strakes

Discussion in 'Powerboats' started by ARGO, May 15, 2005.

  1. ARGO
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    ARGO Junior Member

    I have a question regarding a fast planing boat 40Knots(54'x10'beam).The designer has added 3 strakes a side and the highest one runs into the chine. The chine is 3". All strakes running parrallel with the keel. (The only purpose for this was to add strength to the hull because of only two long. girders inside.)
    Can anyone tell me the factors involved with this strake running into the chine.
     
  2. kach22i
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    kach22i Architect

  3. dereksireci
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    dereksireci Senior Member

    Purpose of strakes

    It is a little known fact that strakes were invented in Miami for the sole purpose of keeping the plug builders from falling off.
     
  4. cyclops
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    cyclops Senior Member

    Derek's statement is the truth. Ask O S H A.
     
  5. kach22i
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    kach22i Architect

    Okay I'm ignorant on this one (not the same as stupid), so educate me.

    What you wrote sounds like a "tounge 'n' cheek" joke (of sorts) which I don't get.

    What is a "plug builder"?

    I know what OSHA is, so maybe this is not a joke.

    Look at it this way, roofing contractors don't leave their walking/working boards on the roof after the shingles are installed, no matter what the pitch of the roof.
     
  6. dereksireci
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    dereksireci Senior Member

    Joke

    A plug is foam/wood/putty/whathaveyou in the exact shape of the part you want to make. A mold is built on the plug. Parts are made out of these molds that look like the plug.
     
  7. dereksireci
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    dereksireci Senior Member

    To answer your question: I think strakes are one of the least understood parts of a running surface. The designer who told me the joke about the plug builders said he just divides the bottom panel into three equal sections and two strakes fall on these boundaries. Doing it this way can lead to an unsightly appearance, sometimes only from certain angles. I've seen them run parallel to the keel and assume the designer figures they work just as well and are much easier to place on the bottom surface. (of the plug)

    Maybe some of our professionals members could weigh in on this topic if it hasn't already been beat to death somewhere else here. Most design professionals feel that their strake design is the best way. I'd love to hear why. I think Lou Codega didn't use them at all on some sizes of hull.

    In my own 17'-30' hull world I use the split the panel in three method and stop them short of the bow so if they're asymetrical you won't notice it. I terminate the inner strake somewhere after it reaches the delta of the bottom. Some will swear that it should run full length to the transom. I've run small boats with high horsepower engines where the water breaks off the inner strake at high speed. The result is a vessel that can become unstable, only having contact with the water between the two inner strakes rather than from chine to chine.

    Opinions?
     
  8. ABoatGuy
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    ABoatGuy Member

    Great topic. Probably beat to death in some past thread, but o' well.

    Don't ignore strake width. A world of difference between a wide strake and a narrow one. Too wide and you can get some instability, too narrow and why bother.

    How about the angle? Flat or turn the spray down? Lift? or break up 'laminar flow' (ie. drag on the hull)? Strakes are definetely faster in the tank (in my experience).

    Derik,

    Installed 'strakes' on my roof when I re-roofed. Kept me from falling off and it broke up rain water racing down the roof. A lot of structural and hydro similaritys between a roof and a boat bottom. Small world.
     
  9. kach22i
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    kach22i Architect

    One claim made was that "strakes" induce cavitation/air bubbles and therefore increased flow because of lowered friction resistance of the water (in one of the article links I gave).

    This describes a "churning" affect.

    What "dereksireci" describes is more of a "foil affect".

    That is to say the strake is a vane straightener holding the water to the hull - and not the opposite.

    Some things are just "tradition" are are not understood until they are left out or off.

    For example I knew a project architect who left off the note and details for "stepped flashing" along the parapet of a sloping roof. He and the contractor (who should of known better) later found out the usefulness of stepped flashing (as seen along chimneys and parapets).
     
  10. Willallison
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    Willallison Senior Member

    It's generally accepted that strakes - or spray rails - are effective on planing hulls, though it's true that there is some debate as to exactly how they should be configured. Most agree that three rails per side, running parrallel with the buttock lines aft then sweeping up and in as they near the bow is the best.
    They serve two main purposes. The enemy of speed is drag, and in a hull - whether it's planing or displacement - this is directly related to the amount of wetted surface. The less wetted aurface area, the lower the drag, the faster the boat goes. Spray rails 'push' water aside as it travels up the vee of the hull, breaking its contact with the boat - and so reducing the wetted surface area.
    Secondly, because spray rails are 'flat' to the water surface - or nearly so - they create more lift than the rest of the vee.
    There can be other advantages too - dynamic stability, increased bottom strength and more....

    Conversely, spray rails on a displacement hull will have the opposite effect - they increase the amount of wetted surface, and so make the boat slower.
     
  11. kach22i
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    kach22i Architect

    Willallison, thank you for putting this topic into terms simple enough for me to understand and visualize. :)
     
  12. marshmat
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    marshmat Senior Member

    There's a lot of mystery and black magic around strakes....

    My understanding is that they evolved on early deep-V hulls to counteract excessive bowrise and chine-walking (a strakeless deep-V will roll around its long axis when planing, oscillating back and forth as each flat face approaches horizontal). Properly placed and sized strakes minimize these conditions. Flatter hulls (look at inboard waterski boats) do not need strakes. Sheet-aluminum hulls have them for structural rigidity. Some hulls have them simply because it looks right.

    The placement of strakes is usually a trial-and-error thing. So far there is no accurate way of predicting what they will do without actually testing a hull. Many builders, before computerized simulations, would build a planing hull with no strakes and tack them on, then move them inch by inch until the boat behaved right.

    Strakes have no business on non-planing hulls. Structural support should be entirely inside the hull; strakes should not be there solely for structural support except on little sheet-aluminum skiffs.
     
  13. PowerTech
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    PowerTech Senior Member

    could one of hull dudes tell me what a strake is.and how it is different to a chine.Thank you
     
  14. kach22i
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    kach22i Architect

    I'm not one of the experts here, but I have assumed Chime and Strake are the same thing.

    Perhaps chimes are pointed or flared, or found on the corners, or some other factor of location and or function comes into play.

    I think its a fair and good question, that I don't have the answer to. :)
     

  15. Thunderhead19
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    Thunderhead19 Senior Member

    A strake is not really a chine. What chines are has been open to some debate, but typically they represent a point in the hull where the hull bottom becomes the hull side or changes drastically to another shape. (simplistic explanation ...sorry). A strake is typically a longitudinal ridge in the hull bottom panel that is more of an intentional interuption of the flat surface of the hull bottom. Think of it as an added on piece, whereas a chine is a physical part (a region) of the hull. Again I'm being perhaps excessively simplistic.
     
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