Placement of seat in a kayak?

Discussion in 'Boat Design' started by magwas, Aug 16, 2010.

  1. magwas
    Joined: Oct 2009
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    magwas Senior Member

    I am designing a one-person kayak for my son.
    He is 42 kg, strong beginner skills. I want a stable, fun, forgiving boat.
    I am not sure where to place the seat.
    What I have done so far:
    - drawn the hull
    - set a draft roughly corresponding to the weight of my son and the hull
    - calculated longitudinal center of buoyancy. It is 1.525m.

    I guess I should place the seat somewhere near the center of buoyancy.
    There are the following questions though:
    Where should I place the back of the seat wrt center of gravity?
    What about trim?

    With some experimenting I have concluded that COG would be some 30-40 cm before the back of the seat.
    I have also concluded that I should have shallower draft on the bow than on the stern, so I have rotated the hull with an amount which seemed to be appropriate to look at (1 degree). The abowe center of buoyancy data is with the rotated hull.

    Considering all the above, I have placed the seat between 1.2m and 1.5m.
    Is there a better placement than that?
    Also I am interested in any comment on the stability of the boat.

    All plans and drawings freely useable under CC-BY-SA.
     

    Attached Files:

  2. alan white
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    alan white Senior Member

    Seems the center of bouyancy would be about the mid-thigh point in plan view from my experience.
     
  3. Petros
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    Petros Senior Member

    I am not sure what your overall length is, it is not clean from your illustrations, could you give us the length and max beam width? In my experience building 10 kayaks of my own design, and paddling perhaps a hundred or more different kayaks by others, that kayak appears to my eye that it will be very difficult to keep upright and control. It will be nearly impossible to keep up right without installing out riggers with floats, and it will not paddle straight at all but want to wander with each paddle stroke. You have too much rocker and not enough water line length.

    I have found when I built several kayaks for my children that when you scale down a full sized kayak the stablity goes away, so you always have to exaggerate the width by quite a lot, and alter the hull design.

    This is what I would suggest: put nearly vertical prow and stern to get maximum waterline length, with only about 5 cm of rocker, make the stern a bit deeper than the bow so it tracks well. You might want to consider adding a small skag in the back, but you can experiment with that after you get the hull in the water for a test paddle.

    Your hull cross section has too much V in it. I would use a nearly flat bottom with hard chine design, keeping the chines as wide as possible for the length of the hull. The side walls will be a little less than vertical. This will give the most statically stable design for a small kayak. Also go with as wide a max beam as practical for the same reason, 60 cm is not too much if it does not interfere with paddling. You might still expect to use some "training wheels", out riggers, until he develops his balancing skills better.

    It also appears your cock pit is too far aft. I always try to get the paddler's belly button at about the mid point of the length (half way from bow to stearn). A person's center of gravity is usually within 5 cm of their belly button. Than allow about 5 to 10 cm of forward movement of the seat to "fine tune" the fore and aft trim. Place the seat accordingly after you test float it. I almost always have to move the seat forward a bit from the "mid point" when building a greenland type hull, but your wide stern may alter this somewhat aft.

    Good luck and have fun with it.
     
  4. magwas
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    magwas Senior Member

    Thank you Petros for the detailed criticism.

    Length is 3.78m, LWL is 2.991.
    The beam is 65 cm, 47cm on waterline.

    The intention of V bottom was to give her better tracking. (
    http://www.boatdesign.net/forums/wo...ation/p-v-luca-szeke-my-first-boat-33763.html is a flatt bottom canoe, and really likes wandering.)

    I redesign it to have longer waterline, and more vertical wall chines. However my son specifically asked for V bottom. Maybe by adding one more strake I can reach a good compromise.

    I intend to build it with origami, so the chines at bow and aft won't be as hard as in the design.

    It is interesting that you both say that the seat is too far aft. I am wondering whether it is because I have given too much distance to the calculated center of buoyancy (nearly 40 cm between CB and seat back), or because this form places the waterline so assymetrically aft.

    I will return shortly with a redesigned version. Here is the output of hydrostatic calculations of the current boat:

    Design length : 3.785 m
    Length over all : 3.783 m
    Design beam : 0.650 m
    Beam over all : 0.649 m
    Design draft : 0.100 m
    Midship location : 1.892 m
    Water density : 1.000 t/m^3
    Appendage coefficient : 1.0000
    Volume properties:
    Displaced volume : 0.051 m^3
    Displacement : 0.051 tonnes
    Total length of submerged body : 2.991 m
    Total beam of submerged body : 0.473 m
    Block coefficient : 0.3573
    Prismatic coefficient : 0.7270
    Vert. prismatic coefficient : 0.4484
    Wetted surface area : 1.222 m^2
    Longitudinal center of buoyancy : 1.523 m
    Longitudinal center of buoyancy : -4.631 %
    Tranverse center of buoyancy : 0.000 m
    Vertical center of buoyancy : 0.071 m
    Midship properties:
    Midship section area : 0.023 m^2
    Midship coefficient : 0.4914
    Waterplane properties:
    Length on waterline : 2.991 m
    Beam on waterline : 0.473 m
    Waterplane area : 1.128 m^2
    Waterplane coefficient : 0.7967
    Waterplane center of floatation : 1.567 m
    Y coordinate of DWL area CoG : 0.000 m
    Half entrance angle of DWL : 17.439 degr
    Transverse moment of inertia : 0.017 m^4
    Longitudinal moment of inertia : 0.601 m^4
    Initial stability:
    Vertical of transverse metacenter : 0.412 m
    Tranverse metacentric radius : 0.341 m
    Longitudinal transverse metacenter : 11.943 m
    Longitudinal metacentric radius : 11.873 m
    Lateral plane:
    Lateral area : 0.207 m^2
    Longitudinal center of effort : 1.609 m
    Vertical center of effort : 0.060 m
    Hull characteristics above waterline:
    Lateral wind area : 0.658 m^2
    Z coordinate of wind area CoG above DWL : 0.077 m
    Distance from bow to wind area CoG : 1.815 m
     
  5. DougCim
    Joined: Jul 2010
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    DougCim Junior Member

    I am not any expert on kayaks, just got one a couple months ago. I looked at about 15 different models though.

    That v-bottom is going to be VERY tippy, like riding a log. Can you go look at some commercially-made kayaks? Their bottoms under the seat area are much flatter, more like this:

    [​IMG]

    To improve tracking add a skeg. Or a rudder, but a skeg is easier.

    I'd agree that a poorly-tracking kayak is not fun to use.
    Mine had mounts for a rudder, but it was not included when I bought the boat,,, the store had to order it. So I used the kayak a couple times with no rudder or skeg. Keeping it pointed anywhere even in only 1-foot waves was a workout. With the rudder it is much nicer.
    ~
     
  6. Easy Rider
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    Easy Rider Senior Member

    Magwas,
    Doug's modified drawing is extreme (I think). Split the difference and retain most of your flair. OR just take half the deadrise out of the bottom and keep your original from the chine up. I have a commercial Kayak that is just about exactly like yours except flatter on the bottom. Good flair gives a narrow bottom where it counts and great secondary stability. A boat with a lot of flair should be wider (over-all) than more slab sided boats.Keep enough rocker so it's fun to paddle but lean toward a long and straight keel so when out in bigger waves there is enough directional stability to maintain control. This balance is difficult to achieve. One should try to minimize windage aft of the seat so beam wind will have a minimal effect of turning the boat into the wind. The end result should be as neutral as possible and I don't know if kayakers have gotten over the no rudder fad but I would plan on having a rudder.

    Easy Rider
     
  7. Wynand N
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    Wynand N Retired Steelboatbuilder

  8. DougCim
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    DougCim Junior Member

    Here is one site that offers kits and plans for wood kayaks, with a section with overview drawings for each plan.

    http://www.guillemot-kayaks.com/guillemot/DesignStyle

    Click onto any model, and the click on the "drawings" tab there.

    The only kayaks that have round bottoms are the traditional Aleutian/Greenland, seal-skin-on-driftwood boats. Guillemot has their "Aleutesque" model based on that design type, and the description notes that it is difficult to get in and out of....

    http://www.guillemot-kayaks.com/guillemot/node/44/draw

    Also if you look in the "photographs" for this model, you can see that one builder added a rudder.... and in the plans, this boat has no skeg.

    --------

    Most modern kayaks are nearly flat-bottomed under the seat.

    Also if you look at these drawings you see that the waterline of your Luna-design is not very long.
    ~
     
  9. magwas
    Joined: Oct 2009
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    magwas Senior Member

    Thank you all for your help and suggestions, especially to Doug and Easy Rider for the discussion of lines, and Wynand for the suggestion of software (I am using Linux, and aim at bigger boats, looking at kayaks as a step in the learning curve).

    Here is the redrawn version.

    Now the center of buoyancy is more at the center of the boat, so using the same CB-30cm have given the seat back at 1.5m from aft.
    I have made the bottom a straight line, nearly vertical prow and vertical back. It is V at the bow and the stern, but much more flatter midship. Maybe the V at stern even works as a skeg.

    What do you think?

    Hydrostatics:

    Design length : 3.785 m
    Length over all : 3.785 m
    Design beam : 0.773 m
    Beam over all : 0.772 m
    Design draft : 0.080 m
    Midship location : 1.892 m
    Water density : 1.000 t/m^3
    Appendage coefficient : 1.0000
    Volume properties:
    Displaced volume : 0.063 m^3
    Displacement : 0.063 tonnes
    Total length of submerged body : 3.664 m
    Total beam of submerged body : 0.602 m
    Block coefficient : 0.3587
    Prismatic coefficient : 0.7200
    Vert. prismatic coefficient : 0.4970
    Wetted surface area : 1.727 m^2
    Longitudinal center of buoyancy : 1.795 m
    Longitudinal center of buoyancy : -1.028 %
    Tranverse center of buoyancy : 0.000 m
    Vertical center of buoyancy : 0.053 m
    Midship properties:
    Midship section area : 0.024 m^2
    Midship coefficient : 0.4982
    Waterplane properties:
    Length on waterline : 3.664 m
    Beam on waterline : 0.602 m
    Waterplane area : 1.592 m^2
    Waterplane coefficient : 0.7218
    Waterplane center of floatation : 1.803 m
    Y coordinate of DWL area CoG : 0.000 m
    Half entrance angle of DWL : 17.514 degr
    Transverse moment of inertia : 0.036 m^4
    Longitudinal moment of inertia : 1.134 m^4
    Initial stability:
    Vertical of transverse metacenter : 0.625 m
    Tranverse metacentric radius : 0.571 m
    Longitudinal transverse metacenter : 17.970 m
    Longitudinal metacentric radius : 17.916 m
    Lateral plane:
    Lateral area : 0.281 m^2
    Longitudinal center of effort : 1.825 m
    Vertical center of effort : 0.041 m
    Hull characteristics above waterline:
    Lateral wind area : 0.832 m^2
    Z coordinate of wind area CoG above DWL : 0.133 m
    Distance from bow to wind area CoG : 1.713 m
     

    Attached Files:

  10. MatthewDS
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    MatthewDS Senior Member

    Most Kayaks have flat or slightly rounded bottoms.

    I am attaching a picture of the interior framing of my kayak. It shows a fairly standard hull form. My plastic kayak is very similar, in fact, I don't think I have ever seen a vee hull kayak, at least not as sharp a vee as what you are showing.

    If you are worried about tracking, make it longer, a long kayak will always track better than a short one. Also, put a rudder on it.

    Also, I concour with DougCim, that boat will be very tippy.

    Updated.. Something that is easily overlooked: The flat bottom allows you to sit directly on the bottom of the boat, or nearly so, which lowers your center of gravity. As the angle of vee increases, you necessarily have to move the seat up, which raises the center of gravity, making the boat less stable.
     

    Attached Files:

    Last edited: Aug 17, 2010
  11. Squidly-Diddly
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    Squidly-Diddly Senior Member

    my Aquaterra Prism SOT had 3 padding positions, only one of which

    was 'factory designed'.

    You had the normal 'seat', but also you could sit up on the deck just rear of the seat to change the angle of paddling or if you had a somewhat too long paddle(240cm), or just wanted a higher viewpoint or mostly to change up what your muscles were doing.

    You could also paddle on your knees.

    I would very much consider including those two additional positions in any kayak design.

    I never tried it but I'm sure it could make any single kayak into a 1+1(like the automotive "2+2", with minimal provision for smaller passenger.
     
  12. Petros
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    Petros Senior Member

    The redesign is much improved and should do well. Put the seat as low as possible in the hull, it will help stablity. I think you are spending too much time with the software and need to go build it.

    Native watercraft do not use rudders. Rudders are an accessory designed to separate the customer from his money, and to compensate for a poor design or lack of skill on the part of the paddler. No native watercraft, canoe or kayak, uses a rudder, when properly designed and the user trained on how to paddle, they are undesirable since the rudder will foul, malfunction, add weight and can compromise safety in heavy seas with weeds, shallow rocks and other hazards. Do not fall for the marketing, learn to use the paddle to control the kayak.

    The deep V hull will not necessarily improve tracking. If your son likes the looks, make the bow and stern deep V (which the altered rocker will allow) and have the center section nearly flat on the bottom. with the lengthened water line and at most a fixed skeg, it should track just fine and still be easy to turn because of the short length. Learn to use the steering stroke with the paddle, also I highly recommend using an Aleut style paddle, it is good for beginners (unlike the Greenland that takes some practice), more efficient for low speed paddling, and you do not develop bad paddling habits by using the fat Euro style paddle. The low aspect ratio Euro paddles are unsuited for sea kayaking and were adaptations from white water or surf paddles. The native kayakers built the boats and hulls for survival, and they developed an amazing little boat for rough conditions, we should take our cue from them.

    The same is true for foot peddles in a kayak, they do not belong in native watercraft. These, like rudders, are "white man" inventions that do not belong in a kayak. The hull should be made or outfitted so the paddler grips the hull with his thighs when he needs to brace or manouver the boat. You wear the kayak, not so much ride in it.

    My first few kayaks I built with foot rests and I kept getting painful cramps and totally numb rear end after more than an hour of paddling. No matter how I adjusted them, it was no use. Than it occurred to me that none of the authentic museum native kayaks I have seen have foot peddles. I removed mine and lo, no more cramps. I can stretch my legs and change positions, but 'grip' the kayak with my thighs when I need to make rapid maneuvers.

    BTW, Mariner Kayaks web page has free kayak design software that calculates rolling moments/correcting moments to help determine stablity. It is easy to use, it is the same soft ware that Seakayker magazine uses to evaluate the factory kayaks they review. The link is at:

    http://www.marinerkayaks.com/

    Good luck. Have fun building it.
     
  13. magwas
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    magwas Senior Member

    Thank you. I am building another one right now, and I think that every minute spent planning the boat returns twice when building. And I like to toss around with programs anyway.

    I did not plan to add a rudder or even a skeg to the boat, I try to keep it simple.

    Here is the layout of one side for origami building.

    When I start building it, I will post about the process here.

    And thanks again to all of you for your help!
     

    Attached Files:

  14. DougCim
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    DougCim Junior Member

    The additions of rudders and skegs is not "authentic", but who really cares?
    If you wanted authentic, you'd be building with driftwood and sealskins.
    Are any of your kayaks authentic? How many sealskins were needed to cover them? My guess is that 99.99% of all the kayaks owned, are not very authentic at all.

    When I was shopping for a kayak, I noticed that pretty much ALL of the mid- to upper-priced kayaks sold in the USA had either a skeg (for sea kayaks) or a rudder (for inland/river/flatwater boats). The only places that sell kayaks with neither are places like Guillemot--who sell hand-made wooden boats, based on very traditional lines. And even they would probably tell you that the Aleutesque model is probably not one of their best-sellers.

    It is true that you can build a kayak without either a rudder or adjustable skeg,,,,, but the boat will not point well in windy conditions without it. You can put a huge full-length fin on the bottom and that will make it tend to track straight, but that's not the same thing. A boat that tracks easily in adverse conditions is a lot easier to have fun in, which is why most people buy kayaks at all. There is no way a fixed fin or hull shape can make a kayak point properly at all wind angles, you need something adjustable for that, based on what angle you are heading relative to the wind--and so that's why the rudders adjust left/right and the skegs adjust up/down.
    ~
     

  15. magwas
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    magwas Senior Member

    I guess the point have been that a very long tradition has shown that one can do well without rudder and skeg in a kayak.

    I think it is as much a matter of personal taste as a technical one. You can cite a lot of arguments for both cases, so at the end of the day judging their relative relevance is depends on the person.

    If you still believe that fun cannot be associated with effort, then you will put a rudder, a skeg and maybe even an engine on your kayak.
    I am riding a fixie, so I won't even begin to think about such things.
     
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