Single Vs Twin Engines and the Get Home Conundrum

jury rig

We’ve been discussing various design considerations with a client headed for Tierra del Fuego and the Antarctic and thought some of the dialog might be of interest. We’ll start with the logic behind single vs. twin engines, and get home systems.

Everyone wants the safest, most reliable yacht. The question is how do you achieve this? Contrary to what you may think budget is usually not a major factor. Clear thinking about the risks and rewards of various approaches is the key ingredient.

Lets start with the risks as we see them, and our approach to reducing or eliminating these.


Engine risks come in several categories. There are reliability issues related to the basic block. These are minimal, especially if you conservatively rate your engine. If you are going to have a piston, valve, or bearing problem it will typically show up in the first few hundred hours.

Next come accessories: fuel injection, sensors, starters, alternators, drive belts, and pumps. Periodic maintenance ahead of suggested schedules, and careful inspection usually takes care of this. If there is a failure, having complete spares and tools (both of which come with the FPB 64) gets you going.

The biggest risk, in our minds, comes from a bad batch of fuel. To mitigate this we have an oversized fuel polishing system, dual pre and post day tank filters and dual fuel transfer pumps. Beyond this, enormous fuel capacity allows us to make even trans ocean passages on half capacity. Making sure to refill when at half tanks means there is range left in the old fuel if the new has a problem.

Now a bit of real world experience. In over 250,000 miles of cruising we have never had a propulsion engine we could not get started within a couple of hours of a problem.

Drive Line

The drive line is arguably more at risk than a properly maintained diesel engine. Problems come in the form of kelp or lines wrapped around the prop, or debris (logs or ice) damaging the system. These risk factors are dealt with in various waves. This starts with an oversized transmission, rated at 400HP in pleasure usage from which we will pull an average of less than 80HP.

Next is a shock absorbing coupling between the prop shaft and transmission. Provisions are made to change the coupling and check bolt torque with a built in prop shaft brake.

The prop shaft is 2.5″ (62mm) Nitronic steel. It is several times the strength it needs to be based on “normal” yacht design calculations.

The propeller itself is cast from Nibral bronze. Its configuration leads to an inherently strong blade root. The prop may be dinged out of balance, but the odds are you could continue at reduced speed. If not, there is a spare prop in the forepeak (standard supply). The prop itself is designed to be used with a simple flat plate puller. Remove the zinc and prop nut, bolt on the puller to the aft end of the prop hub, and you are ready. Quick and simple.

Of course the prop is protected by a full skeg, and there a Spurs line cutter ahead of the prop.

Wind Horse has twin off center props (fronted by skegs) and so are more vulnerable to ice and debris. To date (45,000 miles – two trips to Alaska, and two others to the ice) we have yet to damage a prop.

Single Vs. Twin

This argument has raged for years. At the gross level you can point to the world’s commercial fishing fleet 99.9% of whom use single engines. On the other hand, twin engines offer a built in get home option and better maneuverability.

Our personal preference is for single engine installations. These are simpler, allow mechanical or hydraulic controls, are quieter, and more efficient. But most important to us is the fact that a single centerline prop is better protected from ice and debris, and as higher impact factors of safety than is the case with two smaller props.

As far as reliability goes, our feeling is the biggest risk is a bad batch of fuel. If this occurs the bad fuel affects all diesel consumers.

The next risk we think about is a drift net on the surface. Here the odds to us appear that a single centerline prop is less likely to entangle itself. Twin outboard wheels are prone to both being caught.

Maneuverability initially favors twin engines. But you can compensate with fin configuration, in other words using a big rudder helps. The FPB 64 has a more easily turned canoe body than the 83, a rudder larger in scale (and the same size as the 83!), and more rudder angle with which to maneuver. Bottom line, in really tight spots the single engine FPB 64 at 65 feet in length will maneuver as well as the 83 foot twin engine big sister. Notice we have not yet mentioned the bow thruster which the FPB 64 has and the 83 does not. Between inherent maneuverability and the breast line to electric deck winch system, the thruster should rarely be needed.

Reliability Equation

In our opinion the most important facet of the reliability equation is access. Good access makes it easy to inspect things, and to perform preventive maintenance. Both of these help avoid reliability issues underway. Equally important is simple engineering and systems that are designed from the beginning for owner maintenance and reliability. If there is a choice between simple and complex, it is always best to opt for simple. Of course using the best ingredients helps, as does over specifying items like wiring, plumbing, pumps, and DC systems.

Get Home System Logic

We have saved the hardest part of this discourse for last, get home systems. There are two major components in this decision process. The first is the performance target of th get home system. Does it need to take you across an ocean or just to the dock in a calm bay? The second is the get home system’s reliability and its impact on the rest of the vessel’s reliability.

Lets start with the most important part, reliability. By definition boats are crowded spaces, and typically the most crowded is the engine room. Any get home system that involves a propeller is going to make the engine room substantially more crowded and complex. This results in less access, and more difficult day to day maintenance, which in turn increases the risk of a problem with the primary propulsion system. So, question number one is the tradeoff between the everyday hit your maintenance takes against the potential (small) risk of needing a get home system which is diesel driven.

The other side of this conundrum is the reliability of the get home system, and its ability to get the intended job done. Take an auxiliary prop for example. Is it clean so it will function? If it is offset will it survive impact with ice or debris to which it is substantially more vulnerable? What about the get home prop’s propulsion system, including controls? Will this seldom employed gear work when needed? And how about performance in a breeze and waves?

As cruisers who enjoy out of the way destinations all of these thoughts have been going through our heads for years. We want maximum security ourselves (witness the over spec bottom plate, double bottom, and watertight bulkheads). Like you, we’d like to have a 100% reliable get home system that doesn’t negatively impact day to day systems. The problem is this is zero sum game, with limited space, and a wide range or risks, not all of which can be eliminated without prohibitive secondary costs.

Our own approach is to prioritize the risks. For us these begin with an ocean passage. We want to have a warm fuzzy feeling in our gut that we can complete what we start. Lets say we’re a day out of the Galapagos headed for the Marquesas Islands, with 2700 miles left to go, and something happens. Do we have the tools and parts to fix the problem? Odds are the answer is yes. But lets say we can’t get the engine going, then what?

The issue we worry least about is being without power on soundings, close to shore.

FPB 64 “Sailing Rig”

The get home rig on the FPB 64 is conceived to mitigate the passaging risks. The small jib forward is there for balance and steering, and to get the bow off the wind. The aft mainsail is the propulsor. We are not saying this is fast, but it will get you across the pond on a downwind passage. Obviously it won’t go to weather, but then neither will most get home propellers.

Coming back to the 2700 mile passage to the Marquesas, we are going to drift down wind at around a knot without sails. Add in half a knot of current, and another couple of knots from the sails, and pretty soon we are ripping along at three or four knots over the bottom. It might take a while but we’ll get there eventually.

The close to land scenario is less of a concern in that we have the option of using the dinghy alongside as a tug, or calling for a tow.

Now a question. Facing the need to complete an ocean passage of a thousand or more miles would you rather have a simple jury sailing rig, or a complex, probably poorly maintained auxiliary diesel which needs clean fuel to operate?

Comparing the FPB to Sail

At some point the question of ultimate reliability has to come back to an auxiliary powered sailboat. Having had lots of experience with these animals we can relate our own feelings on this subject in some detail. Initially the lack of a rig made us nervous. But we quickly came to the realization that in all respects Wind Horse was a safer and more reliable cruising platform. Sailing rigs are rife with maintenance and reliability risks. Comparing the issues associated with sailing to those of the Unsailboat FPB we came to the conclusion that the risks of dropping the rig under sail, and fouling the prop with rigging, were greater than of losing our engine on an FPB.


In the end, we look at this a lot like flying, especially in small aircraft. Our lives depend on the reliability of the aircraft propulsion system. Frequent inspection and preventive maintenance reduce the risks of failure to a minuscule and acceptable level. For us, the same holds true with the FPB, except the consequences of propulsion failure are less severe at sea than in the air.

Posted by Steve Dashew  (January 8, 2010)

10 Responses to “Single Vs Twin Engines and the Get Home Conundrum”

  1. Pete Rossin Says:

    Hi Steve

    Indeed this is an age old issue, but the analogy to flying is a good one. In light twin airplanes, the old saw is that the second engine takes you to the seen of the crash. Based on practical (training based) experience this is true.

    On my current boat, I have a hydraulically powered get home drive that is powered from the gensets power take off which drives a hydraulic pump and drives the main shaft with a hydraulic motor.

    Over the years, I have had two situations where a get home drive might have been necessary.

    The first was a prop shaft fouling by a submerged large nylon reinforced tarp in the middle of the Gulfstream. My get home wouldn’t have worked since it relied on the main shaft. It was a calm day so we dove the boat and cleared the tarp, but we could have run slowly at reduced RPM. If it had been truly severe, my get home drive (or one on an exposed spare prop) would not have helped.

    The second was in a strong tidal flow in the Savanah River. The main had to be shut down because of failed belts. This was a design installation issue which would not have happened on the 64. The boat had a wing engine which did start but it did not have the power to move against the current and could only make turns to stbd as the wing prop was to port. My hydraulic drive would not have been helpful as it takes some 20 minutes to set it up. The short answer is that we could have dropped the hook and waited for help.

    Nobody can anticipate every issue, but a reliable diesel with good systems and good fuel is pretty tough to beat as evidenced by your experience.



  2. JMR Says:


    As happened to you, I also spent a good deal of time reflecting about our conversation.

    I tried to understand why I was so strongly reacting to some of your your considerations regarding propulsion systems, even if they where perfectly rational and well technically supported.

    I also read your post on SetSail which is crystal clear, but does not explain why we, non professional sailors, are still discussing this issue.

    I feel that it is mainly psychological and is related to how we deal with our own lack of self confidence in some environments .

    Just after finishing our conversation I asked my self why I was so worried about propulsion backup if in the last 10 years I crossed solo by motorcycle deserts in Australia, Africa and Asia. Obviously I had no backup propulsion systems and carried infinitely less spares and supplies.

    Also I thinked about the flight I took some years ago, on a single engine turboprop ( TBM 850 ) , from France to the US through Greenland over the North sea.

    My conclusion for this psychological issue has the following aspects:

    o as land dwellers, we historically feel uncomfortable and unsafe on water, building whenever possible, technological layers of unneeded ( or even useless ) redundancy on an ever failing attempt to lighten our worries.
    o we think about backup because they are possible to do on a vessel ( versus impossibility on motorcycles ) even if not needed.
    o we forget that by adding backup systems we are adding complexity, weight and operational costs that brings a complete new set of potential problems and performance limitations. ( and the need for new backups on a never ending problem )
    o as members of the affluent society and non professional sailors, we prefer to spend more money on back up systems than becoming really proficient in maintenance,repair and replace. ( the attached pic is from a fishing boat in Brazil. Guess if it has any redundancy at all )
    o sailors tend to be self sufficient and independent personalities, and tend to react to other sailor ideas. I caught myself reacting to you and not to the reasoning behind your ideas.

    Obviously non professional sailors with very large experience ( as yours ) learned how to better cope with this almost anthropological situation.

    I guess the worries are still there, only the reaction is different.

    So it looks that we face a non technical issue that we try to solve with hardware and money and not by experience, training and mind set adjustment.

    Any reasonably knowledgeable boat owner is aware, no matter what he invested, that there is no such thing as an “unbreakable” boat.

    What everybody needs is an easily reparable boat , but for that you need a “repair-able” sailor.

    Much more attention should be given to maintenance trainning and certification , because this is probably the best “get home” system.

    I know owners in Brazi that never went inside of the engine room , and rely 100 % on their crew. And that’s for a 50′ vessel.

    Another issue to consider is that most boat designers and manufacturers prefer to supply all the backup systems required by the client than face the possibility of a lawsuit if something goes wrong.

    Every contract I see has almost 50 % of its content related to disclaimers, liabilities and other manufacturer/designer protection issues. (including yours)

    Obviously this is reality and I fully understand the need for this kind of legal protection

    Well , these are my reflections on our conversation.

    Anyway I am completely convinced about the propulsion solution you designed for the FPB, even if applied to high latitudes cruising.

    Have a nice weekend.


  3. Larry Smith Says:

    I am surprised that you have opted for a single-hull motor vessel with a sail backup after all of your sailing cat experience. Why wouldn’t the most reliable propulsion system be sails with an engine auxiliary? And why wouldn’t you opt for a cat instead of a monohull? Perhaps your high-latitude requirement is part of the reason for choosing a monohull, i. e., capsize recovery. But a sail propulsion system has none of the vulnerabilities that you agonize about in settling on a single-shaft system. Of course, sailing rigs have their own vulnerabilities, but centuries of experience and modern materials have made maintaining standing and running rigging a pretty safe bet, and there are many ways to jury-rig around most failures.

  4. Steve Dashew Says:

    Hi Larry:
    Easy answers first. The high initial stability on cats (and fat monohulls) makes them very uncomfortable at sea. Then there is the heavy weather issue, and finding a berth in port. So, we ruled out cats early on.

    The question of auxialliary sail is a good one. We’ve been doing that for a long time, and it is obviously a valid approach. However, the FPB concept has some advantages: it is easier to handle, substantially more comfortable, draws a lot less water, and less costly than the auxilliary powered sailboat. And you don’t get cold or wet.

    Knowing what we know today, if this were ten years ago and sailing Beowulf were still an option, we’d cruise on the FPB.

  5. Brian Eiland Says:

    But to go back to the idea of just a single engine being on-board, I would bring up one other question that I do not see addressed in your discussion here Steve (of course I’m sure you have addressed it somewhere knowing of your thoroughness) These modern diesel engines are all going the route of electronic control. These ‘electronic brains’ are really producing engines of greater efficiency, and less polution. Combine these electronics with the VERY high pressures of the ‘common rail’ injection systems, and we have a potential problem with realiability in remote locations. What if an electrical event (lighting strike, etc, etc) should disable our engine’s brain?? its not like the old days when you could get the diesel engine required ONLY good air and clean fuel to run. These modern engines may not run at all without a functioning electronics…and no amount of jury rigging will work in most instances.

    So what do we do?…carry spare ‘brains’ in an electrical isolation box….???

  6. Steve Dashew Says:

    Hi Brian:
    We prefer mechanical injection and control diesel systems for their reliability. However, these are no longer available. Our answer is as follows:
    1-Carry a spare injection pump, CPU, and sensor set.
    2-Have some form of ocean crossing get home system.
    3-There is a degree of additional protection from lightning strikes conferred by a metal hull.
    4-To the extent possible, stay away from common rail (the FPB 64 engines have electronic pumps, but are NOT common rail).

  7. Michael Says:

    Hi Steve. How have the electric engine controls ( throttle and gear) been working out. Do you find them to be pretty robust? We have the option of going with Kobelt mechanical or electric for our dual station setup and the pros of the electric system are looking better and better. We love your systems approach so thought the question might be relevant after you’ve had time to test the FBP series out. Worse thing I can think of is for the system to go down during approach to a dock. Offshore it feels like we could manage with adjusting things manually in the engine room without to much trouble.

    Thanks !


  8. Steve Dashew Says:

    Hi Michael:
    If we had the opton we would always go with hydraulic or mechanical controls over electric. We don’t with modern engines, but we’d go with Hynanutics if we could.

  9. Steve Dashew Says:

    Hi Michael:
    If you have the option stay with mechanical or hydraulic controls.

  10. Michael Says:

    Thanks Steve! I’ll be talking with the steering vendor tomorrow and will make arrangements for a mechanical setup.

    All the best and thanks for your insight.