Bilge Ventilation On An Outboard Boat

Discussion in 'Boatbuilding' started by mrintense, May 13, 2017.

  1. mrintense
    Joined: Jul 2012
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    mrintense "Clipper"

    First off, before I say anything, please forgive me if I say something ignorant. I am building a 21 foot wooden cabin cruiser (I have another posting her somewhere on that) and have reached the stage of designing and building the sole under structure (in bilge area).

    I am trying to account for all items that may be placed in this area before I get too far into the construction of the structure so that I do not build myself into a corner. I have accounted for bilge pumps and associated fittings, encapsulation of all wood, and limber holes.

    I understand the need for bilge blower systems on inboard boats, however I am not sure about outboards. In my case, there will be two fuel tanks mounted in the sides of the boat between two of the frames. These tanks will be located above the sole at a point just aft of and outside the cabin. The cabin is an open design with the forward windows capable of being opened, so ventilation of the cabin is not an issue.

    I have no idea what a bilge ventilation system should look like other than pictures I've seen of engine boxes with ventilation blowers built in. It's been difficult finding material that adequately explains this.

    So the questions are: Do I need to have blowers mounted in some way so that they will ventilate the area below the sole? Is it necessary to run ducting under the floor between frames (necessitating drilling holes in the frames)?

    Here is a picture of where I am at as of today. This internal structure (except for the boat frames) is temporary tooling to aid in the final design.

    Note that my boat has frames below the sole and the floor will be constant height, so each section between the frames will be separated from the next with only small limber holes allowing passage of water aft.

    In the photo I have crudely drawn representations of fore and aft supports for the sole so if I were to use this idea, the under sole area would be further compartmentalized. Basically I am looking for a bit of guidance here before I get too far into this phase of the build.

    Floor_Support_Drawing_IDEA.jpg
     
  2. Ike
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    Ike Senior Member

    Are you required to have ventilation in the bilge? No. Is it a good idea, yes.

    First, fuel tanks. The compartment where fuel tanks are located is not required to ventilated, unless there is electrical equipment in the space that is not ignition protected. What that means is, if the electrical equipment (a blower that is not ignition protected for example) can create a spark then the space must be ventilated. However most bilge blowers are ignition protected. Read the label on the box. If the area where the tanks are located is connected to the cabin, that is, air can freely flow from one to the other, and you have cooking appliances or electrical equipment that is not ignition protected in the cabin, then the fuel compartment must be ventilated. This can be natural ventilation, or a blower.

    But almost as important is keeping the space dry and not promoting mold, mildew, and rot. This requires a free flow of air.
     
  3. mrintense
    Joined: Jul 2012
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    mrintense "Clipper"

    So if I were to go with the idea represented by the drawing in the photo, it would be best to provide cross compartment ventilation. I suppose these "ribs" could be built like aircraft wing ribs, giving plenty of ventilation. But to get ventilation from one frame compartment to the next would entail drilling holes in the frames. Not sure if I want to do that. An alternative idea might be to make a few "U" shaped cut outs along the top edges of the frames.

    Another possibility would be to make large ventilation holes near the edges of the sole. These will be located under seating and internal cabinetry, but these will also be ventilated.

    Am I over thinking this?
     
  4. philSweet
    Joined: May 2008
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    philSweet Senior Member

    For starters, you need to read ABYC H-24 Gasoline fuel systems.
    https://ia800208.us.archive.org/10/items/gov.law.abyc.H-24.1993/abyc.H-24.1993.pdf

    Next, we need some idea of what the inside finish is going to be. If the sole is made from 4 x 4's ripped to 3/8 and laid with a 1/4 inch space, that is a bit different to a watertight plywood carpeted floor. The wing tanks will only have fittings on the top. They have to be plumbed iaw USCG, ABYC, and EPA requirements. This isn't as daunting as it sounds if you just buy everything from a single supplier. There won't be that much to worry about then. But the hoses and doodads do take up more space than you might think. Both the tank and the motor supplier will have info on recommended clearance allowances.

    As far as the floor goes, I'd use transverse floors under the sole. All those longitudinals look like a pita. That cuts your parts count by a factor of four. Plywood running longways will like transverse support better, as will boards. I'd design a lift out section at least 20" x 4' for access. That would require carlins to support the edges. You'd be surprised with all the things you can do with a nice flat panel like that. The underside can be a workbench. Make it big enough to lay the outboard on. You could even turn it over and use it when trailering to hold the ob if it had chocks built in. Making these so that they stay leakproof for 20 years is a bit of a challenge, but your work looks pretty good so far. A common cheat is to add some of those round plastic screw-in deckplates into the lift out section to save hardware removal when you just want a bit of airing out.

    <cross posted with Ike and the OP. But a sketch of the GA and more about the interior fitout would still be helpful.>
     
  5. mrintense
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    mrintense "Clipper"

    Thanks for the additional response Phil. And thanks Ike for the original response.

    The plan is to have an encapsulated, carpeted plywood sole, so provisions for ventilation would have to be added to the plywood. I am still doing mock ups (including the plywood sole in the following picture). My aim is to mock up the sole and then use that to establish final cabin profile as well as the internal cabinetry. However, a bit of additional information would be relevant here. The original design of this boat called for an enclosed cabin, however because of the heat here and because of the way I intend on using the boat, I've elected to go with an open cabin. Open meaning that the aft cabin bulkhead is not going to be there. The boat will be open from front to back, however there will be cabin windows on the forward face that can be swung up into an open position. The cabin has been shortened in length as well to increase the aft cabin space as I anticipate that this will be where we spend the vast majority of our time.

    In the cabin proper, I am planning on a simple v berth that can double as seating. There will be provisions for a toilet. The original design had a galley and a sink. Mine will not. There will be a place for a cooler. All of this has to be mocked up first to determine the final arrangement, but I suspect that there will be limited floor space in the cabin when the V berth is in place.

    I would like to use the center space under the sole in the cabin area as storage for a boat hook and oars, so I am hoping to have that part of the floor removable for access. This was one of the reasons I have the longitudinal supports drawn into the previous image.

    Finally, the helm will be just aft of the cabin on the starboard side and will be something like a island console, but dressed up to look nice with the overall design. In the second picture, there is a cushion lying against the side. In the side compartment just forward of that is where I think the fuel tanks will be mounted (above the sole). This is the last outside compartment before the cabin starts.

    And as mentioned previously, everything shown so far (except the actual frames of the boat) is simply tooling to aid in mocking up the interior. The final materials will be higher quality.

    IMG_20170512_160202.jpg

    IMG_20170505_171845.jpg
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Rather than compartmentalizing the below sole areas with plywood partitions, consider using 1x2's with an occasional vertical support. This will be lighter, offer someplace for the sole to land and easy ventilation. Set these 1x2's on edge, for stiffness and piece them in, resting on cleats glued to the lower frame sections. This would be prefered over notching the frames to receive these sole support stringers. Also, tab the perimeter of the sole to the hull shell, then sheath the sole. Cut outs for bilge access would float over a carlin arrangement, for easy removal.

    The bilge in a boat like this needs negative flow, not a pressurize setup. This means it would suck down fresh air through the frame bays, typically behind ceiling pieces, draw it aft to an inline blower (or two), where it gets shoved over the side. This can be passive (Nicro vents) or active (blowers switched at the helm). If the boat will be stored dry and on a trailer, a passive system will do. If she's to see long at berth times, I prefer both passive and active.

    I'd also consider some lightening holes in those frames. They can offer rod storage as well as a good home for oars, nets, boat hooks etc. while lowering weight as well. Hole dimensions should be no more than 40% of the depth of the frame in surrounding areas, centered of course and with well radiused edges. If you prefer, you can make hooked notches, which makes putting things in or pulling them out of the frames easier. In this case, the notch should be well rounded, where it moves away from the inboard edge of the frame, top and bottom and no more than 30% the depth, preferably less.

    [​IMG]
    This is an example of one cut too deep (about 50%), though in this case not as big a concern, because of the build type, this frame is greatly weakened and shouldn't be employed on a build (plank over frame) like yours.
     
  7. Ike
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    Ike Senior Member

    Looks like you've done a good job so far. As usual, excellent advice from phil and PAR.
     
  8. mrintense
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    mrintense "Clipper"

    Paul, thanks for the response. After your's, Phil's, and Ike's comments, I am in agreement that an open fore and aft bracing under the sole is the way to go. This is simpler and easier to do than plywood longitudinals. This provides the transverse ventilation needed between frames. I still need to figure out the best way to ventilate fore and aft (across frames), but have elected to spend some time drawing up ideas (over existing photos) of the interior in hopes that something will come to mind. I am rather reluctant to drill any holes in the frames except for possibly smaller diameter holes for wiring (and not in the bilge area). One thought that comes to mind after reading your comment was having blowers at the aft end of the boat in providing negative pressure as you mentioned across the back sides of any cabinetry and having ventilation holes in the soles under the cabinetry. This is why I want to mock up ideas with drawings for the interior.

    I also feel that I need to get a better idea of how the fuel tanks are going to be mounted, situated, and so forth. Given that they are probably going to be side mounted, they form a fore and aft barrier. Perhaps the blower induced negative pressure ventilation only needs to extend from aft to the fuel compartments and the under sole ventilation can be passive using vents under the cabinetry (in the sole) and vents in the cabinetry doors, walls facing the cabin interior.

    Finally, and I want to add this because I very much appreciate experienced individuals such as yourselves, providing advise to newbies like me. Although I am quite experienced at life and pretty handy, I am a babe in the woods when it comes to boats, so having the help of experienced people really makes a difference and allows me to avoid making serious mistakes. I am determined however to learn from this and perhaps someday, I can lend advise in a similar manner. Thanks again.
     
  9. Barry
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    Barry Senior Member

    I assume and expect that others have assumed as well that you are installing fixed tanks that have a filler to the out side of the bilge and another vent on the highest point on the tank mounted outside the bilge.
    I do not have access to my ABYC manual for several weeks, but I believe there is a distance requirement for fillers and vents to cabin areas.
    What material are you building the tanks out of? There are some specific mounting requirements if aluminum.
    Also and Ike might confirm, that there are now requirements for a pressure relief closed tank system. An item on another thread. It might not be an issue for a home built but perhaps your insurer will require a survey and an up to date surveyor might not approve a "non-conforming" fuel tank installation
    Also you need access to fuel gauges, and tank connections which need to be a draw not a bottom flooded fitting and might require an anti-siphon valve.
    All hoses must meet ABYC specs which auto hoses do not, clamps all stainless including the worm, most auto clamps are not, double hose clamps on the filler hose. If an aluminum tank, no copper/brass fittings
     
  10. mrintense
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    mrintense "Clipper"

    Barry, thanks for your response. I appreciate your taking the time to provide the useful advice.

    I do recall reading something about a distance requirement for the filler and vents. The original design of this boat called for the gas tanks in this same location and the filler openings are located on the side walkways. Perhaps the legal requirements were different when the boat was designed in the 1950s. I will have to check on that. As the cabin will be 18 inches shorter in length than the original design, the designed location of the fillers will be further back from the cabin.

    I intend on aluminum tanks and have quite a bit of useful reference material on the design and mounting of these, so I am hopeful that I can come up with a workable design. I will definitely be mocking the up as well. Fortunately, I happen to know a very experienced boat inspector who has offered to look over my fuel system design when I get to it.

    One thing I don't have a satisfactory answer to is how to keep two saddle tanks in balance short of having to manually switch tanks every so often. It does appear that I might have room below the sole for a very shallow tank, but that location presents more problems with filling, fuel pick up, venting and a reduced capacity. It would of course eliminate the balance problem.

    Just for a little bit of extra information, I plan on primarily using this in lakes for day cruising. Overnight stays on the boat will only occur if I am too lazy to go back to the ramp. I have the ability to work from home so I may spend some of my work days working from the boat instead, provided I can get a decent cell connection on my phone. I don't anticipate ever taking her out on the ocean except maybe for a short jaunt around a protected bay.
     
  11. Barry
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    Barry Senior Member

    The equalization is not an issue
    Most marine fuel filters have two inlets. Near the draw for each tank install a fuel valve so you can isolate each tank but keeping both open will level the tank but you could shut out a particular tank if you needed to
    Re tank design; the tank should be strapped for mounting with plastic or say 1/4 inch neoprene gasket material in strips underneath the tank and between the straps and the tank. The strips must be glued, 3m 5000 is good, baffle the tanks,

    There are have been some good posts on aluminum tank installation that you could search for through the search engine for more info on this forum
     
    Last edited: May 16, 2017
  12. Ike
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    Ike Senior Member

    I do not believe the new EPA standards apply to a home built boats, but ABYC standards have been updated to the EPA regulations so as Barry said you may need to build accordingly to get it insured. see Boat Building Regulations | Boat Fuel System http://newboatbuilders.com/pages/fuel.html and Boat Building Regulations | Boat Fuel System | Fuel Tank Diagram http://newboatbuilders.com/pages/fuel_tank.html. With aluminum tanks they should be mounted so that air can circulate all around the tank. Trapped moisture is a killer of metal tanks, and aluminum is no exception. The best way is to have tabs welded to the tank that can be used to mount it so that no tank surfaces actually touch any wood or other surface. This is not as hard to do as it sounds. As for balancing the fuel in the tanks you can use a crossover line with a valve to transfer fuel from one tank to the other. But really, I think these tanks will not weigh enough to really make a difference. The boat may list a degree or two with one full and the other empty, but that shouldn't be a problem.
     

  13. Barry
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    Barry Senior Member

    I would avoid a tab welded to an aluminum tank unless you can ensure that there is absolutely no flex between mounting points. The flexing can cause work hardening around the tab with subsequent leaks. Certainly lots of aluminum tanks have tabs but we have repaired dozens of tanks steel and aluminum, with cracks around the weld of a tab.
    Straps, with neoprene glued to them permit slight movement and minimize work hardening issues.
     
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