Cartop Sailboat

Discussion in 'Sailboats' started by nbehlman, Sep 15, 2019.

  1. nbehlman
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    nbehlman Junior Member

    I'm looking to design and build a small sailboat for my daughter and I to sail. I would like something that I can throw on top of the car. I had an optimist as a kid, and I'd like to do something that is similar in size and as easy to sail. I find the optimist kind of ugly though, so I'd like to do something more like a catboat. I like the looks of the Cotuit Skiff.

    I also like to hull lines of this sharpie skiff, though I would prefer to put a gaff rig on it.

    My main question is: what are the primary considerations in designing such a small sailboat? Things I'm wondering about... Should center of area on the sail be forward or aft of the center of buoyancy? How much sail area should I shoot for? How do I balance the load on the mast with the weight of the people?

    Any help is appreciated. Thanks!
  2. messabout
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    messabout Senior Member

    Either of these boats will be a bit much to put on the car top. Both of them will be pretty heavy if built to plans.

    Do yourself a huge favor and buy some plans for a boat that you think you'd like. Build it as the designer intended.

    You have asked some questions that suggests that you are not yet sufficiently informed to be able to successfully design your own boat. .....The center of effort of the sail has little or nothing to do with the center of buoyancy. It does have some importance in terms of the boats center of lateral resistance. The position of the crew in a small boat will largely determine where the center of buoyancy is located at any instant.

    A gaff rig is entirely satisfactory but it is not the most efficient sort of sail and not the easiest one to use.

    Tell us about where and under what conditions you might be sailing, the combined weight of you and your passenger, What kind of speed are you anticipating, Do you want to be able to row the boat some of the time, do you anticipate using a small motor some of the time?????

    The experienced members here will be pleased to advise and encourage you. Most of them will also urge you, as I have, to buy a simple set of plans and build according to the plans. It is almost a sure thing that you will save money by doing so and you will have a boat of proven ability.
  3. nbehlman
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    nbehlman Junior Member

    Messabout, I disagree with your statement that I am "not yet sufficiently informed to be able to successfully design my own boat." My last post (which you commented on) documented the construction of a 14ft jet boat that I designed. I lofted the shape, computed center of buoyancy, center of mass, and did the speed and stability predictions. The boat floats on the waterline I predicted, and achieves 38 mph, the exact top speed I predicted. I wrote all of the code for the predictions myself. I even won "best in show - owner build power boat" at the Mystic Seaport Wooden Boat Show. I think I can handle an 8ft sailing dinghy. I asked about the location of center of buoyancy in relation to center of sail area because I know this is crucial in rocket and aircraft design. Rockets and aircraft are dynamically unstable if the center of lift is forward of the center of mass. I take it this is not a consideration for sailboats. Perhaps the center of area of centerboard and rudder are important relative to the center sail area? I've not found a plan out there that I want to build. I'm going to design my own. I'm just looking to understand the basic design considerations.
  4. rwatson
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    rwatson Senior Member

    "the location of center of buoyancy in relation to the center of sail area "

    I think the real question is the location of the centre of Sail Effort to Centre of Effort of the Hull.
    Sail Balance

    Do it the old-timers way. Cut a 3 foot scale of the underwater hull silhouette out of plywood including the board and rudder extended.
    Balance it on a straight edge.
    That will be the Hull Centre of Effort.

    The Sail Centre of Effort needs to be a bit in front.
  5. Dolfiman
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    Dolfiman Senior Member

  6. nbehlman
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    nbehlman Junior Member

  7. nbehlman
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    nbehlman Junior Member

    That kitoo looks pretty quick!
  8. messabout
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    messabout Senior Member

    Nbehlman please accept my apologies for questioning your capacity for designing a sail boat.

    Many people with almost no experience with the fundamentals of boat design post questions here. I made the mistake of presuming that you might be in that category. Your question about center of effort and center of buoyancy led me to that faulty assumption. I am old enough to know that the practice of drawing hasty conclusions is often fraught with embarassment. My bad.

    Rwatson's link to is useful. I have a quarrel with some of the statements made at that site. The quarrel is about the "lead". You are capable of analyzing the forces and resistances of the dynamic actions. Vector analysis ....sort of. When doing so keep in mind that the sail is never in a perfect fore and aft alignment. The CE will move forward as a function of the cosine of the sail angle with respect to the centerline of the boat. If the sail's CE is forward of the boats center of lateral resistance then the boat will try to fall off to leeward. Lee helm is generally a bad idea. An unsafe one in some weather circumstances. Fortunately, the CLR of a very small boat can be moved around by shifting crew weight forward or aft. The boat will need to be designed such that moving the CB and thus the CLR, the boat needs to have had some forethought about the bow and transom immersions at different degrees of trim.
    tlouth7 likes this.
  9. nbehlman
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    nbehlman Junior Member

    Messabout, no worries. Thanks for the input. You make a good point that the CE should be adjusted for the sail angle. It seems to me that saying the center of area is the same as the center of pressure is only an approximation. As the sail fills and curves outward, I wonder how that impacts the center of pressure. I will be sure to account for crew weight in my trim and CLR calculations. This is exactly the info I needed.
  10. messabout
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    messabout Senior Member

    If you intend to build a short boat, like an eight footer, hopefully a bit longer, then you have a lot of decisions to make. A pointy little skiff is cute and boatlike. It will however, be quite sensitive to fore and aft trim. A scow or pram will be less so. Alas.... not as elegant a little boat as the pointy skiff. That the appearance of your boat as an object of derision at the yacht club or public venue ought not to much influence your choice of a tiny sailing boat. The PDR for example, is an awful ugly thing but it works amazingly well , is relatively safe, commodious, and easy to build.

    Your mini Riva is testament to your ability and also suggestive of your tendency to favor elegance. A little sailboat is a whole other ball game. I am thinking of a really small example of an E scow, C scow, M20, or something of that general layout. You have pulled that off with aplomb with the little "gentleman's runabout". I suspect that you can do that with a little sailboat too.

    I will be interested in the your final design and encourage you no matter what the final decision. Messing about in boats with one's daughter is a credit to a special dad.
  11. tlouth7
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    tlouth7 Senior Member

    For the avoidance of doubt, the Optimist is a catboat.

    I do not know the age of your daughter obviously, but a boat that is large enough to be sailed by an adult and child, and yet light enough for one person to lift onto a car, is a difficult thing to achieve. You will have to skimp on the scantlings everywhere you can, which will lead to it not being terribly robust.

    There are some very nice sprit dinghies that you could emulate: check out skerry and gunning dory. The sprit rig gets you a large sail on short and lightweight spars, especially if you go boomless or sprit-boom.

    For a lightweight hull you will probably want to use stitch-and-glue or skin-on-frame, there are plenty of resources online to read about these techniques. Prams are often recommended and get you lots of volume and stability in a small footprint, but obviously can be ugly if done wrong.
  12. OzFred
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    OzFred Senior Member

    I don't think you can build a boat that will meet all your requirements. In particular, being able to "throw [it] on top of the car". A Sabot is the required length (2.4m) and just the hull weighs 43kg. Add a small trolly and it would be over 50kg. An Optimist is a little smaller (2.18m) and 35kg. It's not all about weight, the size and shape make them difficult to lift and place on a roof rack, even harder in a breeze or uneven ground. Both boats can be sailed by an adult and child, but they'll need to be fairly small or you will find it very slow and uncomfortable going.

    Consider something like a Mirror or Herron, I know families who sail with one parent and one or two children aged 6 to 10. Both boats can be car topped but only with help from at least one strong person. Or at the very least try a few boats in the size you're considering to see how much room is available and determine the design features are most important to you. You will have to make trade–offs so having priorities will help with making decisions.
  13. philSweet
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    philSweet Senior Member

    I don't really want to burst your bubble, but if you don't want a Snark, you don't want a cartopper. The two in your original post would have 500 - 700 pound bare hulls if you hope to carry those rigs, plus you'd need removable ballast. Look at the rigs of any of the 20 or so Snark derivatives over the years. That is the sort of sail plan that a cartopper can cope with (about 45 sq feet, not 145 sq feet)


    points to consider -

    Cartopping will add a lot to the weight of the hull over what it requires to be structurally sound as a boat. The loads have nothing in common, so you have to provide structure and hardpoints for both.

    You not only have to get the thing on and off the car, you have to launch and recover it from the water. So how much weight do you really consider practical even if you can disassemble some of the heavier bits?

    There's a reason everybody trailers boats like the Sunfish and Laser - they are too much effort to launch and recover as cartoppers. Also, mileage, if you are travelling any distance at all, is much better with a trailer.

    Unless you have an old DeSoto or 60s Town and Country wagon, your car is probably limited to 75 pounds of weight on the roof. There are a few that are higher, like Range Rover and Hummer. Minivans with factory racks tend to run around 150 pound rating. Obviously, this is a limit you need to work within. And even something as basic as an RS Zest has a bare hull weight of 161 pounds.

    The sharpie skiff you show appears to displace about 1500 pounds. They are specifically designed to be the best load-carrying form for the dollar. They utterly fail if not carrying a load.
  14. sharpii2
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    sharpii2 Senior Member

    I am presently building a boat which will weigh about 85 lbs for just the hull. The rig will weigh around 13 lbs, and the rudder and lee board assemblies will probably add another 10 lbs. I designed it to be built with three sheets of 1/4 inch plywood. It will be 3 by 10 ft and 15 inches hull depth. I believe longer and leaner is the way to go, so you don't have to lift the whole thing at once. One end can be lifted and put on top of the vehicle. Then the other end can be lifted and then the whole thing can be slid into place. I will probably have to design a roof cradle for this thing to distribute the load on the thin-sheet metal roof. I'd rather use a trailer, but there is no place to put it.
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