Air Intake & Ventilation Methods for Engine Compartment

Discussion in 'Downeast Projects and Boat Building' started by Keelboater, Dec 2, 2019.

  1. Keelboater

    Keelboater Captain

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    Because of the additional HP I'm picking up with the Cummins re-power, I have to increase the open area for the passive motor and bilge cooling air on my boat project. Thinking of removing the old school cowl vents in the process which seem to be more of a trip hazard when going up to the bow. I've been thinking about something like the air cutouts that sporties frequently use on the side of the hulls. Cutouts through the sides with a filler piece in the center and dorade boxes on the inside to contain/discharge any water that makes it's way in. These style of vents are not louvered to direct air into the engine compartment when under way, so I'm wondering if these boats use a blower system to draw the intake air into the engine compartment? I was also considering those anodized aluminum louvers made by Vetus. Seems like quite a few D.E. boats have them installed. I don't think a blower is required with them when sized correctly. I do have 4 existing vents in the back of the step up into the wheel house, inside the cockpit, but don't want to add any more there for the sake of noise in the cockpit. I'm aware that some guys often use the aft corners of the cockpit to create 45 degree air intakes that run under the deck to the motor compartment but in my case that might not work very well and I'm still exploring that one. What are you using on your boat? Thanks.
     
    Last edited: Dec 3, 2019
  2. Genius

    Genius Captain

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    don't use blowers, yuck:(. The engine is the air pump, just go by the calculation on boat diesel.com or Tony's site. I prefer the rear house vents or lazarette vents if you don't have watertight bulkheads.
     
  3. Toolate

    Toolate Captain

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    My Sabre has large hull side vents and a box inside to catch spray etc.. It’s a good setup That stops any direct water passing through but it leads to a misty engine room on windy days in my boat anyway. Could be the design of the boxes/louvers (I can send you pics if you want) but I would have preferred to have them on the sides of the trunk cabin and Sabre switched them up on later boats.

    My Sisu had ducts to vents under the gunwales in the rear bulkhead (winter back) and I would vote for that again myself- maybe a rectangular duct space that could be hidden under a seat or cabinet inside and have a louver on the back. This would be in the cockpit but I would vote for that over the hull sides and you could line them with sound down and make them such that they limited the noise reflected up through them.

    I have to also say that I think a couple boxes in the rear corners of the cockpit would dry the bilge and provide combustion air for the engine and probably not be too noisy given the distance to the engine from the transom? Anything back there to capitalize on Like a built in seat, storage box or live well?

    What is the rule of thumb again? 1 sq in of ventilation for ever 10 hp or something? Curious how much free air space you think you will need. Have to take into account the amount of space the louver blocks due to its construction if your want to be picky.

    My boat has fans but they are for exhausting air from the ER not taking it in so think that approach should work fine. Twins in there too.

    Edit to add: soundown used to have a sketch on their website showing a box built under a gunwale that had no sides parallel to each other and was lined with their material. Idea was that no engine sound (or much less) could be reflected up through it.
     
    Last edited: Dec 2, 2019
  4. Diesel Jerry

    Diesel Jerry Captain

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    45’s at the corners of the transom.
     
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  5. BillD

    BillD Captain

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    on my 27 H&H
    fuel tank fills are on top,
    fuel tank vents in the cock
     

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  6. CEShawn

    CEShawn Captain

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    I love this thread, I need to do something. I have 45 at Transom, but I still think I want something midship. I dont think with 17' of deck I get the air flow I want. I wanted to throw in vents over the side, the nice VETUS, but can't squeeze them in the way I want do with a box to prevent water, they make a nice one. I just saw one on the back corner of a boat and captured an imagine. I fear though about engine noise too... will looking for good ideas here. I guess just add as many 90 degree angles to cut down on noise...
     
  7. SlowLoris

    SlowLoris Member

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    When I rebuilt the stringers on my jc Last winter I built them full height and had full
    Knowledge that there was going to be a issue with how much air wasn’t getting to the engine. I did the calculations and even by putting some breather holes in the stringers to promote circulation (mostly for the fuel tanks to stay air dry) the boat would had insufficient air volume available to the engine.
    My solution was to install two blowers that pushed air into the compartment from the helm area and they work great. The added benefit is that it actually cools down the engine compartment as well as the alternator because of the excess air flow.
    In an emergency situation where Both blower go down I can either open the engine hatch, or I have a cutout in the engine bulkhead for service work I can open.
     
  8. Genius

    Genius Captain

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    why go against SKI, JMK and Tony...? Some very knowledgable dudes on the forums.
     

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  9. Keelboater

    Keelboater Captain

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    Thanks for the replies. I'm going with Tony's sizing recommendations of 50 sq. in. per 100 HP seeing that I did purchase the motor from him. That is more air than Dave Gerr's recommended size of HP/3.3 x 1.2 so I'm sure it's fine. Just not quite sure what the best way to get there is at this point. I almost have to double what was originally there so it will take some creativity, especially if I ditch those existing cowl vents. Please keep the ideas and pictures coming.
     
  10. Diesel Jerry

    Diesel Jerry Captain

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    I agree with Tony 100%, but would suggest that be your minimum. With a target 15/20% higher.
     
  11. Diesel Jerry

    Diesel Jerry Captain

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    My day did 45’s on almost every transom of the boats he finished off. Last year someone pulled a deck up of the first boat he finished off (30+ years ago). All the PT was still solid, as well as the plywood. By venting from the aft end of the boat you create a draft the entire length of the bilge. Also the gets assisted by the air flow coming off the cabin down into the deck.
     
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  12. Genius

    Genius Captain

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    any sources for louvered vents? seen some on amazon (non marine anodized aluminum)...don't know if that's a waste of time or what. Thought of making them out of glass or stainless as well. Something I also need to address on the workboat.
     

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  13. BillD

    BillD Captain

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    I cannot find replacements for the two SS louvers I posted.
     
  14. gmac

    gmac Senior Member

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    Like how this one was done.

    Same on each side..
     

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  15. goin4broke

    goin4broke Captain

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    Following many recommendations I put 45's in the aft corners. I fully understand what diesel Jerry has said in this post as well as others on the subject. Venting under decks to me is about as important as getting air to the engine. I calculated I can achieve 49 square inches from each corner which gets me to about 1/3 of where I want to be. Biggest headache so far is finding a louver with the right dimensions and that doesn't look like I stole it off somebody's house. Anyone know of anything out there?
     
  16. Dslpwr

    Dslpwr Senior Member

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    We build our own, aluminum and then powder coat 63A71780-ECF2-4666-B74D-9245F0E601D8.jpeg
     
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  17. Keelboater

    Keelboater Captain

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    After thinking about this a bit, one advantage that I have in this case is that my boat has a liner in the cockpit, and there is a gap between it and the hull. Maybe I can use this area as a simple duct for a couple of nice flush mounted louvers in the back of the cockpit. Don't know just yet but sounds like it should work with no need to build any kind of box on the cockpit wall. :)
     
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  18. cb34

    cb34 Captain

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    If you build your corners in the transom and do not bring them all the way up to meet the wash boards you can create the same air flow as a louver.
     
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  19. Diesel Jerry

    Diesel Jerry Captain

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    That is what I mean by 45’s
     
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  20. Fishnjess

    Fishnjess Senior Member

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    I have to redo my air intakes for the engine compartment as well. Since it is open already I was thinking about putting the vents on the top of the trunk cabin and running with 3”pvc into the engine compartment.
     

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