Sea kayak design -- where does the paddler go?

Discussion in 'Boat Design' started by kengrome, Jun 24, 2008.

  1. kengrome
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    kengrome Senior Member

    I'm starting to design a solo sea kayak for the first time and I'm using freeship. Among other statistics the software pinpoints the LCF -- Longitudinal Center of Flotation -- which I presume is closely related to the *correct* position of the paddler.

    Where do I locate the paddler's butt in relation to the LCF? Do I put his butt about a foot behind the LCF, or right on top of it? Or is there another statistic I should be using to determine the best longitudinal position for the paddler's butt?

    I presume I would want the paddler to sit slightly behind the LCF because I've looked at a lot of kayak pictures and the paddler always seems to be sitting behind the midpoint in the boat. Then again, for all I know this could be just my imagination, and maybe it's just plain wrong.

    If you know the answer to this question please feel free to post it here. I'm stuck on the design until I can position the cockpit correctly, and it helps to know where the paddler will be sitting ... :)

    Thanks in advance!
     

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  2. Eric Sponberg
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    Eric Sponberg Senior Member

    You have to make an estimate of the weight and CG location along with the hull design itself. You know the surface areas and their CGs from your model, and you can estimate a hull weight from the materials that you intend to build it with. You can estimate any additional structure and its weight and CG as you go along. Do a weight and moment balance with respect to the first station and the baseline, and this will give you the weight and CG (LCG and VCG) of the whole boat, less the weight of the paddler. This is called the "lightship weight".

    You also must do the hydrostatic calculations (displacement and CB location), and you can do these at any given waterline. This will give you the weight of the boat for that waterline and the location of the center of buoyancy, CB, both LCB and VCB. Choose the waterline that corresponds to the lightship weight of the boat. In a perfect world, the LCG and the LCB will be the same.

    Now you know the weight of the paddler you want to support, and this increases the weight of the boat as a whole, so it will float at a deeper waterline. Calculate the hydrostatics to a new waterline that matches the new loaded weight. This will also give you the new location for the LCB and VCB for level trim. Do the weight/CG calculations again with the lightship weight plus the weight of the paddler, and keep changing the position of the paddler so that the final weight and LCG of the boat equals the loaded weight and LCB of the hydrostatics.

    This may sound a bit involved, but that is how it is done in all boat and ship design. On a small boat like this, it really does not take that much time to do the calculations, and it gives you a practically perfect answer.

    Regards,

    Eric
     
  3. kengrome
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    kengrome Senior Member

    Hi Eric,

    Thanks for your reply, I think I can use FreeShip's built in calculation capabilities to do most of this, but I will check it manually to see if the answers match up. This brings up a new question of whether or not the bottom of the paddler's butt is on his CG? I see two possibilities here:

    1- The paddler's weight is distributed over both his butt and his feet, so his CG is somewhere in between, which puts his CG forward of his butt.

    2- Since he is typically leaning aft on a backrest his CG may not be forward of his butt, instead it may be almost exactly where his butt is located.

    I tend to think that most of the time he will have some weight on his heels, so his CG will be forward of his butt, but how far forward I could only guess ... a couple inches? Maybe it doesn't matter all that much since there should be some seating flexibility built into the cockpit anyways.
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    It should not matter too much if the seat has some fore/aft adjustment. A human's center of gravity is located at about the belly button. So in a sitting position, there will be some weight forward of the torso. You can probably estimate that about 1/3rd of a person's weight is in his/her legs, and the other 2/3rds from the hips up, you can do a moment calculation accordingly and get a pretty close approximation of the CG while sitting.

    Eric
     
  5. rwatson
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    rwatson Senior Member

    People dont lay down much when paddling, mostly they are vertical or even leaning forward while paddling. After 6 self designed canoes and kayaks, the best success formulae is to make the front of the seat about 2 inches forward of the COB of the hull, so that the legs weight is forrard of the COB, and the main body on or just behind.
    This appears a good averaging rule of thumb for all the different positions the paddler will put themselves in.
     
  6. Petros
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    Petros Senior Member

    The best way to do this is use your own body (if it is typical that is), and sit on a plank in your paddling position. then put a round dowel under it (on a hard, smooth surface). And roll it forward and back until you find the "Balance point". Mark the balance point, the foot location, and the location of the small of your back (an assistant helps here). The small of your back is where the back rest will be located.

    This is a common method for people building skin-on-frame kayaks that are fit to the users body (no fixed dimensions in the design).

    For a pretty good approximation, the belly button on a normal person is close to the CG location. You can use this for preliminary design and you will not be off by more than an inch or two.

    If you are designing a production kayak, i.e. with fixed dimensions, you will need an adjustable seat location anyway for final trim.
     
  7. Bullshipper
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    Bullshipper Bullshipper

    The seating position añso depends on the hull bottom sytle as a lot of fast yaks designs are going to a planing style and rear keels, where the seat will be around 20" aft of center on a 14'6" model.

    This link has side and top views.

    http://www.malibukayaks.com/popImg.asp?src=pix/sideview_44.jpg

    Are there going to be other storage areas for water, gear ?
     
  8. the1much
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    the1much hippie dreams

    in the boat,,instead of under or on top of?,,hehe ;) ,,,sorry KW :D
     
  9. kengrome
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    kengrome Senior Member

    From what I've read, the seating position depends on the Center of Gravity in relation to the Longitudinal Center of Buoyancy, not on a particular hull bottom style.

    Those boats you linked to are not 'fast yaks' they are fishing/diving kayaks. I think the reason their seats are installed so far aft is because they carry a lot of weight forward, so the seats need to be back far enough to balance the forward weight and keep the CG in the correct position.

    No, it's just a fast kayak for paddling in the ocean near shore for several hours at a time. All I need in addition to the boat is a water bottle and a paddle.
     
  10. JEM
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    JEM Senior Member

    One general rule of thumb that has worked for me is find the volume centroid for a particular amount of hull draft, find the center of balance (hull area centroid) , split the difference, and place the paddlers "center of butt" 12" aft.

    Again, that's a general rule.

    It's also why I prefer canoes. ;)
     
  11. Petros
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    Petros Senior Member

    this is totally wrong for a sea kayak. You do not have enough power to plane a sea kayak. This is true for surf kayaks. Do not confuse the two.
     
  12. Bullshipper
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    Bullshipper Bullshipper

    You folks need to read. The model cited is their fast yak, not one their fishing or surf models.

    I would want to keep my tip up in a beam, following sea, and especially on a wave when beaching, or even going out thru the waves if it was my hull.

    River boats can be more perfectly balanced.
     
  13. Guest625101138

    Guest625101138 Previous Member

    Ken

    The hull will be close to balance so will sit with near flat trim.

    The paddler will have the most influence on trim.

    The answer you seek is to place the ******* on the LCB. A little aft if you want slight bow up. If female I guess it depends on chest size but this aside you can make your own conclusion relative to the position of the male.

    Rick W.
     
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  14. messabout
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    messabout Senior Member

    You mentioned pictures of kayaks that appeared to have the paddler fore or aft of the middle of the boat. Kayaks of late have two basic forms. One is where the maximum width is foreward of the midpoint. This is sometimes called "fishform" Alternately the max beam may be aft of the mid point and these are sometimes called "swedeform". Obviously the location of max beam and/or depth will have a profound bearing on the ideal location of the paddler. Thus the pictures may be decieving. Lots of commercially manufactured yaks and many kits, seem to have tended toward the swedeform style.
     

  15. kengrome
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    kengrome Senior Member

    Maybe my final version ...

    Thanks for all the help folks! Based on what I learned here, combined with all my other research, I've got the paddler's position nailed now ... :)

    I've posted some shots of my most recent version of the kayak I intend to build for myself. It is designed more along the lines of Nick Schade's Night Heron rather than my first design which was modeled afterJohn Winter's Q600X.

    As you can see, this boat has a more open cockpit than the Greenland types used for rolling. The last image is a picture of a real Night Heron so you can compare my version to the original. Mine is obviously more like a surf ski with its open top cockpit which I like better for our tropical climate. I also like it better because my knees can be bent at whatever angle I feel like, and some tighter kayaks don't allow me this much freedom of movement.

    This boat is 18 feet long, 20 inches wide, and supposedly about 40 pounds when finished. It is as close as possible to the stitch-and-glue version of Night Heron I could design while making minor adjustments for my weight and personal preferences. I recommend Nick Schrade's website if you like these kinds of boats: http://www.guillemot-kayaks.com and the QCC Kayaks website if you like my original concept better: http://www.qcckayaks.com

    :)
     

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