Cartop Sailboat

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

  1. nbehlman
    Joined: Aug 2011
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    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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    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
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    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
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    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
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    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
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    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
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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
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    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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    tlouth7 Senior 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.
     
  11. messabout
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    messabout Senior Member

    Synopsis: You can build that boat, as drawn for no more than 60 pounds. Use Ocumee not fir. Select any and all stringers carefully for weight and grain consistency. Move the board forward or you will suffer dangerous lee helm. Increase the rocker at the transom so that the bottom is above the design waterline. Make the mast taller so as to get some space under the boom. Increase the height of the sides to at least 16 inches...You are going to have kids in this boat, right? Make the stem higher and let the sides swoop down in a salty curve. Use minimum deck area. That is too small a boat to have much of a deck intruding into the people space. Most of all make the boat longer, ten feet at least. The extra length will make a lot of difference in trim and it will not increase the weight by enough to make a difference in loading. A longer boat is often easier to load on cartop than a shorter one.

    Depending on the shape of your vehicle a simple loading aid is a piece of carpet laid over the rear of the car top. Load the boat right side up and it will slide nicely into the location that you have chosen. Unload it that way too.

    Bast of luck for that noble project.
     
  12. rnlock
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    rnlock Junior Member

    As sharpii2 has pointed out, a daggerboard or centerboard will tend to dominate the CLR. Each square inch of board will provide much more lift (i.e. force opposing leeway than a projected square inch of hull area. Furthermore, the lift from the hull will vary a lot depending on heel angle, displacement and speed. It may be a good,idea to allow for adjustments to mast rake or position, at least at first.
    -----------
    Someone was saying you couldn't carry very much sail on a cartoppable boat. That depends. I'm sure a Snark has this problem. However, my Brick didn't. It had 59 square feet of sail on a mast almost 16 feet long. 20 mph winds were no sweat, except for certain creaking noises, and making impossible to get the sheet back into the slot in the boom after it came out while I was trying to adjust the snotter. This is something to do at the dock when there's that kind of wind unless you like rowing a box against a strong wind. Our Monfort Cartopper 9 probably weighed around 40 lbs stripped, but it stood up to fairly strong winds until the gunwhale started to come apart. (Another boat I'd recommend, at least in relatively small waves, if properly reinforced.) I imagine Michalak's Vole could carry sail pretty well, too.
    -------------
    The aforementioned daughter won't be 50 lbs for long. If you built a Brick you could take her sailing with two or three of her friends when she's twenty.
    ----------
    Has anyone mentioned Bolger's Cartopper? I know someone who built one and liked it. He says other people like it too. If I had one, I think I'd try to figure out some modification to allow rowing with one passenger without being down by the stern. A milk crate in the bow holding a garbage bag full of water helped in another boat, but it was annoying and ugly.
     
  13. nbehlman
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    nbehlman Junior Member

    Spent a little more time on this. I reduced the lead I had before and now CE is 5% forward of CLR. The daggerboard is now fully forward of the CE, so if that really becomes the hinge point, I should be OK. I decided to pick an existing sail design. It is the aft sail used on the Herreshoff Coquina. That pushed the boom up, so hopefully it's more realistic now. It's 47ft2 of sail area, so I bumped my boat up to 8'6" LOA. I played with the lines to make it prettier - borrowing a little from the friendship cat. I raked the transom back and added a coaming. The stem is a little higher now. The rudder is meant to look more like a Beetle cat. Freeboard was meant to be on par with an Optimist. I'll probably use inflatable bags for floatation. The deck adds weight, but I plan to have it double as the mast support. I'm sure I'll have to make some kind of rail system to load it on and off the roof rack.

    skiff rev4 side.png skiff rev4 top.png skiff rev4 iso.png
     
  14. messabout
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    messabout Senior Member

    So far so good except that you have not gotten the bottom, at the transom, raised clear of the waterline. The barn door rudder along with the steeply sloped transom will tend to lift the bow when tacking. That might be helpful in certain circumstances. On the other hand you can gain some waterline length by standing the transom to vertical or near so. The big rudder will add weight at the end of the boat where it it is best to avoid excesses. You might consider a kick up rudder or perhaps a cassette type.

    I get the impression that you are influenced as much by appearance as you are of practical matters. I may not agree with that philosophy but I cheerfully defend your wish to satisfy subjective preferences.
     

  15. ALL AT SEA
    Joined: Nov 2013
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    ALL AT SEA Junior Member

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