How To Repair An Out Drive Skeg

1. 

   Joined: October 2022
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   Alumination Junior Member

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

   What I'm wondering almost is skeg blueprint, shape etc.

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

   

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

   

   This custom racing lower unit is similar.

   

   What is the purpose of the swept back design on newer drives?

2. 

   Joined: Aug 2002
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   gonzo Senior Member

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

3. 

   Joined: October 2022
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   Alumination Junior Member

   Cheers for the info!

   Is it of import for the skeg to finish shut 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 and so it is flat on i side and rounded, similar a foil or wing, on the other. A RH rotation surfacing propeller volition effort to pull the stern to the correct. By shaping the skeg apartment on the right hand side and curved on the left, the horizontal "lift" will beginning some of the propeller "walking" effect.

4. 

   Joined: Jun 2009
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   baeckmo Hydrodynamics

   In that location are multiple perspektives here:

   A/ the chord variation along the span of the foil gives a reduction of the drag due to three-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 menstruum, the propeller is working in an oblique catamenia. A reduction in the gap between skeg and propeller improves the flow.
   D/ Leading border sweepback lets debris slide off outside the propeller disc.
   E/ The Pictures you present testify racing units, designed for Surface piercing operation, where completely unlike optimizations are valid.
   F/ For the normal propeller state of affairs applying to your SX case, the skeg has a symmetrical foil shape, oft closer to a circular arc than a NACA profile, due to the CA's amend cavitation performance.
   Chiliad/ The asymmetrical contour you propose would create an inclined flow into part of the prop disc, producing variations of AoA for the prop blades; it would likewise create different steering performance in SB and P position; not good.

5. 

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   gonzo Senior Fellow member

   The sweep dorsum angle is mainly to continue debris from damaging or wrapping around the propeller. I agree with everything baeckmo posted.

6. 

   Joined: October 2022
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   Alumination Junior Member

   Cheers for the great information.

   If I was planning to employ this Volvo SX-M bulldoze mounted high on the transom, prop shaft well-nigh ii" higher than the lesser of the hull, turning a propeller designed for surface piercing performance, what aspects of the original design should I proceed and what features of the racing version should I try to incorporate?

7. 

   Joined: Aug 2002
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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 demand a remote water pickup.

8. 

   whitepointer23 Previous Member

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

10. 

    Joined: Oct 2022
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    Alumination Junior Member

    Remote h2o choice-upwardly(due south), and a custom stainless propeller are definitely on the list.

    I'thou thinking now to try and maintain the original skeg shape.
    Sharpen the front end leading edge.
    Build up the left side and give it a foil shape then 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 near 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, forth 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 recoup the driver must turn the wheel correct in order to maintain a straight path. Now nosotros accept the undesirable event of "crabbing" the gearcase when the gunkhole travels in a direct line. This creates elevate, only the affair nosotros are trying to avoid. At loftier speeds it also tends to class a chimera on the "shadow" or port side of the gearcase. As the speed increases this bubble trails back further and further until information technology reaches the propeller. This causes the propeller to blow out or ventilate. At present the prop suddenly stops generating thrust and lift and the bow drops and the gunkhole slows. This is known as gearcase blowout and is commonly very dangerous because if one side of the bow catches the water before the other side (or if the steering bike is non direct) the hull will "hook" and change ends violently. And so how does i avoid this "ugliness"? The unproblematic 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 forcefulness 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 h2o is the skeg so it is the single affair left that gives the operator command of the boat. Like flight an plane with but 1 minor command surface, to say it is crucial is a vast understatement.

    The rock and the difficult spot

    Remember that this whole exercise is about reducing elevate 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 overnice, smooth and sparse skeg. Wait… nosotros'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 correct approach but the sacrifice is great, the one crucial affair left to control the boat is now in danger of breaking off (bad things will follow) or not be large plenty to control the boats' path, more bad things…

    Ideally, the skeg should exist thin enough to create the minimum elevate and thick enough to preclude breaking. On high performance skegs like the Mercury Sportmaster nosotros leave the thickness in the heart and thin the trailing edge and sharpen the leading edge. This is achieved past removing textile from the port side, creating a "wing" shaped cantankerous section. The correct side is almost apartment until the rearward section, which flares at the torque tab. Like a wing with the aileron downwards, it creates "lift" in a horizontal leftward direction. The fluid dynamics of air going over an aeroplane wing is remarkably similar to water going over a skeg. The goal in each case is to create "elevator" while generating the minimum corporeality of elevate. On some skegs which have less profile surface area and no tab we add a pre-cast skeg with torque tab to the existing skeg. (See Photos) The onetime skeg is cut such that "teeth" marks are left in the remaining "nub." Now the new skeg is held backside the old and a tracing of the old skeg is made on the new one. The office of the new skeg that was shadowed is cut away on the band saw. The new slice is ground and fitted until information technology overlays the existing skeg. Both pieces are Vee-d or sharpened where the weld articulation volition be accomplished. This may audio 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 potent weld afterwards the new skeg is smoothed. Nosotros use a 5000 series aluminum-welding rod instead of the more common 4043 aluminum rod. Although more than 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 "manufacturing plant like" cease. 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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