Auto a.c. onboard?

So a mechanic friend and I were talking and we can't figure why don't boats use an auto type a.c. compressor and system to cool the boat while underway? I understand the condenser would have to be different sense it is normally cool with a fan and air cooled on the front of a automobile. But onboard a.c. uses water to cool that part. So Thats the only difference we can see. In our minds though Thats the only difference. Seems like that would be better than have to run the genny just to have a.c.?

Are we missing something here. And before y'all hammer me, we aren't liveaboards yet but in the process of searching and if its possible, maybe we could try it out... Just thinking outload. We were having a few drinks and the subject came up. But neither of us could answer this. So a light went on, I'll ask this forum. They know more than us..

Lots have done it, I think there are articles on the web and certainly published through the years.

Much of the time by moving across the water (except in some climates some of the time), enough breeze is generated to satisfy people.

So most of the A/Cs are designed for dock or anchored with genset use.

At some point, space on engines can become a premium for mounting a lot of accesories.

But it can be done by the creative.

Other alternatives are a big inverter and alternator or a bell drive genset.

I think you will find that 90% of the A/C usage time for most folks is at the dock or anchorage. You don't want to have to run a big propulsion engine all night to have air conditioning.

David

djmarchand said:

I think you will find that 90% of the A/C usage time for most folks is at the dock or anchorage. You don't want to have to run a big propulsion engine all night to have air conditioning.

David

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True and I wouldn't, but I was just thinking that while cruising, we wouldn't have to run the genny as well as the main engines to keep the cabin cool. Wouldn't want the wife to have to use the head if its 100+ degrees in there

And we aren't the type of people to sit in one place too long. We are long haul truck drivers currently so sitting in one place for too long drives us crazy...so we plan to be on the move a lot.

Size is also a sizable issue. A lot of difference between air conditional an auto and a house. A boat is closer to a house. Note that most vans require two units. Imagine how many for a boat.

BandB said:

Size is also a sizable issue. A lot of difference between air conditional an auto and a house. A boat is closer to a house. Note that most vans require two units. Imagine how many for a boat.

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Yes we have 2 in our 18 wheeler but it is run off of one compressor. But you do make a good point. Not sure how to figure out the btu's or a auto a.c. system

Prevost motorcoaches have used bus air systems driven off the main engine for years. Nowadays they tend to use large inverters/large alternators to run the roof a/cs. Do some research on the power requirements. The interior size of the coaches is probably comparable with a boat.

I'm curious too. I was hoping someone smarter than I would post some hard numbers: HP/BTU, BTU's from an average auto air-conditioner, etc.

My uneducated guess would be it wouldn't work well when you needed it the most. I base this on the totally unscientific perception that it takes 2, 16,000 BTU marine units to keep my cabin just a few degrees cooler than outside on a hot day, at great expense in terms of power.

Then again, my car cools down pretty quickly after being in a hot parking lot on a summer day. So maybe it would work.

Inquiring minds want to know

CaptTom said:

I'm curious too. I was hoping someone smarter than I would post some hard numbers: HP/BTU, BTU's from an average auto air-conditioner, etc.

My uneducated guess would be it wouldn't work well when you needed it the most. I base this on the totally unscientific perception that it takes 2, 16,000 BTU marine units to keep my cabin just a few degrees cooler than outside on a hot day, at great expense in terms of power.

Then again, my car cools down pretty quickly after being in a hot parking lot on a summer day. So maybe it would work.

Inquiring minds want to know

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Yes I'm looking forbthise same type of answers. My a.c. in my 18 wheeler will make the whole trucks sweat when its really humid outside and that's just a single compressor with two evaporators....

Car ac is cooling a much smaller space. In really hot weather, car ac vents are usually pointed at the occupants. Some vehicles have 2 systems to provide comfortable ac. As I remember, an ac compressor needs about 25hp.

To clear up a few points:

Automotive / truck AC compressors are quite powerful. Most are designed to cool incoming hot moist air as opposed to recirculating cabin air. Cars suffer from lots of heat loading and poor to nonexistent insulation.

The downside will be that cooling production is tied to engine RPM. Not all, but most boat engines are turning lower RPM than truck or automotive engines. Units driven off the engine will necessarily be split units which will be less efficient and more costly to convert to a marine application (think long expensive hoses and custom built water cooled condenser). Unless you're an automotive HVAC tech, finding people to work on your system when your away from home maybe very difficult.

Installing a large second alternator, pure sine wave inverter, and decent battery bank would give you the power to run a normal all in one marine AC and use the inverter for other electrical needs as well.

Ted

Not to hijack your thread Semi, but I've wondered why more people don't try "marinizing" a mini-split home ac. I think the problem with a main engine driven car ac is the volume you are trying to cool and the fact that you mostly don't need ac when the boat is underway, assuming proper ventilation.

I could see how a car ac in a limited function could work, for instance to cool only an enclosed pilothouse while underway. But I don't see much point...and I live in a subtropical climate with crazy heat and humidity.

But for the mini-split variable speed systems, I don't know why it can't work.

Just as a starting point for comparison purposes. Sanden (Sanyo) manufactures a line of replacement automotive AC compressors in 5, 7 and 8 cu. in. sizes. The output of the 8 cu. in. unit is about 27,000 BTU, or 2.25 tons at 3000 rpm using 6.6 bhp (R-134a at a high-side pressure of 235 psig and a low-side (evaporator) pressure of 35 psig.) For a 5 cubic inch compressor, multiply the capacity by .625. For a 7 cubic inch compressor multiply by .875.

Long ago, I seem to remember a number of around 5 tons for large american cars, but I suspect that requirement has diminished significantly with the advent of smaller and perhaps better insulated cars. The range of Sanden compressor sizes above seems to confirm this.

T4Liberty said:

Not to hijack your thread Semi, but I've wondered why more people don't try "marinizing" a mini-split home ac. I think the problem with a main engine driven car ac is the volume you are trying to cool and the fact that you mostly don't need ac when the boat is underway, assuming proper ventilation.

I could see how a car ac in a limited function could work, for instance to cool only an enclosed pilothouse while underway. But I don't see much point...and I live in a subtropical climate with crazy heat and humidity.

But for the mini-split variable speed systems, I don't know why it can't work.

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Have seen it done numerous times on live aboard charter boats and commercial fishing boats. Problem is that the components inside the condenser (outside unit) aren't made for the salt marine environment. They live much shorter lives from corrosion. Also the vibration, bouncing and out of level orientation don't help. Finally, if you're going to run the units all the time, water cooled condensers are more efficient than air cooled. Would you rather try to dissipate heat into 100 degree air or 80 degree water.

Ted

LarryM said:

Just as a starting point for comparison purposes. Sanden (Sanyo) manufactures a line of replacement automotive AC compressors in 5, 7 and 8 cu. in. sizes. The output of the 8 cu. in. unit is about 27,000 BTU, or 2.25 tons at 3000 rpm using 6.6 bhp (R-134a at a high-side pressure of 235 psig and a low-side (evaporator) pressure of 35 psig.) For a 5 cubic inch compressor, multiply the capacity by .625. For a 7 cubic inch compressor multiply by .875.

Long ago, I seem to remember a number of around 5 tons for large american cars, but I suspect that requirement has diminished significantly with the advent of smaller and perhaps better insulated cars. The range of Sanden compressor sizes above seems to confirm this.

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We were considering using a compressor like the one on my semi-truck. I know their built to last a lil longer than a typical car/4 wheeler a.c. but not sure the cu. in. of it though. It is physically a large compressor in comparison to a car/4 wheeler a.c. too

O C Diver said:

Have seen it done numerous times on live aboard charter boats and commercial fishing boats. Problem is that the components inside the condenser (outside unit) aren't made for the salt marine environment. They live much shorter lives from corrosion. Also the vibration, bouncing and out of level orientation don't help. Finally, if you're going to run the units all the time, water cooled condensers are more efficient than air cooled. Would you rather try to dissipate heat into 100 degree air or 80 degree water.

Ted

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I wonder if its possible to integrate a water cooled marine condenser into an auto system to solve that issue. I'm sure their much better than the air cooled type. Next question would be are they big enuff to be effective. Hard to figure which one to use not going the btu's of a custom system.

Semi-Retired said:

I wonder if its possible to integrate a water cooled marine condenser into an auto system to solve that issue. I'm sure their much better than the air cooled type. Next question would be are they big enuff to be effective. Hard to figure which one to use not going the btu's of a custom system.

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You could get a water cooled condenser for a custom system. Part of what's over looked here is that the components are matched to each other to make them efficient. There is some system engineering to make them work and be efficient. BTW, a custom, water cooled, AC condenser isn't going to be cheap.

Regarding size, you're putting the cart before the horse. First you need to get a boat and figure out how much AC is required.

Ted

O C Diver said:

You could get a water cooled condenser for a custom system. Part of what's over looked here is that the components are matched to each other to make them efficient. There is some system engineering to make them work and be efficient. BTW, a custom, water cooled, AC condenser isn't going to be cheap.

Regarding size, you're putting the cart before the horse. First you need to get a boat and figure out how much AC is required.

Ted

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True. We do need the boat first. I was just looking for answers to a conversation a buddy and I were having over a few cold ones...

Semi-Retired said:

True and I wouldn't, but I was just thinking that while cruising, we wouldn't have to run the genny as well as the main engines to keep the cabin cool. Wouldn't want the wife to have to use the head if its 100+ degrees in there

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FWIW, that's very commonly done, especlally in warmer climes (like where you are) and especially hot months in lots of other places.

Genset starts before leaving the dock, gets turned off after return. (Or sometimes at turned off in anchorages where there might be better overnight breezes.) July and August up here, for example, pretty much mean AC running at least all the time during most daylight hours, ditto most overnights in those months... partly for creature comfort, and partly to take a load off the fridges/freezers. In the grand scheme of things. it doesn't necessarily cost an arm and leg...

T4Liberty said:

Not to hijack your thread Semi, but I've wondered why more people don't try "marinizing" a mini-split home ac.

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There are several thread son it, IIRC. Fred (FF) has often suggested those. The main stumbling block often seems to be (from reading) the outside units not being robust enough for saltwater usage. Another issue is where to mount the outside unit...

-Chris

Understand I LOVE my air conditioning, but there is so much airflow under way that I never even think about needing air conditioning. Not to mention that the boat would have to be closed up tight to make it worthwhile. As someone who really doesn't like too much heat, I can totally relate, but you're talking about a totally custom $$$ system which could be easily and far more cheaply taken care of by running the generator. A reasonably well cared for generator will run 10,000 hrs and at something around 1 gph fuel or less, it seems like an easy trade.

Ken

Engine driven marine systems certainly can be made. The engineering is do-able. The issue is most ac load is with boat docked or at anchor, and in that case you would need a complete second system or run main in those instances. Not a practical solution.

It would be less expensive to run a generator the times cooling is desired underway.

I think twice in 2400hrs I started the gennie for AC while underway. Once when wind was exactly at my speed, another time I was attacked by a swarm of biting flies.

Ski in NC said:

Engine driven marine systems certainly can be made. The engineering is do-able. The issue is most ac load is with boat docked or at anchor, and in that case you would need a complete second system or run main in those instances. Not a practical solution.

It would be less expensive to run a generator the times cooling is desired underway.

I think twice in 2400hrs I started the gennie for AC while underway. Once when wind was exactly at my speed, another time I was attacked by a swarm of biting flies.

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What size alternator do u have and what size inverter do u use to run the a.c. unit

This is really an ancient boating idea.

There are lots of boaters with great ideas every day.

This is one with limited success and enthusiasm.

Not saying don't try it...but many have and either gave up or realized the norm was the way to go.

Semi-Retired said:

What size alternator do u have and what size inverter do u use to run the a.c. unit

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I have a small-ish 100amp alt and 1000w inverter, both too small to run the AC. I have a 5kW diesel gen that I fire up if AC is needed.

A large alt and inverter can be sized to run at least some AC, that is more practical than a belt driven compressor, especially if you do not have a diesel gennie. In my case the gennie makes the big inverter system redundant.

Maybe you are thinking that auto AC is run off the car's alternator? The compressor is powered by the car's engine. On a boat, it makes more sense to have an electrically driven compressor, so you don't have to run the genny when you have shore power.

MYTraveler said:

Maybe you are thinking that auto AC is run off the car's alternator? The compressor is powered by the car's engine. On a boat, it makes more sense to have an electrically driven compressor, so you don't have to run the genny when you have shore power.

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No I was just seeing what kind of power it would take to run a typical marine a.c. Trying to see if it would be more feasible to just run bigger alternators and a bigger inverter and just run a regular onboard a.c. while underway than to try to install a diy a.c. from an automobile with all the extra parts or vise versa

The question is not can it work , but how many over laping systems do you want to maintain.

Would be easier (and cheaper) to install a hyd pump and hyd motor on the main engine.

This allows a cruise generator that is not RPM sensitive.

Similar deal with big DN 50 alt and sine wave inverter.

But hyd can have any number of uses , and the white smoke never pops out.