Joining a Beach Cat hull

Discussion in 'Fiberglass and Composite Boat Building' started by gypsy28, May 16, 2017.

  1. gypsy28
    Joined: Mar 2010
    Posts: 218
    Likes: 26, Points: 28, Legacy Rep: 120
    Location: NSW Australia

    gypsy28 Senior Member

    Im currently extending/modernising a 16 ft beach cat hull to 18 ft with the intention of building a fibreglass mold so I can build 6 hulls for myself and 2 mates. We do a lot of camp cruising on our current 16 ft beach cats but could always do with a bit more space hence the 18ft cat. We could just buy 18 ft beach cats but they are designed/built for racing so we are designing/building ours with more camp cruising features (storage).

    Im a fibreglass mold maker myself so modifying the hull and building the mold is not an issue.

    I intend to split the mold on the centre line of the hull to do away with the hull/deck join flange. I have attached a picture of how I intend to do the hull join and am looking for opinions. The drawing isn't to scale or an accurate drawing but it gives the idea. The hull bottoms will have a join flange on the inside and a 2mm rebate on the outside. I intend to glue the flange together with epoxy and add a couple of layers of glass tape in the outer rebate followed by gelcoat over the top. The deck will also have a 2mm rebate on the outside for glass/gelcoat and a couple of layers of tape on the inside. Is this joining method adequate?

    The hull layup will be gelcoat, 450gsm biaxial, 6mm PVC foam, 330gsm biaxial with stringer from midships to bow on each side and extra glass layers on the hull bottom/bow. Does this sound adequate?

    Any other suggestions or tips are welcome.

  2. frenette
    Joined: May 2011
    Posts: 21
    Likes: 1, Points: 3, Legacy Rep: 13
    Location: Southern California

    frenette Junior Member

    You will be operating in some areas with rocks and such the deck joint is a pretty good sacrificial structure to protest the hull. If you're getting rid of this to prevent pitch polling then add freeboard in the bow. Sailing Hobies in the past I don't recall ever walking foreword to land on the beach so you have some real freedom to build a wave splitting deck.

    Use 1/2" (12 or 13mm) PVC and you should be OK with this layup. The Hobie 16 is made for crashing up on the sand so the bottom needs to be REAL strong. Think about bogging out the bottom of the v in the hull the same way you would do a torchered plywood boat.

    For storage think about a V shaped center pod with the ice chest built into. I wouldn't recommend storage in the hulls until you go full on Malibu Outrigger deck layout. At that point the Outrigger would be easier to transport and rig. The newer updates outriggers used inverted box shapes so the side were about 1.5" with quick connects to hold it to the cross arm. This is a pretty easy way to retain the razed above the water feel of the nets when using a these light weight sections. The section is going to be maybe 4 wide to replace the net. The openings will let some spray in but far less then nets will. The boat I looked at used bungee cord with a stopper knot around the cross arm to a fork fitting to hold a ball. These things take less time than walking around the boat to install or remove from the boat. Stack flat. The box sections are spanning more than 8' after you stretch the boat longer so the sides maybe closer to 3" and maybe double up the foam layers. The other option is make the V shaped pod and spand side to center support. The openings running side to side isn't as good for sleeping on.

    What are you going to do about the 8' beam? Stick with it, make it fold a bit, or just go with the same sail plan? I'm thinking you may want a more powerful rig. So maybe a square top main and a larger jib.

    On the PVC 1/2" 5oz is the easiest to find in the USA. The extra weight is a rounding error. The thicker foam makes a much tougher hull.

    So what does what I just described look like:

    Start with the cross arm deck structure. I'd do with 3mm plywood bent in a C shape with lots of opens in the aft bulkhead for storage. The Cross arm could bend in the middle to narrow the boat with a mast base that bridges the fold joint. For longer than you intended passages these will help break the wind and spray. Also it would be real easy to put a pop in insert for the mast.

    Cut the top off the H16 hull. Build a quick splice to get the length. Run an 1.5" stringer on the outside. This will pull any wave backs out of the hull and give additional reserve buoyancy without affecting wetted hull. Make some bulkhead to support the mold. Splice up some plywood to spline the next level hull to add freeboard. You will tapper out the bump to nothing as it get close to the bow.

    If I were doing this I'd find a stiffer mast section and this is small enough where you should be able to scavenge surplus masts vs adding upper spreaders. Update the rig to a square top or bite the bullet and go side by side freestanding masts. You will normally want something with the center of effort in the sail plan closer to the water than a racer.

    I answered more questions than you asked. I hope this is useful.

  3. upchurchmr
    Joined: Feb 2011
    Posts: 3,287
    Likes: 258, Points: 83, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    What is the 1/2" PVC for?
    Can you provide some sketches for what you are talking about? I don't understand your description at all.

    Gypsy28 - what boat are you going to start with?
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,036, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    How do you propose to get inside the hulls once you unite the halves, to tape the join ? :eek:

  5. frenette
    Joined: May 2011
    Posts: 21
    Likes: 1, Points: 3, Legacy Rep: 13
    Location: Southern California

    frenette Junior Member

    First off as soon as I posted I noticed the picture. The joint turned in has that effect of bogging the bottom joint. I don't know if you can pull this off in bows as it's to pointy to get your hands/bag/cloth/glue in there. If it's problematic think about making J shaped joint or an insert. The J shaped joint would be make the shape be a mirror of the other side so you glue the J's together so you get some shape stability and a long glue joint.

    The early F27 ama's have a trailing edge that waves back at you. With the tech 30 years ago they simply filled/faired it off. Now with infusion I would build an insert. When I build this kind of thing the carbon tow is easiest to get ahold of so you fill your V shape with carbon and infuse with epoxy or ??? This should stiffen out the sharp edge. I wouldn't put this share of an edge on your cat project as the rounded joint H16 used is much better for beaching.

    As far as how builders get inside for glue layup? Corsair on the early F27 amas, they made access covers. I've never had anyone comment on less clean lines. Again the way the mold lines adding wider glue line reduce the need for this kind of access.

    On PVC cross linked foams. I like Corecell most. I'm using 5/8" a550 for my 36 cat. For interior I'm using 1/2" a500. M5xx replaces A5xx to continue to offer the 50% stretch before breaking. This is hands down the best stuff I've ever used. If you're infusing the hull there are other foam suppliers in this group. I'm very sensitive to the skin failures with the others Foam product using vacuum bagging. If you're infusing this starved layup problem should be a non-issue.

    I'll get back with some pictures when I have a bit of time. Anything specific?
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.