Electrical Project Notes

There are a few direction that major electrical system upgrades could go in. One direction is simply an upgrade to the existing house battery to offer more reserve capacity and to improved charging. This would be to support instruments, autopilot, and navigation lights for night sailing. A far more extensive upgrade would be required to support an electric auxilliary. Substantially upgraded charging capacity offered by wind generation, water towed generator, or multiple large solar panels would either support continuous sailing with instruments, autopilot, and lights at night, or support relatively quick recovery after use of electric auxilliary. What follows here is some notes on these topics.

Estimated current draw:

Solar panels offer about 1 AH of charge daily per 3 watt rating. For example, a 75w solar panel offers 25 AH on a normal sunny day. This translates to the following net charge (or draw) assuming a 2A draw by the navigation lights.

The table above shows that using solar panels alone is not practical if making passages that involve more than a day or two of both day and night sailing. A battery cannot be left in a state of discharge for more than 24 hours without some damage.

Common water towed generators are rated at 10A to 20A. Since we have already installed a Ferris Power Products 10A water towed generator we will consider the expected output of that product. The advertised power curve is almost linear with 0.5A at 2 knots and 10A at 8 knots. At 4 knots the rated output is 5A. At that speed 120 AH can be added to the battery over a 24 hour period. Even in light air, at 3 knots 1A is produced for 24A per day. Add a 75 watt solar panel or a pair of 40 watt solar panels and enough power to sustain continuous sailing is generated as long as 3 knots of boat speed can be sustained.

The following table gives the number of days that could be sailed before drawing the battery down if winds were so light that less than 2 knots of headway was maintained (the water towed generator produced no power at all). Note that our current battery is 255 AH.

Normally the battery would not be discharged for days at a time since it will cause some damage to the battery. AGM and gel batteries are much less prone to damage than wet cell batteries. The 255 AH battery being used is an AGM.

Another common scenario is anchoring after a sail which has drawn the battery down. At anchor the water towed generator produces no power. The following table provides the time in days to recover completely after a day/night sail when it was overcast during the day (43.4 AH lost).

The only problem with solar panels (besides cost) is finding a place to mount them. A marginally acceptable spot is along the outside of the forward stancions. This location is very subject to shadowing. At most one 50 watt panel could be mounted on each side. A good mounting would be on a pair of fixed dinghy davits protruding off the stern at a reasonable height. The Shell (formerly Seimens) 50 watt panels are a reasonable choice because they are only 13 inches wide, have the highest power per unit area in the industry, and are cost competitive. A davit could support 2 panels quite easily. Currently Defender is selling the Kyocera line of panels which also have good power density (watts/sq.in.) but cost less per watt.

The cost of battery capacity is about $200 per 100AH. The 255 AH battery we bought was about $400. A 50 watt Seimens solar panel is about $300. The davit to mount the panel could overshadow the cost of battery bank and panel. Commercial davits such as Forspar or Kato are intended to swing out of the way and be removeable and cost $700 to $1,200 plus installation. If a simple fixed davit is desired, the cost to have one custom made may be less.

An anchor light consumes about 1A and if run overnight 10 AH. If anchored and not sailing, any of the solar panels would be able to support the anchor light and some cabin lighting.

It is only when underway that daily power consumption can exceed solar panel capacity. A water towed generator may provide the best solution to the problem of sailing multiple days either with a smaller set of solar panels (one or two 40 or 50 watt panels) or on consecutive overcast days. These generally cost $800-900 new. The main contendors are Ferris and AmpAir. Ferris claims 2A at 3 knots and 10A at 6 knots, but Practical Sailor acheived only 1/2A and 7A and about 3A at 4.5 knots. This seems more in line with AmpAir's claims of about 3A at 4.5 knots. The discrpancy was that Practical Sailor's tests were run with batteries already at a high state of charge while Ferris ratings assume a battery at 50% charge. The Ferris 10A water towed generator we bought was a refurbished trade-in for $600.

At this point the only remaining step is to choose a solar panel configuration and install it. The current solar panel is a 6W panel useful only for float charging.