the craft folding up like a pantograph).
Originally the wood was secured with woodscrews. Later these were replaced with 1/4" steel bolts, then with stainless bolts and captive nuts secured within the 2x4" members - the cat is routinely disassembled to transport it on the sailboat deck, both for convenience and also since the construction is not strong enough to withstand significant waves (the original aquacat was only suitable for fair weather, in any case).
There is a wooden block fastened to the aft crosswise member to take an outboard motor - specifically an electric trolling motor. This moves the craft at a couple of knots, adequate to travel from anchorage to shore. The present design is not stiff enough to support a heavier more powerful motor.
Step 4: The Skeg
After assembling the catamaran as described, it became apparent that it would not steer in a straight line. The original configuration, as a sailboat, had a large daggerboard and rudder on each hull as shown in the diagram, but these were not necessary for a landing craft (no need to resist wind on the sail). Some investigation suggested that a skeg would restore stability, and this turned out to be the case. I glued lengths of wood (cut from 2x3) to the sternward section of each hull, rather than re-fitting the rudders, as this allows the catamaran to be dragged up a beach without removal (as a sailboat, the rudders pivoted up against springs when going aground, and had to be secured when going astern, which was time-consuming).
Step 5: Getting Ashore
Getting the motorcycle ashore on a beach is simple. Just drive the cat onto the beach, pull it a little way up, and walk the motorcycle down a ramp onto the beach. I usually secure the cat to a small anchor, or to the motor battery, with its mooring line, else it will float away once the weight of the motorcycle is removed. The ramp is a C-section piece of steel (the blue one in the picture), or piece of scrap aluminium cable tray (which does not rust).
Getting the motorcycle from the sailboat to the cat is slightly more complex. Originally I used a spinnaker pole and a single block (pulley), with a halyard winch. That offered no mechanical advantage, so I had to winch the full weight of the 200lb motorcycle, and later the pole broke. I then made a derrick from a piece of the aquacat mast (tubular aluminium), and a steel hinge. The hinge is mounted on an aluminium runner which is secured in the spinnaker pole track on the mast, supporting the jib of the derrick which is free to move in 2 axes. The end of the jib is supported by the mainsail halyard. 2 blocks attached to the jib provide a 3:1 mechanical advantage, with the line taken to the halyard winch via another block. There is a custom 3-point sling for the motorcycle which goes under the frame by the steering head and is secured to the rear grab handles. The catamaran is secured alongside the bow of the sailboat. Getting the motorcycle onto the cat is then a matter of hoisting it off the deck, moving the derrick jib over the catamaran, and lowering it so that it lands on the cargo platform. I usually secure the motorcycle to the cat with a line on one side so it cannot tip over away from the kickstand. Getting it back aboard the sailboat is a matter of reversing the process.
Step 6: Conclusions
Although this craft was made with found materials, it does actually work fairly well - I have used it now for two seasons, both for taking the motorcycle ashore and as a regular dinghy. I've also taken a couple of bicycles ashore with no problem - they don't get wet, or splashed, and there's lots of room left compared to using my "real" dingy (an Avon Redcrest inflatable). The craft disassembles into pieces easily carried by one person over short distances, and stacks on deck in a relatively small space (about 10x3ft).
Constructing this new from scratch, I would probably still try to start with a used commercial catamaran such as a small Hobie cat, perhaps damaged so it was no longer sailable and thus less expensive. Each hull should individually support the full weight of motorcycle and rider - even though the motorcycle is placed in the centre of the craft, the rider still has to walk around it to operate the motor, fasten mooring lines etc. so must step near the sides and bow/stern to do so. The wooden frame might usefully be replaced with a stronger yet still lightweight design, perhaps in aluminium.