The power/displacement trade-off

From Ocean Navigator #108
September/October 2000

Semi-displacement power vessels, like this Sabreline 36, are capable of high speed as they rise onto a plane and shed wave-making drag, but their higher fuel consumption rate tends to limit their range.
   Image Credit: Courtesy Sabre

Trawler sailors will willingly set out in weather, which keeps the express boats in harbor. They carry substantial ground tackle and often anchor-out, something few other powerboats do. This odd behavior makes them much more a part of the sail group at the yacht club or marina than of the powerboat community.Trawler yacht owners are often asked about their vessel’s voyaging speed. Many who are not sailors or trawler skippers want to know about hull speed. What is it; why does it exist; is it a firm, fixed number; how can it be identified; and are there ways of economically exceeding it? A boat moving through the water is proceeding through a dense, viscous fluid that resists the progress of the hull. The forces acting on a hull being propelled through the water include: 1) friction between the water and the wetted surface of the hull, 2) energy required to create the waves inevitably caused by the passage of the hull, and 3) the energy consumed in creating the eddies constantly shed by the hull, prop strut(s), rudder(s). In addition, the boat’s progress is impeded by the resistance of the air, acting on all above-water surfaces. The sum of all of these forces determines the amount of thrust needed to achieve the desired speed.div class=sectionHeaderLeft>Friction and wave-making effects

The resistance caused by water friction varies with the density of the water, the area of the hull in contact with the water, the coefficient of friction of the hull, and, perhaps most important, the square of the speed of the hull relative to the water through which it is moving. Water density effects are generally beyond our control. There is some automatic compensation for the density difference between fresh and salt water, with marginally less immersed hull surface when a boat is in salt water compared with fresh water. The friction component can be somewhat controlled by ensuring that the hull is clean; however, except for fanatic sailboat racers, the improvement provided by a mirror-like finish can never equal the cost of cleaning the hull.

The restraining force caused by friction between the hull and the water is generally described by a non-dimensional number, the Reynolds number, originally developed in the 1800s by Osborne Reynolds, a British physicist. Reynolds’ goal was to gain an understanding of the manner in which a fluid flows in a pipe, where laminar, non-turbulent flow is highly desired. (As applied to a vessel’s hull, the Reynolds number is the product of water density times speed times hull length, divided by the viscosity of water.) For all practical purposes the flow along the hull will be turbulent; therefore, there is little the designer of a pleasure boat can do to influence the effect of friction. The energy devoted to the creation of waves and eddies are the two largest components of the force resisting the passage of the hull through the water, with the wave-making component typically accounting for more than 95% of the total resistance. Waves are created both at the bow and stern of the hull, and additional wave patterns are created at points of abrupt change in hull section. Naval architects use a measure, called Froude’s number after William Froude, a British naval architect, that relates the speed of the ship, the acceleration due to gravity, and the waterline length of the hull to describe the total drag imposed by the creation of these waves.

Fortunately, we need not deal with calculations of Froude’s number, we can use a simple relationship to define hull speed: the speed at which a further increase in speed demands a disproportionate increase in engine power.

The waves created by the passage of the hull through the water travel at the same speed as the vessel. The speed of a wave is proportional to the square root of its length; therefore, as the speed of the boat creating the wave increases, the length of the wave increases. As the boat speed increases, the length of the wave approaches the waterline length, in effect placing the hull in a hole of its own making. This wave-making effect can be visualized as creating a condition under which the hull must constantly travel uphill.The speed at which this effect occurs is not a precise value. Bow and stern wave interaction may cancel or add to one another’s effects. In general, the result of the wave-making effect is to limit the speed the boat can achieve with reasonable amounts of power to approximately 1.3 times the square root of the length of the waterline, in feet (for speed in knots). This speed is commonly referred to as hull speed. Without some means for diminishing the adverse effect of the wave pattern, any attempt to increase speed will require a very rapidly increasing amount of power. Many large commercial vessels carry bow markings announcing the presence of a bow bulb, a strange-looking appendage projecting forward from the bow, at and below the waterline. In a seeming contradiction of good sense, the bow bulb is designed to produce its own bow wave. Properly designed and operating within a narrow speed range, the wave created by the bulb interacts with the normal bow wave, reducing the amplitude of the bow wave, thereby delaying the deleterious effect of normal hull-induced wave formation. The bow bulb has been gaining acceptance on large trawler yachts and expedition yachts. Prime examples of bow bulb application can be seen on very large cruise ships, whose cruise speeds can be kept within narrow margins.Lifting the hull.

Other than a truly massive application of engine power, the only practical means with which to significantly exceed hull speed is to lift a portion of the hull clear of the water. This can be achieved by creating a hull shape whose wetted surface, when propelled rapidly through the water, develops a vertical force component sufficient to lift a significant portion of the hull clear of the water, reducing wetted surface and drag. Alternatively, other lifting devices, such as surface-piercing foils, can be used to lift the hull clear of the water.

The quest for speed on the water has created a market for boats with trawler-type appearance and accommodation but capable of operating at three or more times hull speed. The availability of lightweight diesel engines in the 200- to 600-hp power range has allowed for boats that look like trawlers but have planing hulls. These vessels are popular with those who need to get to their destinations rapidly. The greater speed achieved by these hybrid trawlers is paid for in a number of ways. High-power engines are costly. Running time at maximum power is restricted, both by engine operating time limits and their substantial appetite for fuel. Planing hulls’ performance can be very sensitive to vessel weight. The need to limit structural weight may increase construction cost. Fuel and water capacity may be significantly less than is common in the traditional trawler. Although these boats can operate at true displacement speeds, their behavior at such speeds is often inferior to that of pure displacement-hull vessels. In addition, the superstructure of a planing trawler creates more aerodynamic drag than the more streamlined designs of other planing-hull boats. Although this hybrid design appeals to some yachtsman, this type of boat has by no means superceded the full-displacement trawler.

Regardless of the design speed of a trawler, it will need an engine or two. Engine choice for a trawler-type pleasure boat used to be a very simple exercise. Only a limited range of moderate-horsepower diesel engines was available. They were often marinized farm equipment or small truck engines. Although cost effective and reliable, these types of engines were not ideally suited to use on a boat. However, thousands of such engines are in daily service, often with well in excess of 10,000 operating hours in the engine log.

The days when engines in the 90- to 200-hp range were great hulking masses of cast iron are past. Today’s engines rely upon a precise understanding of stress levels throughout the engine, allowing the use of precision casting techniques to minimum engine dimensions and weight. Thanks to better understanding of combustion chamber dynamics, improved metallurgy and manufacturing, and greatly improved lubricating oils, the operating speeds of today’s engines can greatly exceed what was practical only a few years ago. The recommended continuous operating speed for some engines exceeds 3,300 rpm.

Traditional trawler engines were normally aspirated and ran at moderate speeds. Choosing a fast-turning, turbocharged, aftercooled engine for a trawler yacht may at first appear inappropriate; however, the savings in weight and the small size of such a power plant can pay dividends in the overall performance and utility value of the boat. Arguments have been made that large, slow-turning engines create less noise than faster turning, turbocharged engines. Such statements are generally incorrect. Producing power from diesel fuel requires the rapid combustion of the fuel in a cylinder. The higher-speed engine burns a smaller quantity of fuel per combustion event than a slow-turning engine. The acoustic result may be compared to the noise produced by a large number of small firecrackers being exploded in rapid sequence versus a few large firecrackers being exploded at a more leisurely rate. The higher-frequency sound produced by a faster-turning engine can be easier to contain in the engine room than lower-frequency sound energy.

A physically smaller engine pays dividends in the engine room, where there is never enough space for equipment and access for inspection and maintenance is always difficult and on occasion virtually impossible. Modern turbochargers are remarkably reliable. In general, the only service they and the associated aftercoolers require is an occasional wash-down with a detergent/water spray. Turbochargers also act as mufflers, quieting the engine exhaust noise.Engine specifications.

The general specifications of three engines of similar power rating, one turbocharged, one normally aspirated, and one turbocharged and aftercooled, illustrate the wide range of powerplant choices appropriate for a small- to medium-size trawler yacht. The Caterpillar 3304B, a four-cylinder, 425-cubic inch turbocharged engine, weighs 1,710 pounds and measures (L x H x W) 55 inches x 45 inches x 38 inches. Operating at its very conservative C rating (Caterpillar’s designation for engines used in ferries, harbor tugs, and displacement yachts) at 2,200 rpm, it delivers 165 hp at a fuel rate of 9.6 gph. A second Caterpillar engine, the normally aspirated model 3208, a 636-cubic inch V-8 engine, weighs 1,685 pounds and measures 57 inches x 40 inches x 40 inches. Operating at its A rating (Caterpillar’s most conservative rating, usually applied to engines used in the most demanding commercial service) it produces 150 hp at 2,400 rpm, consuming 7.7 gph. The Yanmar 4LH DTE, a four-cylinder, 211 cubic inch, turbocharged/aftercooled engine, weighs 838 pounds, measures 42 inches x 29 inches x 27 inches and delivers 154 hp at 3,200 rpm, the manufacturer’s specified continuous operating speed, while consuming 8.7 gph.

Based on these specifications and the manufacturers’ power rating systems, the Cat 3208 operating at its A rating is the most conservatively rated engine. It is slightly more weight efficient than the Cat 3304B (25 pounds lighter) and occupies a bit less space, 52.7 cubic feet versus 54.4 cubic feet. The Yanmar 4LH DTE is the most aggressive engine, extracting 154 hp from 1/3rd the cubic displacement of the Cat engines while turning 1/3rd faster. It weighs about half as much as the Cat 3208 and is also substantially smaller, occupying only 19 cubic feet. Clearly, there is a wide choice of diesel power available for trawler applications.

Most trawler yacht power trains use single-speed gearboxes, driving fixed-pitch propellers. Although this power-delivery system is fully adequate for most users, conversion of engine power into thrust is optimal only at maximum engine speed. At all lesser speeds the engine will be capable of producing more power than the prop can absorb. As a result, the engine will have to operate at a somewhat higher speed than would be required with a more efficient drive-train system. At low rpm, the fixed-pitch prop imposes an excessive load on the engine, slowing acceleration (accounting for clouds of black smokeunburned fuelspewed from the exhaust when the throttle is advanced too quickly). In addition, the minimum idle speed of some boats may exceed the speed limits imposed on some waterways. Two solutions to these problems are available: two-speed gear boxes and controllable-pitch propellers.

Two-speed gear is available in size ranges suitable for many trawlers. Operating in low gear will enhance slow-speed maneuverability. Although not usually a major factor for trawler operations, acceleration will also be markedly improved. When the boat is powering into a head sea, use of low gear allows the engine to deliver all of its rated power, improving handling and allowing maintenance of a more constant speed. During operation in smooth water, running in high gear will allow the engine to operate in its most economical power range. Many commercial vessels, large motor yachts, and large sailboats are equipped with controllable-pitch propellers. With a controllable-pitch prop set to low pitch, the engine can readily accelerate to a speed at which it can produce useful amounts of power. Once underway, the prop pitch is increased, allowing the engine’s power to be precisely matched to the load imposed by the boat’s speed and load. Once underway, engine rpm is set, followed by adjustment of prop pitch. Exhaust gas temperature increases as the load on the engine increases with increasing prop pitch. Exhaust gas temperature, displayed on an instrument panel, indicates the power load on the engine, facilitating precise power adjustment for any voyaging condition. While controllable-pitch props are costly, they can often be justified by the improvement in boat performance, especially in difficult conditions.

The traditional design of the trawler power train (and most other power and sailboats) transfers the thrust developed by the prop to the hull via the engine mounts. The prop pushes on the prop shaft, which in turn exerts thrust on the marine gear/engine, which in turn transfers the thrust through the resilient engine mounts to the engine bed and the vessel’s hull. The need to transfer thrust through the engine mounts limits the allowable flexibility of the mounts. As a result, more engine vibration than is desirable may be transferred into the hull. Vibration transfer can be minimized with use of a thrust bearing, incorporated into the shaft log, which transfers prop thrust directly to the hull. The drive shaft from the engine/gear box is connected to the prop shaft through a set of vibration-absorbing flexible couplings, often including constant velocity (CV) joints similar to those used in front wheel-drive vehicles. With all thrust loads removed, the engine mounts can be as resilient as necessary to isolate engine vibration from the remainder of the vessel. In addition, the system eliminates the normal need for critical alignment of the prop shaft and engine. Some drive systems incorporating CV joints and thrust-bearing systems enclose the prop shaft outside the hull in a non-rotating pipe extending to the P bracket. Doing so eliminates the Magnus effect drag loss created by the annulus of rotating water created by the exposed rotating prop shaft. Enclosing the prop shaft also allows use of an oil-lubricated P bracket bearing in place of the normal water-lubricated cutlass bearing. At present, this type of system is rarely seen in yachts other than really large motoryachts.

Trawler yachts remain popular and in fact may be gaining market share as the boating population matures and sailors opt for the less-demanding voyaging provided by these boats.

Contributing editor Chuck Husick is a marine writer, pilot, Ocean Navigator instructor and the former president of Chris Craft.

By Ocean Navigator