Volvo Penta SX skeg repair and design

Discussion in 'Sterndrives' started by Alumination, Nov 18, 2015.

  1. Alumination
    Joined: Oct 2013
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    Alumination Junior Member

    The skeg on my SX-M is partially broken off. What remains is a triangular shape, I'll try to get a picture.
    It did not break at the bottom of the torpedo so I believe there should be enough left and in the right places to repair it.

    What I'm wondering about is skeg design, shape etc.

    Most now-a-days look pretty much the same to me.

    [​IMG]



    These older style Mercury Racing units have vertical trailing edges, they don't sweep back towards the prop.

    [​IMG]

    This custom racing lower unit is similar.

    [​IMG]

    What is the purpose of the swept back design on newer drives?
     
  2. gonzo
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    gonzo Senior Member

    It helps with steering, particularly a low speeds. That is not a concern in racing drives.
     
  3. Alumination
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    Alumination Junior Member

    Thanks for the info!

    Is it important for the skeg to end close to the propeller?
    Is the gap between skeg and propeller important?

    I've also read somewhere that giving the skeg a "foil" shape helps with propeller torque, especially if the prop is run high and surface piercing. Instead of an actual tab, shape the skeg so it is flat on one side and rounded, like a foil or wing, on the other. A RH rotation surfacing propeller will try to pull the stern to the right. By shaping the skeg flat on the right hand side and curved on the left, the horizontal "lift" will offset some of the propeller "walking" effect.
     
  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    There are multiple perspektives here:

    A/ the chord variation along the span of the foil gives a reduction of the drag due to 3-dimensional flows (irrespective of sweepback).
    B/ It also reduces the bending stress at the root section for a given transverse force.
    C/ When the OB leg is in a "steering position", with an angle of attack relative to the incoming flow, the propeller is working in an oblique flow. A reduction in the gap between skeg and propeller improves the flow.
    D/ Leading edge sweepback lets debris slide off outside the propeller disc.
    E/ The Pictures you present show racing units, designed for Surface piercing operation, where completely different optimizations are valid.
    F/ For the normal propeller situation applying to your SX case, the skeg has a symmetrical foil shape, often closer to a circular arc than a NACA profile, due to the CA's better cavitation performance.
    G/ The asymmetrical profile you suggest would create an inclined flow into part of the prop disc, producing variations of AoA for the prop blades; it would also create different steering performance in SB and P position; not good.
     
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  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The sweep back angle is mainly to keep debris from damaging or wrapping around the propeller. I agree with everything baeckmo posted.
     
  6. Alumination
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    Alumination Junior Member

    Thank you for the great information.

    If I was planning to use this Volvo SX-M drive mounted high on the transom, prop shaft about 2" higher than the bottom of the hull, turning a propeller designed for surface piercing operation, what aspects of the original design should I keep and what features of the racing version should I try to incorporate?
     
  7. gonzo
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    gonzo Senior Member

    You could trim the front of the skeg a bit and streamline it. Also, a nose cone may help. You will also need a remote water pickup.
     
  8. whitepointer23

    whitepointer23 Previous Member

    I knew someone who ran twin 280 legs as surface drives years ago. He said it performed well but used to destroy the volvo props . Custom stainless props should fix that.
     

  9. Alumination
    Joined: Oct 2013
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    Location: Knoxville, Tennessee

    Alumination Junior Member

    Remote water pick-up(s), and a custom stainless propeller are definitely on the list.

    I'm thinking now to try and maintain the original skeg shape.
    Sharpen the front leading edge.
    Build up the left side and give it a foil shape so as to gain some horizontal lift.
    Taper the trailing edge and shape it to compliment the foil shape and create a bit of a tab.

    I found the website I remember reading about some of the stuff mentioned above.
    R and R PropShop
    http://www.randrpropshop.com/tips.htm

    A quote from their tip page.

    Another downside of raising the lower unit is when the prop is raised with respect to the water surface it tends to "paddlewheel." That is, along with pushing or pulling your boat forward, the prop wants to walk on the water like a paddlewheel. This generates a rightward motion to the transom, which causes the boat to turn left. To compensate the driver must turn the wheel right in order to maintain a straight path. Now we have the undesirable effect of "crabbing" the gearcase when the boat travels in a straight line. This creates drag, just the thing we are trying to avoid. At high speeds it also tends to form a bubble on the "shadow" or port side of the gearcase. As the speed increases this bubble trails back further and further until it reaches the propeller. This causes the propeller to blow out or ventilate. Now the prop suddenly stops generating thrust and lift and the bow drops and the boat slows. This is known as gearcase blowout and is usually very dangerous because if one side of the bow catches the water before the other side (or if the steering wheel is not straight) the hull will "hook" and change ends violently. So how does one avoid this "ugliness"? The simple answer is to apply a torque tab to the skeg. This wedge, applied to the right side of the skeg, tends to apply a leftward force to the gearcase which should compensate for the propeller forcing the gearcase right. Now the gearcase doesn’t crab through the water, no bubble, no blowout, no hook and hopefully no accident. Remember when a hull flies the only thing other than the prop in the water is the skeg so it is the single thing left that gives the operator control of the boat. Like flying an airplane with only one small control surface, to say it is crucial is a vast understatement.

    The rock and the hard spot

    Remember that this whole exercise is about reducing drag and improving speeds. In order to lift the lower unit we have to mitigate the effects of propeller torque and gearcase blowout by applying a wedge and thickening our nice, smooth and thin skeg. Wait… we’re going the wrong way again? Wouldn’t it be better to thin the skeg down and decrease its depth and area? Technically, this might be the right approach but the sacrifice is great, the one crucial thing left to control the boat is now in danger of breaking off (bad things will follow) or not be large enough to control the boats’ path, more bad things…

    Ideally, the skeg should be thin enough to create the minimum drag and thick enough to prevent breaking. On high performance skegs like the Mercury Sportmaster we leave the thickness in the middle and thin the trailing edge and sharpen the leading edge. This is accomplished by removing material from the port side, creating a "wing" shaped cross section. The right side is nearly flat until the rearward section, which flares at the torque tab. Like a wing with the aileron down, it creates "lift" in a horizontal leftward direction. The fluid dynamics of air going over an airplane wing is remarkably similar to water going over a skeg. The goal in each case is to create "lift" while generating the minimum amount of drag. On some skegs which have less profile area and no tab we add a pre-cast skeg with torque tab to the existing skeg. (See Photos) The old skeg is cut such that "teeth" marks are left in the remaining "nub." Now the new skeg is held behind the old and a tracing of the old skeg is made on the new one. The part of the new skeg that was shadowed is cut away on the band saw. The new piece is ground and fitted until it overlays the existing skeg. Both pieces are Vee-d or sharpened where the weld joint will be accomplished. This may sound like a lot of extra trouble but since the joint is not made in a straight line it is extremely resistant to fracturing. The pieces are vee-d to provide a strong weld after the new skeg is smoothed. We use a 5000 series aluminum-welding rod instead of the more common 4043 aluminum rod. Although more costly, it provides a stronger weld. The welding is done with a TIG machine and we take the time to assure that no porosity or voids remain in the finished weld. The skeg is filled with a minimum of filler material and block sanded to achieve a "factory like" finish. The finished product is purposely made larger and deeper than necessary. When the boat is tested with the desired prop and engine height the flare can easily be cut down until a minimum of wheel torque is generated at high speeds.
     
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