Marine diesel engines for catamarans
A cruising catamaran has two engines, two drives, two of every service item, and one owner who is often surprised by what that adds up to.
The arrangement
Almost all production cruising catamarans carry twin engines, one in each hull, most commonly on saildrives. Yanmar and Volvo Penta dominate. Some builders — Nautitech among them — use shaft drives on parts of the range, and that is a genuinely different ownership proposition.
The engines are usually small relative to the boat’s size, because a cat is light for its length and does not need much to move it at hull speed. That has consequences.
What is different about twin-engine catamaran ownership
Everything doubles. Two oil changes, two impellers, two sets of filters, two legs, two sets of anodes, two gearboxes. A quote for “an annual service” needs to state whether it is per engine.
Access is often excellent. Engine bays in the aft sections of each hull are frequently easier to work in than the buried installation in a monohull. This is a real advantage and it shows up in labour hours.
Small engines working hard. A cat pushing into a head sea uses more of its installed power than an owner expects. Underpowered cats motoring to windward run at high load for long periods, which is what the engines are for — but it means cooling capacity and servicing discipline matter.
Asymmetric use. Owners often favour one engine for manoeuvring or for charging. The result is two engines with different hour readings and different wear, and the neglected one is the one that has been sitting.
Manoeuvring on engines alone. Cats steer beautifully on differential thrust, which means a great deal of gear changing at low speed. Gearbox engagement and control cable adjustment deserve attention.
The saildrive question, twice over
Two legs means two diaphragms with a time-based replacement interval, two sets of leg anodes in warm water, and two lower seals that fishing line can cut.
On a charter-history cat this is the first thing to establish. On a private boat, if the diaphragms have never been changed, that is the job to plan.
Repowering a catamaran
Two engines, so roughly twice the engine cost and often less than twice the labour, because the second installation repeats the first. Doing both at once is almost always right — a cat with one new engine and one tired one is an awkward boat.
Questions to settle:
- Are the legs included, and the diaphragms?
- Are both propellers in the quote?
- Are both instrument panels and looms included?
- Can the engines come out through the aft hull hatches, or does something larger have to come apart?
Frequently asked questions
Do catamarans always have two engines? Production cruising cats almost always do, one per hull. A few smaller or older designs use a single engine, which manoeuvres very differently.
Is servicing a catamaran twice the cost? Close to it on parts and consumables. Labour is often less than double because access is usually better and the mechanic is already aboard.
Why do my two engines have different hours? Owners tend to favour one for manoeuvring and charging. The lower-hour engine is the one that has been sitting, which carries its own deterioration.
Do catamaran saildrives need the same diaphragm servicing? Yes, and you have two of them, each with its own time-based interval and its own haul-out.
Should I repower both engines at once? Almost always. The second installation repeats the first, so the labour saving is real, and a boat with one new and one tired engine is unbalanced in both performance and maintenance.