Why? (AC)

[Jonah]

Just curious ...

Why is that when my car engine is running, it can power my car's air conditioning system, but my boat's outboards don't power my boat's air conditioning system? Is that true of most boats (and if so, why?), or just something peculiar about my boat?

 

[Fishtales]

If you had as much room under the outboard cowling as your hood for the pully, belt, ac unit and power to do it all, you'd still have the problem of getting that cool air all the way up to the helm and or cabin.... you could on a bigger boat with inboards but the goal if an outboard is power in a compact form factor.

 

[wspitler]

In your car, your AC is a mechanical compressor running off a belt driven by your crankshaft. It probably takes around 4 horsepower from your car engine and only cools a relatively small space. In an outboard boat like the Grady's, the compressor is turned by an AC motor (120VAC). The alternator on your outboards only puts out DC voltage (14.4 VDC) to run DC stuff and keep the batteries charged. An AC compressor driven by an AC motor draws a lot of AC amperage, especially on starting, Converting the outboard DC output to AC would require a large inverter, which would overload the DC system. There are also always some efficiency losses when converting DC power to AC and vice versa. Outboard manufacturers don't have any incentive to add a belt driven compressor that would feed a condensor/evaporator system in a boat. The freon lines would also add an additional degree of complexity in getting to/from the space you wanted to cool. All Grady's require an AC power source, ie generator or shore power to run the air conditioning.

 

[Jonah]

If you had as much room under the outboard cowling as your hood for the pully, belt, ac unit and power to do it all, you'd still have the problem of getting that cool air all the way up to the helm and or cabin.... you could on a bigger boat with inboards but the goal if an outboard is power in a compact form factor.

Yep, I guess that makes sense. But I was thinking that, if the AC unit sits in my cabin (as it does), why can't it be powered by the outboards? Is it that the outboards only generate enough power to re-charge the batteries, and not enough to run something like an air conditioning system?

 

[Jonah]

In your car, your AC is a mechanical compressor running off a belt driven by your crankshaft. It probably takes around 4 horsepower from your car engine and only cools a relatively small space. In an outboard boat like the Grady's, the compressor is turned by an AC motor (120VAC). The alternator on your outboards only puts out DC voltage (14.4 VDC) to run DC stuff and keep the batteries charged. An AC compressor driven by an AC motor draws a lot of AC amperage, especially on starting, Converting the outboard DC output to AC would require a large inverter, which would overload the DC system. There are also always some efficiency losses when converting DC power to AC and vice versa. Outboard manufacturers don't have any incentive to add a belt driven compressor that would feed a condensor/evaporator system in a boat. The freon lines would also add an additional degree of complexity in getting to/from the space you wanted to cool. All Grady's require an AC power source, ie generator or shore power to run the air conditioning.

Got it, thanks very much!

A follow-up question: are there any setups in which the air conditioning system runs off of battery power? Even batteries with an inverter? I assume this is the case for some live-aboard boats, like sailboats?

A friend recently told me that his new Axopar would run the air conditioning off of the house batteries, but we're still waiting for the boat to arrive to find out.

 

[Fishtales]

Comes down to the size of the alternator. I'm guessing they are not as powerful and as boats are equipped differently it is best just to charge the batteries and let the boat manufacturer run the electrical system. When running they are charging the system, it is more when you are on shore power to run the AC.

 

[Jonah]

A friend recently told me that his new Axopar would run the air conditioning off of the house batteries, but we're still waiting for the boat to arrive to find out.

Yep, the Axopar 37 runs its AC system on large house batteries. No generator or shore power needed. Pretty cool!