Openings & Ventilation

Airflow RequirementsLooking up at the companionway from the edge of the galley.

Project Logs

February 6, 2012

Work continues on the cockpit, but along the way I am giving consideration as to how the seahood construction will come together.  I think it will start with a mold of the top of the hood, followed by construction of the sides and face.  That being said, I will have to do some hands on work with this area before I will finally determine how the construction will come to be.

December 28, 2011

The below deck engine ventilation routing is complete.  The next task will be to build below deck dorades and drill through deck vent holes, but this step won’t come for some time, as I will complete this step during construction of the cockpit combings.  Some decisions I made during the installation differed a bit from the research I completed last winter.  Here’s some highlights:

With the key changes understood, let me go into some further detail about the process I went through to add the below deck engine ventilation routing:

  1. Drill Through Bulkhead Holes – I first drilled the holes for which the 3.5″ ABS would fit through.  There were 4 holes total and each hole represented a location where the engine ventilation needed to pass through a bulkhead.   I bought a brand new 4″ hole drill which made very clean and well matched holes for the 3.5″ ABS.  Here’s an image of the hole for the ABS:
  2. Fiberglass ABS– With the holes drilled, I then placed all of the ABS loosely in the holes.  After cutting fiberglass to fit, I thickened the ABS in place, then glassed the ABS as well.  By fiberglassing these through bulkhead connections, I make the connection water-tight around the ABS.  Hypothetically, this means that the lazarette and stern lockers shouldn’t allow any water below decks unless they are filled to the brim with water (an unlikely situation and if it did happen, it’s like the boat would already be lost anyway).  Below I’ve included an image of what the ABS looked like in one of the harder to glass areas.  You’ll notice some areas where the fiberglass overhangs the pipe, but this isn’t much of an issue because after it hardens, I use an oscillating multi-tool and some 60 grit sandpaper to trim off the excess glass.  Here’s the image:
  3. Glue Additional ABS Runs –  With the bulkhead connections in place, I was then able to extend the ABS runs where necessary.  There were three key locations where this needed to happen:  2 in the each sea locker (4 total) and 2 connections in the engine compartment.  Each through bulkhead connection mean gluing additional ABS to the through bulkhead ABS fitting I had previously installed.  In the case of the engine room and through-lazarette connections, this additional ABS was simply meant to allow an easy connection for hose ducting.  However, for the connection point in the lower part of the sea locker, the connection allowed me to raise the height of the air intake.  This way, if water ever flooded a sea locker, it would have to fill the locker all the way to the top before any water would flood into the engine room.  Here’s an image of the lower sea locker connection glued in place  (note how the ABS is sanded roughly so as to prepare the surface for future glass work):
  4. Fiberglass ABS Runs – After the runs were glued in place, I then poured foam behind the lower sea locker runs, cut the glass for all 6 ABS runs and glassed them all in place.  I don’t have an image of these pieces all glassed in at this time, so that will have to wait for a later update.
September 24, 2011

The companionway is still a bit unclear, but the options for how it will come together are being narrowed down (especially with the construction of the cabin sole being completed).  I’m looking forward to building this area out, but I’m going to move to the major deck construction projects prior to continuing work inside the boat.

August 8, 2011

As the cabin sole comes together, I am getting a more and more clear picture of how the companionway will come together.  I’m not quite sure when I’ll actually begin construction on this portion of the boat, but when I do this area will  be updated.

July 14, 2011

At this time, I’m holding off on working too much on the portlights until I’ve completed the construction of the filler pieces.  For most port lights, there needs to be some kind of spacer added to account for the different between a sailboat’s cabin side and the thickness of the port light spigot.  In my case, I will create this spacer out of structural foam and fiberglass.  Often times this piece is made of teak or a hard wood of some kind, however I’ve decided to go with foam and fiberglass because it will greatly strengthen the cabin sides as well as require no maintenance moving forward.  Below is an image show preparation for installing the portlights:

I cut the headliner around the portlight to make room for an eventual ‘plate’ for the portlight.

I’m also only working on the interior of the boat, so construction of the seahood will come at a later date, but I do have a good plan how that construction will come together.


Cockpit Sole Hatch
Deck Hatches
    1. it should be small – not much more than shoulder width;
    2. the weather boards should be solid hardwood or plywood, preferably ¾” thick, or Lexan
    3. as water tight as possible
    4. the retaining channels that hold the weather boards must be very strong
    5. There should be devices to lock the hatch inside and outside.
  • When equipped with good gaskets and dogs, these hatches can act both as skylights and as vents (as, in the case of the forward hatch, as a passageway for crew, sails, and gear). Preferably the hatch should have hinges fore and aft so it can be opened in either direction to be used as either an intake or an exhaust.  (Desirable and Undesirable Characteristics of Offshore Yachts)
  • The hatch sides should be straight (with no flare) and should be close to the deck when the hatch is closed so that lines won’t catch under them…in particular, the corners of forward hatches can easily snag a jib sheet during a tack or jibe. A sheet under great strain may bend up a hatch corner, which will leave a gap…so make sure all hatches are dogged loosed (or left all the way open) before tacking. If there’s any change that a hatch corner will hook a sheet even when the hatch is closed, install a guard at the corners.  (Desirable and Undesirable Characteristics of Offshore Yachts)
  • All hatches should be installed as close to the centerline as possible (in the event of a knockdown, an open, off-center hatch can let in a huge amount of water in a very short period).   (Cruising Handbook, p. 94)
  • I like to see aluminum-framed hatches installed on a molded-in, raised base because the base helps deflect water away from the seals.
  • Mask the deck around the perimeter of the hatch and mask the edge of the flange. You can let the tape stand vertical; its purpose is to keep sealant squeeze-out off the frame. Remove the hatch and completely coat the deck between the tape and the cutout with sealant. Use polysulfide if the hatch frame is metal…Put the hatch back over the cutout and wiggle it gently to distribute the sealant. Insert the bolts or screws and snug them all. Now tighten them, following a patter of each screw in sequence being as opposite as possible to one before (e.g. right side, left side, front right, rear left, etc.) Tighten enough for some sealant to squeeze out around the full perimeter of the flange. Do not over tighten the screws or you will squeeze out all the sealant and the resulting metal to fiberglass joint will soon leak. Allow polysulfide to cure a week, then tighten the nuts on the through-bolts about half a turn to put the sealant under compression. Do not tight screws or you will break the seal on them. Trace a razor knife around the perimeter of the hatch frame to separate the squeeze-out from the sealant under the frame. Peel the tape from the deck and the excess sealant will come with it. Finally, install the trim ring below.  (This Old Boat, p. 95)
Deck Prisms
Dodger Ventilation
Engine Ventilation
  • An inquiry to Perkins revealed that a M60 in a temperate climate should have a duct diameter of at least 16 square inches. In a hot climate, the duct diameter recommendation grows to 32 square inches…Perkins also indicated that the minimum combustion air flow for the engine was 106 cubic feet per minute (CFM). Ideally, the engine would receive 247 CFM of air for combustion and ventilation. The 32 sq. inch cutouts required for proper passive ventilation would be bigger than I would be comfortable installing in the rear…pipes also go through all sorts of convolutions to keep out water – further restricting air flow by increasing pressure drop.  (
  • Once we had installed the blowers, our engine was able to rev 200-400 RPM higher than it could before under no-load, while loaded performance improved remarkably as well. Evidently, the air restriction had held back our engine when it was hunkering down driving both propellers….only criticism I have for the blowers is the noise – they were the loudest thing you could hear in the rear cabins when the engine is idling until we dampened their mounting…install a RC circuit to keep the blowers running a bit longer after the engine was shut down. This is to ensure that the alternator is cooled somewhat after coming to a stop.  (
  • Also, be sure there’s plenty of air space for your diesel to breathe. You don’t want it to suffocate. Additional air space is also good around your alternator, so that it can cool efficiently…at least the overall engine room is large (meaning there’s plenty of combustion and cooling air available regardless of where the front of the engine box is).  (
  • When running, engines act as their own exhaust fans by taking in air from the engine compartment and blowing it out through the engine exhaust system. Still, while requirements for ventilation for diesel engines are much less stringent than those for gasoline engines, it’s a good idea to have a system that includes a blower and two large natural-draft vents leading from deck to the engine compartment through clam-shell vents and large diameter tubing. One vent will serve as intake, the other will serve as exhaust.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 155)
  • …a diesel engine requires a substantial supply of fresh air for combustion. The minimum air vent size, in square inches, can be estimated by dividing 3.3 into the horsepower rating (e.g., 50 / 3.3 = 15 square inches for our 50 hp Yanmar; to find the vent size in square centimeters, multiply the horsepower by 2, e.g., 50 x 2 = 100 sq. cm). Thought has to be given how to duct in this air with the least impact on the insulation  (Cruising Handbook, p. 209)
  • Good insulation also traps heat…(the engine room remains hot long after the engine is shut down, slowly feeling its heat into the rest of the boat), but also because it lowers the efficiency of the engine. Although it is rarely done with diesel engines, sometimes it is worth installing an engine-room blower specifically to get rid of as much of the heat as possible….Should it blow in or suck out? If the blower sucks out, the hot air can be directed away from accommodation spaces. However, in this case, there must be a sufficient surface area of inlet-air ducting to ensure that only minimal negative pressure is created in the engine compartment by the blower; otherwise, engine performance will be impaired. On balance, I would have it suck out, with the blower wired to the ignition switch so that it runs whenever the engine is operating – but also wired to a manual override so that it can be kept running for a while after the engine is shut down.  (Cruising Handbook, p. 209)
  • The Natural Ventilation System must be made up of, at least: A supply opening or duct from the atmosphere or from a ventilated compartment that is ventilated to the atmosphere, and an exhaust opening located on the exterior of the boat….You must have a means to bring in air from the outside and a means to take air out of the compartment, to the outside of the boat. Air in from the outside, air out to the outside. Each exhaust opening must originate in the lower third of the compartment. This is because the fuel vapors collect in the bottom of the boat. So the duct that takes the air out has to begin in the lower part of the compartment.Each supply opening or duct, and exhaust opening or duct must be above the normal accumulation of bilge water. Get it low enough to get the vapors out but not so low it sucks up bilge water.The openings or ducts must be sized to move enough air in and out of the compartment. The absolute minimum size is an opening of 3 (three) square inches….To determine the correct opening size, see graph below and follow these instructions:  First, add up the volume of the compartment. Subtract the volume of the engine, battery, fuel tank and any other large equipment in the space. (Generators, flotation blocks etc). Then reading up from the bottom of the graph (compartment volume) go up to the curve, and the read over to the left side of the graph to get area of the opening. You can have more than one exhaust opening and more than one intake opening to meet the requirement. The important thing is airflow. Here’s the graph you can use to follow from the regulations that will help you to figure out the correct size (

    Required ventilation area for engines (see descriptions of use in research)

  • To help determine the rated capacity of a blower for your system look at this graph of air flow versus compartment volume  (

    Airflow Requirements

    Rated Blower Capacity / Blower System Output

  • Location of vent openings: Vent openings often have cowlings on themand many times one is pointed forward and the other is point aft. Actually, for natural ventilation this is not necessary. Practical tests have shown that when the boat is standing still the wind direction is what determines which opening is the inlet and which is the outlet. When the boat is moving and the blower is on, the duct that has the blower is the outlet. Also cowlings can often create restrictions in the amount of air flow. Use cowlings that do not reduce the size of the openings.  (
  • Preventing Water Intrusion. One thing you do need to know is that placing the cowls on the side of the boat, or back (transom) of the boat, can result in the boat sinking if water enters the opening. This is not a joke. Numerous boats have been sunk because water came in the vents. If the vent is on the side or transom, route the duct up in a loop, an upside down U shape (in the plumbing world called a P trap) and then down, so the top of the loop is several inches higher than the vent opening, or use a manifold inside the opening and have the duct enter the top of the manifold. Provide a drain from the manifold out the side of the boat to get rid of accumulated water  (
  • We have 6 fans installed for ventilation, plus 3 more yet to be installed. We like the Caframo ‘Sirroco’ fans in that they move a lot of air, fold flat when not in use, and don’t use much power…installed a ‘Vetus’ ventilator fan in the bulkhead to the head compartment.  (
  • Electric fans greatly improve comfort below….Small, inexpensive 12-volt, 5 ½” diameter rubber-bladed fans are rated to circulate air at 250 CFM. When located in the galley, main saloon, and sleeping quarters, these fans can provide considerable relief in hot, humid weather.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 153)
Locker Ventilation
  • replac[e] solid cabinet door with louvered ones. Caning should be avoided on the door of any large locker full of heavy objects as a can of tomatoes would have little difficulty busting through…  (Upgrading the Cruising Sailboat, p. 77 )
  • Lockers – whether for anchor rode or for clothing – should have built-in louvers.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 148)
  • To avoid mildew, keep all wet gear in its own lockers, away from dry gear. Louvers also provide an exhaust when set into companionway slat.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 153)
  • Lockers and other storage areas may be dampened by condensation long before the rest of the hull. It helps to pack gear loosely to encourage free air to circulate. Good insulation can be a benefit too, because it keeps engine noise and heat from the living area, and prevents heat from passing through the hull and deck.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 150)
  • …added cane inserts to every locker door on the boat, so that lockers could “breathe” (cane inserts or louvers should be standard on all cruising boats; doors with solid panels are not acceptable).  (Cruising Handbook, p. 124)
  • Bronze port lights are nearly indestructible. I see thousands of aluminum opening portlights that are more than 20 years old and show no signs of failure or weakness.  (This Old Boat, p. 155)
  • Maintenance of opening portlights involves little more than rinsing the frames regularly and applying an occasional coat of wax to retard surface corrosion. Lightly lubricate the screw dogs.  (This Old Boat, p. 155)
  • The part that extends through the cabin side is called the spigot and the flared end of the spigot is called the flange. The frame that slips over the opposite end of the spigot is called the finish ring. Modern designs have the spigot inserted from the outside with the finish ring on the inside. From an engineering standpoint, an outside flange (and inside finishing ring) makes the portlight easier to bed. As a rule you only bed exterior contact surfaces. However, if the finish ring is outside, the sealant must also prevent water from penetrating between the inside perimeter of the ring and the outside surface of the spigot. The best assurance is filling the space between the spigot and the cabin side with a generous application of sealant, carrying the sealant out into the spigot beyond where the finish ring will lie. You must be sure there is sufficient sealant on the ring and the spigot to form a watertight seal between the two components.  (This Old Boat, p. 155)
  • Ports are especially vulnerable. The safest design for a cabin port is an elongated ellipse. Rectangular ports may seem more attractive, but they are structurally poor because of the abrupt transition between the gross and the net.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 106)
  • Portlights aren’t too expensive to purchase and install yourself. I’d opt for bronze ports…They’re a bit more expensive, but much stronger and certainly better looking. A new opening port can be installed where no present port exists, or if the opening port is larger than an existing fixed port, the old one can be removed and an opening one put in it’s place.  (Upgrading the Cruising Sailboat, p. 290)
  • If the port doesn’t come with a template to mark the size hole to be cut, make one out of cardboard. use a pencil to mark the outline and then drill pilot holes large enough to accept a saber saw blade. Small cutting errors of ⅛” or so can be covered by the port flanges. Be sure to seal off any core material…Us a good quality bedding compound to prevent leaking around the port -surely the most common source of leaks. When the screws are tightened, the compound should ooze out everywhere.  (Upgrading the Cruising Sailboat, p. 290 – 1)
  • Rainhoods keep rain out when you want the port open for ventilation. You can form your own by heating thin sheets of Plexiglass with a torch. Simple windscoops can be cut from Clorox bottles or any other plastic container that can be jammed into a port. The open end should face onto the cabin; outside on the forward side of the container, cut a large hole to funnel the wind.
  • Opening portholes always seem to leak after a few years….To minimize aggravation, the external frames should be designed with drains so that when the boat is on an even keel, there is no puddle of standing water in the frame. In addition, a rail should run around the inside of the cabinside to catch any drips that get through. The rail makes an excellent handhold at exactly the right height for most people to hang onto when the boat is heeled.  (Cruising Handbook, p. 94)
  • Portholes can provide good cross ventilation and, when equipped with rain shields, may be left open in light rain or spray. However, heavy vertical rain will splash off the deck and into an open port. Like hatches, they require good gaskets and dogs, and must be dogged down with care so they do not snag sheets.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 153)
  • Keeping the cabin cool in summer is easier and much less costly than heating in the winter. Good cross ventilation is a starting point. It amazes me that more boats aren’t equipped with opening ports to help accomplish this. A major reason is the cost-cutting measures of boatbuilders.  (Upgrading the Cruising Sailboat, p. 290)
  • Opening portlights…contribute almost noting to the ventilation of a boat when at anchor. Their contribution can be improved with individual wind scoops…  (Cruising Handbook, p. 126 -7)
  • (For a full walk through on building a sea hood, please refer to pages 171 – 175 of Upgrading the Cruising Sailboat)
  • (For another walkthrough, please refer to “A Seahood for Magnolia” article from Practical sailor)
  • Aft-facing companionways should be protected with sea hoods and dodgers so that rain, spray, and boarding seas do not find their way below.  ( Desirable and Undesirable Characteristics of Offshore Yachts, p. 120 )
  • All vents, ports, and hatches (including the companionway hatch) require easily installed bug screens. We have a screen that drops into the companionway in place of the hatchboards. For security reasons, it is ruggedly constructed with divider bars that break up the screened areas into spaces too small for someone to squeeze into the boat.
  • All of the screens, particularly the larger ones, need a designated stowage space where they will be protected from damage when not in use.
  • Many portlight bug screens are retained by screws that make the instalation semipermanent. Given that a screen severely limits the airflow (and alos the light), it is preferable to have clip-in screens, which will be used only when needed.
  • Since screens reduce ventilation by about 50%, anybody cruising in a hot, buggy area will have to make a sacrifice…For some reason, bugs do not seem to want to swarm down a vent cowl…

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