1. Single Outboard. Usually used by smaller catamarans up to about 25' or so. The 32' Gemini was an exception with some boats delivered with a 40 HP or so. Unless you link the motor to steer when you turn the tillers, docking under power is pretty challenging. You need to have some speed for water to flow over the rudders to turn the boat. Even if you link the motor to turn, it's still like driving a bus inside a shopping mall at low speeds
- steering under power is interesting
2A. Single diesel + Silette external drive. Prouts and Geminis are the only production boats that I know that have used this combination. A single diesel engine located in the cockpit and a steerable external drive leg is attached to the engine. Visualize a big outboard leg with big 16" 3 blade propeller. The drive legs aren't too heavy (50 kg) but quite costly; around $6500 or so. Owner's report exciting times when they put it in reverse and the leg kicks up suddenly when the lock mechanism isn't working.
- only the weight of 1 engine
- no drag when you tilt the drive leg out of the water. if it tilts enough
- Silette drive leg is pretty agricultural in it's engineering. Reverse lock can fail and so can bellows and steering yoke
- works best on smaller boats that don't need the thrust of twin engines.
- about the cost of 1.5x diesels
2B. Single diesel in one hull. You have to be a good driver in tight quarters with this combination. Only seen on the odd custom catamaran. If the engine is in the port hull, it will turn pretty good to starboard but turns to port take several boat lengths in radius. Advance planning is required or don't even try to dock in tight quarters.
- well it's cheaper and lighter than 2 diesels
- handling in tight quarters is VERY interesting.
3. Single diesel in one hull + small auxiliary thruster. The auxiliary thruster helps offset the unbalanced thrust of the one diesel. It can take the form of a smaller outboard, an electric bow thruster, or even an electric outboard motor.
Ceilydh has this type of system; a 27 HP Yanmar diesel in one hull, and a 6 HP outboard on the back beam near the other hull. Because I'm so familiar with it, I'll give a bit more detail.
The outboard used to be a Yamaha 9.9 high thrust which would push the boat along at 5 knots in calm seas.
That died recently and we replaced it with a Tohatsu "Sailpro" 6. It offers a 25" shaft, remote operating, and quite light weight at 65 lbs. But it won't push the boat faster than 4 knots in calm conditions. That's enough to get into most harbours if the diesel dies but it won't push it against 25 knots of headwinds and it will ventilate in choppy seas. The diesel died in California's channel islands. We sailed back to the mainland and used the outboard to get us the last mile to the dock through the breakwater entrance.
Powering with 1 diesel in open seas is no drama and the rudder deflection to counter the offset thrust isn't really perceptible (it's probably about 2 degrees). It's very common for twin diesel cats to use just 1 engines in most conditions.
It's noticeably low in power compared to a twin diesel installation when you're heading into strong headwinds. But we tend to sail in those situations or at least motorsail if the surrounding land doesn't allow sailing.
We usually anchor with just the diesel. We've learned to live with the boat wanting to turn to starboard in reverse and just snub the anchor and straighten out the boat and then back down hard in reverse.
Our Yamaha was so unreliable we learned to expect it to die coming into the very rare marina or fuel dock and just docked slowly with the diesel. Now we have 2 working engines it's much easier again.
- lighter than 2 diesels
- lots cheaper than 2 diesels
- outboard isn't much use in choppy seas
- less turning ability on really windy days compared to 2 big diesels
Digression: Bollard pull of a motor is the thrust produced at zero knots of boat speed. i.e. the thrust you measure when the boat is tied to a bollard on shore. Useful for comparing tugboat performance, it's also the best measure we have to compare thrust performance of a 'thruster' motor at low speeds. Thrust values below are all are in forward; reverse will be around 1/2 these figures.
Tohatsu 6 Sailpro = about 140-150 lbs (Evan's best estimate)
Yamaha 9.9 = 250 lbs (KatieKat measurements)
Yachting Monthly results:
MinKota Riptide 55 lb trolling motor = 37 lbs (manufacturer lies a bit maybe)
Tohatsu 3.5 = 99 lbs (must have been a good one; seems high)
Torqueedo 2.0 = 119 lbs (manufacture claims only 115 lbs)
Yanmar 27 HP diesl = about 400-450 lbs with 3 blade feathering prop (Evan's best naval architecture guess)
An electric bow thruster type installation is useless as a 2nd means of propulsion, and adds a bit of drag. But it can help turn the boat fairly easily. Bow thrusters are fairly costly for the power they produce and can only be used in short bursts. But for docking that's all you usually need.
- lighter than 2 diesels
- lots cheaper than 2 diesels (but still more costly than an outboard for similar thrust)
- no backup propulsion
An electric trolling motor is pretty low thrust. I know of a PDQ 36 that had one as it's auxiliary thruster. It didn't do much and was replaced with a very costly Torqueedo electric outboard.
- very low chance of backup propulsion unless flat calm
- not enough thrust for real turning unless flat calm.
- extra weight of batteries to get 24 or 36V required for 100 lb thrust motors.
Torqeedo Cruise 4.0R - when the Yamaha 9.9 died I considered this as a possible replacement. (slightly lower thrust at 214 lbs). But the high cost of the motor ($3800 US) + the required 4 batteries to get 48V put me off. (both weight and cost)
4. Two outboards This has been successfully done on lots of smaller cats (Gemini, PDQ 32/36, Seawind 1000). And some bigger cats as well, especially in Australia. Most owners seem to like them.
- lighter than 2 diesels
- much cheaper than 2 diesels
- good steering in tight quarters
- shorter lifespan of outboards. About 1500-2000 hours have been reported for Yamaha 9.9 by regular users.
- harsh environment and maybe lower reliability - it's a tough life for an outboard. You're controlled by electricity and you keep getting regular saltwater baths.
- not as good in choppy seas due to prop ventilation
- bigger than 9.9 HP size and props and gear reduction are optimized for fast speedboats not slower catamarans so efficiency diminishes
5 Two diesels The default solution on larger catamarans.
- better fuel economy than outboards
- reliable and longevity since they're inside the boat. Diesels routinely get 5 - 10,000 hours of life
I've left out hydraulic (shudder) and electric hybrid systems (double shudder). With hydraulics you have noise, heat, leaks, low efficiency, With electric systems you have low efficiency under most conditions and lots of complexity.
Sometimes you just have to take what the builder supplies. If I had the money I would have put a 2nd diesel into Ceildyh. I would have liked to have 2 engines each capable of powering her in all sea conditions. I would have also considered an unbalanced setup; the 27 HP in one hull and say a 15 HP size in the other hull for a bit less weight but still enough power to power her, even a bit slower, should the bigger motor die. Engineers love symmetry but I think this would be an interesting solution.