Cruising Compromises: Air Conditioning

Air conditioning on board: The logic behind the compromises.

Water temperature is 85F (28C), air is a bit a warmer, and we’ve been enjoying lunch in Wind Horse’s salon (it being too hot outside). The air conditioning is coping with the heat load – barely, but enough to keep humidity down and the inside air temperature at 78F (all the glass makes for a huge heat load). We’ve only partially set our awnings, and this job has moved to top of the priority list.

The compromise in our heating and cooling systems, and their balance with genset capacity/use, and the ability to use inverters to provide AC power under way is at the heart of the decision-making process for this boat. You have to make compromises, because just throwing capacity at the problem does not always yield the best answer.

The same holds true for almost every aspect of design and systems, with new boats and old. If you shoot for a perfect answer for all conditions, odds are you end up losing more than you gain. The key is to carefully review priorities, go for what provides the best answer most of the time, and put up with less than ideal on those rare occasions when you are outside the design parameters, like we are right now with our air conditioning.

From the beginning we wanted to be able to run the boat’s AC electrical needs off alternators on the engines, via inverters. We did not want to be tied to a big genset at anchor or under way. Since the biggest electrical load is air conditioning, that’s where the design cycle started.

The forward cabin has a 12,000 BTU unit and the aft cabins share a 10, more than enough even in the present situation to keep them cool and dry for sleeping. The salon, with those aforementioned windows is a tougher situation. A pair of 16,000 BTU units is barely enough, if they run at 100% duty cycle and we have our awnings set. These modest sized units have the advantage of being easily handled by our inverters and small genset. If we went to the next size up, a pair of 24,000 BTU units, we’d need to go up in genset size, and the inverter package would never handle the simultaneous starting loads, unless we substantially beefed up the inverter bank. But if we did that, we could not reliably create the AC power with our alternators, and would be forced to use the genset under way. There would also be a problem on shore power with less than 50 amp circuits.

On the other hand, we are in essence out of season weather wise. We’ve got about 20 days of this hot weather to deal with between here, Panama, and the Bahamas. Once we get to the Bermuda, in mid-May, the environment will have cooled considerably. That’s 20 days in the past three years and when this summer’s cruising is done, 33,000 miles. You can see where we are headed with this logic.

Now, if you wanted to keep your boat in a marina in Texas or Florida for the summer, a different set of priorities would apply. That would be your normal environment, and you would have no choice but to put up with the big genset running 24 hours a day, or never leave the dock.

But if you want to cruise, to “cut the lines and go” to paraphrase one of our favorite artist’s songs, then compromises need to be made – which is why our little salon air conditioners are working so hard.

The same logic applies to the rig, draft, interior layout, and a host of other topics. Outfitting or designing a successful cruising boat starts with a carefully crafted list of priorities, a game plan if you will, for how the boat is going to be used most of the time. Optimize for that, and then see what is left (in budget, space, or complexity) for the fringe conditions.

PS: If we were cruising in the normal sense, moving with the seasons, we would have adapted to the climate. But most our cruising these days is with ice nearby, so we have lost some of our tropical blood.

Posted by Steve Dashew  (April 21, 2008)

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