hydrodynamics of Sharpie type Vaka

Discussion in 'Multihulls' started by Anatol, May 24, 2015.

  1. Anatol
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    Anatol Senior Member

    I've been thinking, drawing and modelling a flat bottom Vaka for construction in persuaded ply stitch'n'glue, glass over. Goal is simple construction which maximises hydrodynamic efficiency. Clearly minimising drag is the fist goal, while minimising leeway comes a close second. Reducing wetted area is a given. But maneuverabilty is, I believe, not the big issue in a shunting boat that it is in a tacker - no quick sharp turns required during a shunt. Rocker would thus seem not a high priority.

    1. So, given a three sided hull, of about 30', with vertical bows, and a flat bottom that is widest amidships - what are the design sweet spots? I'm thinking of max width of 'floor' amidships around 36" - to permit useful space below - tapering to 0.

    2. What is the optimal curve - circular arc?

    3. What angle of flare of the sides is optimal amidships? I'd assumed something in the region of 15 degrees.

    4. Is the hard chine a net cost or benefit in terms of drag? If you could induce horizontal flow along the sides while the floor planed, more or less, this would seem good.

    5. What about chine runners to minimise crossflow under the hull? I know a many here think chine runners are nonsense, but I'm open minded, especially as the level-sailing, double ended Vaka is a special case.

    6. The bow bouyancy conundrum. One wants wave piercing but one wants maximum bouyancy at the bows to prevent digging in and danger of pitchpoling. But a bouyant kninfe edge is impossible, and there are limits to how much flare at the gunwales can be forced out of the ply at the bows

    7. I built a model, from two sides, joined at bows, then spread to accomodate floor, then flared toward gunwales amidship - I found that the bows necessarily rose up, providing a little rocker for free. I thought this was fortuitous.

    8. Considering the bow burying issue, it occurs to me that chine runners at each end - but not in the middle - might work in an interesting way. The fwd runner, following the rocker, would lift the bow, the aft runner would pull the stern down. Seems good. And in addition, the aft runner would minimise induced drag at the optimum location. Have I missed something in my thinking here?

    looking forward to replies! And please point me to relevant threads/pages here and elsewhere
    thanks.
     
  2. Anatol
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    Anatol Senior Member

    help me out here folks.
    107 people have looked at my post but no replies.
    Were my questions unclear?
    Did I transgress an unwritten rule?
    Is this just hazing a newbie?
    Is my question so 'out there' that no-one has an opinion?
    Someone remarked that multihull people are the lunatic fringe of sailing and proa people are the lunatic fringe of the multihull commmunity. Am I out here alone? :)
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Firstly, pateince is a virtue!

    Secondly: -

    So, all you are doing is going to draw a boat and then build it. Great, absolutely nothing wrong with that at all. Good luck :p

    But..if you wish to DESIGN it..that is a totally different proposition. To design it, you need a statement of requirements, SOR. In simple language what is the objective of the design....carry 10 people...go 50 knots....travel the world only by solar, have a spa and jacuzzi on board..etc etc. It is a list of what the vessel must achieve.

    Without such....carry on drawing and building. As it has no DESIGN objective, other than ones own pleasure in building something.

    But wait, I hear you cry...

    Well, that is one objective. There are still many to satisfy. And every design has a different set or SORs. Thus if yours is the above alone....what is the the objective of such a boat that can run at 'hydrodynamic efficiency' only at one speed. Since either side of the "sweet" spot..it is no longer efficient :D
     
  4. Petros
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    Petros Senior Member

    welcome to the forum,

    I think most do not know where to start with your list.

    Sharpies are not very efficient hulls, by necessity they have more wetted area than the same displacement of a multi-chine or rounded hull profile and the hard chines also can be drag inducing. There main advantage is shallow draft and easy to build. In a more conventional configuration they are also usually very stable because of their wider width. Hard to estimate extra drag from hard chines, but if you want the lowest drag hull you would not choose a sharpie. however, on a muti-chine hull, even with 'hard' chines, the drag penalty if very small and not a major consideration. by adding a keel and building up a 4 sided hull, with a V bottom, you will reduce drag consdierably with only the addition of one more seam (down the keel line).

    you have a long slim shape with a length to width ratio of about 10 to 1, with smooth curved sides you will already have as reasonably low drag shape as you can get with a three sided sharpie hull. optimal shape would be something like an ogive, or a simple ellipse.

    putting chine runners at the front and back would be a mistake, they will not offer any additional lee way resistance, they have to be at the widest part of the hull to be effective (they act like wingless on an airplane wing, but the hull is considered a very low aspect ratio "fin"). The buoyancy vs. wave piercing issue is one of those trade offs that a designer must make, however I do not think a flat bottomed sharpie hull will make a very good wave piercer anyway, it will tend to pound in heavy chop. chine runners would also add drag, the most efficient way to have leeway resistance is with a foil shaped dagger board, though that comes with its own hazards when operating in shallow waters as compared to having chine runners.

    I have used 16 degs flair with good results for a lot of small hulls (sea kayaks, canoes and dingy sailors), it provide a good amount of rocker on kayak sized hulls, and a fairly dry ride. 15 degrees will also work, it will be somewhat flatter hull with less rocker, which might be okay for a 30 ft hull, particularly since you will not be tacking into the wind.
     
  5. Anatol
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    Anatol Senior Member

    Ad
    thankyou so much for your reply.

    > Firstly, pateince is a virtue!

    no doubt. I'm remarking now about this thread, because another thread I started the same day - about mast drag etc - is chugging along nicely.

    > it has no DESIGN objective,

    ouch! I don't think that is true, but indeed, it is an exploratory research project.

    > To design it, you need a statement of requirements, SOR.

    OK, I was trying to be succinct. I think my goals (SOR) are quite focussed. My goal is a relatively simple and cheap-build bare bones fast day sailer/coastal cruiser, able to be single handed, comfortable carrying capacity 2-4.

    > what is the the objective of such a boat that can run at 'hydrodynamic efficiency' only at one speed. Since either side of the "sweet" spot..it is no longer efficient

    please bear with my perhaps naive questions ... you *seem* to assume that hydrodynamic efficiency only occurs at one point (of sailing, of speed...). I well understand that boat design is all about compromise. One wants to stretch out the sweet spot and minimise the downsides off the edges of that 'sweet island'.

    thanks again!
     
  6. Anatol
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    Anatol Senior Member

    Petros - thankyou for your reply! I don't know how to cut up quoted passages yet, so bear with the > convention

    >hard chines also can be drag inducing.

    understood, but compared to a semi-circular and an single chine design of same - what - displaces area amidships? - are we talking 5%, 15%, or...

    >on a muti-chine hull, even with 'hard' chines, the drag penalty if very small and not a major consideration

    did you mean 'is very small'? I am considering the relative advantages of a flat bottom vs single chine design.

    > by adding a keel and building up a 4 sided hull, with a V bottom, you will reduce drag consdierably with only the addition of one more seam (down the keel line).

    ok, understood - thankyou!

    > you have a long slim shape with a length to width ratio of about 10 to 1, with smooth curved sides you will already have as reasonably low drag shape as you can get with a three sided sharpie hull.

    you mean - no advantage in this aspect by going with single chine/keel design?

    > chine runners at the front and back would be a mistake, they will not offer any additional lee way resistance, they have to be at the widest part of the hull to be effective

    thats interesting, in monohull designs I see them deployed from aft of midships to transom.

    > buoyancy vs. wave piercing issue is one of those trade offs that a designer must make,

    right

    > however I do not think a flat bottomed sharpie hull will make a very good wave piercer anyway, it will tend to pound in heavy chop.

    I assume you mean because of the flatness. But with a 10:1 ratio, 6ft aft of bow it would still be quite narrow (less than 10"?). I could make a solid triangular keel for this section. Or is it the flatness amidships that will cause pounding?

    I have used 16 degs flair with good results for a lot of small hulls ... 15 degrees will also work, it will be somewhat flatter hull with less rocker, which might be okay for a 30 ft hull, particularly since you will not be tacking into the wind

    wow! I'm taking a minute to pat myself on the back for such a good guess :)

    If I was to go with single chine, would this alter the flare angle consideration?

    Regarding all the above, what do you make of the Berque bros flat bottomed wave piercing Micromegas 5? Very simple hull shape!

    thanks again for your reply

     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Well you better enlighten us, since I fail to see any SOR above.

    Not I, it's a fact. Since if once one of the many variables change, the system is no longer at its maximum efficiency. It is simple hydrodynamics and maths. Every boat is design to operate at its 'sweet' spot..or should be. Once it is no longer in the said condition..ergo..not at max efficiency.

    But it is all poop unless you have an SOR..as you'll be going around in circles attempting to maximise "something"..but you have not even defined it.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Anatol, what Ad Hoc is saying to you is that the optimum hull shape changes with speed, trim, seakeeping requirements and loading conditions. These quantities are what you need to well define if you want to find that "sweet spot". A well-defined target point or target function is, in general, a basis for any kind of optimization.
    Then you also have to add constraints given by the materials you will use, costs, the construction technique and the manual skills of the builder (which is you, I guess).

    Otherwise, you can stick to the traditional shapes which have been tried, tested and fine-tuned trough centuries of field operation. But they have been created to meet the requirements of Polynesian sailors and warriors, and the construction technologies available to them - which are not necessarily similar to yours... :p

    Cheers
     

  9. Anatol
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    Anatol Senior Member

    No, no giant breadfruit trees in my neigbourhood. And working with adzes in flip flops isn't kosher with OSHA :)

    Here is a nice video of traditional proa construction.

    https://www.youtube.com/watch?v=TESimBxah80
     
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