Wave-piercers - marketing myth or design ingenuity?

Discussion in 'Boat Design' started by Willallison, Nov 26, 2009.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    I'm talking about a basis/principal, no hull form or idea, yet...I'm just establishing a bench mark of understanding.

    So, if you re read the statement again, for a "nominal" hull....how would you reply?
     
  2. Willallison
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    Willallison Senior Member

    This too will pass through waves without (major) drama. Clearly it is not a wave-piercer... it is indeed all about intent
     

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  3. Willallison
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    Willallison Senior Member

    My answer would be the same: It depends upon the freeboard.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Ok..before we go too far head of ourselves....what shall occur with a wave with a length much less than the length of the boat and an amplitude much less that the freeboard?

    Will the wave 'pass on by'....or will it do something else?
     
  5. Willallison
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    Willallison Senior Member

    yes, of course, the vessel will happily span the crests. The deck will remain dry and my beer shall remain unspilled
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Ok....so, if the freeboard is now, much less, so that the wave, the very same small wave with small amplitude, is encountered, what happens?
     
  7. Willallison
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    Willallison Senior Member

    I have the very strange sensation of wandering into a trap!:D

    Assuming that the deck is closed, it will pass over the vessel
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    A trap....what....me???? :D:D

    Yes, it shall pass over the hull.

    So, if the entire hull has this low freeboard, it shall pass over the entire hull length. But we are only talking about a small fwd section, the bow.

    Now, what is the purpose, or intent, of having a bow that allows a wave to pass "over it" as you say...?
     
  9. Willallison
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    Willallison Senior Member

    to reduce vertical acceleration would be the main goal
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Ah...now we are getting there :)

    Vertical accelerations result from heave and pitch.

    In both cases, the parameters that effects these motions are:
    1) added mass
    2) water plane inertia

    But, before i come on to that, the easy quick fire predominant influence is length, as i posted recently, see attached.

    So, is the length of the "extension" of the bow small, or large, as a percentage of the total LWL?
     

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  11. Guest625101138

    Guest625101138 Previous Member

    No its not. This debate started because I used the term wave piece with PIERCE being a verb. Some wannabe academic who has never experienced this stated there was no such thing as "pierce" as you see in the attached.

    Clearly hulls do pierce waves - end of story.

    Also if you care to compare the shapes of the lifeboat you posted to the one I posted you will see they are vastly different. In fact the one I posted is a more slender hull than either Earthrace or Maryslim. Its slow speed of operation will not have much bearing on how it responds to an oncoming curler. It will go through the wave. I have been 3ft under water in the cockpit of a beamy sailing boat that "PIERCED" waves regularly in a storm.

    Point is the word PIERCE can quite reasonably be used to describe the behavior of a boat that is 10m long and 500mm wide as passes through waves. That is how I used the word - as a verb

    Wannabe academics are never wrong. This thread will continue until YOU stop posting Will - guaranteed.

    I did not start this debate. I take note of people with experience, knowledge and information. I ignore a certain textbook cowboy whose boating experience is limited to playing with bathtub toys in his backyard swimming pool.

    Rick W
     

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  12. Willallison
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    Willallison Senior Member

    chuckle...chuckle...chuckle... you guys really do crack me up....:D

    I'm happy for the debate to go on - it's an interesting point to consider.
    Whilst I disagree with AH on this occaision, I'm interested in his opinion and reasoning on the matter. I posted the lifeboat as a demonstration of his side of the argument. Clearly any hull can be made to submerge. The lifeboat is designed to withstand that, but is not intended to as a matter of course.
    Actually, I don't care if people use the term as nouns or verbs, I didn't start this thread to get an english lesson!

    Now, back to the last question in this slow death experience...

    Assuming that we add an extension to a hull, in order for it to pass through the wave, it would normally be a small %age of LWL
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Will

    Sorry for the heckling from the back..gate crashers! :eek:

    Ok...so adding a small percentage to its length. As you can see from the data attached there are 2 effects from doing this.

    1) The increase in Lwl, reduces the vertical motions.
    2) It helps to reduce the length-displacement ratio, which, again as noted also reduces the vertical motions.

    So, increasing the Lwl is a positive effect.

    So, lets now image a hull, say 74m in Length, like one of those WPs. If the hull had no "extension", in other words, no small bow section fwd of the FP, the vertical accelerations would be XX. Increasing the Lwl by adding a small percentage say around 5% (3.7m), would help to reduce the vertical accelerations. As noted in the graph. (It is years ago i got this data/graph...i recall -I think- it is from US Navy research on systematic series of hulls and sea trial data)

    Now, take that same hull, but this time, increase the Lwl, just by adding a "normal" bow, not a near non-buoyant bow, what happens. This too has the benefit of reducing the motions, by the same amount. If the Lwl were increase pro-rata by 5% (3.7m), the effects are the same.

    Why do you think Frigates are long and slender with extended waterline lengths greater than they really need, for example??!!

    Ok, now lets address the
    1) Added mass
    2) Water plane inertia.

    So, we can now idealise the bow section as a "bulbous Bow" of sorts. Why, because the BB is just an "add on".

    The extension of the bow, has minor positive buoyancy, otherwise when encountering a wave, that part of the bow will rise, owing to its buoyancy. Ergo, it is slightly buoyant positive, but only for that fwd section of the bow.

    Do you agree with this~?
     
  14. Willallison
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    Willallison Senior Member

    The problem with a discussion such as this, is that you can see the noose before it begins to tighten round your neck....:confused:

    Ok, point taken. If the bow were vertical, rather than a point it would still enjoy the same reduction in vertical accelerations. But I think the point (no pun intended) here is that there is little or no reserve buoyancy. Were the hull not intended to pass through the wave, rather to guide the vessel up and over it, it generally would have.
    Clearly there's much more to being able to class a vessel as a WP than the shape of just the bow.
     

  15. Guest625101138

    Guest625101138 Previous Member

    One of the design challenges with wave piecing hulls is to actually get the bow to lift primarily through dynamic forces once it is submerged as there is very little buoyancy.

    A flat deck in a flared bow is the worst possible shape for forcing upwards through the water. Once it is submerged it will tend to dive as the deck acts as an ideal planing surface driving the bow deeper. You will see all four boats I have shown that are designed to pierce waves have very little flare and the deck forward is not flat. It is peaked or curved to reduce the dynamic downward force. The dynamic lift from the bottom of the hull at the bow needs to be able to overcome the dynamic downward force from the bow. Buoyancy plays a minor role compared to the dynamic forces with these slender hulls.

    I still feel that in the right following sea any of these boats could be driven deeper. You could not power on regardless. The fact that the big tris do pitchpole despite having wave piercing amas may be proof of this.


    Rick W
     
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