Bulbous Bow Canoe

Discussion in 'Hydrodynamics and Aerodynamics' started by solarguy, Nov 1, 2014.

  1. solarguy
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    solarguy Junior Member

    I'm looking into designing a efficient 30 foot canoe at speeds between 4-6 knots. Would a bulbous bow help with efficiency on a boat this size?

    Also by design, I use that word loosely I'm just looking to add a bulbous bow to an existing 30 foot canoe design. The canoe would be a wood strip canoe dragon design. It would be the 40 footer shown here

    http://makeacanoe.com/opencanoesover17.htm

    With 10 foot left off the middle when building it to make it a 30 footer.

    I wouldn't try to make the bulbous bow out of wood, but rater mold it out of foam and cover it in fiberglass than attach it to the front.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    In a word, no.

    The upper limit of your Froude number is the the start of the lower limit of when a bulbous bow becomes worth while. You would need to be doing at least 6+knots for it to be effective basically.
     
  3. rwatson
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    rwatson Senior Member

    Attached Files:

  4. solarguy
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    solarguy Junior Member

    Ad Hoc, thanks for the straightforward answer.

    rwatson, hadn't noticed that before. Will follow you advice.
     
  5. johnhazel
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    johnhazel Senior Member

    for 30 ft long and 4-6 knots you can get a very nice canoe using parabolic waterlines, elliptical cross sections, and flat keel. Just vary your draft to obtain the narrowest beam that you can tolerate.

    You will have some issues with cross winds. A boat this long with only the weight of 2 paddlers will have shallow draft and little lateral resistance. It will get easily blown off course even in light wind.

    Turning may be difficult in small rivers since the boat is longer than the width of some streams you might want to paddle in.

    A displacement of 440lb, 2.5" draft, 30ft long version of this canoe would glide through the water better than any North American Marathon Racing Canoe especially in shallow water. This performance advantage is possible because the NARC is limited to 18' 6" length. Notice though that it is very narrow, 26", and would be too tippy for most people. It would be much more stable than the north american racing canoe though.

    A better boat would be 24ft long with 3" draft 27" beam. This would still give you a faster and more stable boat compared to a racer, but would be so much easier to manage than the 30 foot version.

    Lastly, look at a 3 board canoe version, pirouge style, flat bottom, flat sides. Again make the water lines elliptical and use no rocker to get a low drag hull. see the attached Michlet screen and try the attaches in.mlt
     

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  6. DennisRB
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    DennisRB Senior Member

    Also if you reduce the 40' length by "leaving off 10' from the middle" you will lower the prismatic coefficient and reduce its displacement by much more than the 25% length decrease suggests (as a lot of the buoyancy is in the middle 10' on this design with its parallel mid sections). The Cp decrease might be good or bad depending on the speed range. Possibly good since you want to go slowly. If you want to preserve the original shape you should make it shorter by equally reducing station spacing, which will not change Cp and result in only a 25% decrease in displacement. Or so my limited understanding goes.

    [​IMG]

    Adhoc, at which Froude number does a bulb start to become efficient?
     
  7. gonzo
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    gonzo Senior Member

    These canoes are very efficient at those speeds. We built them to carry people on paddling tours.
     

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  8. Ike
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    Ike Senior Member

    A canoe body is pretty efficient just as it is at those speeds. Not good to mess with a design that has been perfected over literally thousands of years. You won't gain any real efficiency and you will just add unnecessary weight.
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    From all the research on Container ships/Tankers etc, using systematic series of hulls, it becomes clear that the useful range of a BB is 0.24<Fn<0.57.
     
  10. TANSL
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    TANSL Senior Member

    And all these complex study shall apply to the canoes?. It seems strange, right?
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    No, not at all.

    A hull is a hull is a hull. Its purpose is irrelevant hydrodynamically.
     
  12. TANSL
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    TANSL Senior Member

  13. DennisRB
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    DennisRB Senior Member

    I think the point is that the rules of hydrodynamics do no care if the hull later becomes a canoe, destroyer or barge. All must follow the same rules as so are applicable to any hull.

    Going by A 30' lwl hull, an effectively designed bulbous bow shall only be effective between the speeds of 4.4K - 10.5K going by 0.24<Fn<0.57. Is my maths correct? My outcome was lower than Adhocs 6K, however that 6K is somewhere toward the lower end of the middle, so I expect that is probably where it may become the worth the hassle when taken into consideration with the whole SOR in mind?

    This interests me as my boat, a crowther 43 sailing cat has a bow down trim which a bulb bow of sorts may be the easiest solution to rectify. The design of which I would like not to add resistance at normal sailing speeds, and if I am lucky, may actually reduce resistance over the usual sailing speeds. Which seem to be favorable going by the froude numbers given. 5.4K - 12.8K, this is were my boat spends 90% of its time when underway.

    Now my question is, does the range of froude numbers given mean that a single well designed bulb will work over that whole range, or that it will only work at a very narrow speed somewhere within that range?
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Exactly :D

    On an Lwl of 30ft = 9.14m (sorry i don't understand imperial units)

    Fn 0.24 = 4.4 knots
    Fn 0.57 = 10.5knots

    So yes. Thus the lower limit of the BB being worth considering, is the upper limit of the OPs speed range 4-6knots. Thus questionable.

    I would hesitate using any appendage. First why is there a bow down trim?..is this when the vessel is static (i.e stationary in harbour) or dynamic (when sailing ).

    I would suggest moving items about on-board to change the LCG if the former.

    A BB will work in this range, but it still requires some fine tweaking. Such as:

    -The nose to be fwd of the main hull
    -It should be as low as possible (whilst maintaining hull fairness!)
    -The top of the BB must be well below the running water surface

    Then of course a BB at a FN of say 0.24, shall exhibit a very different shape to that of one at a Fn of 0.57.

    But a word of caution too. If you make it too large too, it can affect the directional stability and thus seakeeping in following seas too.
     

  15. DennisRB
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    DennisRB Senior Member

    Thanks Ad Hoc. I also prefer meters, but the equation I found used feet.

    I expect the bow down trim is caused by a combination of design choices VS reality of the build weight and actual fit out. The bows are very narrow and the sterns are very full. So any weight seems to immerse the bows unless the weight is loaded more aft than there is space. Nothing of appreciable weight is stored in the bows (which have a HUGE amount of available space) and all movable items of large mass have been totally eliminated off the boat or moved aft. Bow down is static, and sailing (reduction in clearance increases pounding)

    Since taking ownership the waterline has raised considerably (transom are clear). Elimination of large lead battery bank aft only worsened the issue. However at any displacement the bow remains lower. This issue is greatly magnified with filling the tanks (600L water + 300kg fuel) which are just forward off the mast in the bridge deck. I have a watermaker and try to keep tanks as low as possible.

    I am interested in performance and understand that DLR is the
    (Molland) parameter when it comes to resistance. Which is why I have done all to keep the boat as light as I can. Now its as light as realistic, making it longer by way of a light weight bow extension (as bow is down not stern) seems to give benefits in both DLR and trim.

    I understand this is much more difficult than a stern extension. The easiest and lightest way to gain the buoyancy I thought of was the bulb, which could also extend the bow by 500mm or so, and which I could fair up to the stem with some difficulty to make a reverse bow shape (which is fat enough that it should remain pretty fair without adding too much volume aft of the stem), if only for fashion. The expected gains come from the bulb, DLR and buoyancy not fashion. I just don't like the look of the protrusion. I believe this sort of shape "tulip bow" was used by Crowther on many designs, and is still used today by Catana.

    This is only a thought at present. The boat will be hauled out anyway for a full paint job and fairing job done by myself with in the next year or so, so I would like to do some changes like this while I have the chance. And i have enough time to come up with a well thought out plan.
     
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