Let's face it, we all like to be comfortable!
If you have a climate controlled luxury yacht you can probably skip this post. However, the rest of us might find some useful tips here for keeping our boat cabins a comfortable temperature. The cabin temperature on smaller boats can be notoriously difficult to control.
It is often more difficult to maintain climate control on a boat than it is an RV. Almost all RVs have some kind of furnace or heater for cold days and many have a roof A/C to keep it cool on hot days. But boats, especially small boats like daysailers, have neither and, of course, open boats have none. RVs usually have insulation and many small boat cabins have none. Since boating is mostly a fair weather activity the lack of a heater usually isn't a problem but often keeping cool is. Of course, heaters and A/Cs are not relevant on open boats, but controlling the climate inside any boat with a cabin can be an adventure in itself. Climate control in open boats is pretty much an individual thing -- dress for (or stay out of) the weather. However, you can add a bimini cover provide some shade against the summer sun for many boats and it might help keep the rain off. If you need to keep warm, wear warmer clothes -- or head back to a warm clubhouse, tavern, cabin, or campfire on the shore.
Like any other inhabited area we would like our boat cabins to be a comfortable temperature. Human beings are usually most comfortable when the air temperature is about 70-72F. When it gets hotter we want a cooler environment; when it is colder we seek warmer places. When it is hot outside we want our boat cabins to be cool. When the weather is wet and cold we want a warm, dry place inside out of the cold, wind, and wet stuff. Small boat cabins usually do a pretty good job of getting us out of the elements. However, many times boat cabins are not well insulated making them more susceptible to outside temperatures. Having the hull sit in the water often means a large part of the boat functions as a heat sink to absorb heat from inside the boat and makes it hard to keep it warm inside. Uninsulated hulls transfer heat to the water quite efficiently. On hot days that can be a real boon, but on colder days it means it is going to take more heat to keep the cabin warm than it would to maintain the temperature in an RV or a room of similar size. Rubbing up or leaning against a cold fiberglass or metal hull in bed can really ruin your rest period! Since boating is usually a mostly fair weather sport the most common temperature problem on boats is the cabins are usually too warm , so cooling a boat cabin is often more of a priority than heating. However, an uninsulated hull can still be very uncomfortable to the touch, even on warm days. Rolling over against one on cold nights can be really chilling!
Because weight is almost always a significant factor for boats, they often lack any kind of insulation. Many times you can add some insulation if you have direct access to the inside of the hull as you do any many small boats. Foam anti-fatigue mats are an easy way to insulate boat walls. They are fairly light weight, easy to cut to shape, are flexible to fit curved surfaces, and can be simply secured with basic adhesives. Dark gray or black are standard colors but you can also buy brightly colored tiles that might be more attractive. They can be glued directly to the fiberglass or metal hull liner in many boats. Applying them below the water line will be especially helpful in preventing heat loss through the hull to the water. Having them above the water line will help keep unwanted heat out and desirable heat in. Installed on the walls of the V-berth they give you a padded, insulated surface next to your bed, making it far more comfortable than pressing up against a cold (or hot!) uninsulated hull wall! Using them on the floor also has anti-slip benefits. Adding insulation can also reduce condensation on the inside of the hull and the foam pad is softer than the hull if you bump into it. Make sure whatever you add doesn't significantly increase the displacement. A few pounds won't affect most boats large enough to include any kind of enclosed cabin, but consider that even light weight insulation may add up more quickly than you expect if you are applying it to large spaces. If the weight of your boat and normal gear and supplies is already close to its maximum safe displacement, be careful adding extra weight! The cabins on some boats have an upholstered ceiling that helps minimize heat transfer in and out. I have seen folks remove it for cosmetic purposes usually when it gets old and stained, but doing so ultimately allows more heat into the cabin in hot weather and more heat loss when it's cold. If your boat lacks any kind of ceiling covering you might add some to make controlling the inside temperature a little easier.
A wind scoop can often be used with a ceiling hatch on just about any boat to capture any available breeze and direct it down into the cabin for ventilation and cooling. There are even models that are omni-directional so they will work no matter which direction the wind is coming from. When your boat is in motion there is usually enough apparent wind coming from the bow for a front-facing scoop to work well, but when moored or anchored you may have to adjust the scoop as the wind changes direction. Remember to leave another hatch or window open so the incoming air has somewhere to go! Otherwise you simply pressurize the cabin and you won't get any air movement inside. Wind scoops work pretty well when the boat is in motion but unless you monitor them and realign them with the wind as necessary while anchored they will only work when facing the wind -- unless you have the omni-directional style which is generally more expensive and sometimes a little bulkier.
Sometimes you can keep a cabin cooler by protecting it from the sun with some kind of canopy or awning. A simple tarp suspended over the cabin to shade the cabin and provide a shaded air buffer to keep it out of direct sunlight will help keep things cooler inside. Simply draping a tarp over the cabin might help a little but may also conduct heat if it touches the cabin surface so having it a few inches away from the surface is better. Using a reflective silver or white tarp will keep things cooler than using a dark blue or green tarp. You can usually set up a "boom tent" on sailboats fairly easily using the boom when the mainsail isn't raised. A boom tent can cool the cockpit as well as the cabin but is really only practical at anchor. A more permanent cover called a "bimini" can be adapted to provide shade on many boats. They are more practical on power boats because they get in the way of the very necessary task of watching the sails on a sailboat. Some creative owners have made biminis with clear panels so they get rain protection but can still see the sails.
Portals (windows) on boats are usually much smaller than windows on RVs and campers but they can still let in a lot of unwanted sunlight and heat on hot days. If you find you are getting unwanted heat through windows, cover them with reflective film or even total blackout film or hang curtains over them to block the sunlight. You might be surprised how much cooler your cabin will be on hot sunny days when you limit sunlight intrusion! Of course, smaller openings also don't work as well to help cool things when needed.
On larger boats, you may have the option of installing an air conditioner to cool things down on hot days. Air conditioners are generally too heavy and too bulky to be used on smaller boats plus you need a 120 volt generator or a solar system together with a large and heavy battery bank to power them. I have seen examples of DIY boat A/C using residential window mount room air conditioners but I am not sure they will stand up to the rigors of a marine environment. I also worry about the stress the weight and leverage of typical window air conditioners would put on cabin bulkheads that were not designed for that kind of loading and about the effects of humidity in a marine environment on the electrical components. The weight of an air conditioner is of more concern on smaller boats where the added weight will be more noticeable and affect displacement and handling more than it would on a larger yacht.
Evaporative coolers sometimes used on RVs (and land-based structures) aren't going to work well on boats. Evaporative cooler depend on rapid evaporation of water so they work best in dry climates. They work very well on RVs used for desert camping but since boats are sitting in the water the humidity level in the air is usually pretty high and evaporative coolers only work well in low humidity. Evaporative coolers only really work at a humidity of 50% or lower and work best below 25%. Very unlikely to find humidity that low on a lake, river, or ocean.
Cooling fans. Fans don't really cool, but by moving air they make you feel cooler by evaporating perspiration faster and carrying heat away from your body. Sometimes fans can be used to remove hot air or bring in cooler air to make the interior more comfortable too. Portable 12-volt and even self contained battery powered fans can be use in even the smallest of cabins to create a little air movement that can make a big difference in comfort. Heck, even fanning your face with a hand fan or even a paper plate can help! You can enhance the cooling affect of fans by spritzing your skin with water from a spray bottle. However, any kind of evaporative cooling will be limited by the high humidity in most marine environments. Spraying yourself with cool water when the humidity is already 90 or 100% may help cool you a little but you won't get the same evaporative cooling you would get at lower humidity. Sprays or mist systems generally work well in desert climates but not so well near bodies of water or other places with high humidity. Even so, the cool spray itself can feel pretty good.
Many smaller boat cabins lack any kind of heater. Since boating is often a fair weather activity and weight is a serious factor for boats, it makes sense to leave off heavy fuel tanks and furnaces. The human body generates about as much heat as a 100 watt bulb so just being in a small enclosed space (like the cabin on a daysailer) will usually result in the space warming up fairly quickly, especially if you have more people. My wife and I usually find just the two us us keep the cabin of our Macgregor 25 sailboat comfortable on cool nights without any auxiliary heat. I am sure we would need more heat if outside temperatures dropped near or below freezing. I once had to open doors and windows of our home during winter temperatures in the teens in Utah when I had 20 or so people in my living room even with no heat on! Cooking inside your boat will warm things up but you should never use a cooking stove just for heating and you must always provide sufficient ventilation. Often the heat from cooking an evening meal together with body heat is sufficient if your boat isn't drafty. If you regularly need yet more heat you should obtain an auxiliary heater that is approved for maritime applications. If you have 120 volt power an electrical heater is an easy and usually cost effective way of warming things up. Lacking electricity you will have to consider some kind of marine rated fuel powered heater. Alcohol is often recommended for marine applications because the liquid fuel does not explode and the fumes aren't heavier than air and won't accumulate in the bilge. Propane heaters that use self-sealing disposable fuel cylinders might be an option but you will need to store extra cylinders somewhere outside the cabin. See Safety Tips for Small Propane Bottles from Practical Sailor for more information.
Automotive heaters that run off engine coolant can assist in warming up an RV. I even had one RV that used engine heat to pre-warm the hot water heater while driving. However, since most boat motors use water from the outside for cooling, automotive type heaters that run off hot water from the cooling system are not practical for use on boats. The coolant generally doesn't get hot enough to be very effective and the plumbing to support it would be overly complicated and add unwanted and unnecessary weight on a boat.
Hatches and portals should be appropriately positioned open or closed to help you maintain the desired cabin temperature. On hot days you will want to keep things fairly open to allow ventilation to help with cooling. On cold days you will usually want to keep things closed ("batten down the hatches") unless you are using some kind of heater or lights (lanterns) inside that could put off toxic fumes and consume oxygen, in which case it is essential to maintain adequate ventilation. Open things link the companionway slide will allow a LOT of heat to escape, partially because of the large opening but also because of the location at the top of the cabin (remember: heat rises!).
Since boating is usually a fair-weather activity, cooling is usually the most common problem to be solved. Some days may start out cool, then get warmer as time passes. Dressing in layers so you can strip down to a comfortable level as things warm up is a good practice. You can also use spray bottle to create a mist to help cool your body. As mentioned above, a wind scoop can often provide much needed air flow for cabin cooling. The physical activity often required by a sailboat often helps keep sailors warm. Sometimes engine heat from inboard motors will also provide additional comfort on power boats. If you need more heat be sure to get a heater that is designed for indoor marine use. Catalytic propane heaters used in cabins and campers might work, but be sure to maintain adequate ventilation and store the propane cylinders out in the cockpit or on the deck.
Stay warm and keep your cool!
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