My 40 foot sailboat is only about 100 ft2 of floor space, the surface area is more that of a mobile home - long and narrow, the hull and deck are about an R-5, the glass is all single pane, and there are skylights and hatches that let in ALL of the sun, so how do you calculate the conditioning needs for winter and summer?  Tricky and lots of rule of thumb going on, but here is an interesting article that uses all the techniques we have for homes:

https://www.practical-sailor.com/issues/45_6/features/Calculating-t...

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Personally I wouldn't sail when I needed heat, but if I did I would put in as small a heat sources as I could find.  Your goal is not to provide heat that your minimalist thermal shell can maintain, but simply to heat the air to the required temperature (probably on a fairly continuous duration.) Traditionally larger boats had very small Jotul-type box stoves, but there is always the fire hazard.  Unless you had substantial solar panels (unlikely) and battery storage (ballast?) I would go with a small radiant propane heater.  But really, I would just put on a wool watch cap and heavy sweater.  

As for cooling, I would open a couple of windows.

Why Ventilate my boat?

https://www.minisplitunit.org/

Proper ventilation is important for the maintenance of your boat and is a necessity for the comfort of you and your crew. A good ventilation system will:

Reduce moisture and humidity below deck when the boat is left unattended
Reduce the chances of musty air development caused by trapped moisture and humidity in the boat
Help electronics and fabrics last longer by maintaining a drier climate
Reduce the likelihood of osmotic blisters caused by moist air saturation of the hull
Active and Passive Ventilation
Vents fall into two basic categories: passive or active. Passive ventilators, such as cowl vents, clamshell vents, louvers, grilles, ventilating sails, ports and hatches simply provide an access path for air to enter or leave the interior of the boat. As long as either the boat or the air itself are moving, they work just fine. Of course on those hot, still days when the only things moving are the mosquitoes, they’re not much help.

Active ventilators incorporate a fan to keep air moving even when the boat or breeze is still. Solar energy or ship’s power is used to power the fan, depending on the ventilator. These vents come with both intake and exhaust fan blades for flexibility in creating your ventilation system.


Solar cell and batteries allow 24-hour fan operation with no electrical hookups, and it works for either intake or exhaust directions.

Designing a ventilation system that works
A properly designed ventilation system provides adequate air circulation throughout the boat without allowing water from waves, spray or rain to come aboard. Experts recommend that your ventilation system should provide at least one air change every hour. This means provisions must be made for both the intake of new air and the exhaust of old air. Set up your system to provide a “cross flow” of intake and exhaust ventilation wherever possible. If you only add one active ventilator, use it for exhaust.

A typical 30-footer has about 800 cu.ft. of below-deck interior volume. Unfortunately, this space is often broken up into distinct cabins or compartments that may restrict the free movement of air throughout the boat. Therefore, simply installing a pair of vents rated for 800cu.ft. per hour of airflow may not be adequate to get the total ventilating job done.

Each cabin and head should have some kind of ventilation, especially if the space can be closed off from the rest of the boat. Louvered doors, or vent grilles in solid doors help air circulate into lockers, forepeaks and other isolated areas of the boat.

During wet or rough weather, you’ll need to be able to shut off or remove vents to prevent water from finding its way below. Racing boats may need to use vents that can be removed while the boat is in use in order to keep the decks free of possible snags.

Do you need an air conditioner?
Air conditioning systems will be one of the largest electrical amperage draws when you are under way, but use modest AC current when connected to shore power. At the dock, a 5,000-16,000Btu AC-powered system draws between 4 and 13 amps. Under way in a powerboat, these amperages are manageable.

Under sail or at anchor, you’ll need a sizable inverter and the battery banks to support those loads or a genset. What makes these electrical investments worth considering? Summer cruising locations that combine high heat, humidities in the 80 to 90 percent range, and the presence of mosquitoes or other insects, requiring screened cabin areas that restrict ventilation. Air conditioning opens these locations to comfortable boating.

Split (remote) air conditioning: Central air conditioning systems, also known as split systems, are usually found on boats up to 80' in length. A split system has the air conditioning components split between two separate units. The two units are installed in different locations and connected by insulated copper tubing through which the refrigerant travels. The condensing unit, consisting of a compressor, seawater condenser, and electrical components, is mounted in the engine room or other mechanical space. The evaporating unit includes the evaporator coil and a blower and is installed in the living area.

Sizing your air conditioning system.

Air conditioning systems are rated in Btu, or British Thermal Units, a universal measure of heating and cooling. To calculate the number of Btu needed to cool or heat your space, follow the steps below, then select the unit with the capacity you need.

Step 1 - Required Capacity
Divide your boat into three basic load areas:

Below deck: cabins where the hull slopes inward toward the keel and there are minimal port lights and hatches.
Mid-deck: areas on the main deck with small or shaded windows.
Above deck: areas with large glass surfaces and direct sunlight.

Effective boat ventilation helps air to circulate areas as much as below decks and other areas. Fresh air in and the state air out. A single vent will now allow the air to flow. A minimum and two well-separated vents need to allow dry air to flow in and allow the damp air to escape out. And for more assistance, you need to contact the ventilation expert near you.

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