Engine story - the repowering project

The requirements

Our search for a new engine began at the 2007 Boat Show in Sydney. We trekked from stand to stand, and saw most of the offerings, from Beta and Bukh through Nanni to Volvo and Yanmar. We received expert advice from all kinds of people, looked at everyone else’s engines, and then revisited all the options at the 2008 Boat Shows in both Melbourne and Sydney.

Our primary constraint was the size and weight of the boat. Our own calculations and all the advice we had was that we needed something in the 70-80Hp range to push our 17 tonnes of steel up to hull speed in good weather, and to make good headway against seas and wind in bad weather.

Next constraint was our prop size. All the books tell you that it is the matching of engine, transmission and propeller that is key to getting a good result. We had an 18” Autostream feathering prop, which was relatively small for a boat of Nahani’s size. When we switched from a fixed prop, we had been conservative on prop size as we weren't sure whether the prop would clear the skeg when it feathered. With the 18" feathering prop in place, we were able to mock up a larger blade and check physically that it would clear. We found we could spin 20" without conflict with the skeg and so could match a more powerful engine with a bigger prop.

The final constraint and most obvious constraint was that the new engine had to fit into the existing engine room. From our investigations, it appeared that all the likely candidates would fit, albeit with a bit less room around some.

One more issue where we didn’t have a choice – whatever engine we chose had to have above ground earth because Nahani is a steel boat.

Now we get into the area of preferences. The engineer leant towards a naturally-aspirated engine (no turbo) as there is less to go wrong. On the other hand, we recognised that turbos are getting more common all the time, and there are lots of people cruising with turbo-driven engines, apparently without any issues. A naturally aspirated engine would be a bigger capacity engine, heavier, but probably lower-revving and therefore quieter. We’d also been told that the smaller turbo-driven engines get hotter, which suggested that we might choose to add an intercooler if we went the turbo route. The turbo option was likely to give us better fuel economy, but we were not sure how significant the difference would be.

In the nice-to-have areas, the engineer also wanted

Given all that, we arrived at the shortlist below. [Top]

The options


The “no-brainer” option – leading supplier, thousands of them out there so should be easy to get parts or repair, good reputation for resale. Everyone who’s got one seems happy, although one Yanmar owner told us he finds his a bit noisy.

The downside was that to get the horsepower we need, we had to go to a turbo option. We had also been told that they aren’t as easy to maintain at sea as other options because the sea-water pump isn’t readily accessible. However close examination indicated that the placement of our inspection hatches meant this wasn’t a significant problem on Nahani.

Yanmar are competitively priced and the Melbourne dealer was extremely helpful.


The other international name synonymous with marine engines is the Volvo Penta. However for a period Volvo marine engines had a really bad reputation for quality and maintenance. Although they claim that the latest engines don’t have these problems, the reputation persists and most people advise against. Even if there are no issues, we thought a Volvo engine might be a handicap for resale, so we crossed Volvo off the list.

Marinised Kubota engines

Like our old Perkins, the Kubota is a tractor engine, very common, so highly reliable and parts should be easy to get anywhere. Their suitability for marinisation means that there are three options: Beta (UK conversion), Nanni (French conversion) and Witchard (local conversion done by John Witchard in Pittwater).

The conclusion

We finally placed our order for the Yanmar 4JH4-HTE, the 110HP model with turbo and intercooler. The Yanmar was installed in Nahani in November/December 2008, the faithful old Perkins went to a new life in Dover (Tasmania). The Weblog has a day by day description of the changeover process for those interested in the technical detail. If you are contemplating a similar exercise, it will tell you what you're in for. [Top]