Battery Charging, Maintenance & Safety

General Information

A common complaint from boat owners is that batteries seem to always be going flat and they never seem to have enough electrical power. There are usually two factors that contribute to this problem.

  • A poorly designed electrical system (especially in older boats)
  • A lack of understanding of the electrical system and of how to correctly use and maintain the batteries.

An electrical system that has not been designed by an experienced marine electrician, or electrical engineer, will often suffer from the first problem above.

Most boat owners do not understand how to correctly use and maintain their boat's batteries, or understand the electrical characteristics of those batteries. This results in battery life being significantly shortened (often by as much as 50 - 60%) and a frustrating shortage of power when it is needed. The latter problem is usually more noticeable on longer trips because a daily charging deficit gradually eats away at the level of charge of the batteries. This often causes problems during a summer holiday cruise or other extended trip. Giving your batteries the full charge they need during recharging is often an inconveniently lengthy exercise for yacht owners or in a situation where the engine doesn't run for many hours per day (such as sheltering in a bay waiting for bad weather to pass). A well designed, efficient, charging system will reduce the daily recharge time, but boat owners still need to understand how long it takes to fully charge their batteries.

A basic guide to battery charge for 12 volt flooded cell lead acid batteries (such as many truck and marine deep cycle batteries) is:

  • full charge: 12.6 volts
  • 3/4 charge: 12.4 volts
  • 1/2 charge: 12.2 volts
  • 1/4 charge: 12.0 volts
  • flat battery: 11.8 volts
Double these voltages for a 24 volt system.
Level of charge cannot be read during charging, it must be done at least 1 to 2 hours after the charging process has stopped.

Batteries should never be allowed to be discharged below half charge. To do so will significantly reduce their life. This is a guide for deep cycle batteries. Commercial (truck) or automotive batteries should not be allowed to discharge even that far.

Contact us for more accurate information related to particular battery types.

Many perfectly good batteries have been discarded by boat owners who are reluctant to pay an expert to test their electrical system and tell them what they think they already know. They assume that they have bad batteries and purchase replacements. Very often the new batteries don't solve the problem and the boat owner then has to pay the expert who should have been employed in the first place.

Battery charging information

(For flooded lead acid batteries)

Battery Temperature !

Temperature compensated battery charging is important for battery longevity, particularly for sealed batteries.

Understanding the Physics

An interesting thing happens in the physical world. Many things double with each 10C temperature rise. For example, If a particular mechanism has a determined failure rate at 25C, then the failure rate will more than likely double at 35C. A battery that forever accepts 5 Amps of charge current at 14.4 Volts and 10C, will maintain 10 Amps at the same voltage if the temperature is raised to 20C.

Power (watts) is equal to Volts times Amps (believe me it is). For 14.4 Volts and 5 Amps, power is equal to 72 Watts. If we double the current to 10 Amps as the result of the temperature increase suggested above, the power will be equal to 144 Watts. To help you understand this, have you ever noticed the difference in the heat generated by a 75 Watt light bulb versus one of 150 Watts? Or a bar heater using one bar compared with two?

Didn't understand that? What it says is that a small temperature change can have a huge effect on the power going into a battery and that power increase can in turn have a big effect on battery temperature. In short, it snowballs.

Thermal Run Away

During the 'bulk' battery charging step, the battery can accept most of the alternator current and convert it back to available capacity. Once the battery nears full charge, excess charge current becomes heat. Small at first, the heat begins to accumulate in the mass of the lead plates. As the heat accumulates, temperature of the battery begins to rise. As a result of this, current through the battery begins to double for every 10C. That means more power is dissipated in the battery which means more heat is generated, which means more current flows, which means more heat, and so it keeps going. Thermal run away !!.

How Common Are High Battery Temperature Problems?

We have seen batteries that have become so hot that their cases have melted and two battery cases have welded together. Battery explosions are a very real possibility. We have seen this happen multiple times. Motor vehicles aren't so bad because they often have a good air flow around a battery, which helps to cool it and prevent the accumulation of gas. Batteries in a closed space run a much greater risk of thermal runaway. Batteries in hot areas have an even greater chance of thermal runaway. Another thing that happens is that batteries emit explosive gas while being charged. An accumulation of gas will help to make the bang even bigger !

When alternators are small compared to the battery banks being charged, and automotive alternator voltages (such as fitted to many boats) are typically low because they are designed for starting batteries, thermal run away is not much of a threat. It only becomes a threat when the size of the alternator is larger, and regulator voltages are higher to achieve a full charge. If we don't have these high charge rates the battery will fail due to under charging. High battery temperature may explain why you add water to your relatively new batteries on a frequent basis, or why gel batteries don't give the performance they should. It is important not to let sealed batteries, such as gel, get hot because it is not possible to replace evaporated electrolyte.

Thermal Run Away Prevention

Prevention of thermal run away during charging is easy. As the battery begins to heat, reduce its terminal voltage. This defeats the doubling effect of charge current. As the voltage goes down, so will the charge current. That means less heat buildup. It also means longer battery life and less electrolyte loss. Housing batteries in a cool environment is also very important. A hot engine room is one of the worst places a battery could be situated in a boat.

Smart Regulator with Battery Temperature Sensing

While this solution is slightly more costly due to its battery temperature sensor, it works well. The temperature sensor attaches to a battery post. As the battery temperature rises, the smart regulator decreases the charge voltage. Regardless of the reason for battery temperature rise, the result is a properly controlled, safe and optimum charge.

While any standard alternator regulator can charge batteries, only a temperature compensated multi-stage smart regulator can provide a fast, full charge and also assist to achieve maximum battery life.

Battery Arrangements

Some older boats have a single battery switch that selects between two batteries, all electrical equipment is supplied from the selected battery, and the engine can be started from either battery.

This is not a recommended arrangement for a number of reasons.

The ideal battery type for an engine starting battery differs from the ideal type for a house battery. There are some dual purpose 'marine' batteries available for smaller boats but for most boats the above applies.

High current inductive loads, such as windlass or winch motors and starter motors, can send a voltage spike through the electrical system they are connected to. This voltage spike can damage sensitive electronic equipment. Therefore, it is recommended that electronic equipment is not connected to the same system as these high current loads. The anticipated question at this point is "what about electronic engine controls that are supplied from the starting battery?" The answer is, they are correctly designed for the purpose.

The recommended arrangement for batteries is to have a dedicated battery of the correct type for the 'House' electrical system and separate battery of the correct type for engine starting. Some vessels also dedicate batteries to other applications such as winches, genset starting, thrusters, etc.

When the vessel has a number of separate electrical systems supplied from dedicated batteries, a specially designed charging system is required to individually charge the separate battery banks form one or two engine alternators. Bigger vessels will have more advanced electrical systems but the average vessel will only have two engines to supply charge to all batteries while under way. A similar charging arrangement needs to be available for charging multiple batteries from shore power.

Safety around lead acid batteries

There are a number of safety considerations related to batteries. Probably the most widely known hazard is acid. Battery electrolyte is sulfuric acid, which can cause acid burns on skin and severe chemical burning to the eyes. Be extremely careful to avoid contact with battery electrolyte. We recommend wearing safety goggles and protective clothing when working on a battery

Sulfuric acid is also corrosive to many materials including, steel, wood, fabric, etc. Ensure that battery electrolyte is not allowed anywhere except inside the battery.

Sulfuric acid can be neutralized with baking soda (bicarbonate of soda). A mixture of approximately one tablespoon of baking soda to one litre of water will neutralize any battery electrolyte which has escaped from the battery. Keep some of this solution handy to the battery. If acid comes into contact with skin or eyes, wash immediately with this solution, then wash well in fresh water and immediately seek medical attention.

A baking soda solution can also be used to remove corrosion around battery posts and within the battery box or tray. DO NOT ALLOW THIS SOLUTION TO ENTER THE BATTERY CELLS.... IT WILL KILL THE BATTERY

The next very real danger is explosion. Ideally batteries should be installed in an acid resistant box with a secure lid and good ventilation. Battery terminals, if exposed, should be insulated to prevent accidental short circuits. When lead acid batteries are active, especially during charging, they emit explosive gas. Ensure that flames and sparks are kept well away from batteries at all times, and that adequate ventilation is provided to remove gases. Ensure that your charging system is operating correctly. Overcharging a battery will generate excessive gas.

Battery terminal insulation is important if conductive objects might be able to contact the battery terminals. Battery terminal caps prevent accidental short circuit of battery terminals. If the battery is not in a secure box with a secure lid, the terminals should be insulated.

A mistake often made is to connect or disconnect a portable battery charger while the charger is live (has mains power connected to it). When the clips of a battery charger are fitted to, or removed from, the battery a spark occurs. Many battery explosions have occurred in this way. NEVER connect or disconnect battery charger leads while the mains power is connected to the charger.


Did you know that the most important part of the charge going into your batteries is the last ten percent?

Did you know that with a standard automotive alternator it can take longer to put the last ten percent of charge into your lead acid batteries than it takes to put in the first ninety percent?

Did you know that, while your batteries are on charge, a voltmeter reading as high as 14 volts (for a 12 volt system) does not mean that your batteries are fully charged?

Did you know that many boats, especially American built boats don't have an adequate voltmeter with a scale that will allow even a knowledgeable boat owner to obtain accurate enough information to correctlty maintain their boat's batteries?

Did you know that having batteries in a hot environment such as an engine room (where they are often located in launches) is detrimental and possibly dangerous?

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Fuses and circuit protection

What does a fuse do?

A fuse wire glows red hot then ruptures when excessive current flows through it. The heavier the wire, the greater the current that is needed to rupture it.

If a fuse wire had combustible plastic around it, it would catch fire before rupturing.

If a length of wire in your boat is not protected by a fuse, the wire will act as the fuse and will heat and rupture if excess current flows through it. If the wire's insulation isn't high grade incombustible insulation, it will catch fire.

If the wire is protected by a fuse or circuit breaker that is rated too high, the wire will be the lowest rated component and will act as the fuse. It will heat, and catch fire. If the wire has high grade insulation, it won't catch fire itself, but any combustible material near it, such as plastic conduit, ties, or other wire with low grade combustible insulation, will catch fire. Some automotive wire has combustible insulation.

All wire has a current rating based on it's cross sectional area and length. Calculation of the rating of over-current protection (fuse, circuit breaker, etc.) is based on this.

Every length of wire in your boat should be protected by a correctly rated fuse or circuit breaker. Two common problems are wires with no protection (often some wires are protected, but others are not), and wires that are fed via an incorrectly rated fuse or circuit breaker, allowing them to overheat as though they had no protection at all.

Although fuses and circuit breakers are interchangeable in many applications, there are some that require a particular type of fuse or circuit breaker, in order to provide the correct level of protection. The choice of over-current protection for sensitive equipment should be made by an expert. For example; there is a difference between thermal and magnetic circuit breakers. Each type of device is designed to work best in a particular application. The wrong device could either, give trouble with nuisance tripping, or not trip quickly enough to give the required protection. This applies to fuses as well as circuit breakers. A point worth noting is that manufacturers warranties usually cover faulty parts and workmanship. If a device fails because it has not been given the correct level of overload protection, the manufacturer is under no obligation to repair or replace it.

We see many electrical distribution boards installed and wired by inexperienced people. These boards have circuit breakers or fuses that are rated too high for the wire they are feeding, effectively providing no over-current protection at all. All that money spent on an expensive distribution board that will allow a situation to develop in which the vessel could burn to the waterline if an electrical fault occurred. When this is done by an amateur, it is usually done in an attempt to save money. The attitude is "It must OK because it works". These boat owners will find that, if such a disaster occurs, the money they saved will be lost many times over because most insurers would refuse their claim.

Where should a fuse not be used?

If a fuse or circuit breaker, even a correctly rated one, is not an 'ignition protected' type, it must not be used in a location where potentially combustible gases might be present. Examples of such locations are: Near batteries, near LPG fumes, in an engine compartment or other area where petrol fumes might be present, etc.

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Shore Power Rules and Regulations

Why you need a warrant of electrical fitness

Any vessel that is to be connected to a low voltage (230 or 400 volt) AC supply from a shore mounted power source, must have a current warrant of electrical fitness BEFORE being connected to the electricity supply. The NZ electricity act allows for penalties of tens of thousands of dollars for anyone connecting a vessel to an electricity supply when that vessel does not have a current WOEF. If you plug your boat into a shore power supply when it doesn't have a current WOEF, you risk being prosecuted for doing so.

Those who claim that their vessel is safe and doesn't need to be inspected fail to understand the purpose of the regulations. First, only a qualified registered electrician or electrical inspector is competent to determine whether an installation is safe. Second, if the installation is safe, there should be no problem getting it certified so that electricity suppliers can see that it is safe for them to supply electricity to the vessel. How are they to know which installation is safe and which isn't without inspection and certification? It is irresponsible, and illegal for anyone to supply electricity to an electrical installation that might be unsafe. The only way that an electricity supplier can know that an installation is safe to be supplied with electricity is to see a WOEF.

Almost any arrangement of wiring is harmless before electricity is connected to it. It is connecting an electricity supply to it that can make it unsafe. Therefore, any person who supplies electricity to a potentially unsafe installation is the person who eventually makes it unsafe, and is the person liable to be prosecuted for doing so. Understandably, the marina operators and other suppliers of electricity to boats need to be sure that they are supplying electricity to a safe installation. They can face huge fines for allowing electricity to be supplied to an installation that does not have a current WOEF.

Who can carry out installation of low voltage electrical systems?

Only a registered electrician, or other suitably licensed electrical worker (inspector, etc.) may carry out any part of a low voltage electrical installation. This is 'prescribed electrical work' (PEW). If you are not a registerd electrician with a current practicing license you may not carry out such work on any boat. Even if you live on the boat. An unregistered person may not carry out prescribed electrical work with the expectation that they can get someone else to certify it. If you are not a registered electrician with a current practicing license, you are not allowed to do much more than replace a fuse or change a plug on an appliance or extension lead. Many a dangerous situation has arisen out of a lack of knowledge, or lack of understanding of the legal requirements.

In summary, if you have any electrical work to be done on a system that operates above 50 volts AC or 120 volts ripple-free DC, it must be done in it's entirety by a registered electrician with a current practicing licence. Many marine electricians are NOT registered and not permitted to carry out this work. Before allowing anyone to work on an electrical sytem that operates at a voltage above 50 volts AC (such as a shore power system), you should ask to see their CURRENT electricians practicing license (check the name and expiry date). They must be a registered electrician or working under an employer, or provisional, license under the Electricity Act and be working under the supervision of a registered electrician. They cannot work unsupervised if they are anything other than a fully registered electrician.

If an electrical installation is carried out illegally, you can expect your insurer to refuse any claim you make as a result of any failure of that system. Fire and electric shock are very real possibilities.

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Electrical Safety

Electrical Safety Advisories

The government department responsible for safety of electrical installations and equipment (this changes from time to time, and at the time of writing is the Ministry of Economic Development) issues safety advisories when necessary. These are issued if there has been a serious, or significant number of, safety related issues with a particular type of electrical equipment.

Below is a list of advisories that the boating community need to be aware of. Each list item links to the relevant article.

Shore power lead plugs - 10 amp PDL 56P310 & 15 amp PDL 56P315
PDL-56P310 plug
Water damaged PDL-56P310 plug
Burn damage caused by water entering the plug

These plugs are susceptible to breakage if subjected to side forces, such as those used to 'wiggle' the plug to remove it. To avoid breakage of these plugs they should be removed by a direct pull only. Once broken, they cannot be successfully repaired and must be replaced.

The sealing gasket (rubber ring) inside these plugs sometimes falls out. This ring is necessary to maintain the waterproof rating of the plug. If it is not in place, water damage could occur to the plug and to the socket it is connected to. To ensure that this ring is not lost, we recomment fixing it in place with a couple of drops of contact adhesive (such as Ados F2).

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Corrosion control

One word that we hear repeatedly misused is 'Electrolysis'. Electrolysis is a chemical and/or electromechanical change in a solution, occurring when an electric current is passed through it. This word 'electrolysis' is often used incorrectly as a general description of the corrosion of submersed metals in seawater. There are two types of electric current related metal corrosion that occur in seawater, and to some extent in fresh water. One is 'Galvanic corrosion'. The other is 'Stray current corrosion'.

Galvanic corrosion

Galvanic corrosion is the reaction that occurs between dissimilar metals in an electrolyte, or in impure water. This is similar to the way a flooded cell lead acid battery works. In the battery, plates made of different metals are immersed in an electrolyte (sulfuric acid). The result is that a voltage is generated between the plates. In a given electrolyte, the actual voltage depends on the type of metals used for the plates. If one metal is only slightly more noble than the other, the voltage will be lower than it would be if one metal was a lot more noble than the other. For example, zinc and mild steel are not very far apart on the scale, but gold and magnesium are at opposite ends of the scale. Therefore, if zinc and mild steel were used for the plates in our battery the voltage would be very much lower than it would be if gold and magnesium were used. So we now know that different metals will have a different voltage between them. The next thing we need to understand is that as the voltage increases, so does the rate of corrosion. Also, it is the less noble metal that will corrode. Boats have a variety of different metals below the water line, that are in contact with seawater and might also come into contact with bilge water. Voltage differences will occur between these different metals which will result in corrosion of the less noble metals. There are a couple of simple steps we can take to prevent this corrosion. The first is to connect all the metals together using a copper wire so that the difference in voltage between them is minimized. The second is to introduce another less noble metal into that circuit so that the less noble metal will be sacrificed instead of the metals we want to protect. Typically, zinc is used for this purpose. The piece of zinc used is referred to as a 'sacrificial anode'. There are formulas for the preliminary calculation of the amount of zinc required. The amount of zinc used determines the voltage difference between the protected metals and other metals (or our test probe). This voltage must be within the correct range for the anode to be effective. It is not only possible to have too little zinc, but also possible to have too much zinc. Too much zinc will also cause corrosion. More detailed information is beyond the scope of this article but the information we have provided should help you to understand the principals involved and perhaps recognize a problem or potential problem.

Stray current corrosion

We have looked at the voltage differences between dissimilar metals causing a destructive current flow between those metals. Sometimes a destructive current flow can occur even though we have bonded the metals together and fitted a 'sacrificial anode'. This current flow can be caused by a problem with the boats electrical system. If the electrical system is not arranged correctly or has developed a fault, it is possible for current to flow through the water from one submerged metal component to another submerged component. This current flow can cause the corrosion of metals in a similar way to galvanic corrosion. This is 'stray current corrosion'.

Testing for corrosion or potential corrosion

Testing for corrosion is a specialized job that requires special equipment. This work should be carried out by an experienced person. Every new boat should be tested in this way to verify that the correct level of corrosion protection is provided. If you change the submerged metals or rearrange the 'sacrificial anodes' on your boat, you should have these tests carried out to ensure that the level of protection is still correct.

Impressed current

Impressed current is a means of controlling the voltages between metals by introducing an electrical circuit with a controllable voltage that can be adjusted to maintain the protection voltage within the 'safe' range. It requires specialist installation and will fail if the battery runs flat. It will not be discussed any further in this article.

Galvanic isolator

We have heard people offer the opinion that having shore power connected to a boat can cause corrosion problems "because 110 or 230 volts AC will do that". While it is true that a shore power connection can introduce a corrosion problem, it is not for the reason that most people seem to think. A big contributor to corrosion problems is the shore power's earth wire. For safety reasons the earth wire is connected to various metal parts of your boat. Some of those could be in contact with the water. The same applies to all the other boats that are also connected to shore power. The earth wire is a common connection that connects all these boats together, even when the main switch is off. This common connection between boats allows the metals on those boats to react with each other, causing galvanic corrosion. One solution to this problem, and the least costly for an existing installation, is to install a device called a galvanic isolator. A galvanic isolator is installed in the shore power earth wire to protect the boat from galvanic corrosion without inhibiting the function of the earth wire. A galvanic isolator must only be installed by a registered marine electrician. Never try to install this device yourself.

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Helping to find faults

Fault finding is like detective work. We need clues. Some people seem to think that if they employ an expert. "They will know what to do to fix it". This is not necessarily true. You would not expect a police detective to solve a crime without asking a lot of questions and receiving a lot of information. Your electrical or mechanical detective is no different.

The more information you can provide, the easier it will be to identify the fault. With enough information, an expert in their field will often be able to correctly identify the fault without carrying out any tests at all. Confirming the fault requires tests, but often the initial diagnosis can be made quite accurately from information alone.

The information we need to consider is 'History' and 'Symptoms'.

Useful 'History' information includes:
  • When did the problem start?
  • Was anything added, removed or changed immediately before this problem started?
  • What events took place immediately or shortly before the problem started?
  • What equipment was operating when you first noticed the problem?
  • Has anybody who would not normally operate the boat, or operate the equipment, been operating it before the problem started?
  • If a person unfamiliar with the equipment, has operated equipment, what might they have done differently?
  • Any other history information that you think might be useful.
Useful 'symptoms' information includes:
  • A description of what the fault looks like to you.
  • Anything else that seems to occur at the same time.
  • The circumstances and conditions under which the fault can be seen to occur.
  • Does the fault seem to occur during a particular period of time? (day/night etc.)

When we ask these questions we often find that no attention was paid to the points listed above and therefore, the information is not available. If you ask yourself these questions and record your answers, you might just find that you are able to discover the cause of the problem for yourself. If you can't locate the cause at least you will have the necessary information to help an expert find the problem more easily.

Some faults are intermittent and won't occur at the time that we are looking for the cause. In this case the information that you provide becomes extremely important. With enough information we might still be able to solve the problem with reasonable certainty. Without that information, it would be almost impossible to identify the cause of the fault. You need to understand that if we don't have enough information, and the fault doesn't occur at the time that we are looking for it, then all we can do is to report that "there was no apparent fault at that time".

When you are being charged by the hour for the services of an expert, you can help yourself greatly by supplying all the information you can. Even if you don't think it is relevant. At this point I can't resist offering some very good advice. Provide the information when it is requested, but do not start offering helpful suggestions as to what you think the fault might be or what tests should be made while the technician is working on finding the fault. This is not only very annoying but is also distracting and will usually result in the job taking longer than it needs to. Every trades person hates this.

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Flexible Impellers in Water Pumps

Impellers should be replaced at least once every year or sooner depending on engine duty.

NEOPRENE is the most commonly used material and is the toughest, longest wearing compound.

However neoprene is suitable for use only when little or no oil or diesel is present in the water. NITRILE offers better resistance to oil and diesel fuel, however performance and life is reduced by about 10%.

If the pump body, end cover, back plate, or impeller is worn and not providing a good seal, the pump is unlikely to self prime if it is located above the water line. In many cases this won't be a problem until air enters the pump. Air can enter a sea water pump if the boat is out of the water for any reason, or if air can enter the raw water intake line by any other means such as the intake skin fitting (thru-hull) lifting out of the water when the boat is healed (especially in a big sea), or if the same intake is also used to supply something else via a line that might allow air to enter (such as a worn galley sea water pump sharing the same intake).

A worn impeller might work perfectly well in ideal conditions, and boat owners might be tempted to leave that worn impeller in service, simply because there is still a very good flow of water. However, that worn impeller is likely to fail in the worst conditions. In heavy weather there is a greater chance of the sea water intake lifting out of the water and being exposed to air. That is when air could enter the worn pump and stop the flow of water. In the case of an engine raw water pump, this will almost always result in an overheated engine. The options then would be limited. Shut the engine down, or leave it running and cause major damage that would result in it failing anyway.

Correct maintenance of these pumps is very important.
While there are many incidents, there are very few 'accidents'. Most cases are negligence or carelessness.

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Simple holding tank design

Waste water holding tank installation - Self emptying design

waste water holding tank installation design image

Instructions for use
  • When in a location where waste discharge is prohibited, keep the ball valve (sea cock) closed
  • When you are at sea open the sea cock and allow waste to empty from the tank.

If the bottom of the tank is positioned on or slightly below the water line, sea water will flow in and out of the tank, helping to clean it.
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Engine mount loads

Engine mount life can be extended considerably by limiting horizontal loads. Thrust is the biggest load. Twist of the engine under load creates upward load on the mounts on one side and downward load on the mounts on the other side (vertical loads). Correctly sized mounts easily absorb vibration and handle these verticle loads that are nowhere near as great as thrust loads that are trying to drive the mounts horizontally (fore and aft horizontal load). Fore and aft movement can also cause some shaft seals to leak.

Fore and aft horizontal thrust movement of engines can be limited by ties rods, transmission mounted horizontal thrust mount/s, or a shaft mounted thrust bearing. In an existing installation, tie rods are usually the easiest solution.

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