Keeping it simple. 16' overnighter

Discussion in 'Multihulls' started by wheels, Feb 13, 2011.

  1. wheels
    Joined: Jun 2010
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    Location: Folsom, Ca

    wheels Junior Member

    Hello,
    As with all of us who are just starting out my visions of "Ideal" have changed significantly since I first started dreaming of heading out on the water.

    I've looked at steel, fiberglass, wood, etc... Sail and motor. Singlehull and multihull. I've dreamed huge and minimalist. Blue water and local lake. On and on and I've come to realize a few things: You have to start somewhere and the more complex the more likely to fail. So with that in mind I've started on something simple.

    I will admit that I was mostly inspired by the slider/slipper series of cat.

    I live about 10 minutes away from folsom lake. An 11,500 acre lake near Sacramento California. And an hour from Lake Tahoe. Great fishing and facilities. So my thoughts are a simple flat bottom cat, 16'LOA 8'BOA simple lifting dagger boards, kick up rudders, lug sail, sleeping for 4 (one 7'x2' birth in each hull with folding hoop windscreen/dodger and 2 on the trampoline using the boom as a tent). I'm a paraplegic (motorcycle accident 8 years ago) so I need back/lateral support when sitting so there will be padded seating as shown but I have yet to work out a steering arrangement quite yet. There should be plenty of room in the seating to mount an electric motor and room in the hulls for 4 batteries plus food water camping gear etc for several days (i'm greedy, if i'm camping on the lake I want my phone charged and laptop running for entertainment (music, movies, etc in the evening)

    I understand that this is not the most efficient hull form. As this is my first attempt I'm looking for the edge of absolute simplicity. I know a box would be simpler. and round is the most efficient. But it should provide the stability and space/storage I'm looking for in a simple stitch and glue/ glass over plywood design with the least number of seams and simple curves. 1" pine stringers, 1/2" ply bottom 3/8" sides and deck. Rudders/dagger boards from glass wrapped laminated wood. (I have access to a cnc machine shop so shaping should be relatively precise)

    Placement of the center/dagger boards is still up in the air. I do know that I want it to round into the wind quickly if I fall off. I am quite a competent swimmer still but having to swim for distance is much more difficult without being able to kick your legs.

    I'm thinking balanced lug type sail for ease of reefing. But as with anything that may change in the next 15 minutes.

    So here is my primary "sketch" of what I'm thinking. So avoiding complete hullshape redesign what are your thoughts. Google Sketchup file available for anyone that wants to play with it.
     

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  2. rayaldridge
    Joined: Jun 2006
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    rayaldridge Senior Member

    I'm flattered that you were inspired by Slider. Maybe it would be helpful for me to point out areas where your proposed design is different from Slider.

    One thing I'd worry about with your design is that with those straight topsides, the hulls will not be very fine. Slider achieves 10 to 1 hulls (length to beam) at the waterline because she has flared hulls, like this:

    [​IMG]

    In my opinion, a cat will not be very fast if her hulls are much fatter than this at the waterline. As I'm sure you've discovered, in a boat that short, it's quite difficult to get bunks of adequate width in a hull flared like Slider's. I had to go to 20 feet in the new boat to get enough hull beam to put in a couple of single berths, and still retain adequate fineness in the hulls. About the only way you can do it in that length is to build a knuckle into the hull, but then the problem is that the slender hulls below the knuckle will not carry a lot of weight.

    Another area of concern is your rig. It doesn't look very big. While Slider has a modest rig, compared to beach cats, it still has 140 sq. ft. I'm also not sure that lug rig is appropriate for a cat. I wrote a piece about the reason I chose sprit sloop rig for Slider here.

    Your scantlings are pretty heavy, and you seem to be planning to carry a lot of weight. Slider's hulls are planked with 1/4" ply, and could have been planked in even thinner stuff without becoming too fragile. With several adults, a bunch of batteries, and all your gear, the boat will probably be badly overloaded. Multihulls, to sail well, must be kept really light. For illustrative purposes, Thomas Firth Jones was dubious about carrying a lead plate house battery in anything smaller than a 23 footer, and even then he thought it better to not do so. Another possible problem is weight distribution. In cats, you want to keep weight out of the ends, for several reasons, and you show your crew sitting well aft. For comparison, in Slider we usually sit just a little aft of the center of the boat..

    I don't mean to be discouraging, because if I could luck out and design a good boat, anyone can. But I don't think you can get the boat you want in such a short length.
     
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  3. wheels
    Joined: Jun 2010
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    Location: Folsom, Ca

    wheels Junior Member

    *rig not to scale

    I just put something up there to get the picture in my head. I guess the number one requirement for the rig is ease of adjustment from the seated position as well as reeding and furling.

    All of your points are good and valid.

    Due to my physical limitations my thinking was very shallow draft if I'm gonna be able to get in it I need to be as close to the edge of the water I can get. I know that weight is the enemy in cats and I'm completely flexible when it comes to materials. I was planning on running the slightly thicker ply due to the sheer being vertical with little bow. Also the lakes around here have some nasty bottoms and as folsom is a resovior groundings are common as the water level drops in the summer. I know the hulls are 8:1 and it wouldn't be difficult at all to add length. I'm not lookin for speed at the moment, I'm looking for stability hence the wider hull sections. If the boat heels hard I am more likely to fall off.

    But we all start someplace.

    I guess my #1 thing on my SOR is: can it be singlehanded comfortably by a paraplegic.

    So I'm either stuck with BIG or small. And I don't have the time budget or patience for a floating condo like an offshore cat right now. But I will look at sliders a bit closer and see if I can adapt it easy enough or weather it's worth the effort to design around my specific needs.

    Oh and thanks for reminding me about weight distribution and your other suggestions as well.
     
  4. rayaldridge
    Joined: Jun 2006
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    rayaldridge Senior Member

    You have a difficult design problem, and I can see why you've gone the way you have. Have you been working on the design for very long? I've found that the longer I ponder a problem, the more likely I am to come up with a solution. Withing reason, of course. The world is full of dreamers that never do, and our time on earth is sadly limited.

    Are you planning to board from a dock? If not, freeboard may be a problem. You mentioned that you wanted to be sure the boat would not sail off without you, so I assume you have an idea for reboarding from the water.

    Look into the problem of waterplane loading. If you have big flat hulls, it's true that you'll have a flatter ride, but it will also be more violent, when chop is encountered. One reason I like a boat with flared hulls is that in a modest chop, the boat can still cut through like a fine-hulled craft, but when encountering larger waves, the movement of rising to the wave is less abrupt than it would be in wider hulls. Transverse stability in Slider is pretty good. I can set a beer bottle on the center deck, and it won't tip over even in pretty bad conditions. Breaking ocean waves and really big wakes will knock it over, but you try to avoid that sort of stuff if possible..

    I think one difficulty with a lug rig, as I mentioned in that article I referred you to, is that optimally it is not a stayed rig. It's hard to arrange an unstayed rig on a cat without a bridgedeck cabin. It can be done-- Google Tony Bigras and Miss Cindy. He used a biplane lug rig, with masts stepped in each hull. Another possibility for a trouble-free rig that can be singlehanded from a sitting position is the one used by Sea Pearls. These are open monohulls with a cat ketch rig. The sails are conventional jibheaded sails, but they furl around a rotating mast. Reefing couldn't be simpler. Usually you wouldn't want a two stick rig is a multihull that small, but in your situation might call for it.

    Since you plan to keep the boat at trailerable width, could you move any of your accomodations into a bridgedeck cabin. You might take a look at the various Jarcats for ideas here-- they're the most successful cats of this type that I'm aware of.

    I hope other forums members will chime in. There are lots of folks here who know much more about multihull design than I do, and your design requirements should make for an interesting and useful conversation.
     
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  5. wheels
    Joined: Jun 2010
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    Location: Folsom, Ca

    wheels Junior Member

    I have an irrational fear of bridgedeck structures on small cats...

    :)

    think something like this would work a little better? (still playing with the form, I still need to figure displacement/waterline)

    oh and thankyou again for your intrest
     

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  6. rayaldridge
    Joined: Jun 2006
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    rayaldridge Senior Member

    Wheels, I like the new drawing better, but I would probably change it in a couple of respects.

    One big problem with double-enders that are relatively symmetrical fore and aft is that there is no natural pitch damping. This was an issue that I was greatly concerned with when drawing Slider, because our old Wharram Tane was a double-ender of this type, and one of its flaws was a tendency to pitch. In leftover chop and light wind, it was hard to keep the boat moving and the sails filled. With Slider, I gave the hulls fairly big transoms, and the shape, while very fine forward, was quite fat aft. The result was good; Slider has very strong natural pitch damping. If you look at this video, you can see what I mean-- we cross a couple of wakes, one near the end of the video.

    I can't tell how much rocker you've given the hulls from your most recent sketch. I gave Slider what many designers would consider excessive rocker, in order to get enough displacement for two people and a camping outfit in 16 feet, and still keep the hulls slim enough for good performance.

    But I think you're making good progress. Are you using a design program that can give you displacement figures? This would be very helpful, I think. I use Hulls, by Gregg Carlson and Freeship, by a consortium of Dutch programmers. You can Google these up pretty easily, and they'll give you a lot of info as you work.
     
  7. wheels
    Joined: Jun 2010
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    Location: Folsom, Ca

    wheels Junior Member

    in that sketch there is 0 rocker, but the bottom (orange) of the hull is narrow enough that i'm not worried about slamming, plus the lakes around here get 1.5' swells at the most on gusty days. I do understand about hobby horsing over them.

    the waterline shown is at between 1000-1200 lbs the program I have doesn't calculate volume so I have to run the numbers by hand and I figure in 10% error towards the low side as i try to get close using simple shapes and then guestimating the difference in the curved area vs a straight line.

    I will have the opportunity to get ahold of autocad and solidworks here in a couple of weeks thankfully. The renderings i have here to me are like sketches on a napkin so to speak. Close enough that i get a good picture of what i'm thinking (it took about 30 minutes to throw this one together)

    i guess the reason I keep leaning towards the wider hulls is space and load carrying ability and most importantly ease of build. Ideal for 2 but comfortable with 4 on board. speed isn't really in my equation at the moment. standard usage would be taking it out on saturday with my brother and 2 cousins, dangling feet and fishing line in the water and enjoying life. The lake is about 4 miles across at the longest point so if it takes me an hour and a half to get there that's just time to relax on the way. think small pontoon partyboat with a sail.

    Knowing me, i'll probably fuss around with for several more months before finalizing anything, then once i get into it i'm sure i'll find something that catches my eye and modify it again
     

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  8. dstgean
    Joined: Aug 2006
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    dstgean Senior Member

    Why not build a Gary dierking Tamanu or Wa'apa adapted to your use? The design is already there, it works great, and it is cheap and easy to modify to your particular requirements.

    Dan
     
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