Online Book: Engines For Cruising Boats, Chapter 1 of 12

Better Powertrains For Auxiliary Sailboats and Motorboats


There’s a growing interest in power options, for motor yachts and auxiliary sailing yachts alike, that are more sophisticated than the simple, traditional shaft drive.

Fuel costs are a major factor driving this interest. While they’re unusually low at the moment, they won’t stay that way forever. And even if cost isn’t much of a concern for you, range under power probably is—getting a bit of extra distance out of the same tankage volume can make a big difference to your cruising plans.

Fuel consumption, though, is not the only factor that can spark an interest in more sophisticated powertrains. Perhaps you want:

  • Lots of thrust at low speed for towing and for docking in tight quarters.
  • Or the silent, odourless operation of an electric drive for poking around in quiet, unspoiled rivers.
  • Or an engine that can provide an extra boost for motorsailing just as efficiently as it can drive the boat on its own.
  • Or you just want to stop tearing apart your diesel to re-hone its badly glazed cylinder walls every time the thing acts funny. (Many yacht engines die from being used at low loads.)

Just as often, I see people choosing fancy, high-tech equipment simply for the sake of having the latest, coolest toys to play with and to show off. Honestly, I’m OK with that. It keeps plenty of my friends in business.

Fooling youself about your motivations, though, can be dangerous. If you want a $90,000 hybrid electric drivetrain for the sake of having the coolest engine room at the boat show, that’s fine, but if your intent is to save fuel, money and repair time then I may very well recommend you look at something else instead.

A Closer Look

In this series, we’ll explore the advantages and downsides of various “advanced” drivetrain technologies, and take a look at the logic by which we might select or reject them for our boats.

Some of these technologies—hybrid electric drive in particular— are awash in marketing hype, much of it unjustified or exaggerated. We’re going to try to cut through all that, focusing instead on the questions that really should guide the decision:

  • Does this system make sense, from a technical standpoint, for my boat?
  • Does this system make sense, from a cost standpoint, for my boat?
  • Can I live with this system in remote areas and over many years?

Each of us is in a different situation, so we can’t draw any specific conclusions along the lines of “You should buy System X”. We can, however, get a pretty good idea of what each technology’s strengths and weaknesses are, how these systems are likely to perform in real-world cruising, and how to go about deciding what makes sense for you.

We need to start this discussion from solid, well-understood ground. To that end, the next two chapters of this Online Book are about shining light on things we like to think we’re pretty familiar with:

  • Diesel engines, how their efficiency varies over the operating range (RPM), and why boat engines are often used in a non-ideal way.
  • Propellers, the factors driving their efficiency (or, more commonly, their horrifying lack thereof) and what we can do to improve it.

Then comes the fun stuff. We’ll look at:

  • controllable-pitch propellers;
  • multi-speed and continuously variable transmissions;
  • and the wide range of electric and hybrid-electric systems

to see what, if any, of this technology is appropriate for our cruising yachts, both power and sail.

Book Chapter Navigation:

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Meet the Author


Matt, Engineering Correspondent, is a Professional Engineer and true renaissance man, with a wide range of expertise including photography and all things boat design. He has a unique ability to make complex subjects easy to understand and he keeps an eye on the rest of us to make sure that we don’t make any technical mistakes. Working as M. B. Marsh Marine Design, Matt designs innovative powerboats of all shapes and sizes.

5 comments … add one
  • Jim Kevern Jun 1, 2015, 10:30 am

    This should be an interesting exploration. Too bad I have already made the bed in which I have to lie, so to speak. We ordered an Autoprop for our next boat (after having a 3 blade Max Prop on the last boat). It appears to be the best match to our cruising style given variable pitch did not seem to be a viable option. I’m looking forward to what is to come.

  • Denis Bone Jun 1, 2015, 8:12 pm

    I re-engined my Fisher Freeward 30 some years ago when the forty year old SABB 2GZ (I think) became too unreliable and spares a bit difficult to procure.
    The Sabb, two cylinders giving 20hp with a huge flywheel and a variable, constantly turning, propeller, had proved adequate to drive the boat at 6 knots in calm water and 4-5 in less than smooth. The propeller had a habit of jamming in whatever position it happened to be when it picked up mono filament fishing line. This happened a couple of times and proved to be very embarrassing because the engine kept driving and there was no accessible clutch (a bit of imagination will fill in the details!).
    I intended to replace the engine with a Beta 25.
    I then looked at the fuel consumption curves and discovered that the 35 used less fuel when delivering 20 – 25 hp than the 25 delivering 20+, about 70% of the 35’s available power and therefore within the acceptable load range for the engine.
    I also chose an Autoprop ,the size recommended by Bruntons. I am delighted with the result, apart from an appalling initial installation that I have since had rectified, and that I rather liked the old fashioned off beat of the Sabb (180 degree crankshaft), the Beta being very quiet and smooth.
    The propeller installed is probably a little large for the engine because it will not rev over about 3,000, peak is specified at 3,600, but 3,000 is probably slightly over hull speed and there is little point in climbing watery hills! At 2,000 the boat makes just over five knots and at 2,200, the most I ever use, she is bang on 6Kn is fairly calm water. After 500+ hours I am very satisfied with the combination. The Autoprop also instantly applies a lot of power in reverse, if necessary, can be useful if thinking ahead has not been done properly!
    I am really interested to find out from Matt’s articles, what I could have done differently but, like yourself, I am committed!

    • Jim Kevern Jun 2, 2015, 4:13 pm

      Hi Denis,
      Two boats ago was a Sabre28 that sank at the dock, so I get ‘er real cheap. Repowered myself (retired mechanical engineer) replacing the Volvo with a Nanni, which is similar to the Beta – a marinized Kubota, and was quite happy with it. I met someone, and saw another post of folks who replaced MaxProps with the Autoprop and were quite happy. The last boat (Sabre 38) had a MaxProp, and while adequate I was never happy with how slow it was, despite the survey mechanic telling me it was propped correctly. I was much happier after I increased the angle a notch. Since we will often motor sail in light air, our style involves more using the prop to boost the speed a little, which the Autoprop seems better at. Although there were several magazine prop tests, most don’t compare apples to apples and are less than scientifically rigorous. The best I found is here:
      and you can see in this the Autoprop has a significantly different torque curve from a normal prop. Glad to hear yours is working out.

  • Jim Cockburn May 31, 2016, 5:20 am

    Hello Matt
    I am faced with the rebuild \ replace dilemma on a 1982 vintage Volvo Penta TAMD 70 D which is the original engine on a aluminum hulled Ketch (US built).

    Even with all the mechanical/electrical technical considerations that I have to work through they appear to be child’s play compared to the emission standard reg’s I have to consider.

    Trying to get a strait answer out of the engine suppliers on what EPA \ Worldwide emission regulations I need to comply with when choosing a repower product has been mostly unsuccessful.
    The project will be carried out during the 2015/2016 model year (I hope ) , the estimated cost will be north of 55,000 USD I sure cannot afford to install a non compliant product.

    What I have discovered is my selection is pretty narrow if I have to meet EPA Tier 3 part iii or better, can you shed some light on what the current requirements in fact are for worldwide compliance or direct me to some competent authority.
    Jim C
    Evening Star

    • John May 31, 2016, 8:18 am

      Hi Jim,

      As far as I understand it, as long as the engine you buy is available in the country you buy it in, it is, by definition, compliant. If it were not, the dealer would not be able to sell it. And, again as far as I know, once it’s installed that’s the end of it. No one is going to come aboard and give you a hard time about it in another country. After all there are millions of non-compliant existing engines in boats and vehicles.

      The only exception is that if you subsequently wish to sell the boat in Europe is may be a great deal easier to do so if it has a CE stamp.

      If it were me, I would look at a Beta to replace the Volvo. You will also find Beta very helpful and knowledgeable about compliance issues.

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