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.
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.
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.