Will this hull work for a 12ft beach cat?

Discussion in 'Multihulls' started by ahen, Mar 1, 2017.

  1. ahen
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    ahen Junior Member

    Yes. I don't mean close as in purposefully lowered, I meant as by design a smaller boat will be lower to water surface. I think I will attach the cross beams to the top of the hulls instead of having them flush with the top of the hulls which will give more clearance.

    Also what should I put the thickness of the layer with a specific weight of .6 lbs/ft3?
     
  2. upchurchmr
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    upchurchmr Senior Member

    The crossarms won't really matter. Except for the occasional wave making a lots of spray.

    The forward deck needs height to prevent a gust from tipping the boat up on the bows, submerging the bows, which catch the water on the top, stopping the boat quickly.
    Putting you and the boat upside down.
     
  3. ahen
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    ahen Junior Member

    I am messing around with the displacement at the DWL, and I don't know how I should divide it up between two hulls. Say I have a 350lb total displacement, should I make the buoyant force at the DWL of 1 hull displace half of that amount? Right now the potential buoyant force of both hulls submerged to the DWL will hold around 500lbs. If the boat heels, I assume weight will not be evenly distributed between both hulls, causing one to be more submerged, so I assume I should make the buoyant force for each hull lager than half the total displacement. Is there a certain value that is between 350lbs and 175lbs(1/2) that I should set as the target when adjusting buoyancy? Thanks.
     
  4. upchurchmr
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    upchurchmr Senior Member

    You should be able to sail with all the weight on one hull.
    Boat +sailor +anything you carry (lunch, paddle, drinks, etc.)

    With that you should have freeboard you are comfortable with to handle waves and pitching of the boat due to wind forces.
     
  5. ahen
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    ahen Junior Member

    I adjusted the displacement of one hull to carry ~350 lbs at the dwl. Sailhand mentioned lateral resistance, but I don't know how to calculate that. I looked under the calculations tab and found resistance but I don't know where to go from there. I attached the hull in its current form.
     

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  6. sailhand
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    sailhand Junior Member

  7. sailhand
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    sailhand Junior Member

    got it to open I just have to save the file to my computer first instead of terying to open it from the web, doh!!! check this out for further explanation of balancing a sail boat correctly.

    http://www.schoolofsailing.net/weather-helm-and-lee-helm.html

    a brief explanation of centre of lateral resistance, imagine the boat is sitting on a trailer and the area of the hull normally submerged when the boat is in the water is painted a different colour. now walk a long way from the boat directly out to the side of the boat and look back at the boat. what you will see is like a side elevation of the boat. in freeship this is called a profile view. now look at the area of that view that is under the water and calculate the centre of that area and that will be near enough to your centre of lateral resistance or CLR. copied this info from an eric sponberg post on this site


    "There are two methods to calculate the center of lateral plane: In Autocad, and with scissors, paper, pencil, and a triangle scale (what could be referred to as the kindergarten method)

    AutoCad: Assuming your profile is in AutoCad, a 2-D drawing, using a polyline, trace around the entire underwater profile area. With the REGION command, declare the polyline a REGION (Type REGION on the command line, and select the tracing polyline as the region). Set the Origin of the drawing at some convenient point on the drawing, such as the front end of the waterline (TOOLS-NEW UCS-ORIGIN). Then on the command line, use the MASSPROP command to determine the mass properties of the region (MASSPROP, select the region you just created) and a window will pop up with the mass properties of the region. Included in those properties will be the centroid coordinates of the center of area (or mass) from the origin. Using those coordinates, you can put a point or a set of cross hairs at the centroid location.

    Kindergarten method. Print out to scale a copy of the underwater profile on a piece of paper, and glue it to a piece of cardboard such as the back of a tablet of paper. Cut out the paper and cardboard outline of the underwater profile with a pair of scissors. Bend the profile slightly with a smooth curve from one end to the other so that it is not perfectly flat. Place a triangle square on a flat table, and balance the cutout on the triangle square so that the bow extends one side, and the stern extends the other side. Balance it perfectly, and with a pencil, mark the upper edge and lower edge where the top edge of the triangle is. You should have two little pencil marks on the cutout--connect them with a straight pencil line using a straight edge (the triangle scale, for example). Now you have a line of position, just like in celestial navigation, going somewhere through the center of gravity (area) of the cutout. You need at least one and preferrably two more lines of position. Rotate the cutout about 45 degrees one way, then 45 degrees the other way, and repeat the balancing and marking process, and drawing lines between the hash marks. You should now have 3 lines of position that all cross at the same point. If you have done a poor job of balancing, you will have a little triangle drawn by your 3 lines and you'll have to do it over again until the lines all cross perfectly at a single point. This crossing point is the center of area. You can measure the coordinates to the waterline and the forward end of the waterline.

    I hope that helps."

    eric is a naval architect and this was his post on this forum at

    http://www.boatdesign.net/forums/sailboats/center-lateral-resistance-1898.html

    I am not sure but it looks like the centre of the lateral area below the waterline is marked in the profile view in your drawing near the lcf dipl etc
     
  8. ahen
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    ahen Junior Member

    Can I also adjust the location of the center of lateral resistance by moving the centerboard? Does it matter how far aft I put the centerboard? I don't plan on having a jib so I am trying to get it as far aft as possible.
     
  9. Skyak
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    Skyak Senior Member

    FWIW, Hobie made a 12ft beach cat, the "3.5". I have one. Built like a brick in the 70s it weighs under 130# all up and is still sailing today with all it's original equipment. It was designed for crew of 100 to ~155 lbs. I am over 170 wet and my father was over 220 and it was still a joy to sail without the ability to lift the windward float.

    http://sailboatdata.com/viewrecord.asp?class_id=7860

    You are getting lots of fine advice here based on decades of racing. I would tell you that if you don't have a race class, those tenths on knots are meaningless, and that shaving 15min off the time it takes to get on and off the water, and expanding the season and wind range will yield far more miles and smiles.
     
  10. ahen
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    ahen Junior Member

    Thanks for that, Ill lower the transom and decrease the draft. What do you mean by "expanding the season and wind range"?
     
  11. Skyak
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    Skyak Senior Member

    Just a comment favoring a dryer ride and a versatile sail plan. My history with the 3.5 was that I didn't enjoy sailing through waves, so that tended to cap sailing wind speed in the high teens -less on weekends when there were many power boat wakes. You can carry a tall flat sail in higher winds but the short hulls will pitch too much to keep the wind attached.

    A boat that takes less time and effort to get to sailing, and can sail enjoyably more days a year will put a lot more miles on than one that goes a little faster on that rare perfect day.
     

  12. ahen
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    ahen Junior Member

    Speaking of not being too wet does this freeboard seem correct? At the bow the deck is 1.7' above the waterline and at stern the deck is just over 1' above the waterline. Besides from that I think the hull is almost ready to be built.
     

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