Transom forces with a sterndrive

Discussion in 'Sterndrives' started by Alumination, Jan 20, 2016.

  1. Alumination
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    Alumination Junior Member

    I'll be using a 1999 Volvo SX-M sterndrive for my project aluminum tunnel hull.
    As I breakdown how the drive will act on whatever surface it is mounted to I try to ignore traditional design temporarily to avoid any preconceived style/shape etc. For instance, the need for the top edge of the transom to be the full width of the hull, connect the left gunwale to the right.

    The drive will act like a lever and looks to me that the majority of the force will be at the base of the transom shield assembly and an opposite force at the top. Pushing, pulling and twisting.

    I'm wondering if there are any pictures or diagrams of how this actually looks. How best to go about designing this surface and how the forces should be dispersed into the hull.
     
  2. gonzo
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    gonzo Senior Member

    If you want an unusual structure that doesn't follow standard design, you will need to engineer it. The manufacturers make sterndrives in a standard design so boatbuilders don't need to reinvent the wheel each time they design a boat. Is there a specific purpose the boat needs to fulfill where standard design doesn't work or do you just want to be different?
     
  3. PAR
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    PAR Yacht Designer/Builder

    The transom has little loading on a sterndrive arrangement. The engine beds absorb the vast majority of these loads and transfer them to the hull bottom, where they're put to use.
     
  4. Barry
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    Barry Senior Member

    I would google " Installation, Marine Diesel Engines, Aquamatic and Inboard D4,
    D6 to get some information on the Volvo outdrive
    Or alternatively Volvo SX-M
    The first manual, the one that I am most familiar with has all the information regarding installation for the DPH legs BUT they also include a huge amount of information regarding fuel tanks, ventilation and the list goes on.
    I am almost certain that the transom does take the thrust and torque about the Z axis because their engine mounts are flexible rubber units. For straight inboard or V drive, the engines take the thrust and the mounts are different.
    The installation guide shows the recommended transom angle and other very pertinent measurements especially the cut out of the stringer for clearance of one part of the shield or mount.

    For aluminum, we used a 1/4 inch transom and reinforced the area around the transom shield with I think it was a 2 inch by 4 inch channel around all the sides and carried this out to the gunwale. I do not have any drawings to check

    Almost forgot
    The 1/4 inch transom thickness is not enough and you need to add in a filler plate to get to the thickness required in the installation manual. Do not use wood. Again, too long ago there was a plastic spacer that had to be build to deal with this spacer. You can use some type of non water absorbing plastic, but some plastics are not compatible with aluminum.
     
  5. gonzo
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    gonzo Senior Member

    There is a spline on the input shaft, so the engine does not take any thrust.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Power transmission is different and Gonzo is correct in this regard, but compaired to what is required for an equivalent HP outboard, the I/O transom, handles considerably less and the loading is transmitted through the transom assembly, into the bellhousing, then the block and eventually to the engine beds, even though some isolation exists, nothing compaired to a outboard transom.

    The bottom line for these load transmissions is, to get as much of it to the bottom flanks, as quickly as possible. This is where you want the energy to dissipate, not on or in the transom or any attached elements, such as the transom shield, etc. Engine isolation is for skipper and guest comfort only and race applications, generally don't bother with this accommodation, to transmit as much of this energy to the beds as practical. In a perfect world, you could use a rubber bladder as the transom, as it's need is only to keep out water. In reality, we need some penetration resistance and some modest load absorption, so a relatively skinny transom is employed (comparatively), if only to have a solid place to hang a ladder or pitot tube on.
     
  7. gonzo
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    gonzo Senior Member

    The thrust is taken by the transom, but the torque reaction by the mounts to the stringers. An outboard with equivalent power will stress the transom more. I am not sure what the OP wants to do by having an open transom. I suppose it is possible to build a truss structure of some kind.
     
  8. meren
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    meren Junior Member

    Stern drive unit transmits its mass forces to transom structure so You need to decide or calculate what are accelerations at most (4...6 g?). And then add maximum trust propellers can deliver at full power at some speed which usually is not the top speed.
     
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  9. Alumination
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    Alumination Junior Member

    Thanks for all the good info PAR and Barry.

    As I typed the first post yesterday I didn't expect anyone to comment as soon as y'all did except I did expect Gonzo to respond with doom, gloom and disapproval, thanks buddy, we can always count on you!

    What I had in mind was something like this which shows high and low pressure on a vehicle body at speed.

    [​IMG]

    ...only showing the forces on the stern of a boat, even if it was just a drawing and not an actual measurement.
     
  10. johneck
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    johneck Senior Member

    The forces are much more dynamic with the stern drive. Besides the thrust, there will be steering forces and horizontal and vertical forces to react out as well. The magnitude of the forces will depend on the specifics of the boat, propeller and powering. However, it seems it should be relatively simple to make some (conservative) estimate of the forces and design a structure to transfer them into the shell. Everything needs to go through transom flange, so I guess in some ways it is easier than your car example.
     
  11. Alumination
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    Alumination Junior Member

    Yes, this is what I'm finding out. I've discussed this with some folks from work with varied back grounds and experience.

    If you look at the surface of the transom shield assembly where it contacts the transom, this is where all the drive force is transferred to the boat. I'm not sure the typical transom shape, a 2" thick, 24" x 72" panel, is the optimal plane to absorb this force.
    The sterndrive leg acts like a lever, pushing & twisting.
    Also the engine will be exerting twisting force thru the drive shaft.

    I'm building a new computer so I can download SolidWorks and get a better idea how the whole propulsion assembly should look and attach to the hull. We glued together a cardboard model at work to experiment with, helps to visualize things some.
     
  12. gonzo
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    gonzo Senior Member

    There are several million boats with 2" transoms that work well. Some have been around for more than sixty years. What do you think needs to be improved?
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Sorry PAR, but that is NOT true. All I/O drives have flexible members between the drive's transom plate and the Engine. There is no transmission of longitudinal or transverse forces through these components.

    All forces are transmitted through the transom plate to the transom and from there to the hull via longitudinal members, that may, or may not be part of the engine bed longitudinals.
     
  14. Alumination
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    Alumination Junior Member

    My Volvo SX sterndrive looks to need a transom which is at about a 12 degree angle leaning backwards so to speak. I'd like for the rest of the transom to be at an opposite angle, leaning forwards, I believe this is referred to as a reverse transom.

    Maybe I worded my intentions incorrectly. The transom will extend the entire width of the boat but the part where the sterndrive attaches won't.
     

  15. gonzo
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    gonzo Senior Member

    Outboards and sterndrives are designed for a standard 14 degree angle. It has been like that for many decades. I am not sure who set the standard. Reversing the angle does not require a complete system re-design, only the geometry of it. The upward force component will be in tension instead of compression though. That will require some thinking depending on the rest of the hull. Also, the leg will be much longer, which will increase the moment on the transom significantly.
     
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