Marine Deep Cycle Battery Discharge Voltage Levels

Battery State of Charge

100% = 12.7     Volts

90% =  12.5      Volts

80% =  12.4      Volts

70% =  12.3      Volts

60% =  12.2      Volts

50% =  12.0      Volts

40% =  11.9      Volts

30% =  11.7      Volts

20% =  11.5      Volts

10% =  11.3      Volts

Most experts will agree that for best service life the maximum recommended discharge level for Deep Cycle batteries is 50%. If you go below this I wouldn’t be too concerned but keep this in mind for maximum longevity. Use your battery monitor to keep on top of this. Just make sure that when taking voltage readings that all 12 volt appliances are turned off.

Wood Epoxy Boat Hull Construction

I guess that before we move on from our discussion of fiberglass and composite boat construction that we should include a segment on a technique known as “Wood / Epoxy”. This is a building method that became popular in the late 1960’s with the introduction of epoxy based resins. This was at about the same time that production boat builders were making the switch from wood to fiberglass, utilizing mainly polyester resins as a building material. Until now most boats both production and home / custom built were constructed mainly of plywood, both in sheet form for hard chined hulls and various diagonal strip planking methods for curved hulls.

I remember that as a kid growing up on here in the north shore of Lake Ontario that wood boats were everywhere with sheet plywood and cedar strip being the most popular. Fiberglass was this wonderful new material that made for bright shinny curved hulls and sleek looking boats that almost overnight seemed to take over the lakes and rivers. For while wood was still popular with custom and home builders. It was at about this time that someone decided that it would be a good idea to cover these wooden hulls with lightweight spun glass cloth saturated with polyester resins. It seemed like a great idea at the time effectively turning the exterior surface of a wood hull into a hard piece of plastic. For a few years fiber glassing your wooden boat was the thing to do. It wasn’t a difficult process although a little messy and somewhat smelly.        This went on for a few years until after some time boaters realized that the fiberglass cloth was delaminating from the plywood hull material. It was later determined that polyester resins, in damp conditions, over time will separate from plywood. So much for all the hard work.      At about the same time Epoxy resins, even though they have been around since the 1930’s were beginning to show up as a boat construction material. Their increased cost was prohibiting them from seeing considerable use in production boats however with the custom and home builders who’s predominate material was still wood then began to utilize epoxy resins as a hull overcoat, both with and without light glass cloth re-enforcement. This was possible because of epoxy’s superior bonding strength to wood, it’s superior film strength and it’s superior hygroscopic properties. Remember a few posts back we learned about polyester resin’s hygroscopic properties, well epoxy is far superior in this regard. It does however have a couple of disadvantages. It degrades rapidly when exposed to UV and it also doesn’t like high temperature situations. This means that it makes a poor gel coat in that it must be over coated, preferably with a light colored paint or coating.

Lets’ now skip forward to a few months ago when I was contacted by a potential client to perform a survey inspection on his 26 foot wood epoxy Thunderbird sailboat. For those of you that aren’t familiar with the Thunderbird it is a 26 foot sailboat, designed by a naval architect named Ben Seaborn in 1958. It became incredibility popular with home builders in both North America and Australia as both a racing and cruising sailboat with one design racing fleets and an international class association. They were quite popular in my area here however the numbers have dwindled in recent years. As with many marine surveyors these days I generally shy away from wooden boats for obvious reasons but after a short discussion with the owner a few things came to light. The boat was constructed by the owner in 1977. The hard chined hull construction is GIS fir plywood sheeting over mahogany framing and the exterior hull surface is covered in light glass cloth saturated with epoxy resin. The hull interior is coated out in epoxy resin only.

I then decided to accept the assignment. At the inspection my findings were, that after 37 years of constant use, electronic moisture readings were not far off what I would normally see in dry plywood and percussive tests showed no structural issues. I found this very interesting that the plywood was in very close to new condition. I often inspect fiberglass boats that haven’t fared nearly this well. It looks like this epoxy is pretty good stuff. I guess that it’s just one of those things that just makes you want to go HMMMMMM now isn’t it?????

Diesel Fuel for Marine Engines

I had a couple of enquiries asking me to talk a bit about Diesel Fuel as it pertains to marine usage so here it is.

Diesel fuel is described as any fuel used in diesel engines which can be ignited by the heat of compression pressure (without the use of a spark plug or other ignition source). The most common type is a petroleum distillate but there are other types such as biodiesel that are becoming increasingly popular.

Diesel fuel is classified by its Cetane Number. A higher cetane number indicates that the fuel ignites more readily when sprayed into hot compressed air and provides more heat energy (power) per unit of fuel. A higher Cetane number also means that the fuel a higher viscosity (thicker) and won’t flow as readily at low temperatures. In extreme conditions it can even gel or wax in the fuel lines and pumps. This is why, for cold weather operation a lower viscosity (thinner) diesel fuel is necessary.

In North America diesel fuel is offered in two grades. Number 2 Diesel is the higher viscosity stuff used in the warmer weather and is what we should all be using for marine applications in the summer. Number 1 Diesel is the thinner winter fuel grade that offsets gelling in the cold temperatures. When used in warm weather it won’t produce the same power or provide as good cold engine staring as the Number 2 diesel will.

Each year as the Fall season rolls around and the weather cools off here in the Northern Climates fuel dealers will begin to blend #1 and #2 fuels and as the weather cools even more eventually be offering straight #1 diesel for retail sale.

Fuel Classification           Cetane#

Thick #2                              46-50

Blend                                  41-45

Thin #1                              38-40

Jet A (Extra Thin). You guessed it, used in jet aircraft engines and sometimes sold as #1 diesel.

Farm Grade (Red, quality ?????)

When does this happen? It’s hard to say but it depend largely on your location but to be safe I would consider filling my tanks for winter storage at least by September 30. Leaving #1 diesel in your boat over the winter months will not be a problem because if does gel in the lines it will quickly turn to liquid again as the weather warms up in the spring.

The next aspect that we need to look at regarding diesel fuel is Sulphur Content. Sulphur is a by-product of the refining process used in the manufacture of diesel fuels and it can also act as a lubricant to many internal engine and fuel system parts. Unfortunately it is also a major source of the pollutants emitted by diesel engines. Since 2006 in North America the only diesel fuel sold at the retail level is classified as Ultra Low Sulphur Diesel Fuel (ULSD). Since many of our marine diesels were manufactured prior to 2006 they were not intended for low sulphur fuel and if used it can accelerate wear on fuel system and engine components. This is why a quality fuel additive is recommended. There are many of these on the market today and as such some re-search is required. The best recomendation that I can give is to stay away from any that use alcohol as an emulsifier.

Marine Deep Cycle Battery Size Chart

A short while ago I gave a presentation on marne battery and charging systems to a group at a local marina. One item that we discussed was battery groups and sizes and to that end I compiled a chart of the sizes most common to marine applications. I have since been asked to post that information here.

Batteries Size Codes

Group 24       60-85AH     11X7X10”       50-55 lbs

Group 27     85-100AH     13X7X10”       60-65 lbs

Group 29     100-110AH   13X7X10”       65-70 lbs

Group 31   105-115AH     13X7X10”       70-75 lbs

4D             210-225AH       21X9X9”       135-140lbs

8-D           225-255AH     21X11X10”      160-175 lbs

6 Volt GC   200-240AH   10X7X11.5”      60-70 lbs

I would like to add that this information is averaged and that may be accurate to within about 6mm (1/4″) or so on size and that the capacity may be more or less 10%.

Battery size codes Eg: Group 27, 4D etc. refer to the physical size of the battery only and it is not indicitive of the capacity.

You will also note that Groups 29-31 are all listed at the same size. Again some may be slight larger and some slightly smaller.

At the bottom the 6 volt battery is labelled GC. This stands for Golf Cart and being 6 volt, 2 batteries must be wired together in series in order to produce the 12 volts that is commonly required for marine applications. When you compare a 6 volt battery’s space requirements as apposed to their current capacity the advantage becomes very clear. Another note about 6 volt batteries, that is if you’re considering going this route is that while they have a slightly smaller size foot print and they are taller than their 12 volt counterparts and this needs to be considered in the planning.



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