Hey Guys (Gals?),

I'm a new member to the forum, just signed up but have been reading for a year. I just posted in the new member area and here comes post #2. What prompted me to sign up now is I thought you all might have some insight on an idea proposed to my noisy AC arrangement.

I have a 16k btu unit in the main salon with a 6k btu 110v window unit in the hatch of the rear cabin. I also have an original (1983) 7.5 Onan diesel generator, which works flawlessly and is immensely noisy. Single perkins, with a 120amp alternator , 2-8D house batteries and one each isolated engine and genset batteries.

My personal situation: My wife and I like to go on quiet overnight and weekend trips. We both still work so a few days at a time is what we can take right now. My wife (and truth be told, myself) like to sleep in a cool (70ish) room. We have done this exactly 2 times in this boat. The first time, my daughter, sleeping in the front v berth, begged me to turn off the genset because it was soooo noisy. She didn't have to wake me since by 3am or so, I hadn't slept a wink, worried about the untested (for me) genset and the noise. The second time was because I couldn't believe that it had been *that* loud the first time. It had. So for a while my wife and I would overnight on the boat in the slip on shore power. Being tied in the slip, enjoyable as it may be, is not the reason we bought this boat, it was to take it out and enjoy it while on the hook.

Two paths considered: I am researching sound insulating blankets for the genset. I know of great southern insulation in Fort Lauderdale and am looking for others. If this is reasonably effective, it my just solve my problem but this last week at the ft.laud. Boat show I saw at least 2 12v AC systems that looked interesting. As I asked around a bit I got several "that won't work" kind of answers. It seemed to come down to how long the thing (3.5k-6k btu) would run. I wrote off the idea as not practical and moved on but then a Freind suggested I take another look. My rear cabin is about 475 cu/ft. I have some space under the bed for extra batteries. If the whole thing would cool the space for say, 8 hours, in silence, and then charge using the engine alternator while we cruise the next day, it might be a better solution than the genset. I realize I must learn something new, something alien to me, that is the amp hour/watts/btu/battery capacity formulas. Until I master that, what do you guys think? Why haven't I seen this on another similar boat before? That makes we wonder.

Have mercy.

I should have titled that. Learning

How many amps does it draw?

I have a 9 KW onan generator in a sound enclosure, a metal box with sound deadening barrier foam insulation. It is very quiet when running. Maybe the local Onan dealer can recommend something for you.
good luck

The 3.5btu is "29 amps at 12volts"

I would try the heat blanket route. We have a Northern Lights 12kw with a sound shield and it sounds like another air conditioner. We run ours at anchor and underway.

You would need about 500 Amp hours to cover running for 8 hours. That would keep you within the discharge limit of 50%. Given that you are in Miami you would need to run probably a couple of extra hours to cool the room down so that gets you up to 600 amp hours. That would be a lot of extra batteries and wiring. Also I am not sure if 3.5 BTU would be enough. Someone else may be able to say. I think your most cost effective and successful approach would be to go with the sound shield box from Onan. The sound shield does make a big difference.

It is quite possible to run an ac unit off batteries and an inverter....for night use only. While boats vary widely, my charter boat has a 16K btu unit that runs near 100% on a hot summer day tied to the dock. On a very warm summer night, the duty cycle is less than 20%. So theoretically, 3.2K btu would work very nicely. A continuously running 5K btu unit draws 5 amps 110 ac or about 55 amps 12 volts dc through an inverter. Thats about 440 amps in 8 hours continuous duty, obviously less intermittent. If you want to figure out how much you need, measure the duty cycle of your unit at night and do the math. Having a second smaller ac system for night or just master stateroom use could be a viable option.

Ted

I have to side with the "not practical" camp. Depending on the boat and the conditions, it might not even be possible.

On warm summer days in Maine, my two, 15,000 BTU units going continuously won't cool my boat below the low 80's. It would only reach 70 after the sun goes down, and even then they'd be running 50% of the time or more if it's a hot night. I'd have to fill every inch of space on my boat with batteries to drive that load.

I have the same Onan 7.5 genset and it's a real tooth-rattler. Luckily it's almost silent outside the boat, so at least I don't disturb the neighbors at anchor.

Even if you get enough battery capacity to run two A/Cs all night, I doubt you'd be able to recharge them fully from the engine alternator during a typical day's cruise.

Anyway, my point was not to buck the path this thread is going, but to offer an alternative if batteries don't work out for you.

Get a Honda eu2000i and run it on deck all night. You won't even hear it over the air conditioner, and it has a throttle-down option so it's very quiet when there's no load. It's not totally silent, so you still might get complaints in a quiet anchorage with others nearby who have their hatches open. But if others are running their gensets, the Honda won't even be noticed.

These guys made a great aftermarket sound shield enclosure for my 9KW Onan. Great Southern Insulation | Sound and Thermal Insulation
They had a booth at the Lauderdale boat show.

That 7.5 Onan is a very loud and abnoxious gennie. Two cylinder has horrible balance, so not only is it loud, it shakes like crazy. They can be very reliable, which sucks in a way as this thing must be pretty old and still works.

A sound box will probably give only a marginal improvement. Best to save some money up and get a modern 3cyl unit sized right for the boat. A 5kW might work, but need to add up loads to make sure.

A 3cyl usually is fine with no sound box. The noise it makes is more of a "purr", and lots of folks, including me, find it encourages sleep.

This was the reason I thought it would be a good idea to join the list. A wide variety of ideas, each given with a supporting argument.

I hadn't thought of the inverter idea. That might give me a few hours with existing batteries. I'll have to look into that. I could continue to use my window unit. One of the downsides of the 12v system is that it's so small at 3.5k that when at the dock you have a unit working hard to cool the space and the power is coming from batteries and a charger. That can't be good in the long run.

I'm leaning towards reducing the genset sound, either by a sound shield on the old Onan or a small portable on the fly bridge. I have a dingy with gasoline now so a little more carefully stored is okay. I think the cost of each route is about the same so that's not really a factor. I'm going to research both ways. Thank you for all the good, and better, ideas.

When I put AC on my boat, I looked at running it on batteries and decided it wasn't practicle. I don't have room for a real generator so I bought a Honda 2000. It's only a 5000 btu AC but the Honda wouldn't start it until I put a hard start device on the AC. It only takes about 600 watts to run the AC but it takes over 3000 watts to start the compressor. I think newer AC units have smaller starting loads but you might want to try it before you invest in a portable generator.

In your situation my choice would be a sound shield or quieter generator.

Can't help. Would never spend substantial time east of the western coast of USA or otherwise at an altitude of less than 5000 feet. It's a matter of surviving.

David O said:

These guys made a great aftermarket sound shield enclosure for my 9KW Onan. Great Southern Insulation | Sound and Thermal Insulation
They had a booth at the Lauderdale boat show.

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Each unit varies and some of the older are worse than current but sound shields work wonders. Oman makes Quiet Diesel QD models but not sure if any conversions are available for other units. We have Northern Lights with sound shields and they are incredibly quiet. We also have Kohler with Sound Shields. The Northern Lights have Lugger engines, the Kohler have John Deere and the Oman has Cummins. I think the Lugger are basically quieter to start with.

I'd also ask how noisy your engines are when underway. If noisy, then you might look at not just a shield but at your overall sound deafening to the other areas of the boat.

Both Red Dot and Dometic make 12vDC air cond in small sizes,

but it may take loads more batts to operate , and all day long of running to recharge them.

Greetings,
Mr. BB. RE: Post #15. I think you may mean Onan in lieu of Oman. Have another coffee.

RT Firefly said:

Greetings,
Mr. BB. RE: Post #15. I think you may mean Onan in lieu of Oman. Have another coffee.

Click to expand...

I think you are definitely correct. I don't drink coffee however. It was midnight. Obviously it was time to turn my attention to something other than generators. Perhaps sleep, perhaps....more likely....Ms. B. I'll blame it on her distracting me.....lol

Actually, George Burns once had a skit in which he spoke of coffee during the entire skit but you knew all the time he was talking about something else. So guess what we did could be referred to as having some coffee.

BandB said:

I think you are definitely correct. I don't drink coffee however. It was midnight. Obviously it was time to turn my attention to something other than generators. Perhaps sleep, perhaps....more likely....Ms. B. I'll blame it on her distracting me.....lol

Actually, George Burns once had a skit in which he spoke of coffee during the entire skit but you knew all the time he was talking about something else. So guess what we did could be referred to as having some coffee.

Click to expand...

Wifey B: While he doesn't drink coffee, he sure does like "coffee." hehe

Now see what you started RTF!

Me distracting? Innocent lil me?

Coffee