Transom Step

Discussion in 'Hydrodynamics and Aerodynamics' started by Woobs, Mar 19, 2017.

  1. Woobs
    Joined: Jul 2015
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    Location: Newmarket, Ont

    Woobs Junior Member

    I have been reading over the past few days (probably too much) on a number of sites where people have made a "step" at the transom by cutting out the bottom of their "V" between the stringers and making a small bulkhead between 6"-12" forward. Anyone here have experience with this? I have a few questions.....

    What is the purpose of this "step"?
    What are the benefits?
    How does it work?
    Is it like adding a stand-off box or stern jack?
    Does it clean up the turbulent water coming off the hull?
    Does it compliment a raised "X"?
    What are the drawbacks?
     

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  2. Ike
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    Ike Senior Member

    I am not sure what you a proposing. Some boats, especially bass boats have a flat area at the transom called a pad. It is thought this improves performance.
     
  3. Woobs
    Joined: Jul 2015
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    Location: Newmarket, Ont

    Woobs Junior Member

    No, I'm not looking at a pad. I suppose a more correct term is "transom notch". In reference to the above posted diagram, the section outlined in red dotted lined is removed from the hull. This shortens the running surface and creates a larger gap to the lower portion of the sterndrive leg.
     
  4. Ike
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    Ike Senior Member

    I cannot see how such a thing would improve performance. It would only create turbulence where you really want clean water flow. Maybe someone else here has a better understanding than I do.
     
  5. PAR
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    PAR Yacht Designer/Builder

    It can improve top speed, but the angle of the upper portion of the step is critical, to keep the bow down, compensate for the lose of plane patch, yet not introduce more drag than desirable. As a rule, it effectively narrows the operational speed range quite a bit, but if top speed is the only goal and you can engineer a good step, it's a worthwhile modification. Only some hull forms can receive a benefit from this type of arrangement and again, it will have other, less desirable impacts on the performance envelop.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Presumably the idea is to gain an increment of extra speed, though the height of the drive would need to be right. The gain would come from the prop working in water with less forward momentum. and maybe an overall reduction in wetted area. Potential problems could be porpoising and slower planing.
     
  7. Woobs
    Joined: Jul 2015
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    Location: Newmarket, Ont

    Woobs Junior Member

    From the Donzi site... this is my newfound understanding:

    I'm told this was a common practice for deep V hulls (eg. 24* deadrise). It is often done, in conjunction with a delta pad but, not always.
    It apparently facilitates the raising of the X dimension (which reduces drag) and allows the higher set drive to run in less disturbed water as the distance from the transom to the prop is increased. The upper portion of the notch is kept parallel to the keel and is of little consequence as it is no longer a significant part of the running surface.

    Results of the modification are felt only at speed. The downside is a reduction in running length at speed which is detrimental to handling in big water. The benefit is greater speed through reduced drag while not sacrificing prop "bite". Apparently, re-balancing the boat may be necessary to adjust for the potential of porposing but there is negligible effect on planeing as this is usually only about 24" wide (12" each side of the centerline of the keel and a common distance between stringers) and 12" forward from the transom. So, there is plenty of surface area to help generate lift.

    Lastly, I'm told It is usually done on larger single engine/drive performance boats as the negative effects are proportionally less pronounced. Eg. a 28' hull rides like a 27' hull at speed in rough water vs. a 16' hull acting like a 15' hull in similar conditions. It is more common these days to accomplish the same goals with out the negative attributes by using a standoff or extension box.
     
  8. Joakim
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    Joakim Senior Member

    The effect is about the same as having setback and center of gravity more backwards. So the boat will have higher trim (shorter planing waterline) and the distance from transom to propeller will be bigger.

    Maybe there is some benefit for having the rest of the hull behind the "new transom" at low speeds, especially when getting up to plane.
     

  9. Konstanty
    Joined: Mar 2016
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    Konstanty Junior Member

    Will it be good for the resistance to pitching?
     
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