Outboard Engine Thrust

Discussion in 'Powerboats' started by cheetahmike, Sep 30, 2012.

  1. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Well, a hull has a boundary layer. Starts off with next to nowt up fwd and increases in thickness as you go aft. So, by the time the boundary layer is at the same location as the prop, it has increased in thickness.

    As a hull moves through the water, there are stagnation pressure waves developed at the bow and stern. What this means is that in some locations the flow can actually be the other way around, flows backward as such.

    What all this means is that the ‘wake’ or water flow into the prop shall be different. This is usually termed Va, speed of advance. This is lower than the speed of the hull. Va = (1 – w)xV. Where V = speed of hull and w = wake factor.

    To makes matters worse, as the hull moves it also affects the wake. The water is accelerated through the prop resulting in a low pressure in front of it which produces a suction on the after side. So, what does all this mean?

    Lets say the hull has a resistance, R. The prop, owing to all the above, must exceed the hull’s resistance R, in other words the thrust T, from the prop must be higher. How much is determined by the thrust deduction factor, or t. It is simply R=T(1-t).

    As an example, if a prop produced say 100kN of thrust, and if the thrust deduction factor , t, is say 0.2, the amount of deduction is simply 0.2 x 100 = 20kN. A loss of 20%. Which means the resistance R is 80kN. The prop thrust must be higher owing to the “losses” in simple terms.

    So, now we can say the power transferred to the water by the prop is = T x Va.

    So, when you look at what factors effect Va they shall of course effect the amount of thrust produced by the prop.

    Strictly speaking we can say that length plays a part, since this affects the boundary layer thickness. But fore most hulls, at least in the ‘small’ range’, it can be ignored and treated as a constant.

    The shape of the hull influences how the water flows along the hull from the bow to the stern. Since the shape of the after lines effects how the water flows into the prop. Which also effects Va and the values of , t, and w, noted above.

    Thus in simple terms, the thrust is related to the prop only and what 'it' must do.
     
  2. FishStretcher
    Joined: Oct 2011
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    FishStretcher Junior Member

    There are a few things where I didn't post enough information, or it was very unclear- sorry about that.

    First- I limited the scale of the stress plot. That isn't obvious there. I wasn't terribly worried about local stresses higher than that due to the poor modeling of the load at the bolt hole. I don't have bolts, washers, nuts or the outboard casting modeled. I wanted to look at deflection and strength in other areas of the structure, figuring that doubling material or adding load spreaders at the 4 bolt holes wouldn't be difficult. Plus fiberglass transoms put up with those point loads, albeit with local compression failure over time.

    Second- There wasn't enough information presented for this, but the center of mass of the outboard should be ~300-400mm aft of the transom. If I recall correctly. 5Gs of 170kg mass is a bit more than 8000N, with a .3-.4 meter lever arm. This outboard is a 20" shaft length, or about 500mm lever arm, pushing with 2000N in this example. (20" is really from the top of the mount to the antiventilation plate, but in this case it is close to the measurement from midway between the top and bottom bolts and the center of rotation of the prop.) So at 5G, it would seem that the moment from the G loading is significantly more than the thrust loading. Under running conditions at 1G plus whatever might be seen for heave and pitch induced G forces, the opposite could be true. Especially at full power.

    To my eye, and trying to remember what I was looking at 2 years ago, when I remove the thrust load, the stress plot looks pretty similar to when it just sees 5 Gs plus the thrust load. But I can re-run this at work next week to see if that guess is correct. With the plot legend topping out at 75MPa(corrected), I can't be sure- as you pointed out.
     
    Last edited: Oct 8, 2012
  3. tunnels

    tunnels Previous Member

    Damn it i never knew a simple job could be made so difficult . just get on with it and pull the plug on the puta its always goint to tell you an answer of some sort ! right of wrong !!:eek::idea::(
     
  4. Frosty

    Frosty Previous Member

    Well---thanks for that Ad --I knew as much and thats what I was looking for but that was like instructions for how to find your house in Glasgow starting from the M25.;)
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Well, if those plots are limited, I would be a bit concerned since the implication is that the stress is actually higher than 75MPa!

    Difficult to say, since you have plotted the Von Mises. You need to plot the direct in-plane stress, and membrane at that too and full stress range, not limited.

    I assume that is a typo, 75kN, as it should read 75MPa.

    So long as you appreciated Blenheim Palace, the Cotswolds, the Lake District and Hadrian's Wall on the way, then it serves its purpose :p
     
  6. FishStretcher
    Joined: Oct 2011
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    FishStretcher Junior Member

    I explored different loadings to look at which load contributed how. Next post shows stress at a different scale/legend for the same load.
     

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  7. FishStretcher
    Joined: Oct 2011
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    Location: On the Water

    FishStretcher Junior Member

    Same loading, different legend for stress at a fairly high loading. 2G acceleration and 4400N thrust. Scales/legends are 75 versus 240 MPa- 240 MPa is roughly the yield strength of the material. Again, the stress in the bolt holes and perhaps a few diameters away is artificially raised due to the shortcut of simply applying the forces to the bore, not simulating the real bolt joint.
     

    Attached Files:

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  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    The full scale plot shows a stress around 160MPa iwo the bolts structure, which is a tad high.

    Just take the rotations and displacements from the location of the bolts, in the FEM, and make a sub-model. A more detailed and finer mesh model to examine the bolt hole stress, if that is what you wish to investigate.
     
  9. tunnels

    tunnels Previous Member

    Its interesting what you are doing and the drawings are really pretty !!, its also amazing how people waste there time as well . :)
     
  10. FishStretcher
    Joined: Oct 2011
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    Location: On the Water

    FishStretcher Junior Member

    I don't have any interest, as I stated previously, as I knew the simplified loading I used would over-represent the stresses in the bolt holes. But these same bolt holes that take this loading in old fiberglass with mushy plywood core, so I wasn't worried with what happened under the bolt heads.

    I was interested initially in seeing what would happen to the scratch built bracket as a whole under certain loads- I added the "knee" in the CAD model, it isn't in the existing structure, and it makes a big difference.

    I posted here as someone had a question about the loads from an outboard, and I though this might help visualize it for some people.
     
    Last edited: Oct 14, 2012
  11. tunnels

    tunnels Previous Member

    To go to all the work of making whats drawn i think is a waste of time making it !!:rolleyes:
    Looking at the most practical solution its better to simply make a complete new glass transom !!
    Cut the old transom off the boat 100 mm in from the corners and stitch the new transom on !!,with 100mm left around it can be matted and glassed inside and out then it looks like its always been a part of the boat not an add on . Its stronger ! better looking !, much more practical and would add extra room in the back of the boat !!!. :p
    Could make a wooden mould and mould up almost 80% complete then mount and glass in place inside and out and then its just finishing work .
    Have been there done that a few times and it looks good !!! you get the step back you want , with just a small splashwell !!, its advisable to use a 25inch outboard so when you slowdown and the stern wave catchs up with the boat it dosent swamp the outboard totally even to the point of drowning it so its stops when the water gets inside the covers . :)
     
  12. tunnels

    tunnels Previous Member

  13. FishStretcher
    Joined: Oct 2011
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    FishStretcher Junior Member

    One more thing- 4400N (just under 1000lb force) roughly equates to a 175-200HP outboard pushing a boat a 40 knots- assuming it is in the power peak RPM range for that outboard, and assuming a very good prop match.

    It also roughly corresponds to ~140HP pushing a boat at 30 knots at peak power RPM and a decent prop. Regardless of the hull shape/lower unit ratio.

    Maybe all this will motivate me to work on my bracket soon.
     

  14. tunnels

    tunnels Previous Member

    I still think you should make a complete new transom and forget the add on bracket !!:confused::eek:
     
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