There are quite a few factors to consider when it comes to marine engine cooling systems and which types of pumps and systems are most appropriate for your application. In this guide we will review:
- The advantages of flexible impeller pumps (FIPs).
- The cooling systems most used for marine propulsion engines.
- Safety margins.
- Guidance on suction systems.
- Pump types and what to consider.
- The different pump drives available.
IMPORTANT: The data given for engine cooling systems and pump selection is for guidance only. It does not cover high-performance engines. You should always consult the engine marinizer.
It is important that Jabsco engine cooling pumps and systems are matched during the design of ships' engine installations. Once installed, safety margins must be safeguarded, i.e., the emphasis must be on preventive, not corrective maintenance.
The Advantages of Flexible Impeller Pumps
Flexible impeller pumps are versatile. They combine the priming feature of positive displacement type pumps with the general transfer ability of centrifugal pumps. Flexible impeller pumps will pump either thin or viscous liquids and can handle more solids in suspension than other types of rotary pumps. They operate at low or high speeds, can be mounted at any angle, and pumps in either direction with equal efficiency. Additionally, flexible impeller pumps are:
- Self-priming. FIPs pump instantly with dry suction, lifts up to 10 ft. (3m) — up to 25 ft. (8m) when wetted. This permits cleaner, safer installations, and no foot valve is required.
- Simple. FIPs have one moving part — a tough, long-lived, wear-resistant flexible impeller, lubricated by the liquid being pumped. No metal-to-metal pumping action means no gears to jam, clog or become noisy.
- Capable of handling more capacity. FIPs, in general, require less space because they deliver greater flow for weight, size and price than other types of pumps.
Pressure Ranges
Standard impellers are suitable for continuous operation up to the following limits:
- 1/4", 3/8" and 1/2" port sizes to 40' (12m) head, 17.3 psi (1.2 bar)
- 1" to 2" port sizes to 70' (20m) head, 30.3 psi (2 bar)
Special high-pressure impellers are available for certain models. Impeller life will be extended by operating in the lower portion of recommended pressure range.
Temperature Range
- The temperature range is: 45° to 180°F (7° to 80°C). For fluids less than 45°F (7°C) consult with the factory
Operating Speeds
Below are the operating speeds for ball bearing pumps by size:
- 1/4" or 1/2" ports: 3600 rpm max
- 3/4" or 1" ports: 3000 rpm max
- 1-1/4" ports: 2200 rpm max
- 2" ports: 2200 rpm max
Cooling Systems Most Used for Marine Propulsion Engines
Heat Exchanger Cooling
This is also known as a closed system, i.e., the primary cooling circuit is isolated from the surrounding atmosphere.
A centrifugal pump circulates fresh, treated water through the cylinder block passages and around the tube stack of a heat exchanger. The Jabsco raw water pump draws raw water (from sea or lake) through the ship’s hull inlet and pumps it through the heat exchanger tubes where it removes the heat transmitted from the primary circuit before discharging overboard.
The heat exchanger, which can be single-pass or two-pass type, should be capable of handling approximately 10% more than the maximum engine heat rejection rate and may be fitted separately or as an integral part of the expansion/header tank. This tank allows venting of air or combustion gasses absorbed by the cooling water during engine operation and provides a positive pressure on the freshwater pump inlet. This pump, fitted in the coldest part of the primary circuit, has a capacity which will maintain a water temperature differential of approximately 45°F (8°C) across the cylinder block at full load. The engine operating temperature is regulated by a marine thermostat to about 185°F (85°C) on most engines. To provide an adequate safety margin for commercial diesel engines, which may be expected to operate more than 2500-3000 hours per year, size and rpm of the Jabsco raw water pump should be selected to give a flow capacity of approximately 15 GPM (57 LPM) for every 100hp maximum engine load and rpm.
If an exhaust manifold is fitted in the raw water or freshwater circuit, the raw water pump flow capacity should be 10-15% higher.
Additional coolers, such as oil coolers or charge air coolers, must be fitted after the pump, but in a compromise between reliability, size and weight, and pump delivery. Head should be about 10-13 psi of water at maximum rpm.
Safety margins in pump inlet systems are extremely critical, so any restrictions or bends, other than seacock and inlet strainers, are to be avoided at all costs.
Raw water from the exhaust manifold is often injected into the exhaust pipe (after an exhaust elbow to prevent raw water flowback into the cylinders), which may then be ducted through areas where a hot pipe would create a hazard. This is known as a “wet exhaust” system. In addition, mixing of water with exhaust gases will reduce exhaust noise. However, on small engines the raw water flow should not be less than 2.5- 4 GPM (9.5-15 LPM) for adequate silencing.
Keel Cooling
In principle, this is like a heat exchanger cooled system, but the raw water circuit and heat exchanger have been replaced by pipes attached externally to the vessel’s keel.
Pipe bore and surface area must be adequate for effective dissipation of heat from the primary circuit to sea or river water. In some installations where the flow capacity of the centrifugal circulating pump is insufficient due to system pressure losses through keel pipes, cylinder block and exhaust manifold, a Jabsco pump may be used (flow capacity approximately 30 GPM [113 LPM] for every 100hp at maximum engine load and rpm).
Alternatively, the ship’s steel hull may be used as the cooling surface and the heat transmitted directly to the surrounding sea or river water from a tank welded inside to the bottom.
The “dry exhaust” can be made a “wet exhaust” by a separate raw water pump.
The expansion/header tank may be fitted separately. In which case, it must be connected directly to the circulating pump inlet and the primary circuit adequately vented into the tank.
Direct Cooling
Raw water is pumped directly through the engine block. However, as the temperature of the water entering the block is ambient, the outlet temperature needs to be much lower than in an indirect cooled engine to reduce the formation of scale and salt deposits and thermal stresses in the cylinder block.
Lower operating temperature means that engine performance will be considerably less efficient, and direct cooling systems should, therefore, not be used on commercial craft engines.
For weekend pleasure craft engines which operate no more than 50-100 hours per year, reliability margins will probably be maintained for several years until deposits in the engine cooling passages begin to affect heat transfer to the cooling water.
Engine operating temperature is best controlled by a marine thermostat rather than a manually operated valve.
For cold engine start, the thermostat will be closed and most of the cooling water (about 10 GPM [38 LPM] for every 100hp at full load and speed) will bypass the engine via a spring-loaded back pressure valve and discharge into the exhaust manifold. A small bleed hole in the thermostat will ensure a slow circulation of cooling water through the cylinder block to prevent “hot spots” while the engine is warming.
Safety Margins
The raw water flow capacities indicated earlier are for diesel engine cooling systems and include a safety margin of some 30% to ensure adequate engine cooling under adverse operating conditions.
As gasoline engines, unlike diesel engines, have a high heat rejection rate at idling speed, an increase in pump flow capacity of some 10% at maximum rpm is recommended.
While flow capacities can be affected by salt deposits or scale in pipes due to operation in tropical regions, seawater pollution or age and wear of pumps and systems in general, this is usually a gradual process with ample warning that preventive maintenance can counteract.
Far more serious is a reduction in cooling water flow due to adverse conditions caused by an inadequate system.
When marine vegetation such as seaweed gets stuck in seawater inlet strainers of insufficient mesh or hole size, this can cause a significant reduction of the cooling water supply to the pump.
On the other hand, no amount of pump flow capacity will safeguard against a sudden but total blockage — for example, a plastic bag covering up the hull inlet. System safety margin can be substantially increased by providing two separate hull inlets usually on either side of the keel, joining the pump inlet pipe below the water line. In actual practice, the likelihood of both inlets being covered simultaneously has been found to be insignificant.
As heat exchanger and cooler manufacturers’ recommendations for a given engine performance and corresponding pump flow will include appropriate safety margins for minimum and maximum flow, it should be kept in mind that excessive water velocities through the heat exchanger pipes due to “overkill,” i.e., fitting oversize cooling pumps, may cause pipe erosion and accelerated pump wear through excessive pump operating pressures.
Low water velocities through heat exchanger tubes, on the other hand, could result in the formation of sludge. Therefore, too small a pump (insufficient safety margins) may cause inefficient operation of heat exchangers and coolers for much of the time.
Suction System
To provide reliable performance under seagoing conditions, a raw water-cooling pump must operate in a correctly designed cooling system. It is, therefore, essential that cooling water should reach the pump without having to overcome undesirable resistance or restrictions.
The following general rules are given for guidance:
- Suction pipe bore must not be smaller than the pump inlet connection, but if total suction pipe exceeds 10 ft. (3m) in length, the bore should be one size larger, particularly if the pump is operated at high speeds. Use the same size bore pipe throughout, i.e., avoid sudden enlargements or contractions. Use long tapered sections of pipe at any change in pipe bore.
- The suction pipe run should be as straight as possible, i.e., avoid unnecessary bends. Do not use square or standard elbows, instead use long sweep bends.
- Do not fit gearbox or engine oil coolers in the pump suction system. Always install after the pump.
- Seacocks of the same nominal size as the suction pipe work should be of the ball or plug type, giving full through bore in the open position. The handle position should clearly indicate whether seacocks are open or closed.
- Seawater inlet strainers should have a hole or mesh size of at least 1/8" (3mm) diameter and up to 3/16" (5mm) for larger pumps, but smaller than the heat exchanger tube bore.
- Check frequently that inlet strainer is not clogged. If in doubt clean thoroughly.
- Fast boats (over 12-15 knots) and planing craft must be fitted with inlet scoops positioned in a permanently wetted area of the ship’s hull to create sufficient inlet pressure at higher boat speeds. Flush inlet fittings are not suitable for fast boats.
Pump Selection
Most proprietary marine engines are already fitted with specially adapted flange mounted Jabsco cooling pumps driven by gears or couplings of various descriptions.
A belt drive from the crankshaft pulley will enable almost any standard foot mounted bronze Jabsco pump to be selected, provided that the required cooling flow rate is obtained without operating the pump at excessive speeds.
Global cooling water requirement for each 100hp at maximum engine load and rpm is at right.
Diesel Engines
Heat exchanger cooling
18 GPM (68 LPM) — raw water
Keel cooling
36 GPM (136 LPM) — fresh water
Direct cooling
12 GPM (45 LPM) — raw water
Gasoline Engines
Increase diesel engine figures by about 10%.
If exhaust manifolds are water cooled, increase pump capacity by a further 10-15%.
Pump Types
Selection may depend upon pump speed, space available and type of drive.
Ball Bearing Pumps
For pump speeds in excess of 2500 rpm, ball bearing pumps should be used. Depending on size and duty, speeds of up to 5000 rpm are possible, but special attention to suction system conditions is then required (consult Jabsco).
Ball bearing pumps are available in heavy-duty and compact types.
Heavy-Duty Pumps
This range is designed with two spaced bearings and mechanical rotary face seals to meet high reliability criteria under adverse operating conditions, i.e., excessive belt tension, high operating pressure, abrasive conditions found in shallow waters or in-shore operation. Pump bodies are either bolted to a bearing housing or body and bearing housing are a single, integral casting. These types of pumps are often unique to the application, but some varying pump types can share common bearing housings.
Compact Pumps
This pump range is fitted with double row ball bearings, providing a low cost compromise between overall pump length and a limited heavy-duty capability.
Bearings are shielded, ensuring permanent lubrication by the special grease.
Pump Drive Failure
Most pump failures are caused by badly designed pump systems. Secondary causes of pump problems are unsatisfactory drive arrangements.
Pump Drives
Direct Drives
Misalignment between pump and drive shaft is a common cause of pump failure.
A flexible coupling is only “flexible” within limitations specified by the manufacturer. Excessive misalignment, particularly at high pump speeds, can cause noise, knocking, vibration, premature bearing failure, usually followed by pump leakage.
Belt Drives
The pump speed is determined by the engine pulley diameter and the need to maintain the pump pulley at a practical maximum size. Some engine crankshaft pulleys incorporate vibration dampening membranes resulting in large diameters at the V-groove. To lower pump speed relative to engine speed, it is necessary to use a pulley on the pump that is larger than the drive pulley. High engine speeds may, therefore, require oversize pump pulleys which preclude the practicality of a belt drive. Calculating a suitable size pump pulley (example):
Excessive drive belt tension will cause rapid belt wear and may result in premature bearing failure. It should be possible to deflect a correctly tensioned belt between pulleys approximately 1/2"–3/4" (13–19mm) by applying finger pressure.
While excessive belt tension may be due to inadvertent overtightening, it should always be verified that it did not result from an inherent fault in the installation, e.g., if pump flow is insufficient due to a slipping belt caused by insufficient contact area between belt and pulley, any amount of tightening would not solve the problem permanently. Ideally, the contact area should be about 120° but not less than 90°.
If the engine is installed on flexible mountings, a belt driven pump must be mounted on the engine, never bolted to the vessel’s structure. This is to avoid tensioning or slackening of the drive belt due to relative movements of the engine.
Gear Drives
Drive gears fitted to “heavy-duty” flange mounted pumps are supported by the pump ball bearings and driven by one of the engine gears.
Crank Pulley Mounted Drive
A number of Jabsco pumps specially developed for high-speed operation on gasoline and diesel engines are fitted with a multi-positional bearing housing bolted directly to the crankshaft pulley, thus eliminating mounting brackets, belts and pulleys.
As the pump body is supported by the ball bearing only, it must be prevented from rotating by means of a torque arm, designed and installed so as to avoid any side load on the bearing. Inlet and outlet hoses must be flexible, i.e., of adequate length (at least 10 times the hose diameter) so that the pump can be rotated freely over a few degrees by finger pressure only.
