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Super Yacht Engines: We Come to the Subject of Engines

Updated: May 5, 2023

So we come to the subject of yacht engines, and we find ourselves in a quandary. Not all yacht engines are the same.  Put rather crassly; there are big engines, reliable engines, heavy engines, high-performance engines, engines that need less or more maintenance, engines with and without accessories, emissions regulated, and those that are not.  And on goes the list, but through all this, it is necessary to make a decision.

The first part of that decision was easy.  We wanted two engines for redundancy.  We will fit two engines and two service tanks, each with independent fuel filtration.  So in the unfortunate event of fuel problems, we can at least continue on our way on one engine if we don’t co-mingle bunkers.  Looking at the power required, we run into our next issue.  Vanguard boat has a hydrodynamically very efficient hull shape, it makes compromises in other areas, but it should be pretty good for fuel consumption.  We need an estimated 120kW for a theoretical 12 knots.   That’s not much at all as motor yachts go, more like a fast sailing yacht.

So two engines, each capable of propelling Vanguard boat at service speed. The following criteria are efficient propellers tend to be slow ones – one reason why large merchant vessels run shaft speeds sub 100 RPM.  We can’t go down that far and have a 700/750mm diameter limit on the propeller diameter.  Your typical ZF or TwinDisk gearbox at this power rating has options for 2.5:1 reduction, so we look at 2500 RPM max at the crankshaft.  That rules out a whole genre of the higher-speed types of diesel, if only for shaft speed.  We have a few more criteria to add.  For use in the US, we need US EPA Tier 3 Emissions Certification in this power range. Thus ruling out older but otherwise beautiful engine choices such as Gardner used on Mobius. We also wanted an option that is supported worldwide.

So now 120kW continuous rated max, 2300 RPM max, Tier 3, w/w support. Our choices whittled down to CAT (Perkins), John Deere, Yanmar. There were some excellent popular engines (Scania, Cummins …) ruled out on minimum power.

John Deere 4045 AFM85 120kW @ 2300 RPM. M1 rated.

So the coup de gras was a referral to For the uninitiated, that is the site reporting progress with his FPB design series.  Not much is new under this sun so let us learn from the past. All fitted with the ubiquitous JD engines though in their case, they were the six-cylinder line-up, and we will go for the smaller four-cylinder units on power considerations.JD 4045 AFM85 M1 rated 160bhp continuous at 2300 RPM (can be operated to 2500 for short intervals, such as charging), Tier 3 rated to keep the US EPA happy.  I believe we have a reliable choice of engine. Economic in its performance and strongly built as the same frame produces more than 700 bhp in other configurations.   It has some downsides, mainly in complications as they are turbocharged, aftercooled, and have common-rail fuel injection, all necessary for the emissions certification.

However, we have two, so reliability should be where we need it to be.

Now what to pick for generators? The answer is not what you may expect and will be the subject of a later blog. 

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