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Vertical stainless strake behind bulb?

Discussion in 'General Yachting Discussion' started by captholli, Mar 31, 2018.

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  1. captholli, Mar 31, 2018
    #1
    captholli

    captholli Senior Member

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    Any ideas as to what these stainless steel vertical strakes at the waterline above the thruster tunnel are used for ? I'm seeing this now on a few Italian builds.


    SYIQ_30539.jpg SYIQ_30869.jpg
  2. SeaLion, Mar 31, 2018
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    SeaLion

    SeaLion Senior Member

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    To protect the bulb from the anchor?
  3. YachtForums, Mar 31, 2018
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    YachtForums

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    Looks like a laminar disrupter. First time I've seen this but pretty sure the strips are being used to diffuse the bow wave. Should be an effective means of reducing drag.
  4. captholli, Mar 31, 2018
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    captholli

    captholli Senior Member

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    The one photo with the tender garage has threaded holes so I thought maybe screw an eye bolt in to hold the tender line but then thought of it being useless for that after a couple of months of bottom growth fouling the threads. The other photos has no holes in the strake.
  5. HTMO9, Mar 31, 2018
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    HTMO9

    HTMO9 Senior Member

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    I would agree with Carl, it is a method of reducing / controlling interference drag. This metal strip acts as a spoiler to disturb the flow at a given position. In aviaton related aerodynamics it is called the laminar transfer point. This area is one of the most difficult areas in hull design. The area, where the bulb meets the bow. The strakes on the bulb are for the protection of the bulb from abrasion from the chain. But in this area several flow and drag patter meet. The laminar flow along the bulb meets the downward flow of the bow wave from the ships bow and the drag caused by the bow thruster tunnel with its fairing.

    Most likely, the position of the bow thruster tunnel caused to much drag and this added spoiler was the court plaster for the problem. I would have placed the thruster in the bulb or more to he aft.

    This problems are the reason, white sheet yacht hull design likes to use towing tanks during the design phase and yards always try to use and adopt proven hulls ever and ever again.
  6. captholli, Mar 31, 2018
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    captholli

    captholli Senior Member

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    First off, "Laminar Disrupter" will certainly see a spike in Google searches today! No idea what the heck your were referring to Carl. Knowing your background as a hydrodynamic engineer I don't doubt your hypothesis on these strakes. However, in an industry that's had decades of tank testing and fluid dynamic simulations and real time testing. I'm curious as to why commercial carriers don't have this strake seeing that the commercial fleets always search for efficiencies in fuel savings. Certainly a yacht builder wouldn't be leading the efficiency charge on slippery underbodies would they?
  7. AMG, Mar 31, 2018
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    AMG

    AMG YF Moderator

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  8. YachtForums, Mar 31, 2018
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    YachtForums

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    Sorry about that Mark. I should have given a better response, but I think HTMO covered it well. I'm not sure how effective these disrupters would be on a cargo ship. The pressure and volume of the bow wave on a narrow entry yacht hull is different from that of a freighter.
  9. HTMO9, Mar 31, 2018
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    HTMO9

    HTMO9 Senior Member

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    The effectiveness of an laminar disruptor is not dependent on the speed of the fluid or airstream passing it or it is influencing. The shape of the body and its angle of attack towards the medium passing is most influencing.

    On high performance airfoils like on gliders, the effect of those laminar disruptors can be dramatic.

    31-Mi-SF-002.jpg
    This little zigzag tape on the rear part of the vertical tail of this glider acting as a laminar disruptor, improves the rudder effectiveness by more than 30% and reduces the overall drag of this tail accordingly.

    A hull of a ship / yacht can be perfectly designed and balanced but adding one item after the hull design is completed, things like protection strakes or impropper thruster tunnel position or design can have dramatic influence on the overall effiviency of the complete hull.

    This little piece of metal really looks like a court plaster to cover a problem or a mistake made during the design.

    A design flaw on a bulbuous bow can be counterproductive to such an amount, that the boat would be more effective without a bulb at all.