Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 23
  1. #11
    Join Date
    Mar 2002
    Posts
    17,608

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by CLB View Post
    You are misquoting me.
    Ops, guilty as charged.
    Maths still wrong though, see next point (on top of throttle) ...

    Quote Originally Posted by CLB View Post
    A four stroke engine will suck in every four revolutions
    I don't think so.

    Regardless, I just couldn't resist challenging your math, but as I said all this is just academic, and not helpful for the OP.
    If she were my boat, the check suggested by JL still would be my first course of action.

  2. #12
    Join Date
    Mar 2002
    Posts
    17,608

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by James L View Post
    I don't think you need these blowers to run all the time, you need to run them for a few minutes before starting the engine to get rid of any petrol fumes that could be ignited by sparks when you start the engine.
    Once the engine is running, it is doing the job of the blowers.
    There are actually different schools of thought on this.
    I used to own a US speedboat, big block powered and pretty fast.
    Both her blowers extracted air from the bottom of the engine compartment, which is obviously how they must work, in a petrol boat (@ CLB: they would be useless for preventing an explosion, if they would push air into the e/r).
    But while they were not wired to run permanently when the engine was on, the manual recommended to keep them running not only before startup, but also whenever idling for some time (I can't remember the exact wording, but this was the sense).
    The rationale is that even if the air sucked by the engine normally grants enough circulation, petrol vapour are heavier and could cumulate at the bottom whenever running for some time at very low rpm, also because the engine air intake is usually much higher, so a little air exchange at the top of the e/r could be insufficient to prevent explosions.

  3. #13
    Join Date
    Jul 2012
    Location
    London / West Cork
    Posts
    235

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by moresparks View Post
    I agree … but the blowers were designed that way by Fairline, I am guessing they felt it was necessary so I am reluctant to rewire. The Third blower is switchable.
    I'm no expert but that seems like a strange arrangement. 1 should be plenty to remove fumes.
    Are you sure of the original direction of the 2 permanent blowers, any chance they were blowing fresh air into the engine bay, if as you say there is no other ventilation this would seem like a more sensible arrangement. 2 blowing air in all the time the engines are running and 1 to run before startup to remove any petrol fumes.
    www.fimtra.com - Distributed software, Innovative solutions.

  4. #14
    Join Date
    Mar 2002
    Posts
    17,608

    Default Re: Petrol engines – engine compartment ventilation.

    Agreed, the opposite would make more sense.

  5. #15
    Join Date
    Jul 2011
    Posts
    449

    Default Re: Petrol engines – engine compartment ventilation.

    Maths being my strongest weak point.
    2.5l 4 stroke fires every 2 revs. Every 2 revs the engine has fired every cylinder.
    5500 rpm / 2 = 2750 cycles x 2.5 l = 6875 l x 2 engines = 13750 l = 14 m2 per min. Add to this air suction fans

    Take of engine cover at tick over and feel how easy / hard it is to lift.
    Take off cover at flat out and feel if it has resistance ie being sucked on and if revs increase
    Look for tell tale black streaks at edges / joints of engine cover where air is being sucked in.

    The “bilge fan” or the (incorrectly named) “engine blower” is normally (if pipe is at bottom of engine bay or bilge) a suction fan to remove petrol fumes from the bottom as they are heavier than air.
    If pipe is around the air intake or may blow from front to back of engine to give cooling air as well, and it blows in this is then forced air for engine consumption

  6. #16
    Join Date
    Jun 2013
    Posts
    3,521

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by MapisM View Post
    Ops, guilty as charged.
    Maths still wrong though, see next point (on top of throttle) ...

    I don't think so.

    Regardless, I just couldn't resist challenging your math, but as I said all this is just academic, and not helpful for the OP.
    If she were my boat, the check suggested by JL still would be my first course of action.
    Yep, fair cop. I was thinking of the four strokes, but that is, of course, achieved over two revolutions. Basically double my figures.
    If found in the Brexit forum, please return to the real world.

  7. #17
    Join Date
    Oct 2010
    Location
    London no more
    Posts
    2,655

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by moresparks View Post
    “I don't think you need these blowers to run all the time, you need to run them for a few minutes before starting the engine to get rid of any petrol fumes that could be ignited by sparks when you start the engine.
    Once the engine is running, it is doing the job of the blowers.”


    I agree … but the blowers were designed that way by Fairline, I am guessing they felt it was necessary so I am reluctant to rewire. The Third blower is switchable.
    You can always fit a petrol vapour alarm - I think I would on an older boat, especially if I was planning to sleep on it.

  8. #18
    Join Date
    Apr 2007
    Location
    Westwick - Essex
    Posts
    119

    Default Re: Petrol engines – engine compartment ventilation.

    Hmm... a lot to think about, many thanks for your suggestions.
    The blowers that are on all the time are connected via one of the engines ignition switches, so it seems that the builders made a conscious effort to ensure the blowers work effectively.
    All the louvered vents are at the stern; to have both “suck and blow” from the same area I thought would have been dangerous as you may be pushing extracted vapours back into the carburetors?
    I can check next time I am out and prop up the hatch a little and test.
    I will also look into a petrol vapour alarm specifically for the locker. I currently have a CO alarm near the helmsman position which is next to the main cabin door.

  9. #19
    Join Date
    Feb 2008
    Location
    Farndon
    Posts
    3,037

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by moresparks View Post
    . I understand that petrol engines do not perform as well when they are hot and can lead to shorter lived engines. With that in mind I thought it may be better to improve the ventilation. .
    The engines are water cooled so dont rely on air for cooling.
    At slow speed , after a fast run , the engine temperatures should fall.
    I am surprised there are no air intakes .

  10. #20
    Join Date
    Apr 2011
    Location
    Boat- Western Med
    Posts
    5,321

    Default Re: Petrol engines – engine compartment ventilation.

    Petrol sources with old engines are from in no particular order .....
    The lead carb plugs seeping , carb gaskets leaking seeping , poor fuel lines deteriorated. Poor adjustment of the idle jets or stuck choke not quite closing .
    So it makes sense with an aged carb boat to run the bilge extraction fan all the time when the engines running and what ever ( suggest 5/10 mins ) before start up .
    Hot starts too .Because the residual block heat will evaporate off any float chambers into the air filters and out into the ER .

    The OP ought to fit some sort of fwd facing ram air scoops, the faster ( more rpm ) the boat goes the more air is stuffed in to feed it .
    Vetus do some nice SS air vents
    A row of theses for example https://www.vetus.com/en/ventilation...ntilators.html facing Fwds on top of the cover ?
    Last edited by Portofino; 10-09-19 at 20:33.

Page 2 of 3 FirstFirst 123 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Latest YBW News

Find Boats For Sale

to
to