Cartop Sailboat

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

  1. nbehlman
    Joined: Aug 2011
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    Location: CT

    nbehlman Junior Member

    My daughter is 2 years old. In terms of displacement, I'm sizing for myself at 150 lb, plus a 50 lb 8 year old. Not completely unreasonable for a beamy 8ft dinghy. Below is a picture of what I've got so far. It's a flat bottom, 2-plank lapstrake skiff. It's very similar to an optimist in terms of size, it looks a little nicer to my eye. I think my hull weight estimate is conservative at 120 lbs. I assumed fir plywood, but maybe I can get lighter with okoume. I'll need some kind of system to get it on and off of the roof rack, but that was kind of expected. I'd like to put a little deck and coaming on it to make it a little prettier. That will probably bring the weight back up on me.

    In the images below...
    black X = center of effort
    red circle = center of lateral resistance
    blue triangle = center of bouyancy
    inverted blue triangle = center of gravity

    The waterline shown is based on 200 lbs of human payload and my estimated weights for hull and rig. Looks OK to me. I was thinking about finding a sail that's close to this size that I can order. Anyone use sailrite.com?
    skiff1.png skiff2.png
    sailarea=35 ft^2
    LOA=8ft
    beam=4ft
    displacement=328 lbs
    CE 12% forward of CLR
    Hull weight=120 lbs
     
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Way back when. I regularly car topped a sunfish On a Toyota hatchback. Sorry no pictures. I mounted a homemade wooden bracket to the car's roof rack. It consisted of cross members below two for/aft vertical carpet covered 2x4s with rollers mounted at aft end. It was easier to launch solo than with "assistance". The looks I received while backing down a boat ramp.
     
  3. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Looks OK. But the Boom seems a little low. Remember, this is an 8 ft boat. Do you plan on sitting on the very bottom? If you are, I wouldn't advise it. You will be sitting in a constant puddle of water. This was one of the characteristics I liked the least about the Super Snark(r) I once owned. In the boat I am building, I will be sitting about 5 inches above the bilge. Your Boom looks too low for even bilge sitting. I would allow at least 2.5 ft (30 in) clearance.

    I'm not sure the lead you drew is a good idea. Far better to end up with weather helm than lee helm, as long as it isn't ridiculous. With too much lee helm, you may never get out of it. The board so dominates the Effective Lateral Area (ELA) that no matter which way you trim the boat, the lee helm will stay. This is especially true with a deep board and a short rig.

    You will almost certainly want a double halyard for your sail, but it could be quite simple. It could be a dumb sheave for the throat and a dumb sheave for the peak. The peak halyard could simply lash to the gaff somewhere near its mid-point.

    You may be able to get away with 3/16th or even 1/8th inch plywood for the side panels to save a little weight. The gunwales will need a little beef. You could go with a double timber gunwale or a double inwhale with spacer blocks. These, along with a short fore-deck and a short aft-deck, will certainly add considerable rigidity along with plenty of potential hand-gripping and tie down points. The decks need not be thick if you don't sit on them and as long as you put a mast partner block in the fore-deck.

    I think you're off to a good start.
     
  4. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    They were probably thinking you were about to launch the car to get the boat off ;)
     
  5. nbehlman
    Joined: Aug 2011
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    nbehlman Junior Member

    I’m confused. Are you saying my CE is too far toward it too far aft? This thread started off with CE and CLR. The reference rwatson provided (Sail Balance https://www.diy-wood-boat.com/sail-balance.html#Center_of_Effort) says to place CE forward of CLR to achieve weather helm. I was targeting 10ish% lead.
     
  6. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    I'm saying the CE may be too far forward.

    One puts Center of Effort (CE) forward to reduce weather helm. On beamy boats, weather helm is often a big problem, especially those with a long Bow taper. This weather helm is caused by the change in the bottom shape as the boat heels. It is also caused by the CE of the sails moving to leeward also as the boat heels. So, a boat with a very tall rig will have more weather helm than one with a shorter rig for this reason. Being that your boat is a dinghy, it will probably not be allowed to heel enough for the weather helm to get that bad. Another thing to keep in mind is that your boat will have a relatively deep 'board which will dominate the effective CLR, because it will produce its maximum lift at a much lower angle of attack than the immersed hull profile will. And your boat has a short rig, on top of all that.
     
  7. rnlock
    Joined: Aug 2016
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    Location: Massachusetts

    rnlock Junior Member

    I think a lot of you guys are being pessimistic about weight. With clever design work and persistence, remarkably light weights can be achieved. For instance, I used to own a boat that could sail just fine with 4 adults on board. I could also hold the whole thing over my head, so it was easy to put on tne car by myself. It weighed maybe 75 lbs stripped. Obviously, a Bolger Brick is kind of ugly. I designed and built a dingy for a friend which came out at around 45 lbs, though it was made with crummy underlayment which I think was yellow pine. Also, I put very light glass and epoxy on it so it would hold up to outside storage. I suppose adding rudder fittings and a mast step would make it a few pounds heavier, but better plywood could take care of that. I also had a Bolger Nymph, which would row fine with 3 adults. It was very manageable. I never put in a mast step and rudder fittings, but I'm sure it would still have been very easy to carry around. Unlike the other boats I've mentioned, it's actually kind of cute. One modification I did was to rip out the internal framing. I'm sure this would have made it far more comfortable for sailing. To keep the boat rigid, I made the gunwhale a little thicker and then capped it with a wider board to make a c shaped beam (as oppose to an I beam) out of it. This worked well, and probably would have helped with strong winds when sailing.
    What all of these boats SHOULD have had was flotation. A light way to do this might have been with those canoe air bags sold for that purpose. Or buoyancy chambers covered with aircraft fabric instead of plywood. A la Monfort.

    Speaking of Platt Monfort, with his designs you can get elegant shapes and very light weight. We had his Cartopper 9 for a while. Very light, and sailed very well with two fairly large adults until the gunwhale started to come apart. A gunwhale like the one I had on the Nymph probably would have fixed that. If the Cartopper is too mundane, he did a traditional looking sailboat design which I think was 11 or 12 feet long. I seem to recall it was supposed to weigh 75 lbs. I saw the hull of the prototype while he was building it and thought it was a beautiful shape.

    While the aircraft fabric he covered his boats with is remarkably strong, I'd consider using something heavier, or maybe 2 or 3 layers. He had some ideas about that for the pretty sailboat. Also, I'm sure they can hold a lot more leaves and dirt inside and would obviously be more work to clean. If you didn't like any of his designs, (which seems unlikely) you could always adapt the techniques for some other shape. Hereshoff and Chappele had some good looking dinghy designs. Bolger said his Defender dinghy had good lines for sailing, though you could probably shrink it a bit and still carry enough,weight. With any of these skin on frame boats, I imagine you'd be trading a fair amount of extra work to get the elegance and light weight.

    I wonder how much Bolger's Fieldmouse would weigh if built stock, but with light wood. Right now I forget who designed the Auk and Auklet dinghies. Also, there are designs out there that can be separated into two lighter pieces. However, the most famous one, the Folding Schooner, would probably be a bit much!
     
  8. rnlock
    Joined: Aug 2016
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    rnlock Junior Member

    A further thought. When I was a kid, I sailed an O'day Sprite. Hull shape was very pretty. Under the right conditions, it could actually plane, though I doubt that would be possible upwind. I remember circling a 50 foot schooner going downwind. My brother once actually asked me to slow down after he slid across the bottom when turning. It was an easy boat to manage, in tbe water, but out of the water it was quite heavy. Maybe it came out of a chopper gun. It could definitely have used bigger flotation chambers. In a chop, it would slam into waves upwind, which slowed it down some. It might have been even faster with a vang.
    Anyway, I imagine a boat of similar shape and size, but in some other, lighter kind of construction and flotation tanks, would be fun for you. Especially with a better rig, centerboard, and rudder. Was it an Uffa Fox design?
     
  9. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    For my boat, I am using 12 plastic milk bottles for self-rescue flotation. Each one is a separate flotation chamber and each will displace about 8 lbs of water. And they don't weigh much. Originally, I had planned to use blue or pink construction foam but worried about two things. One was fire, as verticle surfaces of this foam burn spectacularly. And two was moisture getting between the foam and the hull sides and encouraging rot. Plus I would have to pay for the foam. I can get the milk bottles for free. The milk bottles will be mounted to the insides of the sides with twine in four groups of three.
     

  10. tlouth7
    Joined: Jun 2013
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    Location: Cambridge, UK

    tlouth7 Junior Member

    A couple of thoughts:

    Your freeboard looks very low, you will dip a gunwale at very little heel which will be catastrophic.

    Your boom is far too low to get under during a tack. Also the aspect ration of your sail is a bit low to my eye.

    Dragging the transom will kill your performance, but equally you already have quite a lot of rocker. Can you not increase length a little?

    Finally you won't be able to get anywhere near the mast to raise and lower sail, so you will need to run all the lines aft.
     
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