BTUs In Marine Applications

I get asked about BTUs in marine applications.  I hear folks installing 16,000 BTU, 24,000 BTU air conditioners on their boats. But the question is, do you need to keep your boat at 68F?  That’s a choice.

Personally, if your boat needs to be at 68F, maybe boating is not the right activity for you. 

The magic of 15F degrees – 

Our body begins to detect ‘cool’ when there is 15F temperate change in the environment.  If outside temperate is 95F, and a cabin down below is reading 80F, your body will feel cool.  

How many BTUs, you ask?

There’s a lot of hype out there about BTUs.  But you don’t necessarily need more BTUs in your marine air conditioner to get relief from the summer heat out on the water.  I hear folks installing 18,000 BTU, even 24,000 BTU air conditioners on their 40-50 foot or even smaller boats.   Here is food for thought: an 18,000 BTU air conditioner equals a 1.5-ton residential air conditioner.  That would be typically installed in a 700-800 sq/ft house (with 10 foot ceilings)!   That’s overkill for a 40-foot boat.  In fact, the AC needs of a boat are measured by one leading marine AC company’s calculations which became the generally accepted standard for sizing BTUs needed for a boat – the same calculations used by companies to determine the AC needs of a house – which has square rooms, good air flow, and is not surrounded by water(!).  Not surprising, the core business of many of these companies is household appliances and residential air conditioning (https://www.dometic.com/en-us/us/about-us/our-brand/history). 

Apples and oranges, we say.  To determine efficient air conditioning of a boat, you need to factor in all the consequences of its environment, including the water temperature!  What boaters really need to be more comfortable down below is removal of humidity. That’s achieved by installing a longer running variable speed compressor in a smaller unit that actually goes through its full cooling cycle.  This way, opting for a few smaller units in one boat will be more economical and makes for a less energy-hungry boat.  As we all know, the interior design of a boat usually has irregular corners and nooks and obstructions which would prevent good air flow in the cabin, so more than one unit would be a better choice.