Powerboating - Part Six

In previous issues we've touched on the extra convenience of having access to AC (household) power while afloat. Whether it be to power airconditioning, a microwave or a thumping sound or entertainment system, most powercruisers are prodigious users of current!

That said, the only way you can access to such mod cons when you're away from home base is to produce the power yourself...

GENUS OF GENSETS
AC generators for boats come in two basic categories: portable and fixed. Let me state at the outset, the latter is by far the best. Indeed, mounted in the engineroom, normally using the same fuel as the main engine and often surrounded with an effective sound shield, good gensets are completely unobtrusive in operation.

There are a number of brands that are instantly recognisable as solid reliable equipment. Indeed, today's powerboat manufacturers offer auxiliary gensets as an option if not standard equipment in most non-trailerable cruising craft As a rule of thumb, it normally requires 2hp of engine to generate one kilowatt of power. Thus a 6kVa unit would be powered by a 12hp engine.

Such powerplants are usually simple diesel or petrol set-ups in mid-sized craft, but can be larger as the situation demands. In most cases, the genset's powerplant is basically comparable in complexity to the main engine with regard to cooling. Although the pumps and other components will be smaller on the generator than the main engine, both the salt and freshwater cooling circuits will be similar and will require commensurate maintenance.

Normally, starting is by starter motor, so the generator's engine will have its own battery and charging system. Generally the starting is by a remote key switch located near the craft's main switchboard.

The usual sequence of events for starting a fixed-installation genset is as follows:

WATSON'S WONDER
The advantages of fixed proprietory gensets are many, but are there any disadvantages? Unfortunately, yes, and not the least of which is the cost - both purchase and installation. In smaller vessels there is a weight issue - particularly if the engine is diesel - and fixed gensets can also crowd a small engineroom.

Thus for some boaties portable units are an attractive proposition. These generators are relatively cheap, robust and furthermore are extremely light in weight. But, and this is a big but, when placed on a boat the noise and vibration output is almost always unacceptable - particularly as the units increase in size over 2kVa.

There is, however, a home-grown installation that can combine the best of both worlds. At least in some applications...

Have you ever wondered why the main engine on the boat is so quiet? Mainly because the exhaust is water-cooled. By introducing water into the exhaust pipe, the exhaust gases are instantly reduced in temperature and this reduces their velocity. Presto, most of the din disappears.

This simple solution, if it could be applied to a small (portable) generator, would seem to go a long way to overcoming the biggest objection of noise. And it's achievable - we've done it!

By way of explanation, this writer had already installed a 4kVa cruising alternator to the main engine (see more below) and while it was perfectly satisfactory when the boat was underway, running the mains in an anchorage was a pain. Thus the decision was made to modify a portable genset to make it suitable for onboard life.

The portable generator we selected was a small unit of 3.5kVa (or 3500W). The low weight (55kg) and cost (under $1000) were also deciding factors. Powered by an air-cooled Honda OHV four-stroke petrol engine, the unit has the added advantage of an automatic shutdown in the event of oil pressure failure.

Unfortunately, small generators of this size do not come with electric start, but because there was plenty of room around the installation, the recoil starter (like a mower) was not deemed to be a disadvantage. Meantime, its 3500W output allowed us to comfortably run essential loads with a bit in reserve.

With the portable in our hands, the next step was to find a home for it onboard. As it was air-cooled it needed plenty of airflow around it. The logical spot onboard was my craft's engineroom. The next step was modifications.

Firstly, most portable gensets come with an integral fueltank mounted to the engine. The idea of petrol being stored in the engineroom or lazarette did not appeal to me, so the tank was removed and discarded. A stainless steel tank of 20lt capacity was fabricated and mounted remotely.

(In our case behind the steps to the flybridge. This is the area where the outboard motor is stored, so all the inflammable liquids are in the open air in the one place.

Now, the frame within which the genset was mounted was unbolted and the standard muffler was also removed. Using hardwood we created an engine bed onto which the genset was bolted remembering to position the set-up in a location below that provided both access and airflow.

Before finalising this positioning we worked out the routeing of the exhaust system using rubber exhaust hose. As a principle this should ideally slope downwards to the outlet and is best kept as short as possible.

CUT AND SHUT
Now for the fun part. Taking the original muffler and using a hacksaw we cut the exhaust pipe from it. Next a piece of 32mm stainless tube was bent into an inverted U shape and fitted to the exhaust pipe (see Fig One).

The bore of this tube will depend on the size of the generator standard exhaust system - it should always be equal to or greater than the exhaust pipe. Before welding this tube to the exhaust pipe, fit it to the engine and check that the rubber exhaust hose will run fair to the tube without kinks.

At the side of the U opposite to the engine exhaust port, a 10mm hole was drilled into the tube and a 50mm length of 10mm stainlesstube inserted. This is the point of water injection and should angle downstream (away from the engine). The tube was then welded into place when the exhaust pipe was welded to the stainless tube.

To combat vibration of the engine, the new exhaust set-up was also braced using struts welded to the pipe and bolted to the engine.

As a rule of thumb, when the exhaust tube is bolted to the engine the whole machine should be able to be lifted by this pipe without fear of breakage. This indicates a good firm exhaust system.

At this stage the engine bearers were bolted up using stainless steel bolts, washers and nyloc nuts to overcome the problem of the nuts becoming loose through vibration.

To mount the machine I used four flexible mounts purchased from an engineering supply house. These were expensive ($70), but the result well worth it.

The exhaust hose was now run using top quality rubber, wire reinforced hose to a skin fitting no more than 100mm above the waterline if possible. [Ed: the installation of skin fittings is beyond the scope of this feature]. Hoses were clamped securely using stainless steel hose clamps.

On some installations it may be necessary to fit an exhaust waterlock (ie. if the exhaust runs uphill) but whatever the set-up, it's important to break the hose and fit a suitable ball valve as near as practicable to the generator. The valve is important, as when this is closed it will make sure no saltwater can enter the engine.

A 240V water pump was used to pump the exhaust cooling water. Small magnetic drive pumps are readily available and only a low capacity is required provided the pump is not located with too large a lift. Position the pump as near to the generator as possible.

On the installation described, a separate water inlet fitting through the hull was not deemed necessary; I simply teed off the fitting supplying water for the eutectic refrigerator.

From the outlet of the pump, we used good quality hose to run to the water injection point on the exhaust pipe. We also fitted a small ball valve in the output to control the amount of water from the pump. Too much water increases the exhaust back pressure with a consequent loss of power, but by slightly closing the valve, the flow can be controlled.

When the motor is in operation, feel the exhaust pipe just after the injection point and reduce the flow until the pipe is slightly warm to the touch and the exhaust note is still quiet. Increase the flow a fraction more for safety.

Power to the pump is supplied direct from the generator. We removed the cover and hard-wired it to the generator output. This ensured that the pump will start as soon as the generator comes online. To electrically protect the pump, a 2.5-amp circuit-breaker was fitted in the end housing of the generator.

Regards fuel supply for the generator, we installed a ball valve on the remote tank outlet so the petrol could be shut off at the tank. To stop the generator, I turn off the fuel supply and allow it to run out of fuel, ensuring the fuel line is drained, a further safety precaution. A small filter was also placed in the line after the ball valve, similar to the type fitted to vehicle petrol tanks.

GETTING QUALIFIED HELP
This installation is within the realm of most capable boaties. What isn't is the electrical output side of the equation.

Most portable generators have one or two power sockets already fitted, and if these are suitable for your use, then the installation is complete. However, wiring the generator into the boat's existing AC system is a job for a qualified electrician alone. Remember 240V is just as lethal coming from a small generator as from the mains supply.

Basically, what the electrician will do is feed the generator output to a SHIP/SHORE selector switch, and from there to the switchboard (see Fig Two). This allows the boat to be powered from either the shore or the generator, but not both.

By using meters, the frequency, voltage, and the load can be monitored. (I also installed an hour meter to keep an eye on the generator hours for servicing).

DOES IT WORK
After all this effort, does it work? I can only speak from the point of our installation, and the answer is a most resounding yes - the noise level is extremely low and the vibration negligible.

Admittedly, the installation is in an insulated engineroom, but most of the noise generated is the bark of the exhaust, which is now reduced to a murmur. It is possible to sit in the cockpit and carry on conversations at a normal level.

A further bonus from the point of safety is that the generator forms a completely independent charging system in the event of flat starting batteries. Two or three hours of running will put enough into the bank to get a start, and that sure brings peace of mind.

Fuel consumption on the generator we selected is less than 2lt/hr running at average loads. As far as cooling goes, after two hours of operation on full load, the engineroom temperature was raised by only one degree.

After use, I stop the generator by first turning off the main circuit-breaker, then shutting off the fuel supply. When the motor starts to run out of petrol after 20 seconds or so, I stop the water pump and allow the motor to continue for a few more seconds and then I operate the engine stop button and finally close the exhaust seacock. This ensures a completely dry exhaust and with the cock closed, no water can enter the system in rough weather. The fuel line will be empty of petrol as well as the carburettor, leaving the whole system safe.

Final all-up costs will vary from boat to boat, depending on how much you are prepared to do yourself. I did all the work and it took about a day and a half. Allowing for the pump and fittings the whole job was under $2000!

CRUISING ALTERNATIVE
The fitting of an AC generator (or cruise-gen) to the main engine on a craft is a reasonably straightforward task, however, I doubt if the average person would bother if the boat already had an auxiliary generator.

As the frequency of the supply is dependant on the speed of rotation, this means that the engine can only be run at one set speed if the output is to be correct. Nonetheless, for people who envision long-range cruising they are a real asset and well worth the cost as they can reduce the demand on the auxiliary genset by as much as 50%.

By far the biggest advantage gained is in the area of refrigeration which is probably the most important of all the necessities on a cruising craft. Refrigeration is such an important subject that it merits almost an entire article, however, at this point it is enough to say that the two most common methods of powering cold plate or eutectic refrigeration is by AC condensing unit or compressor driven off the main engine.

The latter system is popular, particularly with the dedicated cruising fraternity, but it does have the disadvantage that even when shore power is available, the engine still has to be run for one or two hours per day to keep the icebox down at temperature.

An AC alternator mounted to the main engine and an AC-powered refrigeration system would seem to deliver the best of both worlds - refrigeration being operated off the main engine as the boat is cruising along, yet freedom to operate it via shore power when available. The only disadvantage is that the engine has to be run at the one fixed speed while the alternator is online, otherwise the frequency and the voltage of the supply will be incorrect which can have serious consequences for any equipment that is connected to it. More on this point later.

UP THE REVOLUTION
If it is decided to install a cruising alternator, the first step is to purchase the right model for your requirements and these are determined in exactly the same manner as when determining the size of an auxiliary genset.

We decided on a 4kVa unit - a common and readily available size.

It is important to know the number of poles of the machine and this will nearly always be two or four. Most alternators sold today can be connected for either 110 or 240V by simply changing the connections in the junction box of the machine.

To calculate the RPM of the machine that will provide the correct frequency, use the following formula:

N = f x 60/Number of pairs of poles
Where: N = speed of rotation, and f = frequency.

Thus to calculate the speed a two-pole machine will have to rotate to produce 50Hz will be 50 times 60 divided by 1 which equals 3000rpm. A four-pole machine would rotate at half that speed (ie. 1500rpm).

The alternator will be usually driven from the front of the main engine utilising a V-pulley bolted to the crankshaft. Often a spare pulley is provided by the manufacturer and sometimes you can get lucky and this will be suitable. If not, one will have to be machined by your local engineering shop.

First a word of warning, there is generally a maximum load that can be placed on the front crankshaft bearing when driving ancillary loads, and it always pays to refer the situation to the engine manufacturer. Also, V-belts have a maximum horsepower that can be transmitted per belt, depending on the size of the belt, diameter of the pulleys, speed of rotation, etc, etc.

The next step is to determine the correct ratio between the driver and the driven pulleys, but first a decision must be made at what engine speed we want to generate the AC power.

This decision is a bit of a compromise as several factors come into play, the first being overspeed. This is the maximum rpm that the alternator can be subjected to without damage.

Taking a two-pole alternator as an example with a max overspeed figure of 5000rpm and a main engine that has a rev ceiling of 2500rpm, the relationship between the two figures dictates that the ratio between the driver and the driven pulleys cannot be more than 2:1. Thus in this particular design, we will only be able to generate AC power at the correct frequency at a main engine rpm of 1500. That is when the alternator will be rotating at 3000rpm.

THE WHOLE TRUTH
The actual mounting of the alternator to the engine can be difficult in some situations. In the case of my own cruiser, I took the easy way and mounted the alternator to the engine bearer and the hull using plywood for the bracket and glassing all this in position. Slots were cut in the bracket to allow for belt adjustment, and the pulley on the alternator is of the type that can be moved along the shaft and tightened in position.

Since my engine is solidly mounted to the beds there is no vibration between the two and it has not given any trouble. This set-up greatly simplified belt alignment.

Is such a set-up worth the effort? The answer must be yes if this is your craft's only possible method of generating high-power AC. That said, if you have a reliable generator, then I wouldn't bother going to the trouble of fitting a cruising alternator as well.

Finally, some cautions to be observed in the operation of cruising alternators. Remember that the correct voltage and frequency of the output will depend on you and that once the alternator is online the throttle must not be altered unless the alternator is disconnected from the load. Failure to do this can feed incorrect voltage and frequency into the various appliances onboard and severe damage, even fire, can result.

ON ITS EAR
There is an alternative to gensets and cruise-gens - the inverter. Put simply, an inverter is an electronic device that takes DC power from the battery and converts it to AC at the correct voltage and frequency. The most obvious question that arises from this is, why bother with alternators and generating sets if these can do the same job? Like everything, if only it were that simple.

Firstly, let's look at the inverter output. Remember when we were talking about sine waves during the introduction on AC power? We said that certain electronic equipment would not operate correctly if the shape of the wave form varied from a true sine wave. This holds true for inverters as well as for generators.

Unfortunately, it is technically difficult, and therefore expensive, to make inverters that have a true sine wave output. The more usual output is a sawtooth wave or a square wave or a jumbled combination of the two. In advertising techno-speak it's called a 'modified sine wave'. If you buy an inverter that has an output other than a true sine wave, expect trouble in some equipment that is connected to it - particularly televisions, video equipment, computers and microwaves, etc.

On the bright side, more and more inverters are being made with true sine wave output and the price is coming down.

The second problem to be solved in inverter selection is one of size. The larger the output (and this is measured in watts) then the larger the equipment they can run, however, remember that an inverter is essentially supplying us with power from the battery.

Every hundred watts of power is eight amps from a 12V battery, so it is not too hard to imagine just how tough a 1200W hair dryer is on the system. That's nearly 100 amps being pulled from the battery bank. If your system has been set up with these sort of discharges in mind then there shouldn't be too much trouble, but it will require some hefty recharging.

Another thing, inverters are notoriously poor at starting inductive loads, motors especially, and a factor of five should be allowed. Thus a small refrigerator with an operating load of 200W would generally need an inverter of around 1000W to get it away.

The rating of inverters is generally stated in Continuous Output Power in watts. Often another figure, Surge rating is quoted. My advice is to ignore this second value completely when comparing brands as I have found some wildly optimistic claims.

Finally, stick with a reputable manufacturer.

INSTALLING AN INVERTER
For those who want to wire the inverter into the ship's system, installation is generally straightforward.

Manufacturer's recommendations should be followed, but good rules of thumb are: