A 20 foot cruising catamaran

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  • slidercat
    Senior Member
    • Jul 2008
    • 808

    A 20 foot cruising catamaran

    I hope folks will find this interesting. For my next attempt at boat design, I'm going to try to build a fast 20 foot cruising cat for two. The rig is a donor from a Nacra 5.2, and the boat folds to trailerable beam, but sailing beam is 12 feet.

    I've been posting a series of articles on the various design decisions I've been making. The drawing below is from one of the accommodations articles-- about the two single berths, which are to be in the starboard hull. A small galley, dinette, and space for a porta-potty are intended for the port hull.



    I'm trying for the minimal amount of cat that will allow two people to cruise the Bahamas. Among other design goals: good performance, trailerable behind a compact car, inexpensive, simple and quick to build, and maybe a little wholesome beauty.

    More info here:

    I should add that there will be no plans available for years, if ever. I'll have to build and test the boat extensively before I could consider offering plans.
    Ray

    http://slidercat.com/blog/wordpress

    sigpic
  • Dan St Gean
    Senior Member
    • Nov 2004
    • 1138

    #2
    Re: A 20 foot cruising catamaran

    Great start Ray. I've been enjoying watching the design process!

    Dan

    Comment

    • wtarzia
      Senior Member
      • Feb 2007
      • 2104

      #3
      Re: A 20 foot cruising catamaran

      Looking forward to more. The hulls seem to be the 'best of' the Wharram and the dory design but with more appropriate rocker to reduce Wharram cat hobby horsing (which I have only heard about, not experienced). -- Wade
      Last edited by wtarzia; 02-01-2011, 09:23 AM. Reason: spelling

      Comment

      • peterchech
        Senior Member
        • Apr 2010
        • 796

        #4
        Re: A 20 foot cruising catamaran

        20 ft seems like a really practical size for a cruising cat, a catamaran "pocket cruiser".

        Short enough that cost should stay very low, cheap and easy to find rigs/etc., easy to trailer, yet large enough to be relatively seaworthy. But, you gonna be able to squeeze space into those hulls, even for single berths? Seems like they will be really tiny, unless you increase the width of the individual hulls alot, but then you have other problems... any other pics with some more specs and angles? What length/beam ratio are you going for on the hulls?
        “The difference between an adventurer and anybody else is that the youthful embrace of discovery, of self or of the world, is not muted by the responsibilities or the safety-catches of maturity.” Jonathan Borgais

        Comment

        • slidercat
          Senior Member
          • Jul 2008
          • 808

          #5
          Re: A 20 foot cruising catamaran

          Thanks, guys.

          Wade, you're right about the excessive pitching you sometimes get with a Wharram. Our old Tane was a great boat, but in the right circumstances (say, light wind and leftover chop) she would pitch the wind right out of the sails. These hulls are very similar to Slider's hulls and Slider is really resistant to pitching, for a fine-hulled cat. There's a video here that shows us crossing a wake, and you can see how quickly the pitching damps out. I think the difference is that Wharrams are double-ended and Slider is not. The run aft flattens out and has a good bit of displacement all the way to the transoms.

          The rocker is a leftover from Slider, too. I had to add a little to get enough displacement for two people and a camping outfit, and still keep the hulls fine enough. But it didn't seem to hurt Slider's speed, and I hope it won't hurt the new boat's. I do think the extra rocker helps to make Slider as handy as she is.

          Peter, it isn't easy to get even modest accommodations into a tiny cat. What makes it possible in this case is that the new boat is dory-hulled. The flare gives it reasonable fineness at the waterline. These hulls are 10 to 1, which I regard as the minimum for a reasonably fast cat. However, the flare also allows single berths of reasonable width higher in the hulls. The forward berth shown in the drawing above is 28 inches wide at the head. Th lower berth, which is less luxurious, is 24 inches at the head. Both are wider than the absolute minimum width for a single berth, generally held to be 22 inches. Both taper significantly at the foot, but this is okay.

          This is a drawing that shows some more information:



          You could probably shoehorn a double berth into these hulls, if you were willing to build a knuckle into them, but I like the simplicity of build, lower weight, and (I think) drier ride of the dory hulls.
          Ray

          http://slidercat.com/blog/wordpress

          sigpic

          Comment

          • AnalogKid
            Senior Member
            • Aug 2008
            • 3680

            #6
            Re: A 20 foot cruising catamaran

            Have you got all the details of your folding system together Ray?

            It looks like the centre longitudinal section will go forward with the mast as the beams hinge at the hull and on the centre line. What about the decks?

            Andy.
            'When I leave I don't know what I'm hoping to find. When I leave I don't know what I'm leaving behind...'

            Comment

            • Chris Ostlind
              North of the Pier
              • Jun 2002
              • 730

              #7
              Re: A 20 foot cruising catamaran

              Some things, Ray...

              I see that you have moved the daggerboard to one of the hulls, rather than the previously argued center beam structure. What prompted this deviation when you were so adamant about a surface piercing centerboard in previous conversations?

              I also see that the boat is just shy of six feet tall at its deepest point. I had this impression that it was going to have very low profile, slippery aero structures that did not create large drag signatures. The way these cabins are drawn, they turn out to be but a small difference from the Gato Especial after all is said and done. Could you tell us why you have such tall cabins considering the previous discussions on this topic?

              I, too, would be interested in the details of the folding system.

              Comment

              • Woxbox
                Senior Member
                • Feb 2006
                • 9923

                #8
                Re: A 20 foot cruising catamaran

                Looks very promising to me. If it were to be my boat, I might be more interested in a low aspect, simple rig like Slider's. I do also wonder where all the gear and goods for a cruise around the Bahamas would get stowed. Maybe a forward beam that's a chunky box section that could take odds and ends like anchors, fenders, dock lines, etc.? How about a boom tent? All that stuff that gets wet or grungy that you don't want to stow below.
                -Dave

                Comment

                • slidercat
                  Senior Member
                  • Jul 2008
                  • 808

                  #9
                  Re: A 20 foot cruising catamaran

                  Thanks Wox. I think that weight is a more limiting factor than stowage. There is separate stowage in the buoyancy compartments for light stuff, and I'll probably put anchor chocks just forward of the forebeam. There seems to be plenty of volume in the hulls. The deal with the rig is that I acquired a complete Nacra 5.2 rig-- mast, boom, jib, main, blocks, travelers, sheets, standing rigging, halyards, etc. for less than half the cost of Slider's loft-made mainsail. The rig is from a 17 foot cat with an 8 foot beam, so I think at 20 X 12 Slinger will be pretty stable. I'll need to build a couple of reefs into the sail for cruising off soundings.

                  Andy, I think I have the folding system worked out. I've never seen it before, and it might be patentable, so I plan to keep it under my hat for a while. The system I'd worked out previously required a fairly heavy central spine in order to work, which is why I'd considered putting the foils on the centerline. But the current idea can use a much lighter central spine, so I moved them back to the conventional positions.

                  I can never tell when Chris is kidding. Here's the above-waterline profile of Slinger.



                  I'm not seeing any resemblance at all, but maybe Chris will post a similar profile from his boat.
                  Ray

                  http://slidercat.com/blog/wordpress

                  sigpic

                  Comment

                  • Dan St Gean
                    Senior Member
                    • Nov 2004
                    • 1138

                    #10
                    Re: A 20 foot cruising catamaran

                    Originally posted by slidercat
                    Thanks Wox. I think that weight is a more limiting factor than stowage. There is separate stowage in the buoyancy compartments for light stuff, and I'll probably put anchor chocks just forward of the forebeam. There seems to be plenty of volume in the hulls. The deal with the rig is that I acquired a complete Nacra 5.2 rig-- mast, boom, jib, main, blocks, travelers, sheets, standing rigging, halyards, etc. for less than half the cost of Slider's loft-made mainsail. The rig is from a 17 foot cat with an 8 foot beam, so I think at 20 X 12 Slinger will be pretty stable. I'll need to build a couple of reefs into the sail for cruising off soundings.

                    Andy, I think I have the folding system worked out. I've never seen it before, and it might be patentable, so I plan to keep it under my hat for a while. The system I'd worked out previously required a fairly heavy central spine in order to work, which is why I'd considered putting the foils on the centerline. But the current idea can use a much lighter central spine, so I moved them back to the conventional positions.

                    I can never tell when Chris is kidding. Here's the above-waterline profile of Slinger.



                    I'm not seeing any resemblance at all, but maybe Chris will post a similar profile from his boat.
                    Your boat has a real workboat flavor to it in profile. I like the look. Have you thought of an interior chine flat to expand the accomodations a bit? It would add and extra step in the build, but the K650 has quite the interior volume for individual hulls. I sailed for a week on a Tennant Tourissimo 10 that was stretched to 36' and had inboard pods for the bunks. I think the port hull had a queen! You could have a twin in each hull with such an arrangement even on the short waterline. They'd have to be pretty far above the waterline to avoid slamming though...wasn't that the earlier working name?

                    I'm loving the creativity and bringing yet another interesting multihull microcruiser into being. Continued success!

                    Dan
                    Last edited by Dan St Gean; 02-03-2011, 09:21 AM.

                    Comment

                    • JimD
                      Senior Mumbler
                      • Feb 2002
                      • 29704

                      #11
                      Re: A 20 foot cruising catamaran

                      Originally posted by slidercat
                      ... The deal with the rig is that I acquired a complete Nacra 5.2 rig-- mast, boom, jib, main, blocks, travelers, sheets, standing rigging, halyards, etc. for less than half the cost of Slider's loft-made mainsail...
                      Presumably there is room to play with the rig as other builders might find deals on variously available sails.
                      There is no rational, logical, or physical description of how free will could exist. It therefore makes no sense to praise or condemn anyone on the grounds they are a free willed self that made one choice but could have chosen something else. There is no evidence that such a situation is possible in our Universe. Demonstrate otherwise and I will be thrilled.

                      Comment

                      • peterchech
                        Senior Member
                        • Apr 2010
                        • 796

                        #12
                        Re: A 20 foot cruising catamaran

                        who will be the first to slap a polytarp junk rig on this? lol
                        “The difference between an adventurer and anybody else is that the youthful embrace of discovery, of self or of the world, is not muted by the responsibilities or the safety-catches of maturity.” Jonathan Borgais

                        Comment

                        • slidercat
                          Senior Member
                          • Jul 2008
                          • 808

                          #13
                          Re: A 20 foot cruising catamaran

                          Dan, you're right about the flavor, I do like the looks of Western workboats a lot-- probably for the same reasons WoodenBoat is a popular magazine. One of the things that I'm trying to do with these designs is to try to meld that esthetic with modern catamaran ideas. A knuckle would give considerable more interior volume, but at the cost of some weight and complication in building. I also wonder about the behavior of boats with a pronounced knuckle in chop. It seems ideally placed to catch a wavetop and thump the boat a little. That may not be a real problem, but I think that for the same reason that bridgedeck clearance needs to be as high as possible, that knuckles may put additional stress on a hull. I'm probably wrong about that, as many excellent designers have taken that approach. I guess I'm committed to keeping the boat as light and simple as possible-- to have only enough stuff to take two people to the Bahamas, and no more.

                          Jim, that's correct. Because the mast is stepped on the central spine-- in order to allow the rig to stay up when the boat is folded for a narrow slip or for dry-sailing-- the mast placement is independent of the crossbeam positions. You can see in the drawings that the mast is well aft of the forebeam. Anyway, what this means in practice is that the mast can stepped anywhere along the central spine. Should my various calculations go awry, and I have too much weather helm or even worse, lee helm, I can just move the mast, which is a much easier proposition than moving a daggerboard case. If it turns out to be a decent little boat, it would probably be ideal for someone who wanted to experiment with different rigs.

                          I'm working on the drawings for the port hull, which contains the galley and the dinette. Hard to believe it can fit, but I think it will.
                          Ray

                          http://slidercat.com/blog/wordpress

                          sigpic

                          Comment

                          • Chip-skiff
                            Wolves Without Borders
                            • Jan 2008
                            • 22781

                            #14
                            Re: A 20 foot cruising catamaran

                            Having lived for years in various camps, I'm looking at the mundane logistics.

                            You can probably fit a galley/dinette into one of the hulls, but won't it be pretty cramped with two people for reasonable comfort while cooking, talking, charting, etc.?

                            What about bridging the house forward of the mast, so there's a communicating space between the two hulls? I don't think it'd add much windage, and it would certainly make the boat more liveable, i.e. you could pass food to the other hull, converse at anchor, and so on.

                            Comment

                            • Dan St Gean
                              Senior Member
                              • Nov 2004
                              • 1138

                              #15
                              Re: A 20 foot cruising catamaran

                              Along with what Chip-skiff mentioned, that could be a dodger/bimini arrangement a la Jim Brown's setup. Make each independent of the other yet it'll give lots of utility either way. Easy to fold up and stow as well! Little big boats seems to favor multis as long as one can keep them light. One of my faves in the category is Wood's Sango with it's slick folding system.

                              Dan

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