Catamaran Nacelles

Discussion in 'Multihulls' started by redreuben, Jan 29, 2017.

  1. redreuben
    Joined: Jan 2009
    Posts: 1,999
    Likes: 223, Points: 63, Legacy Rep: 349
    Location: South Lake Western Australia

    redreuben redreuben

    Over on the central daggerboard thread the utility of nacelles has been mentioned as a place to mount a central board but I have a whole bunch of ancillary questions so I have started a new thread.
    From that thread.

    1) My interest is in cats in the 7-8m range.
    Do you think there is a wet deck height where the effect of a nacelle is negligible and a height where it is going to be a major hindrance ?

    2) Load distribution, would a nacelle actually help with mast and forestay loads ? I just assumed they would as its a deeper structure ?

    3) Heavy objects like batteries, fuel, water, beer etc; Is it not better to have them in the hulls assuming even distribution than in a nacelle ?
    What are the pluses and minuses ?
     
  2. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    This is to be a pure coastal cruiser? With bridgedeck cabin or w/o?
     
  3. redreuben
    Joined: Jan 2009
    Posts: 1,999
    Likes: 223, Points: 63, Legacy Rep: 349
    Location: South Lake Western Australia

    redreuben redreuben

    Up On Stands, cabin wise I was thinking Richard Woods style at most, not a full bridgedeck.
     
  4. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    RR, trouble is, the boat is rather small for providing any decent storage space along the mid-line of the bridgedeck.
    You have fore and aft cross beams but between them you'll want to use for living and headroom is tight inside or outside the cabin.
    Fore beam has the anchor locker and aft beam has the outboard (assume you use only one).
    Lithium batteries are what, 6.5" wide and 9" tall, length depends on capacity.
    Since you want to fully use the internal hull height over keel to maximize headroom how about 2 batteries or 2 battery sets on the longitudinal center on either side of the bridgedeck, i.e. hard against the inboard hull sides. . Keeps the weight out of the ends.
    If the cat were bigger you could put a lot stuff in the bilges but its not.
    what type of hull profile do you have in mind? flat canoe or pronounced vee?
     

  5. guzzis3
    Joined: Nov 2009
    Posts: 847
    Likes: 159, Points: 43, Legacy Rep: 42
    Location: Brisbane

    guzzis3 Senior Member

    It might be useful to think of the tunnel area rather than just bridgedeck height. Like you this is where my interest lay, but on small cats there is always going to be situations where you get significant impact of water on the connecting structure. In a beam sea waves will potentially hit the nacelle side on for example. The 14% rule attempts to avoid hard vertical slamming, but the other biggie is sneezing or choking, where the whole area hydraulic locks. So on an 8 meter cat for example with a 5 meter beam your looking for bridgedeck clearance of about 750 mm. If you're taking say 20% of that area with the nacelle I'd be looking to increase the bridgedeck to restore that total area between the hulls.

    Only if it bridges most or all of the gap between the hulls and that negates the advantages of a nacelle. Almost all the load on the mast beam is bending around the center. Dolphin strikers and thick mast beams attempt to convert that into compression. A nacelle could be the center post of a dolphin striker but it won't in itself spread the load much.
    Ideally you want them as low as possible and as close to the center of gyration, which is usually centerline and mid length of waterline. Because the buoyancy is at the sides I'd say putting them low and near the mid length of the hulls is better than higher and mid bridgedeck.

    As always opinions worth what you paid for them...
     
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.