Marine Batteries

- Introduction to Marine batteries -

Modern ocean trawlers have large and complex electrical systems, with large battery banks as the foundation for the system. The batteries generally supply two types of loads:

  • High burst loads, which are characterized by short-duration and very high current—typically hundreds of amps for a few seconds to a few minutes. Engine starting is the most common such load.
  • House loads, which are generally much smaller, longer-term loads. These include lighting, electronics, and small appliances.

Virtually all marine electrical systems are based on lead-acid batteries. This is because lead-acid batteries can store very large amounts of power in relatively small space. While there are a variety of technologies used to construct lead-acid batteries, they all share the same basic components. Lead plates or grids are immersed or sandwiched in an acid electrolyte. A reversible chemical reaction between the electrolyte and plates generates electrical energy. Charging the battery reverses this reaction.

There are three primary battery technologies in common use today, and any of these technologies can be used to build either starting or house batteries. The three primary technologies are:

  • Flooded-cell batteries
  • Gelled electrolyte (commonly referred to as "gel cells")
  • Absorbed glass mat (AGM)

Jet Tern Marine has chosen to equip Selene Trawlers with AGM batteries for both starting and house applications.

- What is an AGM Battery? -

An AGM battery is built by packing a glass mat, similar to fiberglass, between the battery's positive and negative plates. The mat is saturated with acid electrolyte. This packed structure gives the AGM battery one of its greatest strengths: vibration and shock resistance. Flooded cell batteries, and to a lesser extent, gell batteries are subject to damage when their relatively fragile plates are subjected to vibration and shock—which are quite common on an ocean-going trawler.

Sealed, positive pressure relief valves within the battery re-direct excess hydrogen and oxygen vapors back into the glass mat, where the vapor returns to liquid. Thus, the AGM battery also requires no regular maintenance to maintain electrolyte levels. In addition, because there are no vents or fill holes, the batteries will not leak dangerous acid, even if they are inverted.

The dense packing of plates with the glass matt also lowers the internal resistance of the battery, which allows AGM batteries to recharge faster, discharge longer and deeper, and generate larger bursts of current for engine starting and other high-current applications, such as electric windlasses or bow thrusters.

Of course, there are always tradeoffs. The primary one is cost. AGM batteries are more expensive than similarly rated flooded or gel-cell batteries. In addition, if AGM batteries are severely overcharged over long periods, they can be damaged and the electrolyte cannot be replaced as it can in a flooded cell battery. AGM batteries are also somewhat heavier than similarly rated flooded or gel batteries.

- Advantages of AGM Batteries -

  • No maintenance (except periodic external cleaning)
  • Batteries will not spill or leak, even if cracked or inverted
  • Dual purpose; can be used for starting or deep-cycle applications
  • Can be installed at any angle (except upside-down, which could restrict valve operation)
  • Shock and vibration resistant
  • Minimal gas release when properly charged
  • Low self-discharge (3% per month at 77F)
  • Submersible without damage
  • Long cycle-life when properly charged
  • Can be shipped via UPS
  • More efficient charging—less heat generated during charging

- Disadvantages of AGM Batteries -

  • High initial cost
  • More weight per AH than flooded
  • Electrolyte cannot be replaced if severely and continuously overcharged

- Summary -

Lifeline AGM batteries are standard equipment on Selene Ocean Trawlers because they represent the safest, most reliable, lowest maintenance battery technology. They will provide many years of trouble-free service. For more information on Lifeline AGM Marine Batteries, see:

Sound Insulation

Modern ocean-going trawlers are virtually all powered by diesel engines, which are very reliable and efficient, but also quite noisy. Controlling engine noise is an important part of a safe and comfortable passage. Excessive engine noise is not only annoying, but it can impair communication between crew members, cause headaches, and hinder sleep. Inadequate rest can lead to fatigue and poor decision making at sea. Long-term exposure to excessive noise can damage hearing.

- How Does Sound Travel? -

Engine noise travels by two primary means. Each requires specific steps to control and contain the noise.

  • Airborne noise transmitted directly off the engine casing.
  • Structure-borne noise, induced by vibration from the engine through the hull and bulkheads.

As the name implies, airborne noise travels through the air, and is transmitted most readily through holes or other openings that allow air to pass through. Controlling airborne noise is primarily a function of sealing up any paths that would allow the noise to be conducted into the living spaces in the trawler.

Structure-borne noise, on the other hand, is best isolated at the source by isolating the engine vibration from the hull and other mechanical structures. Notice that vibration noise can actually become airborne noise when it vibrates a bulkhead or other structure, and the noise of that vibration is then transmitted through the air.

Sound insulation is a primary weapon in the battle against noise. It is designed to defeat noise in three ways:

  • By creating a barrier to block noise from escaping the engine room
  • By absorbing noise from within the engine room
  • By dampening vibration in the walls and ceiling of the engine room

- Selene Engine Room Noise Insulation -

The engine rooms of all Selene Trawlers are equipped with a system of Soundown insulation designed achieve all three of the above goals. The sound insulation consists of three layers:

  • The innermost layer, which is visible from within the engine room, is perforated aluminum. It's primary purpose is to protect the inner layers from dirt and damage. It is painted white and provides a clean, bright surface in the engine room.
  • The middle layer is 3 inches of foam insulation. This layer absorbs and dissipates engine noise in the trapped air pockets within the insulation.
  • Finally, the outer layer is a sheet of high-density IMT20 tuff-mass lead barrier. It reflects any noise energy that passes through the foam insulation back into the insulation and engine room.

The cabin bulkhead or floor is installed outside this outer insulation layer, and provides additional sound isolation and vibration damping. The floor, for example, typically consists of 1-3/4" of balsa sandwiched between layers of plywood decking. Refer to the drawing for a typical cross-section of the insulation and floor layers.

- Results -

The SOUNDOWN insulation barrier substantially reduces both airborne and vibration noise. The salon, staterooms, and pilothouse on the Selene Trawlers are all quiet enough to allow normal conversation even under way. This noise control allows crew to sleep, communicate and work more effectively, resulting in a safer and more enjoyable passage.

For more information on the Soundown Insulation system, see

Fuel Polishing

Ninety percent of all diesel engine failures are fuel related. Microorganisms, dirt, water and other contaminants can cause everything from annoying plugged filters to major engine damage. Passage-making trawlers typically carry large quantities of fuel, and often store that fuel for relatively long periods of time with little turnover, compounding the problems of fuel contamination.

Selene Trawlers are equipped with an ESI fuel polishing system as standard equipment. The system is designed to keep fuel clean and eliminate the sources of most fuel-related problems.

- Microorganisms in diesel fuel -

There are many types of microorganisms that thrive in diesel fuel, including various types of fungi. These fungi can grow into long strings, forming slimy mats or globules in tanks or other fuel system components. They use the diesel fuel itself as their primary food supply, and require only small amounts of water and other minerals to grow. As they grow, they produce water (which further supports new growth), sludge, as well as strong sulfurous acids, which are corrosive to metals in the fuel system and engine.

There are several telltale signs of microbial contamination:

  • Slimy buildup in filters, tanks, and other fuel system components
  • Rotten egg smell
  • Corrosion in fuel system components
  • Darker, more opaque fuel

These microorganisms and the byproducts they produce can cause a variety of problems. The diesel engine's injectors may not properly atomize contaminated fuel, which is required for proper combustion. Initially, this may only result in lower performance and efficiency. However, as contamination increases, so do the risks of serious damage to the injection pump and injectors, as well as other fuel system components.

Of course, the most critical concern is the potential of complete engine failure underway. Unfortunately, the conditions in which a loss of power due to fuel contamination may be most dangerous are also the conditions where it is most likely to occur. The heavy seas that usually accompany foul weather can stir up sludge and other contaminants from the bottom of fuel tanks and cause filters to plug or other failures at a time when a loss of power may be most dangerous.

- Eliminating the bugs -

Until recently, biocide dosing was the most common method used to control microorganisms in fuel. However, the biocides are composed of highly toxic chemicals that can be hazardous to both humans and the environment. In addition, adding biocides to the fuel system can actually cause problems. Once killed by the biocide, the dead cells from the organisms collect on the bottom of the tank and may actually increase the short-term likelihood of clogged filters or other fuel system components. Further, over time, biocides lose their effectiveness as microbes build immunities to the chemicals.

- The Selene Solution -

Selene Trawlers now come equipped with the ESI Clean Fuel system as standard equipment. The ESI system not only cleans and decontaminates fuel, but provides a convenient way to transfer fuel between tanks. It is installed as a stand-alone system, directly connected to the fuel tanks independent of the engine's fuel supply system. Fuel is circulated using the system's fuel transfer pump through the filter/water separator and the "De-Bug" fuel decontamination unit, and back into the tank. The system can even be setup to run automatically at pre-determined intervals, maintaining clean fuel even when the vessel sits idle for extended periods.

The De-Bug decontamination unit is based on technology that was developed over ten years ago in New Zealand. De-Bug is not a filter or a chemical treatment process. This patented device uses magnetic fields to destroy the fragile membrane of single-celled microbes, reducing them to sub-micron size debris that easily and completely passes through filters and pumps and is burned with the fuel. It has an effective kill rate of 97.6% kill rate for a single fuel pass.

The De-Bug unit requires no power, as it uses permanent ceramic-coated magnets, and has no moving parts. The only maintenance required is occasional cleaning.

- Results -

The ESI Clean Fuel system, installed as standard equipment on every new Selene Ocean Trawler, virtually eliminates fuel contamination problems. Clean fuel means higher reliability, better efficiency, cleaner combustion, and lower repair and maintenance costs. Since the same fuel supply is usually used to supply diesel generators, as well as heating systems, clean fuel also means better reliability and lower maintenance for these systems as well. Selene Trawler owners can go to sea with confidence, knowing that their fuel is free of damaging contaminants that are a primary cause of diesel engine failures. For more information on the ESI fuel polishing system, see

Bottom Paine

Osmotic blistering is the nightmare of very fiberglass boat owner. Modern materials, such as vinylester resins as well as careful process control during layup have substantially reduced the incidence of osmotic blistering. However, the high cost of repairing osmotic blisters justifies extra diligence in protecting the hull.

Selene Trawlers are built using vinylester resins in the outer four layers of the hull for resistance to osmotic blistering. In addition, Jet Tern Marine, the builder of Selene Yachts is only of the only yards in China to receive ISO 9001 certification, which demonstrates a commitment to world-class process control and quality standards.

As a further protective measure, all Selene Trawlers come from the factory with International Paint's "Gelshield" two-part solventless epoxy barrier coat. Five coats are applied. Finally, three coats of Micron Extra anti-fouling bottom paint are applied.

Hydrostatic Data

Jet Tern Marine publishes detailed specifications for all of the Selene Trawlers. Most of the published specifications are pretty obvious—providing key dimensions for the Trawler, such as LOA: Length Over All. There is also a wealth of information in the hydrostatic data, but the definitions of many of these terms is not as widely understood, let alone their significance to the performance of the vessel.

This article will define the key Hydrostatic specifications and provide a brief discussion of their significance. We can't possibly cover these specifications in any detail here, so considerably more detailed discussion is available in the references listed at the end of this article.

- D/L Ratio -

D/L Ratio is the ratio of the displacement of the vessel to its length. The formula for D/L ratio is:

Displacement is the weight of the water the boat displaces in pounds—which is essentially the same as the weight of the boat. In this formula, the division by 2240 is to convert the weight in pounds to "long tons". The length (L) in the formula is the waterline length of the boat in feet—the length of the boat measured along the water surface, which is typically considerably shorter than the overall length.

Heavier vessels of the same length have a higher D/L ratio. There is no single "right" D/L ratio, but a heavier vessel will typically be able to carry more fuel and provisions and will have a more comfortable motion at sea. Capt. Robert Beebe, in his classic "Voyaging Under Power" recommends a D/L ratio of at least 270 for a 50-foot ocean-capable vessel. D/L ratio may be somewhat smaller for larger vessels, since relatively small changes in waterline length have a large effect on the D/L ratio.

- A/B Ratio -

A/B ratio measures the ratio of the side-view area of the vessel above the water to the side-view area below the water. For stability and sea-worthiness, this ratio should be as small as possible, since more area above the water means more area on which wind and waves can exert pressure to capsize the vessel. In practice, it's very difficult to build a yacht with an A/B ratio below 2.0 with reasonably comfortable accommodations. Once again, Beebe recommends an A/B ratio of 2.6 or less for an ocean-going trawler.

- S/L Ratio -

S/L is the ratio of speed to waterline length, and is defined by the formula:

The S/L ratio is tightly tied to the concept of "hull speed". As a displacement hull moves through the water, it creates a wave. The faster the hull moves, the longer the wave gets until the length of the wave matches the waterline length of the vessel. At that point, the displacement hull has reached "hull speed." Thus, a longer hull has a higher hull speed, since it can go faster before the wave it creates matches the length of the boat.

Hull speed is reached at an S/L ratio of 1.34. For a boat with a 50-foot waterline length, the hull speed (S/L = 1.34) is about 9.5 knots. At S/L ratios below about 1.2 (about 8.5 knots for a 50 foot waterline length), the amount of power required to move the boat is fairly linear—in other words, doubling the power doubles the speed. However, as the boat approaches hull speed, the amount of power required to move the vessel faster rises dramatically.

The key to the long range of a passage-making trawler is operating at low speeds, well below the hull speed, where the engine can push the hull with maximum efficiency. For most trawlers, maximum range is achieved at S/L ratios under 1.0. For most specifications, even the published "cruising speed" is not the most efficient speed, where maximum range is achieved.

However, many voyages don't require the maximum range of the trawler. Careful hull design (including the Selene's "disappearing chines" discussed in another article) allows the vessel to operate at considerably higher S/L ratios—with a corresponding decrease in range, but a much higher speed for shorter voyages.

- Prismatic Coefficient (Cp) -

The prismatic coefficient compares the volume of the submerged part of the hull in cubic feet to what the volume would be if the largest cross-section of the hull was extended all the way to the ends of the boat. In other words, find the spot along the hull where the underwater cross section is largest, and multiply that cross-section area by the length of the hull. Then compare that volume to the actual submerged volume (See the illustration).

In simple terms, the prismatic coefficient defines how fine the ends of the boat are. Vessels with a narrow bow and narrow stern will have a small prismatic coefficient than a boat of the same length, but a broader stern are bow area.

Like D/L ratio, there is no one "right" Cp. However, tank testing has shown that there is an ideal prismatic coefficient that makes a hull most efficient at a given speed. The ideal Cp changes with speed (or S/L ratio), so the value of Cp suggests a sort of "sweet spot" for a particular hull where the hull is most efficient. The Selene 53's Cp is 0.67, which corresponds to an ideal S/L ratio of about 1.55, which corresponds to a speed of about 10.8 knots—just over the published "cruising speed." It is also better to err on the high side of the idea prismatic coefficient, since the performance impact of too low a Cp at high speeds are worse than the impact of too high a Cp at low speeds.

- Block Coefficient (Cb) -

The block coefficient is similar to the prismatic coefficient except that block coefficient measures the volume of a rectangle that encloses the largest dimensions of the submerged hull to the actual hull volume of the submerged hull. A block coefficient of 1 would suggest that the submerged hull is simply a rectangle with length equal to the waterline length, width equal to the beam at the waterline, and depth equal to the draft of the vessel. See the illustration.

Higher block coefficients suggest indicate a hull with more interior volume, but very full ends and a flat bottom—great for a cargo vessel, but probably not ideally suited for a sea-going yacht. Smaller block coefficients suggest a hull with more rounded bilges and/or finer bow and stern sections. Hulls with large block coefficients also tend to have poorer directional stability. In other words, they are harder to keep going on a straight course.

- Waterplane Coefficient (Cwp) -

The waterplane coefficient is the ratio of the "waterplane" of the hull to the rectangle that encloses the waterplane. The waterplane is essentially the shape of the hull where it touches the water—as if you drew the shape of the hull at the waterline on a flat piece of paper. Now enclose that shape in a rectangle that is the same length and width as the waterplane. Calculate the area of the waterplane and the area of the rectangle that encloses the waterplane. The ratio of the waterplane area to the rectangle area is the waterplane coefficient. See the illustration.

A Cwp of 1 is a hull that is a perfect rectangle at the waterline with blunt ends. Lower Cwp suggest a hull with finer "sharper" ends, which tends to be more easily driven, requiring less horsepower for the same speed. However, all things being equal, a lower Cwp also means lower stability.

- Metacentric Height (Gm) -

Metacentric height is a measure of the vertical distance between a yacht's center of gravity and its "metacenter". In simple terms, the center of gravity is the point along a vertical line drawn through the exact center of the hull where the hull would be perfectly vertically balanced. There is an equal amount of weight above and below the center of gravity. The metacenter is the point along this same vertical line around which the boat will heel for small angles of heel (less than about 5 degrees). This point could be thought of as the "hinge" point around which the boat tilts as it heels at small angles (at larger angles the change in shape of the submerged hull has a more significant effect). The metacenter is always above the center of gravity. Otherwise, the boat will capsize, since it's weight is centered above the "hinge" point.

The distance between these two points is called the Metacentric height, and is a measure of stability (resistance to heeling). A yacht with a large metacentric height will tend to be more "stiff"—it will be more resistant to rolling. However, it will also have a more violent motion and shorter period when it does roll because of the larger force (called "righting moment") that is exerted to right the boat. A smaller metacentric height means lower stability—less resistance to rolling, but a gentler rolling motion as well.

Metacentric height is also affected by loading—both the amount of weight and it's location, so both unloaded and fully loaded Gm values are sometimes provided.

- References -

For a more complete and detailed description of these and other hydrostatic terms and their effects on yacht design, see:

"Voyaging under power" by Captain Robert P. Beebe, International Marine, ISBN 0071580190" "Naval Architecture for Non-Naval Architects" by Author: Harry Benford, Society of Naval Architects, ISBN 0939773147" "All About Powerboats: Understanding Design and Performance" by: Roger Marshall, International Marine, ISBN 0071362045"

Hull Design Considerations

- Bulbous Bow -

Anyone who spends much time around large ships has noticed the bulb-shaped protrusions that are common on bows of large ships. To understand the purpose of the bulbous bow, we have to briefly review the characteristics of a displacement hull as it moves through the water.

Displacement hulls push water aside as they move. The water that is pushed aside forms a wave that begins near the bow. The length of this wave is proportional to the speed of the hull through the water—the faster the hull moves, the longer the bow wave. At some speed, the length of the bow wave increases to the point that it matches the length of the hull, and the hull operates in the trough of the wave, with a peak near the bow and another peak near the stern. This speed is called the "hull speed" and it is approximately 1.34 times the square root of the waterline length of the hull.

The purpose of bulbous bow is to change the nature of this bow wave to reduce the drag it induces on the hull The bulbous bow creates it's own wave that is farther forward and "out of phase" with the natural bow wave created by the hull, effectively subtracting from the normal bow wave and reducing it's drag-inducing effect.

In this illustration, the green line represents the natural bow wave of the hull without the protruding bulb. The blue line represents the wave created by the protruding bulb. The red line is the sum of these two. Notice that the height of the bow wave is substantially reduced, which in turn, reduces the hull drag associated with the bow wave. This improves fuel economy and increases range.

The Selene 62 offers an optional bulbous bow with an optimized bulb shape, in place of the simple cylindrical bulbs commonly used on trawlers. Instead of a simple cylinder shape, the bulb is an inverted teardrop shape helps damp pitching motion and reduces slamming at the bow rises and falls in larger waves.

- Disappearing Chines -

Modern trawler yachts are used in two different scenarios with conflicting requirements. The first is the classic passage-maker scenario, where a yacht makes a long passage in the open ocean. This scenario typically emphasizes range over speed, and the trawler will likely be operated at speed to length ratios of 1.0 or less for maximum range. At these speeds, the hull operates as a true displacement hull.

However, many trawler yachts are more regularly used for shorter distance passages where range and fuel economy may be sacrificed for additional speed. Getting to and from a favorite cruising spot may motivate the captain to operate at or even well above hull speed.

The Selene Trawler line incorporates a "disappearing chine" concept designed to blend the favorable fuel economy and range of a full displacement hull with the high speed capabilities of a semi-displacement or planing hull. The Selene hull has very typical round bilges in the forward half of the hull, while the aft part of the hull shows more pronounced chines and a flatter section.

Where a full displacement hull will tend to "squat" in the stern as it approaches hull speed because of the trough from the bow wave, the flatter aft section of the Selene hull provides more lift and allows the hull to begin to plane, enabling higher speeds.

The Selene design also incorporates slightly concave or "hollow" bottom near the stern. This hollow bottom tends to increase the water pressure under the hollow, which is precisely where the propeller is. The higher pressure discourages propeller cavitation, which increases efficiency. See the illustration.

The most common drawback for a hull with flat sections aft is it's tendency to yaw in following or quartering seas. The Selene hull design counters this tendency with a deep full ballasted keel and a large rudder for added directional stability.