Mailport October 1, 2003 Issue

Mailport: 10/01/03

More on Oversized Alternators
This is a follow-up to Roger Bohl's letter in the August 15 issue. Many cruisers fit oversize alternators to minimize recharging time while at anchor for extended periods.  That's why I have one.  But there is no free lunch. As Mr. Bohl points out, energy being transferred to the battery has to be transmitted to the alternator by way of the belt, so belt wear is increased. Higher output also increases the alternator's internal temperature and aggravates the electrical stress on its components.

I've suffered belt failure as well as alternator failure.  Here are somehints I learned the hard way...

Don't use standard alternator belts. Go to the local speed shop and buy thehighest grade Gates belts they will sell you. These last much longer. (I learned this one from a friend who builds race cars.) Carry at least one spare belt and keep it and the few tools required for changing the belt ready to hand, not buried in the bilge. I bought extra wrenches of the requisite sizes just for this purpose, and consider them cheap insurance.  You'll need the same tools handy for maintaining belt tension, anyway. (Don't over-tighten it!)

Inspect the alternator belt regularly, and replace if in doubt.  Better tosuffer the pain of replacement than have a failure at a critical time.

Large alternators are capable of providing higher currents, but the torque required to drive an alternator is determined by actual current output and alternator speed, not by its size. For a given output current, a larger alternator requires the same torque but will run cooler.  Using a regulator such as the Heart 2000R, set the maximum charge current well below the maximum rating of the alternator.  I suggest 50% or less. Increase this setting only when a quick recharge is necessary.

Carry an alternator repair kit with spare diodes and bearings.  Even ifprofessional help is at hand when failure occurs, spare parts or areplacement alternator may be days away. As I discovered off a desertedbeach in the Exumas, it's easy to disassemble an alternator, follow theinstructions that come with the kit and get the alternator back on line.

-Robin C. Moseley, PE
S/V Trenarren


Roger Bohl’s letter raises important issues regarding the inadequacy of many alternator installations. There is a practical limit to the power that can be delivered by a single V-belt; if that limit is exceeded, then the belt will slip, producing heat in the alternator shaft that will eventually cause failure. No alternator will tolerate this abuse.

Nevertheless, this kind of failure is not inevitable on a single-belt installation if good equipment is installed and appropriate guidelines are followed when setting up the belt drive. When we can, we prefer to install high quality "hot-rated" alternators. These units are specially built to run continuously under full load without burning up. We have been installing these single- belt driven 124/106 amp alternators for 15 years. In that time, we have not seen a single failure in an alternator of this type.

This kind of installation requires a 3/8" V-belt, and the pulleys must be machined to fit the belt. The V-groove dimensions depend on the width and type of belt, and will vary with the pulley diameter, as the section of a belt will change as it is "bent" to different diameters. The Gates Corporation and SAE are good sources of information for setting up a belt drive system. The belt will have to be set up tight, but not too tight. This requires some judgement and experience.

Most alternator fans are designed to rotate clockwise; if the engine has CCW rotation, then a bi-directional fan must be installed. The belt and pulleys do need to be maintained in good condition. Whenever a worn belt is replaced, the pulleys should be deglazed—otherwise a new belt may slip as soon as it is installed.

Of course, larger alternators must be driven with two belts (to be purchased as a matched pair) or some other form of power take-off. A nice alternative is a power take-off shaft separately mounted on pillow bearings in front of the engine, driven by a flexible coupling. Multiple pumps, alternators, and compressors can be driven off such a shaft without creating bending loads on the crankshaft. Frequently, an economical installation will require that the alternator share a single belt with the circulating pump and the raw water pump. This is not ideal, but, for the weekend sailor, the water pumps will still last a very long time, and the cost of a water pump will be less than the cost of fabricating the extra brackets and pulleys for a separate alternator drive.

On the other hand, a long-distance sailor who will be putting a lot of hours on his boat away from port should not be trying to save his pennies by taking shortcuts with the drive belt system, nor should he be running the boat with an undersized alternator—fast efficient charging, a reliable electrical system, and long life from the water pumps bearings are all critical.

As Mr. Bohl discovered, higher torque at lower rpm is unavoidable. Since power is proportional to the product of torque and rpm, torque will increase as rpm is reduced for a given alternator output. The greatest demand on the alternator will usually occur while recharging the batteries and hold-over plates while at anchor or while sailing. To minimize recharging time while an engine is running at idle speeds, the sailboat alternator should be optimized to develop power at lower rpms. A well-designed three- step regulator will offer features that mitigate problems with high belt loads at low rpm—after the engine is started up, the regulator will call for alternator output to be ramped up in steps, thus avoiding a sudden application of full load to the belt and pulleys. The regulator may also offer a current-limiting function if it is determined that the alternator should not be loaded beyond some limit. A larger alternator pulley will help, but both driving and driven pulleys must be increased in the same proportion to maintain adequate alternator rpm.

A high-output alternator with 3-step regulation should not be blamed for reliability problems.

One of the benefits of high-performance charging is improved reliability, because the batteries are properly recharged rather than being subjected to the long-term abuse that is characteristic of standard charging systems. Modern electrical systems require more capacity than is available from the standard alternators that are supplied with today's sailboat engines.

We wish that manufacturers of sailboat engines would acknowledge the fact that their engines must be called upon to drive multiple devices, including more powerful alternators that are retrofitted when owners discover that the factory alternators are inadequate. The automotive industry solved this problem years ago with better and simpler V-ribbed belt drive systems: single serpentine belts that deliver significant amounts of power to multiple devices, sometimes with very small pulleys. Facility for additional belt drives and other forms of power take-off should be engineered into marine engines at the factory. Appropriate brackets, belts, and pulleys should be available as accessories so that aftermarket installers do not have to "reinvent the wheel," sometimes with costly and undesirable results.

-Paul Muenzinger
Brewer Post Road Boat Yard
Mamaroneck, NY


More Tinkering With Pumps
Chip Marsh's letter in the August 15 issue caught my attention. There are a number of ways to detect the pump's operation.

An op-amp circuit could use a very low value resistor without the voltage drop at the cost of circuit complexity. op-amp is electrical engineer jargon for operational amplifier, usually an off-the-shelf, low-cost IC (integrated circuit) that can perform a variety of tasks, including amplifying a very small DC voltage to a level that can operate a transistor or LED. In this application, the series dropping resistor could be made very small (say .05 ohms) with a negligible voltage drop. The op-amp requires a power source and, in the hostile marine electrical environment, some care in circuit design.

A simpler approach to monitoring pump current occured to me. I wound about 1 foot (approximately 40 turns) of #24 wire around the body of a magnetic reed switch. The reed switch would operate with less than .5A through the coil. In the pump sense application, the coil would be connected in series with the pump and the reed switch contacts would be in series with the LED/resistor. The voltage drop in the coil with 24 gauge wire is negligible, but to allow for start-up surge, momentary locked rotor, etc. the wire diameter should be as large as possible while still allowing sufficient turns around the reed switch, thus 22 gauge is probably a better choice.

The reed switch is sensitive to a magnetic field and the strength of the field depends on the product of the current and number of turns in the coil. Since the sensitivity of reed switches varies with manufacturer and design, some experimentation will be necessary, but 60 ampere turns will probably work fine. Some reed switches used for alarm circuits on window and doors may be a normally closed switch, i.e. the switch opens in the presence of a field, and this type is to be avoided.

-Curt Canavino
Via e-mail


In your Aug 15 issue, Chip Marsh of Santa Barbara inquired about running a pilot light on Jabsco Max VSD fresh water pump. A company called Borel Manufacturing ( makes a unit for bilge pumps that will meet his needs. The unit does not require any modifications to existing pump and works by detecting current draw. The unit costs about $50 so this would be a expensive pilot light. But if he really wants it, this would be a solution.

-F. Grace
San Francisco, CA


Adding an annunciator light to the Jabsco Sensor Max VSD pump is easy to do and doesn't even require opening up the pump, which might void the warranty. Anytime the pump is running there will be a voltage drop in the supply line from the battery to the pump.  With this particular pump, the greater the water flow the more current it draws and therefore the greater the voltage drop.  A simple comparator IC such as the LM339N (29 cents each from can detect that voltage drop and turn on a panel lamp or LED.  You can adjust the sensitivity of the circuit to turn on only at high flow rates if you wish.  Since the 29-cent chip includes four comparators, you could have it turn on a green LED for low flow rates, yellow for moderate flow rates, and a red LED for high flow rates and still have a spare comparator for something else. There are all kinds of boating applications for these inexpensive logic chips.

-Mike Miller
Goffstown, NH


In response to Chip Marsh's request, your friends at SHURflo have a solution. The SHURflo Extreme 5900 series pump has wire access, albeit a small length, available between the circuit board (which is potted into the baseplate to prevent corrosion) and the overmolded wire grommet leading into the motor.

Make sure the LED set-up is rated for 12 VDC or higher, depending on generator voltages. We even tried bulbs set up for 28 VDC.

As the circuit board drives the motor with pulse width modulation, the RMS voltage to the motor can be downwards of 6 volts DC when the motor is turning very slowly. This will result in a dimmer, but still noticeable, light.

We have also had good success with the neon gas bulbs available.

Our engineers don't like you cutting into the wires, either, but from an applications standpoint, as long as you seal your connections, electrically size it right, and don't cross your wires, we don't see a problem.... ...unless your pump dies, then its warranty is void. Ha ha. Just kidding.

-Chris Beh, Applications Engineer
RV/Marine Division
SHURflo Pump Manufacturing Co.


Farrier Weighs In On Corsair
I have long been an admirer of your magazine, apart from the subscription reminders that seem to arrive more often then the magazine. But that is another topic.

I have just read your article on the Corsair 36 [August 1] which tends to put a different slant on what actually happened at Corsair Marine in the early days, and which I set up in San Diego from 1984 to 1991.

I decided to part ways from Corsair in December 2000, and am now in competition with them to some degree. An item of major concern in your article is the statement that TPI built six F-31s. This is incorrect, as I had to demand that my name and 'F-31' trademark be removed from this boat due to numerous unapproved changes, to where it was no longer representative of how I believed a trimaran should be. It should thus be called the TPI or Walton 31. One of the reasons for my decision was the high build weight of around 5,300 lbs., leaving little load-carrying capacity. F-31s built correctly to my design and specifications weigh from around 3,100 lbs. to 3,900 lbs., depending on model, giving them a very usable load-carrying capacity.

In regards to the Corsair 36, I had started designing a new F-35 for Corsair in July 2000, but a visit to their factory later that year showed someobvious differences in how the boats should be built, so I decided to discontinue any further involvement. Rather than leave Corsair in the lurchI allowed them to purchase the F-35 lines, giving them the freedom to redesign and build however they wished, but under their name only. They then apparently added a foot to the transom and called it the Corsair 36. I have had no further involvement with Corsair Marine.

Your article also gives the Corsair 36 displacement as only 5,500 lbs. This is actually the apparent weight, as having done the original lines I know the full-load design displacement is more around 7,000 lbs.

The statement that the C-36 is only 900 lbs. heavier than an F-31 also seems incorrect, as hull no. 4 was weighed at 5,660 lbs. This is actually over 2,000 lbs. heavier than the average F-31 weight of around 3,500 lbs. when built to my specifications, while the first F-33 launched, with a diesel inboard, weighed in at 3,730 lbs.

-Ian Farrier, Farrier Marine
Bellevue, WA

The feeling is mutual—we've long been admirers of Mr. Farrier and his designs. However, we decided not to use boat review space to discuss the relationship between Mr. Farrier and Corsair Marine. Too tangled and confusing. We used production information directly from Corsair, the C-36 builders, and there should have been no slant in either direction.

Mr. Farrier is correct about those weight differences between the F-31 and C-36. We regret the error.

Readers can have a look at current Farrier designs at In particular, multihull lovers should check out the new F-33.


Binoculars: Eye Relief
In your August 1 article on 7x50 binoculars you left out any information about eye relief, an important plus for people who wear glasses. Here are the eye reliefs for all of the binoculars tested. 

Bushnell: 18 mm
Celestron: 26 mm
Fujinon: 23 mm
Nikon: 22.7 mm
Steiner: 22 mm
Swift: 19 mm
Tasco: 20 mm
West Marine: 27.4 mm

Gleaned from the Celestron and B&H web sites, and various catalogs.

-Robert G. Curtis
Sudbury, MA

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