According to U.S. Coast Guard statistics, for incidents involving serious injury requiring medical treatment beyond first aid, on average, when something went wrong with the vessel’s mechanical equipment, 43 to 45 percent of the time, the problem was engine failure. Interestingly, steering system failure was the next most frequent incident.
Admittedly, a well maintained diesel engine, in a favorable operating environment, will run forever (almost). But there are so many other factors — plastic bags, jelly fish and seaweed that can get sucked up into the intake; dirt, sediment, algae in the fuel, pot warp, ghost nets, etc. — all of which can, and do, disable engines, that it’s not a question of if, but when you’ll experience an engine failure. One of my favorite authors, Roger C. Taylor, in his excellent little manual, The Elements of Seamanship, said, half tongue-in-cheek, “Even if you are a mechanic who knows how to repair engines, still you may run up against a troubled machine that can only be cheered up with tools or parts that are far away on land. In such a situation, the more engines you have in your vessel, the better. The chance of two, three or four engines simultaneously needing tools or spares that are not on board is remote.” Well, maybe you don’t want or can’t fit four engines on your boat, but how about two?
The question of twin versus single engines in power voyaging boats is a controversial and much debated subject, and we’ll not resolve that issue here, but we will discuss their pros and cons regarding getting home.
Twin engines
The advantages of twin engines are well known — safety, maneuverability, and shallow water access. Twin engines give you the safety of a backup propulsion plant — a get-home engine. In addition, you have the ability to cannibalize parts from the comatose engine to keep the surviving engine running, should you need to. There’s also the added benefit of greater maneuverability in close quarters, often the primary reason for twin engine installations. And finally, there’s the ability to bring the boat into more shallow, more interesting coves and inlets.
The advantages of twin engines are well known — safety, maneuverability, and shallow water access. Twin engines give you the safety of a backup propulsion plant — a get-home engine. In addition, you have the ability to cannibalize parts from the comatose engine to keep the surviving engine running, should you need to. There’s also the added benefit of greater maneuverability in close quarters, often the primary reason for twin engine installations. And finally, there’s the ability to bring the boat into more shallow, more interesting coves and inlets.
However, these advantages come at some additional cost: two engines, shafts, props, rudders, cooling and exhaust systems, etc., plus increased fuel consumption, and, since the shafts and rudders are off the centerline, they are more exposed to damage from floating and submerged debris and groundings, unless they are to be protected by skegs or bilge keels, and thus more cost.
Single engines
Single engine installations also have their advantages, lower cost, less noise and vibration, greater cruising range and superior prop, shaft, and rudder protection.
Single engine installations also have their advantages, lower cost, less noise and vibration, greater cruising range and superior prop, shaft, and rudder protection.
Regarding vessel handling and maneuverability offshore and in rough seas, the advantage goes to single screw boats due to the larger rudder area. It’s only in close quarters and around the docks that twin engine boats perform better. However, with the addition of a bow thruster to the single engine installation, vessel handling and maneuverability is as good or better (and cheaper) than a twin installation.
So, which is better? In terms of get-home capability, it’s the twins — unless you add some form of emergency backup propulsion system.
Get-home systems
There are three options for backup propulsion systems in common use: sails, wing engines or a genset-based auxiliary drive.
Sails: Sails are a simple, reliable, never-fail backup system and thus attractive as a backup get-home system. Robert Beebe, widely acknowledged as the father of distance voyaging in powerboats, chose to use sails on Passagemaker, his famous 50-foot, wooden, round-bilge, full-displacement motorsailer. Beebe cruised Passagemaker some 60,000 miles from its launching in 1963 to 1975 when he published his book, Voyaging Under Power, the bible for passagemaking power boaters. However, Beebe concluded, after all this sea time, that its performance under sail was barely acceptable, reaching speeds of only 2 to 3 knots, broad reaching or running its only practical points of sail and that some form of diesel-driven auxiliary was recommended. Still, they are sometimes used.
There are three options for backup propulsion systems in common use: sails, wing engines or a genset-based auxiliary drive.
Sails: Sails are a simple, reliable, never-fail backup system and thus attractive as a backup get-home system. Robert Beebe, widely acknowledged as the father of distance voyaging in powerboats, chose to use sails on Passagemaker, his famous 50-foot, wooden, round-bilge, full-displacement motorsailer. Beebe cruised Passagemaker some 60,000 miles from its launching in 1963 to 1975 when he published his book, Voyaging Under Power, the bible for passagemaking power boaters. However, Beebe concluded, after all this sea time, that its performance under sail was barely acceptable, reaching speeds of only 2 to 3 knots, broad reaching or running its only practical points of sail and that some form of diesel-driven auxiliary was recommended. Still, they are sometimes used.
Wing Engines: A wing engine is a small, low-power, completely separate propulsion system, independent of the main engine, with its own shaft and prop — usually a feathering or folding type — and with its own electrical and fuel systems. The engine is located in the engine room off the centerline, as is its drive line. The horsepower of the engine and the size of the fuel tank depend upon the size of the boat and the nature of its intended usage. Proponents of twin engines question whether these small power plants could really get a big yacht home if the main engine failed in mid-ocean. However, Jim Leishman, vice president of Nordhavn Yachts, in his revised edition of Beebe’s book says, “We have found that a 28-hp three-cylinder Yanmar engine swinging an 18-inch Martec folding prop will push our Nordhavn 46 at almost 6 knots and maintain headway into a significant head sea and wind.” Not all that bad!
The advantages of the wing engine are that it can be brought on-line immediately and can also be used to power hydraulic systems for bow and/or stern thrusters, anchor windlass, etc., and can also be used to maneuver in close quarters. Wing engines are extensively used by Nordhavn, Shannon, Kadey-Krogen and others. Too many variables in this conundrum to solve here, but there is no question that the little devils will provide headway and the needed steerage for the vessel while work is underway to get the main engine back on-line.
Generator-based systems: A third alternative for single-engine boats that do not have the space for a wing engine installation is the generator-based auxiliary propulsion unit. These systems use the vessel’s genset to provide the power which is then coupled — belts, chains, hydraulics, electric, etc. — to the prop shaft. The principal drawbacks are the lack of available horsepower in the genset and losses due to friction in the transfer of power from the genset to the prop shaft. These have been mostly custom systems. That is, until the development of the Bayview Auxiliary Tug (BAT) drive by Ronald Voegeli in 2003. In 2005, Voegeli, together with several partners, formed Bayview Edison Industries (BEI), to manufacture and market the BAT drive. To date, BAT drives have been installed in some 60 vessels.
The BAT drive is a hydraulic motor powered by a pump driven from a power take-off (PTO) on the vessel’s generator. It is installed between the prop shaft coupling and the transmission. The space required for the unit depends on the size of the prop shaft. For a shaft diameter of 2 to 3.5 inches, about 13 inches is required behind the shaft coupling and for 3.5- to 5.5-inch shafts, 15 inches is necessary; 15 inches of width and 13 inches of height are required for all shaft diameters. It can be activated and controlled — vessel speed, forward, reverse — from the bridge in the event of a main engine failure. Should the failure be in the transmission, modifications to the shaft coupling must be made at the unit.
Estimated boat speeds vary from as little as 3 knots to as much as 8 knots, depending on the size of the boat and that of the generator. For example, a 40-foot boat with a 10-kw generator can be expected to push the boat at about 3 knots. A 100-foot boat with a 50-kw genset may achieve speeds up to 8 knots.
These systems have other attractive features as well, such as quiet, low-speed trolling, sight-seeing, late-night or early-morning departures. Another very attractive feature of these units is low maintenance; no lubrication is required, just a periodic visual peek-see and a one-hour run every six months or so to prevent the poly chain drive from taking a set.
Last September, Wesmar (Western Marine Electronics, Woodinville, Wash.), purchased patent rights from Voegeli to manufacture and sell the BAT drive under the Wesmar APU (Auxiliary Propulsion Unit) brand name. Voegeli is working with Wesmar on product development and new features. BEI no longer sells the BAT drive but provisions have been made to support existing installations.
Last September, Wesmar (Western Marine Electronics, Woodinville, Wash.), purchased patent rights from Voegeli to manufacture and sell the BAT drive under the Wesmar APU (Auxiliary Propulsion Unit) brand name. Voegeli is working with Wesmar on product development and new features. BEI no longer sells the BAT drive but provisions have been made to support existing installations.
Catastrophic engine failures — seized pistons, bearing failures, cracked heads, etc. — are relatively infrequent occurrences in a properly maintained diesel engine. The starting problems most frequently encountered are injector failures, inoperative glow plugs, faulty fuel flow solenoids, restricted fuel or air flow, air in the fuel line and low battery power (poor connections). With a proper set of spares and a skipper or crew with the necessary skills, these little auxiliary power plants can give you the time and sea-room needed to make these repairs. A short course in diesel engine maintenance would be a good investment for distance voyagers. n
