What type and how many AC BTUs for a 27' Watkins

[akeegan]

Hello y'all,

I would like to install an economical (Frys, Home Depot, Lowes, etc.), free-standing AC unit into my friend's 27' Watkins moored in Merritt Island, FL. I know the formulas, but does anyone have any real-life success AC stories about this particular boat, including suggested BTUs?

 

[TFrere]

Are you thinking about a portable unit with the hose exhaust? If so, go larger than you think. I had a Hunter 27 with a 9,000 BTU and it would not keep up during the day in South Louisiana. Whytner makes a nice compact 13,000 btu for about $500.

 

[akeegan]

Are you thinking about a portable unit with the hose exhaust? If so, go larger than you think. I had a Hunter 27 with a 9,000 BTU and it would not keep up during the day in South Louisiana. Whytner makes a nice compact 13,000 btu for about $500.

TFrere,

Thanks for the quick reply. Yes, a portable unit. Hadn't seen one with an external exhaust, but of course that makes sense. And bigger is better, I agree. I will investigate Whytner.

 

[Ross]

You cooling one uninsulated room in a very humid enviroment. When the doors are openned you will be starting over each time.

 

[Andy1237]

We put a 5000 BTU window unit in the companionway of our previous 29.5 and it cooled great. The top hatchboard would fit in and we cut some styrofoam insulation board pieces to trim around the angles. It was a hassle to crawl over or drop through the V-berth but it kept the boat cool.

 

[Bad Obsession]

How do you calculate the square footage inside of your cabin? I am just wondering what my square footage would be for a Catalina 30.

I had a 7000 BTU window unit that I made hatch boards to fit my hatch. The problem was that it was a pain in the butt to remove, step over, or carry since it would leak water.

The stand alone units are less efficient than a window unit, so I have heard. It would help to know what the square footage of the cabin was and then go a size bigger. I am thinking that between 10,000 and 12,000 btu unit would work for my cabin....Any thoughts?

 

[Bill Roosa]

Your square footage is length of the room times the average width. You then add up all the footages to get a total.
So for a Vee birth it would be length from the (pointy end to the bulkhead)*(width of the pointy end + width at the bulkhead end)/2
For a salon you can get a better guess by measuring at three locations, both ends and the widest part and dividing by 3.
I believe you actually need the cubic feet to calculate BTUs needed though. Once you get the footage just multiply by the height of the cabin(s)
If they ask for an R-value for the walls use 5

 

[Bob J.]

When it comes to air conditioning larger is not always better. You do not want the a/c unit to short cycle, better to have it run for a longer period of time. This is what provides the dehumidification of the space. When an a/c unit shorts cycles, it cools the space but it doesn't remove moisture so it feels clamy.

 

[wflowe2]

I put one of the free standing units in a 28' Beneteau. The exhaust hose put off so much heat that it defeated the cooling . I subsequently cut a second set of hatch boards to accommodate an inexpensive window unit and it worked fine. See photo. Another advantage of the window unit was I didn't have to move or secure it when I sailed.

 

[Ross]

Square foot calculations will be3 misleading because they figure standard rooms with 8 or 9 foot ceilings. calculate cubic feet. Figure 76 pounds of air per thousand cu.ft. and one BTU per pound per degree f. Water vapor, i.e. humidity, requires 1000 BTU's per pound to remove, The dew point will be the threshold that you must cross to cool the space lower. Dry air is easy to cool but humid air has a lot of energy that must be removed.

 

[zeehag]

here in banderas bay i am having beautifully cool huge uninsulated boat with 5000btu lg unit. plus one fan to move it around works perfect.

use tarps to keep sun off coach house roof--makes a huge difference.

 

[akeegan]

Your square footage is length of the room times the average width. You then add up all the footages to get a total.
So for a Vee birth it would be length from the (pointy end to the bulkhead)*(width of the pointy end + width at the bulkhead end)/2
For a salon you can get a better guess by measuring at three locations, both ends and the widest part and dividing by 3.
I believe you actually need the cubic feet to calculate BTUs needed though. Once you get the footage just multiply by the height of the cabin(s)
If they ask for an R-value for the walls use 5

Bill,

You're right; BTU calculations need cubic feet (or meters) in order to be accurate. I saw a boat manufacturer's thread on this a while ago, and it suggested 14 BTU / cubic foot as a minimum value, going up to 19 BTU / cubic foot depending on insulation (R-value), direct sunlight, etc. Interestingly, the 14 BTU value comes out to about 10000 BTUs for the Watkins 27 cabin volume. But from what I read here, 10000 BTUs is a bit undersized.

Also, just found a free iPhone app, rated 4+, includes R-value, that appears way too complicated for this problem, but looks fun. Here's the link:

http://itunes.apple.com/us/app/btu-heater-calculators/id415594032?mt=8

 

[Bill Roosa]

Engineers just use conservative estimates and then turn the temp down. That solution ALWAYS works.
Also you can dramatically reduce the BTUs required by installing sun screens over the deck. I did this for the salon and foredeck (bimini handles the stern) and noticed a 10 deg F drop in about 15 minutes. It was in the high 90s and full sun. It worked so well that I did not have to turn on the AC and the cabin cooled down more rapidly once the sun went down. An added benefit is you can leave the hatches open in a rain storm!

 

[Stu Jackson]

Bill's right. There are two basic "engineering" loads: solar and the difference between your hull and the outside temperature - the heat difference between inside and otuside.

Cutting down on the solar load through your portlights will go a loooong way to reducing your A/C load.

 

[Bob J.]

You would not want to use the temperature difference calculation for a boat. This equation is used in conjunction with solar/equipment heat gain calculations to determine the additional gain for an interior adjacent unconditioned area(s) within that given space.

Both the Watkins & the Catalina could get away with a 9000 BTU unit. In the case of the Watkins being in Florida, sun shading as others have noted would be helpful. Going with a 9000 BTU unit would allow the a/c to dehumidify the space so it didn't feel like a meat locker.

 

[kloudie1]

The other huge load at Al's location is the dehumidification load.. Condensing water out of the air in a humid location like Merritt Island is a big load.. Occupants contribute to the load as well.. Each person (seated at rest) is generates a total load of 330 BTU/Hr... Every pound of water condensed out of the air requires about 1060 BTU.. All that said, it is important to shade the decks if possible and reduce the ventilation as much as possible and shield the ports/hatches/companionway as much as possible..
9000 BTU on a 27 foot boat sounds about right.. It will run a lot during sunny days, but it will be comfortable... and at night, it will be able to freeze ya out of there.. I wouldn't use the wheeled portables in a location that is very humid because it forces intake of humid air from outside.. The built-in one is best but the companionway mounts are next .. their downside is access to the cabin and hot air and noise in the cockpit.

 

[Stu Jackson]

WM Advisors, sizing suggestions at the bottom of the article

http://www.westmarine.com/webapp/wc...Id=11151&catalogId=10001&page=Climate-Control