Choosing Engines

The choice of engines was a bit confusing at first, as the 55hp standard engines offered by Outremer seemed a bit small. They also offer a 75hp engine as an option, so I had to do some research to determine which was best for us. We decided to go with Volvo D2-75hp engines for Wildling. Here’s why:

First of all, it’s great that Outremer is comfortable recommending 55hp engines for a 59 foot boat. This further confirms the performance of the 5X design, in that it can be effectively propelled by a 55hp power plant. Contrast this with our previous 47 foot Catana, which had 50hp Volvo engines, and that Catana is installing 110hp engines in their new 59 foot catamaran.

In order to make the decision, we had to determine the selection criteria that were important to us:

1. Weight and hydrodynamic performance of the hulls – heavy and less efficient designs need larger engines
2. Windage – larger hulls have more wind force against them and require more powerful engines
3. Manufacturer and reliability of the engine model – For our purposes we considered Yanmar and Volvo and both normally aspirated and turbo-charged engine models
4. Ease of maintenance
5. Access to trained service technicians in our intended cruising grounds
6. Weight of the engines
7. Fuel efficiency
8. Engine load / RPM at cruising speed
9. Ability to handle extreme situations – overcoming high wind and current, handling difficult bar crossings, docking in windy conditions

VOLVO vs. YANMAR

There are many opinions on which of the two major marine engine manufacturers is best. Some folks hate Volvo and swear by Yanmar and vice versa. We had Volvos on our last boat, and they were fine for us. They were pretty easy to work on and very reliable, although the cost of parts was high. When I talked with Outremer about the reasons they had chosen Volvo, it had to do with the level of after sales service and warranty repairs in the Mediterranean cruising area. They had some bad experiences with Yanmar, and have had much better service from Volvo, so that drove their choice. Since I didn’t have too much of a bias either way, I decided to stick with Outremer’s recommendation of Volvo.

ENGINE SIZE

When I did the test sail on the 5X Addiction, I was able to experience the performance of the Volvo D2-55hp engines that Outremer provides as standard. They did a fine job, and pushed the boat along at 8-9 knots at around 2200 rpm, with hardly any vibration from the 3 blade folding propellers. I’m sure that the 55hp option would be good enough in most cases, but I was a bit worried about how well they would do in more extreme conditions.

The 5X is a big boat, and when pushing into large seas, against high winds and strong currents, it felt to me that the extra power of the 75hp option would be a better choice. We also have some difficult bar crossings on the east coast of Australia, so having some extra power to keep in front of the breaking waves, when coming across the bars could give some safety margin. That was my assumption, but I had to study the performance data for each engine to validate it.

WEIGHT AND COMPLEXITY

The big advantage of the 75hp Volvo engine is that it is actually the exact same engine as the 55hp, except it is fitted with a turbocharger, a more robust saildrive leg and a 4 blade (higher torque) propeller. This adds 20kg per engine, which isn’t much given the large increase in power and torque. From an operation and maintenance point of view there’s not really any difference between the two engines, so it comes down to the reliability of the turbocharger.

RELIABILITY OF TURBOCHARGED ENGINES

Based on the research I did, it seems that modern turbocharged diesel engines are very reliable. It’s also reassuring to know that each of the boats competing in the Volvo Ocean Race is equipped with a D2-75 Volvo engine as the sole source of power generation and emergency propulsion. Given the power benefits of the turbo it seems the extra complexity is worth the risk.

ENGINE PERFORMANCE COMPARISON

Because the D2-75 produces more power and torque at lower revs, it will be under a lot less stress than the D2-55, which will lead to less noise and vibration when running, and the ability to power larger alternators for rapidly recharging the lithium battery bank.

It’s a well known fact that running marine diesel engines at low load is not good for them, but in talking to the engine experts, I found that by keeping the engine high on the torque curve and at 1,800 rpm and above when running for extended periods, they will do fine.

Here are the performance curves for the two engines, which show a big difference in engine load under cruising conditions:

 VOLVO D2-75 Performance curves

The D2-75 engine develops peak torque at 1,800 rpm. When fitted with a high torque, 4 blade folding propeller, this engine will provide the same level of performance as the D2-55 at much lower revs. This equates to less stress on the engine, less noise, less vibration, less heat and essentially the same fuel consumption as the D2-55. In addition to these benefits, the D2-75 has extra power available when needed in difficult conditions. All of these factors added up to the D2-75 being our preferred engine choice for the 5X.

A NOTE ON ENGINE DRIVES & LOCATION

The ideal location for engines on a catamaran from a performance and motion perspective is at the center of the vessel in each hull. This makes a big difference in the motion of the boat at sea. Less motion = higher efficiency = more speed (and comfort). The other great thing about placing the engines in the center, is that you can install a shaft drive instead of a saildrive.

Saildrives are complicated to maintain, as they require the boat to be pulled out of the water every one to two years to service the seals that prevent seawater entering the gear case. They will also corrode if careful attention isn’t paid to preventing electrolysis.

When going over the 5X design with Outremer, we had a good deal of discussion about the engine placement and drives. They were quite willing to move the engines forward and install shaft drives for us, but in the end it came down to Robin’s decision!

The big compromise with shaft drives in Catamarans is the engines have to be mounted further forward, which places them either under the aft bunks or under the companionway floor. Compare this to the saildrive option which places the engines in nice, big engine rooms at the aft ends of the boat, that are completely isolated from the living areas.

Pulling our beds apart every time I need to work on the engines is a pain for me, and a non-starter for Robin! We were also concerned about the extra heat and diesel smells that are hard to avoid when the engines are located inside the living areas of the boat.