Catamarans these days have many options for their engine based propulsion, even more than monohulls due to the more diverse configurations of a catamaran. The traditional twin diesel engines is a popular arrangement, and dual or single outboard configurations are also becoming popular. When choosing an inboard engine, you then often have a decision to make in drive shaft configuration: traditional straight shaft, or a saildrive.
The drive type has long lasting consequences for how you use and maintain a boat, so it’s an important factor. It affects how much power you have for motoring into strong winds or against current, fuel economy, reliability, maintenance needs, and purchase plus maintenance costs. And let’s not forget noise level and liveability – the choice between diesel inboards vs gas outboards can have a big impact on cabin storage areas and engine noise level.
Single Gas Outboard
Outboards have always been common in smaller sailing boats (under about 30 feet) that don’t require a great deal of power to get them moving. However they’re now becoming popular with larger catamarans as well, up to about 40 or 45 feet. They offer several unique advantages over inboard diesel engines. First and foremost, they’re significantly less expensive than a diesel engine – a 30hp outboard might be around $4,000 while a 30hp diesel could easily be $20,000. They’re also smaller and lighter, and take up less space inside the hull – freeing up storage or accommodation space.
Smaller catamarans or lightweight ones can often get by with a single outboard. The advantages are in the weight and cost savings of only having one engine instead of two, and only one engine to maintain. The downside is it may make maneuvering in a marina a bit trickier, and there’s no backup engine if the one outboard has an issue.
Twin Gas Outboards
Catamarans have a unique advantage over monohulls in that they can have more than one engine. Two engines provide many advantages – redundancy in case one fails at sea and greater maneuverability in marinas. Many catamarans can motor reasonably fast on only one engine – which some owners do to save fuel and reduce engine hours – but two are nice to have for the redundancy and for pivoting easily in tight quarters.
Twin outboards are usually mounted at the aft end of each hull, or in drop-down lockers built into the cockpit. Some catamarans with twin outboards have them located in lockers under the cockpit seats (ex, Seawind 1160 lite and PDQ 36) which is handy because they can be raised to eliminate drag yet are easily accessible to work on, and don’t add an unsightly appendage to the stern.
Speaking with an owner of a Seawind 1160 lite catamaran with dual outboards located in a well under the cockpit seats, he noted appreciating all the space they freed up vs having inboard diesels installed at the aft of each hull. In the space normally occupied by diesel engines, they store two bikes, a kayak, a water heater and other gear.
He also loves that they’re quiet, fuel efficient, emissions efficient, and can be raised with electric tilt for zero drag in the water while sailing. One disadvantage he noted is the impellers are harder to replace, and he’s hauling out to make it easier to replace the gear oil along with impellers.
Outboards may not have as long a life span as diesels, but when it comes time to replace, he can do so relatively easily anywhere in the world, with much less work than replacing a diesel. One possible disadvantage is in rough seas or steep swell, outboard props may come out of the water – whether this can happen will depend on the design and size of the catamaran.
Single Diesel Inboard/Outboard Sillette Drive
A somewhat unique option is the Sillette Sonic drive (common on Gemini 105 catamarans) which is a type of saildrive. It allows using an inboard engine with an outboard drive leg exterior to the boat. The drive leg can be raised while under sail, providing the same no drag advantage as outboards, while having the engine inboard which protects it from saltwater. The drive leg is typically mounted from the bridgedeck or in a pod underneath.
Diesel Inboards with Direct Drives
Inboard diesel engines have historically been the most common option in mid to large size monohulls and multihulls. Diesels excel in delivering high working power while being exceptionally reliable over a long service life. Since a diesel is located inside the boat, it’s exposed to a lot less seawater than outboards are. Diesels can also support a powerful alternator to charge the house batteries, reducing the need for alternate power generation such as solar, wind or a generator.
The straight shaft is the more traditional option and is a tried and tested design from work boats to pleasure boats. In this design the propellor shaft connects from the engine (via a shaft coupling), passes through the hull of the boat (via a shaft seal) and connects to the propellor. Typically there will also be a strut forward of the propellor to stabilize the spinning prop shaft.
The advantages of this design are that it’s robust and easy to maintain. If your propellor strikes a submerged object, it may get damaged but isn’t likely to rip a huge hole in the boat. The main disadvantage in a catamaran is the shaft placement limits where the engine can be placed – it must be fairly far forward, and takes up more space due to the shaft and transmission arrangement. Additionally, since the shaft isn’t perfectly horizontal, the propellor works at a slightly lower efficiency.
Diesel Inboards with Saildrives
Diesel inboards paired with a saildrive are quickly becoming the most common installation option on newly built catamarans. With a saildrive there’s no shaft or strut needed, just a drive leg attached to the hull of the boat underneath the engine. A couple advantage of saildrives is that they’re easier for manufacturers to install, and allow more flexible positioning of the engine to free up greater interior space. Saildrives can also have higher efficiency due to the vertical orientation of the propellor. Saildrives often run quieter, with less vibration than a conventional straight shaft system.
There are a few possible disadvantages that boaters typically cite, relating to maintenance: watertightness of the saildrive seal, maintenance of the drive leg oil, and corrosion. The saildrive seal must be maintained or the boat could potentially flood. The drive legs also have gear oil which usually requires hauling out to replace (except for some newer saildrives which support changing it from inside the boat). Neglecting the maintenance could cause seawater to start mixing with the oil. Lastly, corrosion is a concern because the drive leg contains metal components which must be protected with sacrificial anodes.
Gasoline inboards, such as the Universal Atomic 4, are sometimes found on sailboats but are becoming much less common. A gas engine has greater risk of fire or explosion due to fuel vapor build-up. Fuel efficiency can also be slightly lower than a diesel engine, and engine longevity may be less than with diesel engines.
Electric Drive Type
Electric propulsion options are still a niche market in boating but are becoming increasingly popular as battery technologies advance. Electric systems can be other hybrid (electric recharging is assisted by a generator) or full electric (recharging only via solar and other non-fuel sources). Hydro generation can be a good recharging source on catamarans since their higher sailing speeds improve the output of hydro generators.
The advantages of an electric drive are: environmentally friendly, very quiet when operating, frees up space that a large engine and fuel tanks consumed, and relatively easy maintenance compared to diesel or gas engines. The main disadvantage of electric drives is the motoring range may be more limited, especially in the full electric option.
It’s great to have so many options in today’s catamaran designs. Having these choices allows you to prioritize what’s most important in your sailing and maintenance plans and adopt innovative technologies that make your boat best suited to your needs.
You can’t go wrong with the traditional choice of diesel inboards, but in mid-size lightweight catamarans you may also want to consider outboards for the space they can free up, lower initial cost, and quiet, drag-free sailing.