Why electric AC?

[capn_billl]

Your average AC takes 10-15 Horsepower of drag on the engine to operate.

This load is intermittent as the AC cycles.

A car AC is designed to cool 40-60 cubic feet of living space. An average cabin boat is several hundred cubic feet.

That is why a 300HP car engine has no problems cycling a 10HP drag with a clutch.

Scale that up, and you end up with a fairly large engine to run it, half the time idle, and half the time full load.

My boat has two 20,000BTU AC's that draw 20 amps each.

SO running an AC from a generator that charges batteries in between AC cycles makes more sense.

Even more sense is the newer DC powered Air conditioners from an electric car that can at least crack the sweat from a small well insulated cabin so you can sleep.

With your generators keeping the batteries topped off, and powering the AC at night, and solar panels during the day.

Several Youtubers have done this with varying results.

[Lake-Effect]

Originally Posted by EngineerRetired

With a genset the diesel runs at at a constant speed to turn a generator that produces electricity that is used to spin an electric motor, that turns a compressor. I am proposing driving the compressor directly with the diesel. It's RPM can can easily be altered to meet demand. Get the boat cooled off or only cool one cabin and the RPM can drop to a fraction of full RPM.

Less fuel, longer life, not nearly as expensive. When not being used used for the AC, the same small diesel can charge house batteries or run the water maker directly.

There's more than one kind of efficiency. There's running efficiency (work done divided by energy input), but there's also efficiency of use... is this custom system going to be in use often enough to justify its purchase and upkeep? Will it be good enough at charging or watermaking that you wouldn't also need a conventional AC generator?

Again, this is all off the shelf tech and should be fairly low cost to build.

What you've outlined is intriguing, and it's not hard to imagine the yacht of the future having a "swiss army" power plant that performs all those functions. But for now, as custom assemblies... you'd be a guinea pig, and how do you get it repaired in the middle of nowhere?

[EngineerRetired]

Your average AC takes 10-15 Horsepower of drag on the engine to operate.
This load is intermittent as the AC cycles.
A car AC is designed to cool 40-60 cubic feet of living space. An average cabin boat is several hundred cubic feet.
That is why a 300HP car engine has no problems cycling a 10HP drag with a clutch.
Scale that up, and you end up with a fairly large engine to run it, half the time idle, and half the time full load.

Simply put, you are wrong. The average mid-sized auto AC compressor is about 5 tons. (60,000 BTU.) (Larger cars up to 7 tons) It's needed to be able to bring that 120 degree interior down to 70 in three or four minutes. Source, Society of Automotive Engineers. https://www.google.com/url?sa=t&rct=...Fsc-oMYsw9323S

There simply is no way the average boat interior has or needs that much cooling power. (You can cool a very large home with 3 tons.) Much more typical is around 20,000 to 24,000 BTU. A smaller compressor (in the 1.5 to 2 ton range) is easily driven by 5 to 6 hp and will do a very good job of cooling the boats interior in the same time frame as it would a house. (Not as fast as a car.)

My boat has two 20,000BTU AC's that draw 20 amps each.
SO running an AC from a generator that charges batteries in between AC cycles makes more sense.
Even more sense is the newer DC powered Air conditioners from an electric car that can at least crack the sweat from a small well insulated cabin so you can sleep.
With your generators keeping the batteries topped off, and powering the AC at night, and solar panels during the day.
Several Youtubers have done this with varying results.

If you are using 40,000 btu to cool the interior of your boat.... WOW That's 3 1/2 tons of a/c, something a 2000+ square foot home would use in Florida

I also have to disagree that changing a rotatory motion, into electricity, into a battery charger, into batteries, into an inverter or directly into an electric motor to create a rotary motion is more efficient than using a rotary motion to drive a rotary load. It just isn't and that's the entire point.
Smaller, less thirsty, long life power to get the job done. Being able to throttle back on the engine once cool down has happened is much more efficient than a genny running at full RPM.

[rslifkin]

Interestingly, our boat has significantly more A/C capacity than our house. The house happily stays cool enough in 90 degree weather with about 12,000 BTU running flat out. In that same weather, the 16,000 BTU unit in the salon on the boat will keep it around 74 in there running flat out, but the staterooms are warmer and you need to turn on those units as well to actually get the whole boat cooled down. Installed capacity is 36,000 BTU, although the forward stateroom unit is a bit oversized (so 32k total is probably in the ballpark for what it needs).

Boats are generally insulated poorly, much like a car (especially with large, single pane windows). Houses are insulated better.

[sfnz]

if you must do this then best go with carrier or thermoking, they've been at it for years

[skipmac]

One point that I haven't seen mentioned. AC powered air conditioners have a fully sealed motor/compressor. Externally driven compressors (like belt driven compressors for auto air conditioning) the compressor has a shaft through the housing requiring a seal that will fail sooner or later. They have gotten better but I keep cars 12-15 years and usually end up replacing the compressor at least once. Sealed units like compressors in refrigerators, home air conditioners, etc., for me have lasted at least 25-30 years, some after 40 years are still going strong.

[mako]

Originally Posted by rslifkin

Interestingly, our boat has significantly more A/C capacity than our house.

Boats are generally insulated poorly, much like a car (especially with large, single pane windows). Houses are insulated better.

Heat load and amount of insulation- those are they factors. I lived in a 2,000sf villa in the Middle East and we had 12 tons of air conditioning. In America it would be about 3 tons.

[rwidman]

When I read a question like this one, I factor in the thought that it's highly unlikely that someone hasn't already though of this before.

If powering an air conditioner directly with a diesel engine was a practical idea, someone would already be selling units configured this way.

[capn_billl]

Originally Posted by rwidman

When I read a question like this one, I factor in the thought that it's highly unlikely that someone hasn't already though of this before.

If powering an air conditioner directly with a diesel engine was a practical idea, someone would already be selling units configured this way.

Not necessarily.

There is a size of boat that this is practical.

A car AC powered by a diesel (or gas) engine to cool a 20 to 30 foot boat would definitely work.

That size of boat rarely has any AC at all.

Car AC's leak freon, and require constant topoff which is a significant maintenance issue.

The EV cars have DC powered compressors that should work better, and cost will come down as they become more widely used.

These compressors are already optimized for minimal power, and are small enough they shouldn't drain a battery pack too quickly.

[Dockhead]

The elephant in the room!


We use electric, and not direct drive AC compressors, NOT for any reason of efficiency.


But because boat AC system are typically used 90% off shore power if not more. Who the hell would want to run a diesel engine at the dock to run AC when there is shore power available?


I don't have AC on this boat, but did on the previous one. We didn't have a generator and didn't use the AC off shore power. Even in the tropics, this was perfectly satisfactory -- seemed there was always a breeze at anchor, but in a marina you could die without AC -- and voila, there was shore power there.


So an AC system which required running a diesel engine -- I think for most sailors that would be a total non-starter.

[mako]

Originally Posted by rwidman

When I read a question like this one, I factor in the thought that it's highly unlikely that someone hasn't already though of this before.



If powering an air conditioner directly with a diesel engine was a practical idea, someone would already be selling units configured this way.

There were gas (CNG, LP) residential aircon units developed. They achieved efficiencies of about SEER 26 if I recall. But those have pretty much disappeared as we have developed electric aircons which reach just about the same.

[skipmac]

Originally Posted by Dockhead

The elephant in the room!


We use electric, and not direct drive AC compressors, NOT for any reason of efficiency.


But because boat AC system are typically used 90% off shore power if not more. Who the hell would want to run a diesel engine at the dock to run AC when there is shore power available?


I don't have AC on this boat, but did on the previous one. We didn't have a generator and didn't use the AC off shore power. Even in the tropics, this was perfectly satisfactory -- seemed there was always a breeze at anchor, but in a marina you could die without AC -- and voila, there was shore power there.


So an AC system which required running a diesel engine -- I think for most sailors that would be a total non-starter.

My experience as well for 99.9% of the time. But then I can still recall a few nights at anchor when it was pretty warm, no breeze and the bugs descended upon us. Then it came down to go below, close all the hatches and die of heat stroke or leave the hatches open to be eaten alive by the skeeters and noseeums.

It was so bad one night we pulled anchor in the middle of the night and motored out to open water to get away from the bugs.

Only happened to me a few times in many years of cruising but those few times I would have paid almost anything for a generator to run the air conditioner.

Now have a Honda and the EasyStart on the air con I can run my 16kBTU unit just fine if I'm ever again in this situation.

[rwidman]

Originally Posted by capn_billl

Not necessarily.

There is a size of boat that this is practical.

A car AC powered by a diesel (or gas) engine to cool a 20 to 30 foot boat would definitely work.

That size of boat rarely has any AC at all.

Car AC's leak freon, and require constant topoff which is a significant maintenance issue.

The EV cars have DC powered compressors that should work better, and cost will come down as they become more widely used.

These compressors are already optimized for minimal power, and are small enough they shouldn't drain a battery pack too quickly.

My 28' Trawler definitely has AC, a 18,000 BTU self contained unit.

Cars do not typically leak Freon. A Freon leak in a car AC system is pretty rare.

If a better system comes along, you can be sure the major boat manufacturers will be among the first to begin using it in their production boats. So far, this has not happened.

[Dockhead]

Originally Posted by skipmac

My experience as well for 99.9% of the time. But then I can still recall a few nights at anchor when it was pretty warm, no breeze and the bugs descended upon us. Then it came down to go below, close all the hatches and die of heat stroke or leave the hatches open to be eaten alive by the skeeters and noseeums. . .

Why God invented screens?

[skipmac]

Originally Posted by Dockhead

Why God invented screens?

Screens work for mosquitoes and flies but they can't keep out noseeums, they're too small.

And screens block a lot of the breeze, so if it's already hot, humid and calm the screens will take away what little ventilation there is.

But again, it's not often this combination occurs. Two+ years liveaboard in the Bahamas and Caribbean I can only remember 4-5 nights that it was unpleasant. For me that isn't enough to justify a large, diesel gennie but I like having the little Honda anyway and it works for occasional use.