Lifting strakes do they produce lift in the submerged sections

Discussion in 'Hydrodynamics and Aerodynamics' started by Lynton Banks, Jan 27, 2023.

  1. Lynton Banks
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    Lynton Banks Junior Member

    Hi guys Newbie here ,

    I am trying to understand exactly what theses strakes do. Particularly if those wider more pronounced ones running to the transom create any lift. In a adding a flat surface to a deep V, or is it more of a double chine effect the excessive width of the strake taken all the back to the transom?

    I think I have excessive lift.

    The reason I ask is say up on the plane running at 26/27 knots . The boat responds to rudder like any other.
    The boat runs very flat requiring zero tab. If a push the sticks Fwds more ( max speed is 38 knots ) the hull rises higher up , you can feel it lift higher and it low running angle say 4 degrees ( guess ) gets lower with out tabs . Tabs on Zero.
    In fact the stern rises so much, along with the rest of the hull so it makes those rudders pretty ineffective.
    For quick emergency type turns I have to pull the sticks back to effectively dip the rudders so they bite.
    At 35 knots full rudder just adds drag the boat still tracks true or if anything deviates by a tiny degree hardly worth mentioning.

    So where is the additional lift coming from?

    Yes I have been reading Reggie Fountains stuff. He used theses strakes underwater to the transom. Far faster boats so not as wide due to the speed advantage. But nether the less more lift means less drag of the hull in the water = more speed = win races. All the others like Donzi etc and most Ray hunt designs run the strake to the transom or very close. They call them lifting strikes, strips .
    So as the name implies ……. do they create lift ?

    Also above a certain boat weight and size the lifting effect I understand is limited .Something to do with froude numbers
    I quote this from a naval architecture forum.

    “Another reason you don’t always see lifting strakes running all the way back . Or very pronounced.

    Boats with a volumetric Froude number less than 4.0 don't need lifting strakes... for example, a 55-foot boat weighing 60,000 pounds should only need strakes if it will go faster than 42 knots. Location and shape, again, is about finding the balance of the design.” [ end quote ]

    Tend to see them running back to the transom / more pronounced, wider on deeper V more performance orientated hulls .

    I realize something like this stuck on the stern underwater section about 2 ft down at the transom on say a San Lorenzo 95 chugging along at 18 knots isn’t gonna create anything.


    The boat in question is a Itama 42 , 14 m with a 4.2 M beam and 23 degree dead rise, so into Magnum territory it’s a Med version . Twin 700 Hp MANs.
    At wot it reaches 38 knots about 18 ton on a hoist .

    pics of the hull here.


    8C883E41-81E1-4E7B-8694-409E04231176.jpeg 21E5350A-2051-4AF3-BBD9-8E8F39FEABFD.jpeg
    Does this create any lift between 30-40 ‘knots ? The strakes get wider and flatter as they go back .
    That’s my Q .
  2. Lynton Banks
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    Lynton Banks Junior Member

    Hi guys I had an initial helpful reply by rwatson from Tasmania .It came to me at my e Mail address .
    Yet does not appear on here .I haven’t figured out how to contact him or retort to his kind post .

    What am I missing ? Forum probation period ?
  3. fallguy
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    fallguy Senior Member

    Your post is confusing because you are talking about lift and loss of rudder. I am not your expert, but perhaps my comment will stir some others to dialogue.

    As any planing hull increases in speed; less of the boat is in the water. It lifts up. When you go faster and the boat is up higher in the water; you have less rudder in the water. Adding more lift means even less rudder. Basic stuff, right?

    So, adding something that promotes lift effectively reduces the amount of rudder even more. You do not need or want to discuss lift.

    Almost certainly, your rudders are improperly tuned. But you seem somewhat wise, so people are afraid to speak.

    Some of the contributers here may want to see pictures of rudders even, but my guess is the boat is performing well, the steering not so much.

    Take some pictures of the rudders and report back the toe in or out and find out what design spec is for original boat.

    For now, at least, let's assume the boat was properly designed. My guess is the rudders are not tuned correctly or even that they or the boat or drives were modified, but not the rudders or v/v.
  4. Lynton Banks
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    Lynton Banks Junior Member

    Yes the rudders are pretty ineffective at its higher speed .I am a keen Amateur.
    Lift is what I want to get to the bottom of .
    I am enquiring if the excessive lifting strakes running to the transom at speed and due to the deadrise 23 degree contribute to stern lift .It runs very flat btw with neutral tab .

    In other words the designer Mario Amati added back lift by utilising pronounced strakes carry them to the transom .
    Redirecting water over the edges ,
    F6A54CDE-175E-4F99-B286-0361741434F9.jpeg 4D7F6959-1309-405D-BECE-21A02CF2876F.jpeg
    Edges of the strakes effectively reducing the deadrise so flatting it off = more lift .Due to the pronounced height .

    Will there be any kinda flat surface added back lift as well?
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  5. Barry
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    Barry Senior Member

    The effective depth of the rudder is the distance from the bottom of the boat to the bottom of the rudder
    It is unlikely that this boat would be able to get so much lift that the rudders come up out of the water

    At increasing speed smaller changes in rudder degree will provide more sideways force due to increasing speed, from the green water and/or the prop disc

    The op says that he is running 4 degrees. For a 23 degree deadrise a 6 degree up is more lift to drag efficient

    Lynton, the strakes will provide increasing lift with increasing bot speed. The strakes “turn” the water that is moving in the direction between the keel and chines. A change in direction produces a lifting force. Simplified

    I am intrigue by the pockets in the hull
    Are they exhausts?
    If they are quasi steps, the chine side edge should have been ventilated to atmosphere to produce any benefit, like a regular stepped hull

    A concern that I would have is that the stern side edge/ face of this pocket would act like a brake and push the bow down
    Note: I am only making these comments based on what I can see in the picture
  6. fallguy
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    fallguy Senior Member

    I suggested his rudders are not tuned.
  7. Lynton Banks
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    Lynton Banks Junior Member

    Some more pics of the exhaust.Not sure if everyone knows but Amarti ( Itama ) was the first designer I understand ( happy to stand corrected ) that introduced under water exhaust.
    The motive not entirely clear .About 1999 with the 38 .Obviously noise reduction, but there may be a performance advantage.By all accounts he was as fractions guy. One tiny thing , but he put the sum of tiny parts together to make a difference .His history allegedly was in the 70 making plywood fly bridge up to 40 ftr s sales were languishing.plastic was gradually taking over ,He met up with Dick Bertram , Don Aronow , Ray Hunt , the Theodali guy from Magnum at a the Monaco to Venice motor boat race late 70 s . Clicked winning races sold boats , or folks like to be associated with race boat tech .He wanted a slice of that marketing glamour .It wasn’t seen in Europe then .
    So his first attempt was this .
    In new fangled plastic / resin 1999 .

    Took EU sales by storm .
    Anyhow the UW exhaust have fart pipes for low rpm back pressure issues .
    As said he was a detail guy .
    A 54 below of the same genre to show the section aft of the UWE ….. note the pronounced strake near .
    Back in the day this was / is a 40 knot boat with the V 12 mans on a 22 degree deadrise.

    The rest are my boat .
    There is a slight deflection flap anterior to the UWE .^ ….Is this arrangement to air rate this section = reduce drag , or utiles exhaust gasses for a performance benefit? If so exactly what was he thinking ?
    Stern 1/4 fart pipe ,
    The internal view it’s just a box .The WL is approx where the top of the racor sits .^ 17E49058-82C7-45AC-8348-A124B8F9A979.jpeg
    Strop covers the UWE .
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  8. baeckmo
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    baeckmo Hydrodynamics

    There is a general misconception about the "rails" here. Their function on this size and weight of hull is twofold:
    • They are spray-rails in the forward part of the hull, where there is an outward spray generated at the stagnation line. If the spray flow is allowed to follow the bottom, it creates additional drag and creates a wet shower coming onboard. Correctly designed the spray rails add some lift forward, by deflecting the spray down and aft. They have no function aft of the stagnation line, since there is no transverse spray!!!
    • Over the rest of the bottom, there is practically little-to-no transverse flow! The rail function here is more related to their vertical surfaces adding to the lateral surface, resulting in increased course stability.
    For lighter and faster vessels it's different; they operate at speeds where the chines generally are above the waterline, with the consequence that there is a continuous stagnation line from stem to stern, ie diagonal spray is generated all over the length. Here the rails have a major function both in reversing the spray and creating lift plus reducing drag. In addition, they react on small changes in both heeling and trim by creating differential lift; resulting in increased dynamic transverse stability.

    From the pictures of your hull it seems that there is a "hook" in the bottom in front of the openings close to the chines, which Barry also has noted. This hook will have the function of a trim surface, generating quite some lift. This can be reasonably well calculated.

    But your steering problems lie elsewhere. The root of this issue is the protruding forward edge of the shaft bracket mounting plates. At a certain speed the flow is detached from the surface and deflected downwards, creating a lowpressure "bubble", extending from the bracket leading edge all the way to the rudders. The void is aerated from atmosphere aft of the transom and this aeration is causing a loss of lift when your rudders are supposed to create a transverse force.

    This mounting is bad practice; there should be nothing protruding beyond the hull surface, if it cannot be avoided, then there must be a looong and smooth chamfer plus rounding of all edges, the leading and trailing edges in particular. There may be addditional measures required to further reduce rudder ventilation, but the available pictures don't show the critical zones.

    Edit: You managed to add a couple of pics while I was writing; the new ones showing what we call "hangover" rudders.....! You can see that as long as the outflow from the bottom surface is not disturbed, it covers the top of the rudder blade. But when there is a detached flow from the brackets leading edges, there is a free aeration path straight down the rudder's suction side.
  9. Lynton Banks
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    Lynton Banks Junior Member

    Thx guys this is really helpful . Especially the lift .Answers a lot of Q s .
    Thought it’s different just by “ yard “ walking .

    So it’s sits steerable around 25 knots .If I go up a bit to say 27/28 it’s steering response substantially diminishes .30 - 35 might as well not have rudders .Full rudder just adds a bit of drag and knocks 2 knots off .It kinda arcs gently say 3 / 5 degrees off course .For emergency manoeuvres I have to pull the sticks back let it drop down and slow down to under 25 knots to make a tight evasive turn .
    The plus side once it set running at say 30;knots there ain’t nothing gonna deviate it off course , wakes waves etc .
    It minimally rolls sorry heels on corners as well , turns quite flat .Maybe dips about a foot running at 22- 25 knots .
    The slower it goes the more heel .
    At 30 + knots full rudder it hardly heels and what heel it does do I could by way of unilateral flap upright it .

    I do notice the extra lift at this 27/28 knot transition ….yes it feels like the whole chines in the air and it runs flat .
    Any tab I have down ( 2000 L carried behind the mid mounted motors ) I have to lift to zero .
    low tanks it’s zero anyhow .I only need serious flap down in big ( relative to boat size seas ) to get more bow V cutting .

    I am not complaining.Just curious. The benefits of the seakeeping in rough at speed make up for the idiosyncratic steering .Any other comments welcome.
    I think Amarti did the hanging rudder thing lower the shaft angle .Where I stand on the helm is basically on the motors header tank as they are mid mounted.There is an aft cabin the boats accommodation is split in two by the machinery space .

    More pics of the rudders and exhausts set up .

    3BE16441-988D-4856-BEE9-B47765FB6996.jpeg 61765E40-10C5-43B7-9398-B82B46D2E910.jpeg 71936B61-0EC6-4229-B0A7-66E82F7DCAD2.jpeg 61C8CEBF-E6A0-4E26-A9C2-2B28D860795C.jpeg AFB6EFCC-BAC7-4DF6-8CAE-8EF53A4AEAB8.jpeg
    Prop wash on the rudder …..any significance ? Is this the aeration , cavitation maybe cleaning them .They are always polished metal here .
    P 5C4BF5A5-41BB-4F32-83C4-EA40962FFCAE.jpeg 514FAC13-BAA6-4926-AC08-7B0659DE592F.jpeg C5084AC0-ACEB-442F-A922-F7A331DA33FA.jpeg 4CA937FC-4BB4-4E87-B4F3-CF45ED575D9B.jpeg E4898EFF-D7EB-4994-8FAD-63616EBCBADA.jpeg
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  10. baeckmo
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    baeckmo Hydrodynamics

    The rudder cross-section shape used there is unsuitable for high speed where ventilation may occur. They may even cause a force reversal in certain conditions.
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  11. rwatson
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    rwatson Senior Member

    Hi mate.

    I deleted my post, as I felt it was too generalised based on your complex questions.

    It was just a simple Google search, which you can do yourself.
    I am glad that some of the links were useful.
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  12. Ad Hoc
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    Ad Hoc Naval Architect

    In addition to the comments from Barry & Baeckmo
    They serve 2 basic functions.

    1. The promote the reduction in beam, thereby increasing the L/B ratio of the lifting surface,...since the vessels lifts onto the those stakes in progressive fashion,
    2. And hence, reduce the wetted surface area....which reduces drag.

    I must say the keel is not an ideal shape, this being rounded, - this does not promote the separation of flow, as required:


    And as for the rudder issues, Baeckmo nailed it.
  13. Lynton Banks
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    Lynton Banks Junior Member

    Thx guys .
    Yes what baeckmo says about the rudders makes perfect sense, new info for me new info .

    My primary Q aimed at the underwater strakes being very pronounced, in certain circumstances the strakes can produce lift submerged has also been answered by baeckmo but was also answered by rwatson

    I hope you don’t mind if I add back the text of your original post regarding the lift element?
    To aid others following.

  14. Lynton Banks
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    Lynton Banks Junior Member

    Yes it’s only above a certain speed .I have gotten used to it .TBH I never attempted full rudder or anything over a 1/4 on the rudder indicator cruising.Up to a 1/4 of its indicated range is sufficient for course corrections at 28/30 knot the eco cruise speed . As said in emergency manoeuvres to add more bite I pull the sticks back drop it down to under 25 knots .

    Its only a pad at the last 1/2 to 1 m or so between the props .Where the shafts pop out , or the hoist slings sit in pics it’s pretty sharp .
    I would imagine the water twixt the propellors this far back and low is better running parallel to the shafts , or minimally diagonally disturbed ??
    The props are further back than most boats I see in the yard too .
    Being flat I would imagine that pad also contributes to lift .

  15. skaraborgcraft
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    skaraborgcraft Senior Member

    I would say the fact a portion of your rudders are above the "fast" water flow does not help. You would probably get an increase in rudder performance if the entire rudder blade was below the hull and not partially hidden. Just an observation from one who knows nothing about fast boats.

    I believe Dag Pike wrote about Nelson 42s being used by the Italian coast guard, and said something along the lines of "judicious use of the throttles were required in certain conditions, due to the small surface area of the rudder blades".
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