Selecting components for use on a boat.

Copywrite 2010 by Boat-Project.Com



When it comes time to select electrical or electronic components for a boat, a few factors must be considered. If the item is contained within the engine room or any area that gas vapors could come into contact with, it must be ignition rated. This is actually a requirement of the USCG, and the intent is to prevent a spark from the electrical component from igniting the gas vapors. Ignition-proofing is usually accomplished in one of two ways; suppression, and containment. In the suppression method, the components are prevented in creating a spark in the first place. In containment, any spark is isolated within the equipment so that it cannot come into contact with gas vapors.

Waterproof Rating


Another consideration is the item's Ingress Protection (IP) rating. This basically determines how waterproof the item is. The IP rating, or code, is an international standard as defined in IEC (International Electrotechnical Commission) standard 60529. Individual components, such as switches, as well as complete assemblies may have an IP rating, which means the item has been tested by the manufacturer.

The standard IP rating is a two digit code; the first digit is the items protection against objects, such as dust, and the second digit is the protection against water.

Many waterproof "marine" products have an IP-67 rating, as well as certain electronic products. This Hammond Mfg Series 1554 enclosure is an example of a (National Electrical Manufacturer's Association) NEMA-4x industry standard. These enclosures generally have an IP66 rating, which means they are watertight, but not necessarily waterproof (at least not down to 1 Meter under water). Still, they are generally suitable for many of the wet-area electrical or electronic projects you may have. And they are releatively inexpensive - at least by marine standards (provided you buy them from Mouser, Allied, etc). They can be purchased in many shapes and sizes, with or without a clear cover, and in several colors.

Having a water-tight enclosure is no good unless you have a means for cable entry that is also watertight. For this purpose, look to Heyco 3000 series Cord Grips. They come in a variety of sizes and shapes, and generally carry a rating of IP67. What is interesting is these items are re-packaged by a popular Marine electrical supplier, and are about $10 each from the marine stores. But you can buy the very same item for $1 from the electronic supply warehouses (again, Mouser, Allied, etc.).

 

 

The above examples are only two of the many devices that are available in a watertight or waterproof package. switches, LED assemblies, connectors, and a host of other devices are also available, and will usually carry an IP designation as to how waterproof they are. Generally all of the items available from Mouser, Allied and like supply houses have datasheets that can be downloaded. You will have to read the item's description or datasheet to determine the IP rating, if any.

Corrosion


Corrosion of metal is always an issue on boats. While it is usually more of a concern in salt-water, fresh-water environments are also subject to corrosion - but at a lower rate. Corrosion occurs anytime two dissimilar metals are joined together and in contact with an electrolyte. For purposes of the marine environment, this means water. In effect, when this happens, you have created a battery, and a galvanic current will flow from the less Noble metal (anode) to the more Noble metal (cathode). In fact, a battery is sometimes known as a galvanic cell.

So how to you minimize corrosion? The determining factor as to how "active" the galvanic current is depends on how far the two metals are on the Nobility scale. Metals adjacent to each other are generally compatible, and exhibit minimal activity, while metals far apart will be more active. Try an experiment. Take a copper plate, put an aluminum screw in it, and let it sit outside for awhile; or better yet, put it in a jar of seawater. Soon, the aluminum will be corroded, as it is lower on the Nobility scale than copper.

Some common metals on the Nobility scale

MetalGalvanic Index (volts)
Gold0.0V
Silver0.15V
Nickel0.30V
Copper0.35V
Brass and Bronze0.45V
Stainless Steelvaries
Tin-Lead solder, Tin plating0.65V
Steel and Iron0.85V
Aluminum0.90V
Galvanized Steel1.2V
Zinc1.25V

This is not an exhaustive list, and many alloys, especially stainless and aluminum have varying indexes, depending on the composition of the alloy. But a search on the internet should provide an answer for the metal you are working with. As you search the internet, you will find different Galvanic Index values, which can be attributed to the test methods and the electrolyte used (water, alcohol, etc). However, the relative position and differences in voltage of the metals should be a constant. Therefore, it is not as important to know the exact index number, but more importantly, the relative position and voltage difference.

It is suggested that metals that have a voltage difference of 0.15V or less can be considered inactive - that is, minimum corrosion will occur. In well controlled temperature and humidity environments, up to .05V can be tolerated. But a boat is not a good example of that! Therefore, you should always strive to stay within an Index differential of 0.15V for any metals that come into direct contact with each other. Of course, painting or other means of insulation will also prevent galvanic corrosion, but you knew that.

So how do you put this chart to use? If you buy a nice new aluminum Fortress anchor, then attach a stainless steel chain to it, then throw it into the water, you should expect the aluminum to corrode around the chain. Using a galvanized chain, while still greater than 0.15V, is far better as the two metals are closer galvanically.

How about an example that baffles most people? Compare tin plating to gold. The difference suggests a farily active corrosion potential. Now think of a common gold/tin junction. How about a stereo system with tin-plated RCA jacks, and someone spent high dollars and used gold-plated cable connectors? That is right, its worse to do that then buying the less expensive tin plated cables. Of course, if the stereo connections are relatively protected from moisture there won't be much of a problem, but this example shows the bias that sellers of the more expensive gold plated cables have promulgated.

The advice then is to match metal-to-metal whenever possible. If you are adept at making your own cables, you can attach a gold plated connector to one end, and a tin plated connector at the other end if required. But what about the copper wire and solder. The chart suggests those can be active as well. True. However, I am trusting that you will propely seal the connection by shrink wrapping it, using a waterproof connector, or similar.

So then, using a mechanical joint on coax rather than a solder joint does make some sense, given the tin-lead solder will be more galvanically active than say, the copper wire and gold plated connector.

From the chart, you can see that you should avoid using zinc plated hardware on stainless steel fittings, and so on.

Up until now, we have considered only the interaction between metals, but how about self-corrosion? For example, everyone knows that plain steel will corrode if not protected. For the marine environment, metals such as stainless steel, aluminum, and brass offer reduced self-corrosion. Corrosion could be a topic within itself, so I am not going to cover much here. You are welcome to research further on your own. It should be sufficient to say that for the most part, if you select marine-grade fittings and hardware, you are using the best material for the task at hand. Just avoid mixing metals if you can avoid it, or especially, running down to the hardware store and using zinc plated fittings.

Using a zinc plated bolt to attach a sailboat's stainless steel back-stay cable to a stainless-steel clevis is asking for trouble! But after reading this, when the mast comes crashing down due to a corroded bolt, you'll at least know why it happened.

USCG Marking


Critical safety components are often required to be USCG certified, and in some cases, marked as such. The best example of this that I can think of is Type A-1 Fuel Hose. 33CFR183 requires that all fuel hose be tested, certified, and marked according to it's type.

33CFR183 has a whole host of items that must meet minimum standards, or down-right certified. In addition to the mentioned fuel hose, electrical wiring, and engine parts are two areas that must also be considered. For this reason, anytime you work on your boat, especially in an area where safety could be comprimised, you will do well to have read 33CFR183, or the USCG publication "Boatbuilder's Handbook", which presents 33CFR183 in an easy-to-understand format.


In summary, we've just touched the surface when it comes to selecting boat components. Be aware of the USCG requirements for using the proper component, and ensure it is certified when required, and put some thought to what parts are compatible (galvanically), and be aware of the environment you put your projects in.

Happy Boating

Reader Comments

Home    Return