How much air cond is enough?

So, it is Monday of Memorial Day weekend and I'm sitting here wondering about systems in our boat to be, the American Tug 395.
As I can't contact anyone with "authority" this morning I thought I'd fire away with my question here...

It appears that American Tug equips these boats with 2 air conditioning units. Certainly the older 34/365's have 16,000 and 12,000 BTU units installed (from my research on yachtworld) ... The 395 comes standard with a 6kw Northern lights generator too and that would suggest a maximum of 2 air conditioner's. The AT website lists 26,000 BTU's of capacity for east coast models.

Is 26,000 (or 28,000) BTU's enough? Seems like a lot of boat for that amount.
We have a 16,000 BTU system on our 38' sailboat and it is fine but I'm simply wondering.... We will certainly take the boat south into Florida once or twice.

Any thoughts?
Bruce

Many factors are in play...

No one cruiser a ND certainly not heat gain formulas can answer you question ...to a point.

Boat spaces tend to be small and things like frequent cooking, more people. How you manage air exchange, window coverings, awnings, etc etc all factor in as does climate at its worst to include hunidity.

Basic BTU calculations are good but then only fudging works. You don't want too big or too small per unit, so even location matters.

My experience is that more units the better to balance what you need. You need more than enough for the worst conditions, yet split enough that one unit isnt so much it frequently cycles.

Boat layout not withstanding.

Bruce B - There is an American Tugs Owners website open only to owners that might be of help. Now that you have officially ordered one, you probably qualify.


https://www.bigtent.com/groups/atugoa

psneeld said:

My experience is that more units the better to balance what you need. You need more than enough for the worst conditions, yet split enough that one unit isnt so much it frequently cycles.

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Agree.

The only thing I would change in that regard on my boat is a smaller unit in the staterooms area. Would like that one to cycle less frequently at night. If I'm on the boat by myself, I leave the stateroom door open and only run the saloon AC . The cold air flows down hill to the staterooms. With a guest on board and the stateroom doors closed, it short cycles. In a perfect world my master stateroom would have its own remotely located 5,000 btu unit. Thought about it briefly when refitting the boat; should have done it.

Ted

Here's a sizing guide from Dometic. I'd think the major players in the marine AC market would have something similar.


Marine by Dometic Group - Planning guide

It's much like asking what temperature should you keep your home in summer. However, when in doubt, I'd always go for more. Borderline, I'd choose more.

First, I've never heard owners complain that they have too much a/c, that it keeps the boat too cold. Second, as the systems age they tend to not cool as well as they did when new.

Also, while you're in RI, the next owner may be in Galveston. There's also the reverse side of heat and the next owner may be someone deciding to live aboard year round in Boston.

Looking at the Dometic charts, they list air temperature, water temperature and humidity on each of the classifications. I'd say if any of the three elements exists to move up to that level. We're in South Florida. Our air temperature would be in the Temperate level but our water temperature and humidity definitely in the tropical level. For Texas, I'd think Arabic Gulf. lol

One thing to consider...are they going to be used as reverse cycle heat? If so the the equation changes yet again. The placement and ducting become very important then.

As to whether you can gave too much....sounds good but manufactures I believe have a different opinion and you should read their tech stuff on it. Especially if you plan to use the humidity control.

Humidity control can be in portal even if heat and air are not that big of a deal. Especially for those that cook an shower a lot.

BandB said:

It's much like asking what temperature should you keep your home in summer. However, when in doubt, I'd always go for more. Borderline, I'd choose more.

First, I've never heard owners complain that they have too much a/c, that it keeps the boat too cold. Second, as the systems age they tend to not cool as well as they did when new.

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Air conditioning needs to be sized properly. While a bigger unit may sound like a good idea, they don't lower the humidity as well. The greater the percentage of time the compressor runs, the better the unit will lower the humidity. When air conditioning, lower humidity for the same temperature is more comfortable. I much prefer a dry slightly warmer temperature over cold and humid.

Ted

I went minimal when I spec'd my AC's. 12k in salon, 6.5k in two staterooms in the fwd turtleback. On a hot day in full sun the heat coming in the unblinded windows can overwhelm the salon unit. I could put up shades, but boat is usually on the move so want the visibility. But salon remains comfortable to me. Can it keep a constant 68F?? No, but it does well enough. Fwd unit does pretty well because there is less solar heating due to less window area.

When underway, I rarely run any ac, just keep boat opened up unless spray is real bad.

Biggest thing was whether it would be comfortable for sleeping. And it is. No solar heating then. And no short cycling.

I'm happy with the sizing. Another advantage is everything can live off one 30A shore cord with minimal load management required.

O C Diver said:

Air conditioning needs to be sized properly. While a bigger unit may sound like a good idea, they don't lower the humidity as well. The greater the percentage of time the compressor runs, the better the unit will lower the humidity. When air conditioning, lower humidity for the same temperature is more comfortable. I much prefer a dry slightly warmer temperature over cold and humid.

Ted

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I didn't mean to imply to go overboard but "properly" is difficult to define. So I'd say adequate to take care of all conditions likely to be experienced. I'd also say that I still haven't known of complaints regarding too much and have heard many wishing they had more. There's a greater tendency to undersize than oversize so that's what I'd protect against. Now, for the reasons you point out, I prefer more units to larger and fewer units. I do find humidity control most important in both summer and winter as the dry air experienced running heat in winter bothers my sinuses and kicks off my allergies.

Our 16K cruisair heat pump split system is sufficient when the sun is not blasting heat onto the boat through clear windows and no bimini top up. Most all our windows can slide open. If you could add up all the open spaces including ceiling vents, I imagine it is like more than 1 sq foot wide open space to the outside.

If the system was doubled in capacity, I think it could easily cool the whole 37 foot boat.

Our lower forward section, the cold air flows there. Other upper salon half of boat is much warmer, if the sun is out.

I need to fix the top, and put on window shades for the forward salon windows.

My sliding salon windows, I need to replace the inner felt seals. The fit onto or into a SS groove, and they seal window to window (where the windows slip past each other) . Anyone know what I am talking about? Must be some kind of felt strip, self adhesive? If you have seen the double window track where a window glass slides back and forth, that is what I have. About 3/16 inch gap between sliding panes.

Me, I'd buy whatever American Tug says you should buy for hot weather climates, say a boat that will be in FL year around and whatever that is will also keep you cozy in northern climes in fall and winter. You can indeed have too much and as well as not enough.

I doubt that a 6 KW generator is big enough for serious air conditioning plus boat loads. I would carefully evaluate your powered needs given other electrical equipment such as water heater, battery charger, stove, oven, microwave, refrigeration etc.

caltexflanc said:

Me, I'd buy whatever American Tug says you should buy for hot weather climates, say a boat that will be in FL year around and whatever that is will also keep you cozy in northern climes in fall and winter. You can indeed have too much and as well as not enough.

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This is what I will most likely do!
I still love going through the exercise of planning a system and the comments and input I get here is helpful. I find that the comments help to set expectations and sort of define the range of possibilities.
Many thanks to everyone who takes the time to contribute!
Bruce

hmason said:

I doubt that a 6 KW generator is big enough for serious air conditioning plus boat loads. I would carefully evaluate your powered needs given other electrical equipment such as water heater, battery charger, stove, oven, microwave, refrigeration etc.

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That's an easy exercise to go through. In the end it is simply math!
The idea of keeping the mechanical injection diesel (The generator) under a load makes me happy though. It is not a case of my not trusting the builders decisions but rather my wanting to understand the compromise's taken. Nothing says compromise like a boat after all...
Bruce

There's a plethora of things to consider if you're coming to Florida vs. the PNW. Hull color, boat layout, window shades and canvas to begin with. My boat's volume is likely more than the 395 but is on a single level, air moves easily. Multi levels that are connected can depend on air falling down into the lowest areas so one might have to regulate air flow a bit more. My boat has an older single Cruiseair 16K BTU but runs through very well insulated ducting. 90 degree days here are manageable but much more and it begins to fall behind. I'm planning to install another 12K BTU unit for the pilothouse, but not before I do the new windows with clear 3M UV blocking film. This has been done on at least three new KK's that I'm aware of, and it made a huge difference in pilothouse comfort in all three. Lastly, the cracks and crevices that was noted in previous posts. There's a world of difference between an efficiently sealed space and a leaky one.

healhustler said:

There's a plethora of things to consider if you're coming to Florida vs. the PNW. Hull color, boat layout, window shades and canvas to begin with. My boat's volume is likely more than the 395 but is on a single level, air moves easily. Multi levels that are connected can depend on air falling down into the lowest areas so one might have to regulate air flow a bit more. My boat has an older single Cruiseair 16K BTU but runs through very well insulated ducting. 90 degree days here are manageable but much more and it begins to fall behind. I'm planning to install another 12K BTU unit for the pilothouse, but not before I do the new windows with clear 3M UV blocking film. This has been done on at least three new KK's that I'm aware of, and it made a huge difference in pilothouse comfort in all three. Lastly, the cracks and crevices that was noted in previous posts. There's a world of difference between an efficiently sealed space and a leaky one.

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That is interesting..
Our Sabre 386 sailboat got its first air conditioning last year with a 16,000 BTU unit and it makes such a difference I can't imagine how we did it before! Maybe the 26,000 BTU's is plenty...
I'm guessing that the people at American Tug have this figured out and that they will provide a great system. I know that everything I've looked at is well though out and very nicely executed...
Bruce

It depends on your location. Here, all is needed is sometimes to have no doors or windows open, but one or two or maybe three doors provides plenty of cooling breeze. Like last Friday, one pilothouse door open was sufficient. (Perla often complains there is too much ventilation as in both pilothouse doors open. No genset on this boat, and definitely no A/C.)

One of the other considerations is underway versus at the marina. I don't use the AC underway very often as the pilothouse has lots of airflow and there's almost always a breeze on the water. However, in a protected marina there can often be minimal airflow, more humidity, and higher temperatures. This doesn't speak specifically to how much AC you need when cruising and are limited by the size of your generator, but how much you may want when shore power is available. From my limited cruising experience my AC requirements were greatest at the dock. I think I can run everything on my boat with 50 amp shore power; would have to load switch with the 8 kw generator,

Ted

hmason said:

I doubt that a 6 KW generator is big enough for serious air conditioning plus boat loads. I would carefully evaluate your powered needs given other electrical equipment such as water heater, battery charger, stove, oven, microwave, refrigeration etc.

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With a little load management, it should be fine. I have a 5kW NL and can run the 12k and 6.5k AC's and a 1500W water heater and a 1500W air compressor all simultaneously. That is how I load test my machine. It is super rare that all are on at the same time. If load gets too high, easy enough to open a breaker and shed some load.

harbor950 said:

Bruce B - There is an American Tugs Owners website open only to owners that might be of help. Now that you have officially ordered one, you probably qualify.


https://www.bigtent.com/groups/atugoa

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Hey, thanks for pointing me in their direction! They did indeed let me join and it is a wealth of information and insight.
Thanks again,
Bruce