Inverted chines, strakes and a question oh no!

Discussion in 'Hydrodynamics and Aerodynamics' started by Lawrence101, Jul 14, 2012.

  1. Lawrence101
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    Lawrence101 Junior Member

    I'm nearing the completion of the hull of my Sea Angler boat project (still some sanding to do) and I'm about to install strakes and inverted chines.
    My question is does it matter at what angle the chines and strakes have with the waterline ? .....or should they just be left flat ? why or why not ?
     
  2. sottorf
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    sottorf member

    You are likely to get a thousand different opinions here.

    I assume you are referring to the transverse angle (like deadrise) and not the longitudinal pitch angle. Normal guideline are to angle Chines down by 0-10 deg. and spray strakes are usually horizontal.

    As important as the angle is to ensure the outboard edge of chines and rails is nice and sharp to ensure proper water separation from the hull.

    Attached is the best work on the subject that I know. Anybody have other good references on spray rail design?
     

    Attached Files:

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  3. tspeer
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    tspeer Senior Member

    Would you repost the Whisker Spray paper, please? The first page appears to be corrupted.
     
  4. midnitmike
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    midnitmike Senior Member

    Lawrence,
    I've been building spray rails for the local fishing fleet for a number of years. As you can see they're triangular in shape and set at 90 degrees to the hull which utilizes the draft of the hull to decline them approx. 10 degrees. I've only built two flat sets (my first experiments) and was very disappointed with there performance. As the boat rolled in heavy seas these flat versions became aerated and lost contact with the water greatly decreasing their stabilizing effect.

    It's always been my practice to go for a ride on the boat prior to them being outfitted with a set of my spray rails to help me determine the size, shape and length. With no formal training I have to trust my instincts on what is an appropriate fit for any particular boat.

    To answer your questions specifically:
    Yes, it does matter what angle the inverted chines are installed. As you can see I use the chine (not the waterline) to determine correct placement.

    In my experience a flat spray rail or inverted chine has little damping effect on the roll characteristics of hull.

    Here are some shots of one I did last summer.

    MM
     

    Attached Files:

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  5. sottorf
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    sottorf member

    here it is again...
     

    Attached Files:

  6. tspeer
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    tspeer Senior Member

    Anyone else having problems with this file? About half the pages in the repost are blank. There are pages in the previous file that can fill in for some of the blank pages, but there are still pages in common between the two files that are blank.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Yes, same here - that pdf file appears corrupt. I'm attaching one from my library, which should be ok.

    P.S.:
    Oddly, the Boatdesign.net server has decided to crop the last word in the title from "hulls" to "hu". You can correct it manually.
     
  8. Lawrence101
    Joined: Oct 2011
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    Lawrence101 Junior Member

    Thanx sottorf for the PDF files.

    Wow it never occured to me how indepth hull design can be, seams abit overwhelming. This is my very first boat build and i have very little design knowledge. Right now im more concerned about stability than planing hull
    resistance, but its something ive bin thinking about.

    MM
    Mike, Yes I too use the chine to determine sprayrail location. I was surprised
    at your answer tho, in that a flat chine had very little damping effect on hull
    roll. So im to assume that a chine that is tapered down at some optimum
    angle would improve the damping effect ? Also my sprayrail/chines are in-built.
     
  9. midnitmike
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    midnitmike Senior Member

    Hi Lawrence,
    If you look at the pictures I posted you'll notice that the spray rail / inverted chine I built for this boat is triangular in shape with side against the hull and the bottom of the rail forming a 90 degree angle. The flair of the hull tips the rail downward, with this angle varying slightly from bow to stern.

    I use 4 lb foam to construct the core of my designs which allows me to easily hand sand them into to their final shape. Once that's done the entire area is resin coated and allowed to cure. My standard layup is an 1 1/2 oz CSM followed by two layers of 1708 biaxle with all the joints staggered. The majority of the glass work is done overhead so experience and good technic are essential to the process.

    Although some might argue that the lay-up schedule is too light I've been building these rails on commercial fishing boats for over 15 years, and haven't had a single repair job in all that time. I've built rails that range in size from 2 1/2" all the way up to 6" depending on the hull and customer requests. In most cases the average speed increase at cruise is 1.5 knots with only two vessels reporting a 3 knot increase. It should be noted that both of these boats previously ran with a bow down attitude which I was able to correct.

    Without a scientific or engineering background to describe mathmatically how or why they work the way they do all I can do is observe and refine the design as I deem neccesary. The two flat rails that I built initially would not maintain contact with the water when the chine rolled upward, and would instead become aerated. This allowed the hull to lift with almost no resistance to that movement. My later versions almost completely eliminate roll while at rest with the boat only gently rising and falling with each passing wave. For fisherman that spend days, weeks or even months on the water this more then a matter of comfort.

    MM
     
  10. Lawrence101
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    Lawrence101 Junior Member

    MM, Ive taken your (among others) advice and ive re-designed/rebuilt the reverse chines from true flat to about 12 degrees from the horizontal, which is about the same as the strakes now. I will post some updated pics of the hull in my gallery within the next couple of days or so.

    Im at a loss has to what kind of performance this will give me. I have the beginnings of the hull drawn up in Delft ship, but for the life of me I dont know how to install strakes and inverted chines .:confused:
    (maybe i should post that question in the Delft ship forum) :idea:

    AFAIK I can use the software to test the hull when the design is completed.

    Thanks for replying.


    Lawrence.
     
    Last edited: Jul 30, 2012
  11. SHayward
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    SHayward New Member

    Hey midnitmike
    I know that this is a very old string, but I thought that it is worth a try :)

    I added a small pilot house to my Boston Whaler Montauk and it is no longer as stabile as it was. It still works well, it just takes some coordination while on board. However if two guys go to one side it tips to that side more than I would like. Do you think that adding spray rails like yours would help with this?
    Any suggestions that you may have would be appreciated.
    Yes I know that this is a small boat. I like it and am willing to put some more work into it to make it more stabile.
    Thank you
    Stefan
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

  13. midnitmike
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    midnitmike Senior Member

    I'm afraid that my spray rail design would have very little effect in correcting a static buoyancy issue such as you described.

    MM
     

  14. PAR
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    PAR Yacht Designer/Builder

    Spray rails are what the term suggests and control spray. They also can affect directional tracking and to a much lesser extent, longitudinal stability underway, but they simply don't offer enough volume to help lateral stability.

    The Boston Whaler hull type is such, that placing much weight above the CG is going to dramatically affect its roll moment, making it feel tender (tippy). You need additional athwart volume or more beam to fix this issue. If you like the idea of foam or inflatable training wheels on your boat, there are products (like the link above) that will help a little. In reality, the cabin you've built, likely needs to be a lot lighter. Let me guess, some 2x4's and 3/4" plywood? A small cabin structure should be built of 1x2's and 1/4" plywood on that size boat. The real fix is to make it much lighter. The roll moment will still be less than a stock Whaler, but much better than what I imagine you've got. I'm not trying to insult you and your modifications. In fact, I'm all for making changes to boats, but seeing all the realities or implications of these modifications, isn't always so obvious.

    Lastly, designing effective rails isn't an easy thing. You can slap on things that work to some degree, but if you're looking for gains across the SOR and not just some of the SOR, chine flats and rails need to be well shaped and placed within the performance envelop the SOR defines. To most designers, spray rails and chine flats, as well as chine placement and shapes themselves are a bit of black magic, most aren't willing to surrender very quickly. We develop these shapes over time (lots of it) though trial and error (lots of them), so getting much information that is application specific will be difficult. It's a bit like asking an engine builder, to reveal why his engines produce more HP, than other engine builders that are using the same pieces. They'll give you some general answers, but they will not tell you how many hours they put into unshrouding the valve bosses.
     
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