Modifying a Nacra 5.8

Discussion in 'Multihulls' started by gypsy28, Dec 1, 2013.

  1. gypsy28
    Joined: Mar 2010
    Posts: 218
    Likes: 25, Points: 28, Legacy Rep: 120
    Location: NSW Australia

    gypsy28 Senior Member

    Hi All,
    I have just purchased an old cheap Nacra 5.8 catamaran. The hulls are structurally ok, just a bit neglected (mouldy). The rig, sails, beams, deck gear and trailer are all in good condition.
    As I am already going to be doing work on the hulls (minor repairs and a full respray) so I would like to lengthen the bows a bit (add 0.4m to make 6.2m LOA), so as to qualify the boat for local trailerable multihull races that usually exclude beach cats. I also plan on adding wing seats as trapezing is also banned.
    I have experience with timber/epoxy work and also poly/vinylester mould construction, but I haven't ever lengthened a catamarans bows before ;) I figure I should use foam to shape the new bow and sheath with epoxy/glass.

    Im just not sure what type of foam to use or how much glass would be enough (I thought maybe 400 to 600gsm?)
    Any help would be appreciated.

    Here is a pic of a similar catamaran with lengthened bows that Im trying to replicate.
    Cheers, Dave
     

    Attached Files:

  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 472, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The laminate schedule depends on which resin you'll use. The foam should be no less than 5 pound (2.2 kg) density, preferably heavier, especially if you sail in rough conditions. A polyester laminate would be a few alternating courses of mat and cloth or roving, finished with some mat, so you have something to fair on. An epoxy laminate could be a couple layers of 1208 to gain bulk quickly, though the lighter and stronger arrangement would be a straight biax schedule with a 1 ounce (34 gsm) mat on top. Lastly, you'll likely want to stiffen the centerline and bow of this nose piece with additional fabric. If you use good quality, higher density foam, you can go fairly thin on the schedule, but if you use a modest density core, the skin has to be thicker.
     
  3. TANSL
    Joined: Sep 2011
    Posts: 5,739
    Likes: 185, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Indeed, the foam should serve only to shape the new bow. The foam can not be accepted as a resistance element, thus the final thickness of the skin is independent of the weight of the foam. Should you get the new bow a thickness equal, or very similar, to that in the keel of the boat.
    Moreover, what PAR proposes seems very correct, except that I do not think a centerpiece of reinforcement is needed.
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 472, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    My thoughts on the bow reinforcement are for abrasion resistance and modest impact, as it is a beach cat.
     
  5. TANSL
    Joined: Sep 2011
    Posts: 5,739
    Likes: 185, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Yes, that is what is called a "protective Bow", which is achieved by increasing the thickness of the laminate, never putting an inner reinforcement.
     
  6. Corley
    Joined: Oct 2009
    Posts: 3,740
    Likes: 173, Points: 63, Legacy Rep: 826
    Location: Melbourne, Australia

    Corley epoxy coated

    If you just extend out the bow it will be out of the water and not be of any value to your waterline length. One way to integrate the new bow would be to spring a batten from the centre of the hull and extend it forward to the current waterline/bow height to give you the new outline. Then you can fair it into the existing hull.

    I know it's more work but you could add a small sugar scoop to the rear of the boat. I could see how you could make a nice stern extension with a cutout for the rudder and it would make an easy boarding step too.

    Just a few random thoughts, good luck with your project :)
     
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 472, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    [​IMG]

    The bows on the Narca 5.8 will be "engaged" most of the time, even with a 15"-16" extension. Even with the slight rocker in the bows, just extending them forward, along the same curve will be beneficial as well as conform to the rules. I'll be interested in reports on balance and rig tune with this modification.
     
  8. bruceb
    Joined: Nov 2008
    Posts: 1,207
    Likes: 39, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    keep it light

    I used to repair beach cats of all brands, and except for the bows and keels, they were usually built of two layers of fairly thin glass on the outside, 1/4-3/8" foam, and one layer of glass on the inside. Plenty strong, but not very puncture resistant. I can't see why an extension needs to be any heavier, even if it broke off, the boat would still float. The 5.8 is a bit of a tank already, it certainly doesn't need much more weight. I suspect the extended bows will help some in heavy conditions, but they will make a wet boat even wetter- bring a snorkel:D
    B
     
  9. gypsy28
    Joined: Mar 2010
    Posts: 218
    Likes: 25, Points: 28, Legacy Rep: 120
    Location: NSW Australia

    gypsy28 Senior Member

    Thanks for the advice everyone.
    I will outline my current plan, please agree or shoot down in flames my proposal :D
    I intend to cut a profile of the shape of the new bow out of 4 mm Okoume ply and glue it to the centreline of the bow. Then I will build up either side of the ply with enough foam to fair back into/onto the hull sides. Then I will sheath with upto 600gsm (too much? or not enough?) and do final fairing. I will be using epoxy throughout. I have attached a picture of a mock up of the shape of the new bow.
    Please correct any of my above plan or suggest a better way.
    Would a high density polyurethane foam sheet be OK to use (something similar to these http://www.afpt.com.au/afpt_polyurethane.html ?

    Thanks again for the help.
     

    Attached Files:

  10. luckystrike
    Joined: Feb 2010
    Posts: 226
    Likes: 14, Points: 18, Legacy Rep: 92
    Location: Germany

    luckystrike Power Kraut

    Hi,
    for foam I would use styrofoam (xps?) this is a insulation foam with a density of around 35 - 40kg/m³. This is the same foam ( a little stiffer for the boat) the european surfers use (30kg/m³) for there home made windsurf boards. It soakes no water if you damage your laminate.

    A woven 560 - 600gr/m² glass fabric with the fibres aligned in 0/90° and the second layer diagonally at 45/45° will do the job perfectly.

    Best Regards, Michel
     

  11. petereng
    Joined: Jan 2008
    Posts: 581
    Likes: 21, Points: 28, Legacy Rep: 252
    Location: Gold Coast Australia

    petereng Senior Member

    Hi Dave - Don't get too carried away with the reverse bow. They are a bit of a fashion at the moment and probably will go back to vertical soon. The dreadnaught bow is exceptionally wet and if done you will need to put on a spray rail. I follow the 5.8's around the race course doing rescue and have to right them occasionally. The more bow volume the better!! Originally the reverse bow comes from making the fwd stations an upside down pear shape. Which means as the hull goes forward you run out of geometry at the top first. This was done so that when a hull dives the usual flat deck does not drive the bow in due to hydrodynamic effects. It then became a fashion, so as you have done the designers leave a small vertical at waterline but add the reverse for fashion at the top. The reverse looks aggressive and does its job in terms of asthetics but doesn't function well .(IMHO) The other side of bow design is that a modern bow is designed to go over the top of the water not through the water. Your extension gives you the opportunity to create this. So don't try and get the knuckle in the water, keep it high. Even monos are doing this now and AC have been doing it for decades. Look up Davidson bow. Cheers Peter S

    35kg PU is perfect and 600gsm is probably too heavy, 450 would be fine. Hobies and small cats use 250gsm for their hulls so 600gsm is hugh!
     
    Last edited: Dec 5, 2013
    1 person likes this.
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.