Wave Piercing - What vertical profile to split waves

Discussion in 'Hydrodynamics and Aerodynamics' started by BlunderBus, Dec 8, 2014.

  1. BlunderBus
    Joined: Oct 2013
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    BlunderBus Junior Member

    Hi there,
    I'm in the process of designing a Javelin Class sailing dinghy/skiff, and I have a question...
    For the boat between the water line and the gunwhale (the free board), if vertical, what would be the best rate / curve from the bow to the stern to pierce / split the waves apart?
    Would it be something like this...
    Distance back from the bow | Distance from centre line
    0 | 0
    1 | 1
    2 | 1.5
    3 | 1.75
    4 | 1.875
    etc | etc

    The class rules have a minimum beam at the chine at the mid length, so I'm looking for what is the best way to transition from the bow cutting into a wave and pushing the water aside and out to the minimum beam at the mid length with the less drag, and then transitioning past the rest / 2nd half of the boat.

    Thanks in advance,
    BlunderBus
     
    Last edited: Dec 8, 2014
  2. JRD
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    JRD Senior Member

    BB,
    The late Frank Bethwaite theorised in his book High Performance Sailing that the water line bow wedge angle was important when it came to minimising drag in waves and proved this by towing full size NS14 dinghy's at real life speeds.
    There were of course many other factors explained which would be worth reading. They didn't tend to refer directly to the well understood principles of naval architecture. They appeared to run parallel with these but were drawn from intuitive thinking, testing and sailing experience, his designs were successful so its hard to argue with the reasoning.

    There is very little reading material on the design of racing centreboard dinghies particularly with regard to efficient use of displacement and planing modes.

    Its not possible to draw any conclusion from the offsets you suggest as it depends on the underwater shape and volume distribution.

    International 14s have near vertical topsides to minimize drag when punching through waves, they don't need flare for righting moment as they use carbon fibre racks. This has been proven fast but they have no reserve stability. Existing Javelin designs seem quite well developed at this point without departing from the rules. Could you describe what improvements you are targeting as most design improvements carry a level of compromise. A couple of ex Jav sailors used to design and build their own boats, they would probably be happy to discuss your ideas.

    I have a copy somewhere of a technical interview conducted with a number of successful small boat designer's which may interest you. I think the interviewer / writer is a member of this forum.

    Cheers,
    Jeff
     
  3. gonzo
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    gonzo Senior Member

    What you are asking would be best one by a wave piercing type, which is not good for the application. Dinghies usually go over the waves rather than through them. Also, the crew has a lot of influence on how the hull and the waves interact because they usually weigh the same or more than the boat.
     
  4. BlunderBus
    Joined: Oct 2013
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    BlunderBus Junior Member

    Hi Jeff,
    Thank you for the feedback.
    If you are able to dig out that technical interview you mentioned I'm sure it would be a great read.

    I've had some good chats with Phill McNeill about Javelin design, and hope to do so some more at New Year at the Sanders cup. But all the more feedback / thoughts I can the better to improve my thoughts. I realise I'm not boat designer, but I'm keen to give things ago. It appears to be what a lot of people before me have done.

    My thinking is pretty similar to the i14 guys, and I'm hoping to gain an understanding of how best to part water if we just consider it in 2d on the horizontal plane.

    How best to transition the water from being split by the bow (push out side ways) and then as the boat travels past the water to avoid wasting energy by shoving it in the water that is just outside the full width of the boat. I imagine this would happen if the sides of the front half of the boat was a wedge and the back half of the boats sides were parallel, and thus a sharp transition.

    Perhaps if the bow pushed/split the water quickly at first by having a blunter wedge, it would mean the water being pushed away from the keel line would have some momentum by the time it was further along the boat and then that part of the boat wouldn't need as blunter wedge shape, and would not part with so much energy to part the water out to the full width of the boat.

    Now there is obviously a whole bunch of stuff I haven't considered.
    1. Boats current mode: Planning, or Displacement
    2. Wave height and length
    3. Volume distribution and what pitching would take place, and in each mode.

    This relatively 2d thought pattern only really applies to short chop while planning, which we do sail in often.

    So what is the best way to part the water?
    1. Progressively (blunt bow tapering off to parallel sides)?
    2. Straight sided wedge (with some sort of transition to the parallel sides)?
    3. Or something else.
     
  5. Ben G
    Joined: Jun 2013
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    Ben G Junior Member

    Hi,
    A year or two ago I designed and built my own 12' skiff. Took a whole season to get her on pace.

    Keeping in mind you're building a Jav, not a 12', and I know nothing about Jav's, but, I've been looking keenly at 12's for a few years I offer the following:

    - keep a straight wedge from the stem to the widest point at midships. I have found this to be surprisingly straight ie within an inch or so
    - From the midships keep a straight run aft.
    -this makes it quite tortured around midships, but if you manage it well I think it works.
    -pay careful attention to the rocker and lines of the chine (I'd recommend looking at existing boats for this), so the forefoot is neither too deep nor too shallow, and the corners of the transom don't drag.

    Take a look at the Matthew's Cherubs, they do this well. The Matthew's 12's also do this and pierce the waves excellently.

    If the boat has time to rise and fall over the waves you're going too slow or the boat is too fat ;)

    On my boat (and some 14's) I had a kind of dreadnought stem profile which I truncated back to make the stem a blunt parabola. 50mm back it is 40-50mm wide. I think this works well.
    This 'dreadnought stem' approach also removes the hollow which is developed at the waterline, usually about a foot back from the stem. In LOA constrained boats I don't see the point of doing this, as you're wasting surface area for no extra bouyancy.. but it does make the design more sensitive to rocker.

    Here's a link to my construction thread / blog:

    http://z10.invisionfree.com/12ft_skiffs/index.php?showtopic=504
     
  6. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    You might find this recent article intersting especially the photo one from bottom of a Merlin Rocket hull. These are wider 7'2" and less kite area (not assymatric) but 14' long and apart from having to be 'clinker' not a million miles away.

    http://www.yachtsandyachting.com/news/180581/Radical-Thinking

    Personally, when you play with the bow 'wedge' area, I tend towards trying to get the water under the boat not to the sides. Tricky thing though and depends where in the range you are looking for performance ie marginal planing, flat out, or upwind in a chop. Lots of things happen shape wise when subtly moving the waterlines or rocker - just try it!. Go with what you feel will work as long as the basics ie Curve of areas etc are reasonable.

    Key upwind seems IMHO to be the total ammount of rocker for the weight. Again personal but I'm not totally sold on absolutely flat aft sections in a fore/aft plane. Very very dependent on class, weight, rig etc so I'll stay open minded on it. Seen and sailed a few very quick boats completely different there but generally sub 15-20 knots.
     
  7. Gary Baigent
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    Gary Baigent Senior Member

    Why don't you bite the bullet and forget about wave piercing bow on a Javelin (because the dinghy sharp bow is a contradiction in terms on what is a very buoyant hull, not a slim multihull) - bite bullet and go for a scow/mini-transat winning-type bow?
     
  8. Ben G
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    Ben G Junior Member

    This video might help decide whether or not dinghy bows pierce waves.. sometimes you do sometimes you don't!



    This is more an action reel but was in very choppy water nevertheless:
     
  9. BlunderBus
    Joined: Oct 2013
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    BlunderBus Junior Member

    Hi Everyone,

    I'll consume and ponder your thoughts over the next couple of days. Just finishing off the mad Christmas work rush.

    Ben I've checked out your blog. It looks amazing, some great food for thought. Thanks.
     
  10. PI Design
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    PI Design Senior Member

    Any idea what the wedge angle is on a modern Merlin?
     
  11. SukiSolo
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    SukiSolo Senior Member

    Not too sure about the wedge angle on a current Merlin, but do have a couple of Canterbury Tales 4s' (Winder built) that I could measure!. Used to sail a very quick NSM 2 (Turner built) in the early 90s' and am familiar with quite a few designs from that era. We did OK, 4th in Silver Tiller with several Open wins, 7th Inlands etc

    My instinct is that the shape of the second plank ie one beyond the garboard is one of the most critical for this class. That's after looking at, and sailing a reasonable number of them and noting the difference in behaviour on the water inland and sea. The Canterbury Tales 1 is pretty different to the newer 5s' btw.
    Also important to avoid any nosediving tendancy offwind - the NSM 1 was a real sod for that and slow too.

    There's a great bit of Youtube video of the Merlin Champs at Looe where Stu Bithell is driving the boat at max on the reach. Really motoring for one of these beasts. Sometimes Mr Craig brings the Merlin down for a test run....;)
    and shows us how to do it.
     
  12. rcnesneg
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    rcnesneg Senior Member

  13. PI Design
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    PI Design Senior Member

    I'd be really interested if you could measure the angle SS. I know Bethwaite claims the Tasar has a half angle of 11 degrees but that is a slightly longer and much lighter boat. I wonder whether a modern Merlin is that narrow, maybe even more so now? I might try and put a protractor the photo of Shabazzle in the Y&Y article!
     
  14. SukiSolo
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    SukiSolo Senior Member

    OK I'll give it a go. My only assumption is that the W/L is pretty much flush to garboard at transom and I'll set a hull up horizontal (fore/aft) to that. Hopefully I can get the 'caliper' to give some meaningful nominal w/l points at level and the resultant wedge angle. Might be a week or two so don't panic. My experience of PM designs is that there may be some hollow in the entry w/lines, the early Holt ones seem straight. Hence all the bow rocker on the Tales 1.
    It is possible that I could get detailed measurements of a 'Tales 4 as I know one of the owners well and he would probably be just as interested in getting a set of lines. In this case we could turn the hull upsidedown and level it and measure thoroughly. The 'Tales 5 is a small modification though.

    My 'caliper' will measure from almost 0 to the max beam at deck level if required. I've checked a few Cadets at Section 5 for owners as well as points on my own hulls. Winder are good builders so I expect the hull to be pretty fair and symmetric.

    The Callaghan, Richards and other new designs all look quite useful too. Margins are so small in this, as so many classes and the rig is very, very important.

    Nice videos Ben G. Just how windy was it on that second one? - it looks almost scary. More scary seeing than being in it I would suggest, but your hull actually handles it very well, nice work.
     

  15. BlunderBus
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    BlunderBus Junior Member

    Gary, that is very very tempting. I'm surprised I haven't seen a 12 with it as they are so short for how much sail they carry.

    Probably part of the decission of skiff or scow is about do I have enough dynamic lift up front or not already. If not then I'll do down investigating the scow idea more, but I'm pretty sure I can get enough dynamic lift up front using a fairly sharp vertical stem (Javelin's must have a vertical stem)

    cheers,
    BlunderBus
     
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