FPB 781 Cochise Solar Update Spring 2019

It is late spring in the Bahamas, water temperature is 83/85F and air that or more. Humidity often is in the 80% range. We are making water, staying comfortable with air conditioning in the evening, generally leading a carbon neutral existence. Welcome to the new world of solar panel cruising.  What follows is a bit of data and several suggestions that might help on your own vessel.

FPB 781 Cochise has 16 solar panels with an average output of 340 watts each. The six on top of the Matrix deck roof are unshaded 100% of the time. The other ten, at each end of the Matrix deck roof we think are producing 65% of the time. Our present output varies between 18.5 and 31.9kWh per day. That is a lot of electrical power.

If you discount HVAC (air conditioning) from the energy calcs this solar array easily handles all of the rest of the loads. With temperatures and humidity levels a touch lower we are energy neutral. But now that it is warming up we need a bit of air conditioning for cruising equanimity.

The FPB 78s have excellent natural air flow at anchor. Couple this with massive insulation and normally just the sleeping cabins need air conditioning in the evenings. Our 1600 amp hour traction battery bank has the capacity to take us through several evenings. But when the breeze becomes light, as it is now, natural air flow needs augmentation. Step one is to run the air conditioning during the sleeping hours. With one guest cabin and the master suite in use our basic solar array takes care of 95% or more of our needs.

The last couple of days have been warmer, with light winds, and high humidity. This calls for more air conditioning. Before we bring the genset online aboard Cochise we:

  • Drop window shades or rig awnings to keep sun from making direct contact on hatches and windows.
  • Assuming six knots of wind, or more, we open the foredeck hatch, swim step door, and great room door to enhance natural air flow.
  • The four large in coaming Dorade vents in the great room are opened.
  • Air flow in the great room is enhanced with a 16″ Vornado fan, powerful and quiet.
  • We may spend most of the indoors time on the Matrix deck which has the best air flow aboard due to its elevated position and multiple opening, doors and windows.

When the breeze drops off in the evening some  nights we run the great room air conditioning. We start off with the two 12,000 BTU units, and once at a comfortable level often drop to a single 12,000 unit. During the period the sleeping cabins are being used these are air conditioned. In this mode the solar array has power left over for a bit of clothes washing, possibly with electric drier in use. Depending on vessel orientation and cloud cover the genset will not be used, or if needed run for no more than an hour a day.

The last couple of days have been warmer and more humid with the breeze often below six knots. We also have some cruising friends aboard. From mid-day on we have been running one of the 12,000 BTU air conditioners in the great room. The dorade vents are closed. The air con dehumidifies, moves air around, and drops the temperature a couple of degrees.

If we go exploring in the dinghy and close the boat up while we are away, Cochise is much warmer when we return. In this situation we will fire up the second great room air conditioner (another 12,000 BTU unit) and possibly the 24,000 BTU unit as well. These in concert will quickly bring the temperature down to the 78/80F range, and the extra BTU capacity drops off as temperatures come down.

Keeping electrical loads neutral now requires an hour or so of genset time. With the large traction battery bank – 1600 amp hours/24 volts using a C2o discharge rating, we can go three or more days without running the genset, using battery capacity to offset lack genset operation. We will sometimes defer the genset if we have a passage coming up, as the engine mounted alternators can bring the batteries up on capacity.

At some point comfort makes a bit more genset time necessary. What we often do is to run all the air conditioners at once, late in the evening, dropping temperature and humidity below norm. When its time for bed the genset is shut down and we run just the cabin air conditioners.

We are back in Beaufort now. It is much cooler. Here the solar array easily handles any air conditioning needs – so far.


Posted by Steve Dashew  (May 18, 2019)




2 Responses to “FPB 781 Cochise Solar Update Spring 2019”

  1. marcus Says:

    nice update.

    would you be comfortable being off the boat with it closed up and aircon running to keep it cool whilst away? – that is just on inverters and without genset running

    at what point do you bring the genset on? – when batts are down to perhaps 50%?


  2. Steve Dashew Says:

    Hi Marcus:
    Whenever we leave the boat we turn off the fresh water pressure pump. We do not leave air con running, even for a few hours. As to the genset use, we play that by ear. We typically use it at night when dinner is being prepared, so we can load it with cooking, making water, and air con.