Thoughts on differntial Cp foreward/aft

Discussion in 'Boat Design' started by Tim Hall, Apr 29, 2011.

  1. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 147, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Ah! The mystery is solved! because the hull is summetrical I only had intersections defined for half the hull. Of course, adding the other half completed the curve. I feel slightly foolish, but it will wear off - it always does. Phil: great link for portaging. I was thinking of attaching a bike helmet to the floor . . .
     
  2. Tim Hall
    Joined: Apr 2011
    Posts: 57
    Likes: 1, Points: 8, Legacy Rep: 13
    Location: Fort Worth

    Tim Hall Junior Member

    This is a solo canoe, and I do plan to partially deck it. Bow stem is 17" (above max draft) and stern is 14".

    Good point. I haven't figured this out yet, but I want kneeling to be an option...but I also intend to portage.

    Regarding Cp range :
    Yes, I'm curious too. I was perusing the Bear Mountain catalog of designs and noticed many of the canoes and kayaks are in the 0.56 range. This seemed a little high to me, as it indicates more wetted surface for a hull that's probably not going to pushing its 'hull speed' all that often.

    My overall Cp unwittingly came to 0.57, and I feel like I need to drop that down a little.
     
  3. philSweet
    Joined: May 2008
    Posts: 2,582
    Likes: 348, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    I can't hope to help you choose between .57 and .56 Cp with regards to a single design point on an original linesplan. Check it out at a few different displacements a see how it trends. That would be more constructive IMHO. Don't be shy about the displacement interval, you want the big picture. Change the disp by 20% or so. I have very little experience in kayaks, rather more in canoes, but I haven't designed and tested a whole series of them, so I would refer to as many other designs as I could and see how I sat with regards to established norms. Sounds like you are doing that already. Someone pointed out that its ok to err on the high side a bit (must of been in another thread). It sucks to find out your boat won't plane down a wave face when you're hanging in the air suspended by the ends of your boat in the middle of a big train of waves.
     
  4. DCockey
    Joined: Oct 2009
    Posts: 5,199
    Likes: 598, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Be careful equating Cp and wetted surface area. For a given displacement and length a shape with a higher Cp may well have lower wetted surface area. It depends on what the differences are that are increasing the CP.
     
  5. DCockey
    Joined: Oct 2009
    Posts: 5,199
    Likes: 598, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Use a removable thwart or yoke for portaging. Adirondack guideboats are portaged using a carved shoulder yoke. The ends of the yoke fit into brackets just below (with the boat upright) the gunwales and is removable. Or if you prefer use a straight thwart or tube which fits into brackets below the gunwale.
     
  6. LP
    Joined: Jul 2005
    Posts: 1,417
    Likes: 58, Points: 58, Legacy Rep: 584
    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    Phil, I hope you don't mind me asking. Where do you pull this data from. I like the concept, but would be interested in it's source. Can you elaborate?
     
  7. Tim Hall
    Joined: Apr 2011
    Posts: 57
    Likes: 1, Points: 8, Legacy Rep: 13
    Location: Fort Worth

    Tim Hall Junior Member

    Phil, if you're still following this, I was wondering if you could point me toward any references regarding speed/Cp relationships also. I've seen a couple different curves relating Cp to SLR, but of course this if from sailing design sources. I imagine it's relevant to paddling also, since a canoe travels in displacement mode(?)

    I'm putting the finishing tweaks on my curves, and I'd like to examine my hull with the procedure you've outlined above...seems like a sound approach.
     
  8. philSweet
    Joined: May 2008
    Posts: 2,582
    Likes: 348, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    The stuff below is pasted from Eric Sponberg's document that can be seen in its entirety in post #264 of "Center of flotation calculation and implications?" thread.


    It turns out, after some 150 years or so of analysis, that performance is closely related
    to Cp. That is, there is an optimum range of Cp for various speeds of the boat traveling
    through the water. You can see a table of speed/length ratios versus optimum Cp in
    Skene’s Elements of Yacht Design (by Francis Kinney, 5th ed.) pg.284, which I repeat
    below:
    Speed/Length ratio Cp
    1.0------0.52
    1.1------0.54
    1.2------0.58
    1.3------0.62
    1.4------0.64
    1.5------0.66
    1.6------0.68
    1.7------0.69
    1.8------0.69
    1.9------0.70
    2.0------0.70
    Larsson/Eliasson shows a similar range in their book on page 83, Fig. 5.22, in which
    they plot optimum Cp against Froude Number. Froude Number is very similar to
    Speed/Length ratio, and if you convert Froude Number to Speed/Length ratio, you will
    find that Larsson/Eliasson’s curve is a bit lower than Skene’s curve tabulated above. As
    is true with many things, therefore, there is some wishy-washiness in the guidelines.
    Nothing is hard and fast.
     
  9. Tim Hall
    Joined: Apr 2011
    Posts: 57
    Likes: 1, Points: 8, Legacy Rep: 13
    Location: Fort Worth

    Tim Hall Junior Member

    Phil, thanks. Just want to let you know in analyzing Cp this way, forward and aft Cp's were almost dead on to what you described. Very minor tweaking to angles at the sheer and keel made everything match up perfectly, and I feel I've arrived at a very sensible hull form...with the help of forum users here. Thanks to everyone for your input.
     
  10. Paddlelite
    Joined: Feb 2013
    Posts: 35
    Likes: 3, Points: 0, Legacy Rep: 42
    Location: Maryland

    Paddlelite Junior Member

    One last question on fore and aft Cp

    I'm late to this discussion, but really hope someone can answer an unresolved issue without creating a new thread.

    Phil explained earlier in this thread, in reference to human powered craft, that "regarding the merits of a different fore and aft Cp [with dividing point at the max sectional area]. . . if you envision two hulls, one consisting of the fore section mirrored front to back and the other consisting of the aft section mirrored front to back, you can look up the ideal Cp of each based on length for each and the design speed."

    Now I must ask the extent to which this analysis should be employed or disregarded. For instance, I'm looking at two different recently designed "race style" paddleboards of 12.5 ft length x 29" beam each, one designed by NA's Melvin & Morelli and the other by Tim Kernan Yachts, the first with max beam at 63% and the latter with max beam at 61.5%. Both are meant to maintain speeds around their theortical hull speeds with strong paddlers.

    But if I use the suggested analysis, the stern section of either hull, along with with its mirror image, together comprising a virtual hull with its own hydrostatics, would be so short that it would be driven far above its "hull speed" and be holding back the more favorable bow.

    So what's going on here? Is the stern section Cp far less relevant because it operates in turbulence and its wave is not as big as the bow's? It seems silly to totally disregard the fore and aft Cp as separate quantities, but equally silly to treat them as though they reflected identical functions in identical conditions. Maybe there are acceptable proportions for bow and stern Cp for small craft near hull speed? Or maybe some much broader or higher range for the stern? Need to know because I'm designing a paddleboard similar to those described, with max beam about 62% aft, good Cp for the bow, but way high for the stern at .67 and pushing the stern (as a "mirrored hull") way above its hull speed. Any insights at all, please.
     
  11. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    Mr Hall's boat is (in my opinion) much less skewed Cp wise than many or even most production kayaks. EddyLine kayaks are mostly (at least) skewed the opposite way w the greater Cp fwd commonly called a "fish form" hull. Not sure why but the EddyLine kayak's performance is highly regarded but that may be more for handling that the lowest drag.

    I'm basically on this thread as my Willard 30' boat is considerably fuller aft than fwd and a very similar boat (the British Fisher) seems to be the opposite as a fish form hull. Fisher for "fish form?????

    Basically balance in a boat is almost nearly the holy grail in my mind so I'm always curious about variations.

    Paddlelite,
    Perhaps the fact that paddlers of kayaks almost always paddle well below hull speed requires some variations to differentiation in Cp variations that commonly apply.
     
  12. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member


    Regarding balancing the windage it's very difficult w a kayak. With the paddler amidships (CG wise) his body above deck is considerably aft resulting in un balanced windage abeam requiring a rudder or skeg to easily paddle a straight course. With a beam wind the kayak pulls strongly into the wind. With the paddlers aft position the asymmetrical kayak w a full stern is almost a must for the CG balanced kayak w a "less" compromised unbalanced beam windage. This is why kayaks have such low freeboard aft and frequently lofty bows.
     

  13. Paddlelite
    Joined: Feb 2013
    Posts: 35
    Likes: 3, Points: 0, Legacy Rep: 42
    Location: Maryland

    Paddlelite Junior Member

    Now days, more paddlers are interested in racing and "performance paddling." They often paddle within a few tenths of an MPH of where the craft seems to "hit the wall" where the bow simply surges upward instead of forward with more power.

    I read Leo's paper on kayak hull asymmetry, squat, and resistance. But see the attached pictures which I wish someone would explain. Here is Matt Broze's Mariner kayak with extreme assymetry. At least Matt had plenty of length to play with. But also see the 12.5 foot racing paddleboards by NA's Melvin & Morelli and the other by Tim Kernan Yachts. Notice their max beam at least 60% aft and truncated sterns.

    So, how does this comport with Phil's suggesting separate analysis of bow and stern Cp and hydrostatics, or Leo's work on kayak hull assymetry?
     

    Attached Files:

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.