Advice on Preventing Prop Ventilation

Discussion in 'Powerboats' started by BCD, Jan 11, 2015.

  1. BCD
    Joined: Jun 2013
    Posts: 16
    Likes: 2, Points: 0, Legacy Rep: 30
    Location: Australia

    BCD Junior Member

    I have a 42ft Power Catamaran which is prone to prop ventilation. This usually occurs at top revs and especially in a beam-on seaway. I'm looking for ways to fix this problem from people who have had experience with it. Attached are some photos of the underwater area.
     

    Attached Files:

  2. whitepointer23

    whitepointer23 Previous Member

    Interesting hulls. Similar to a seabright skiff box keel. What design is it. It looks like the prop is very close to the surface and that may be the problem .running with a quartering sea it would have trouble keeping the props submerged all the time. Do you have pics of the full length of the hulls. What horsepower and cruise speed.
     
  3. michael pierzga
    Joined: Dec 2008
    Posts: 4,862
    Likes: 114, Points: 0, Legacy Rep: 1180
    Location: spain

    michael pierzga Senior Member

    The trailing egde of the keel looks a bit crude.

    Dont know much about cats. They are so shallow and wide that in a beam sea the windward prop might break the surface.

    Perhaps a different style prop , one that eats air like the racing powerboats use, might work
     
  4. BCD
    Joined: Jun 2013
    Posts: 16
    Likes: 2, Points: 0, Legacy Rep: 30
    Location: Australia

    BCD Junior Member

    It's a cat. It draws under 1m so the props are close to the surface. 2 x 110hp, 16knot @ 90%.
     

    Attached Files:

  5. michael pierzga
    Joined: Dec 2008
    Posts: 4,862
    Likes: 114, Points: 0, Legacy Rep: 1180
    Location: spain

    michael pierzga Senior Member

    You might contact the designer...he might have some suggestions.
     
  6. whitepointer23

    whitepointer23 Previous Member

    I realise that but it doesn't make a cat special. I have had vent issues on mono's that draw under a meter. Usually when the prop is to close to the transom. Is it semi displacement. What is your cruise speed. 12 to 15 knots. Does it trim level or bow down. You may be able to improve it a lot just by trim. Sorry mate I was typing didn't see your last post.
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,680
    Likes: 265, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    A vexing problem no doubt, but do you know which prop is letting go, at which stage of the hull/wave interaction ? It doesn't occur in a head-on situation ?
     
  8. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 41, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    Not a solution, but perhaps fairing the trailing edge of the keel would reduce some of your problem. The turbulence coming off those flat ends can't help.

    :cool:
     
  9. gonzo
    Joined: Aug 2002
    Posts: 13,558
    Likes: 374, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    A flat plate over the propeller may help. I fixed a similar problem on a monohull that ventilated the prop and rudder on quartering seas.
     
  10. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A shroud would help, but the hulls are long and skinny, so enough vertical movement in these shallow bodies are going to "uncover" the wheels from time to time, especially in certain conditions, like running into or before a beam sea. Fairing the aft end of the box keel will not help the ventilation issue, but will very marginally improve efficiency. The problem is physics and some wave trains are simply just going to expose the props. Tacking across the conditions that cause these issues is the only simple solution, though a well fitted shroud will delay the onset of ventilation.
     
  11. BCD
    Joined: Jun 2013
    Posts: 16
    Likes: 2, Points: 0, Legacy Rep: 30
    Location: Australia

    BCD Junior Member

    The boat semi displaces bow up a bit. Would trim tabs pushing the bows down help? What does a shroud look like? Would 300mm vertical boards about 2m long along the length of the boat help?
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,680
    Likes: 265, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    It might be helpful to travel alongside in another boat in the conditions where the problem shows itself, just to get a better impression of what is happening, but I'd be inclined to fit a plate like gonzo suggested, there seems to be plenty of room there to do it, just allow the regulation prop tip clearance. Could be enough to prevent air being sucked down, when it is close to leaving the water. It would be a simple enough experiment to do, and should not alter performance adversely. Might even dampen pitching, if that is part of the problem.
     
  13. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I suspect trim changes are the issue (pitching) and a plate is an easy thing to try.
     
  14. BCD
    Joined: Jun 2013
    Posts: 16
    Likes: 2, Points: 0, Legacy Rep: 30
    Location: Australia

    BCD Junior Member

    How does the stern going down and the bow going up affect ventilation? When the boat is going 16kn, if you look over the side where the prop is, there is a hollow in the wash extending under the boat. Is this the bow wave or the boat trying to get on the plane?
    Would fairing the rudder help with efficiency?
     

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,680
    Likes: 265, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Whenever the hull(s) adjacent to the prop clear the water, you have the possibility of air being dragged down into the prop. A plate can delay that happening.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.