Planing hull - sail vs power

Discussion in 'Boat Design' started by Will Fraser, Sep 18, 2018.

  1. Will Fraser
    Joined: Feb 2014
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    Will Fraser Senior Member

    Thank you for all the insights so far.
    I just realised that ideal steady state trim is achieved in practice simply by moving the cg, regardless of thrust-line height.
    My initial thought regarding the rockered hull is that there might be some pressure recovery aft to reduce form drag. This should still be the case but comes at the expense of not generating the same amount of dynamic lift as a flat bottom, or if it does, would require a less than ideal trim angle.

    Insight on the fore-aft placement of the propeller would still be appreciated.
    In an article by Dave Gerr, values of speed for a given power obtained by his and Wyman's formulas should be adjusted upwards by 5% for stern drives and outboard motors (I assume transom mounted). Is that just and inboard vs outboard drive-train efficiency difference or does the location of the prop itself play a role?
     
  2. Will Gilmore
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    Will Gilmore Senior Member

    I know this is a two year old thread, but in reading through it, I have, what I think is a pertinent thought to the last and unanswered question.

    The answer to this last question is, yes it makes a difference. Consider the difference to a drag racer with rear-wheel drive vs front wheel drive. The torque vectors for the rear-wheel drive can overpower the cg of the body and "pop a wheely", where the front wheel drive torque vectors work to keep the body of the vehicle down. In the former case, the vehicle is essentially lightened with regards to downward pressure on the wheels while the latter case increases the pressure on all wheels.

    The same would be essentially true with a power boat. With the thrust located below the stern, the vector forces contribute to the lifting of the bow, where in the case of a bow drive, the vector forces help "squat" the stern.

    As for rocker on a planing sailboat, you should consider the hull shape when on a sailing tack. While a profile or elevation view may reveal no rocker, the boat heeling at an 8°+ angle probably has rocker to account for. Keep in mind that the center of effort for a planing sailboat is well above the center of mass and center of resistance while the opposite is normally true of a planing powerboat.

    -Will (Dragonfly)
     
  3. gonzo
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    gonzo Senior Member

    That is not correct. Draw a free body diagram and it will show you that the front wheel drive also lift the front. However, it can't pull a wheely because the traction (and applied torque) will be zero when the wheels lift.
     
  4. Will Gilmore
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    Will Gilmore Senior Member

    Where the cg is behind the torque force instead of the in front of it. The driving axel will never drive under, over or around the cg. It is already there.

    It isn't an issue of traction. It's an issue of pulling versus pushing on the center of mass.

    -Will (Dragonfly)
     
  5. gonzo
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    gonzo Senior Member

    Accelerate hard on a motorcycle and it will drive under the CG and right on your head. Draw a free body diagram and think about the applied forces.
     
  6. Will Gilmore
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    Will Gilmore Senior Member

    Not that I want to have an arguement over this with you, gonzo, but I think you are mixing my statements a little. The motorcycle analogy doesn't relate to front wheel drive.

    You are dead-on with your thinking and understanding around these problems. I have developed a growing respect for you.

    In this case, I think we are not really saying different things, just failing to fully understand each other's statements.

    To get very detailed in the dynamics of high powered front wheel drive, yes, it would be conceivable to lift the front and pull the back of the car under the front wheels. This isn't about traction, it's about rotational inertia. The torque on the axle will try to spin the body attached to that axle in the opposite direction of the axle's spin, with relation to the vehicle. This means the back end of the vehicle is driven downward against the ground. If the forces on the axle overcome the gravitational and inertial forces of the vehicle pushing against the ground, the front end will flip right over backwards. That dynamic would not be the same for a powerboat because the rotational forces acting in that sense are actually perpendicular to direction and the length of the boat. This is just the torque of something that is essentially a flywheel causing the vehicle to rotate, not the vector forces of off- centered lever overcoming the inertial movement of the center of mass.

    However, I wasnt really meaning to get into detail about torque as it applies to a boat. I was only trying to relate the outcomes for a drag racer to the position of drive forces on a powerboat. There are differences, of course, but the basic forces behave similarly.

    If you still disagree, maybe this should have a thread dedicated to working it out. I'm interested in learning more.

    -Will (Dragonfly)
     
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  7. tspeer
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    tspeer Senior Member

    I think you have this backwards. In the static condition, prior to acceleration, the weight of a dragster is shared by the front and rear wheels. When a rear-wheel powered dragster accelerates, the load on the front wheels decreases, which increases the weight on the rear wheels because the total load has to equal the weight of the car. The load on the rear wheels can actually exceed the weight of the car for a short time as the front end is accelerated upward in a wheelie. With a front-wheel drive dragster, the load on the driving wheels is decreased by the torque reaction increasing the load on the rear wheels, because again, the total vertical force has to equal the weight of the car as there is no vertical acceleration.

    You can increase the shifting of the load to the rear wheels by moving the center of gravity forward, and by increasing the moment of inertia of the car for the same weight by making it long and narrow. Hence the rail dragster.

    The pitching moment due to thrust is the same, no matter where the prop is located along the thrust line. What is different between the fore vs aft prop location is the effect of the vertical component of the thrust. If the thrust line is angled from the horizontal to produce a bow-up moment, there will be an upward force on a bow-mounted thruster and a downward force on a stern mounted thruster. The different vertical forces will contribute to lifting or sinking the boat.

    Another way to think about it is to consider the thrust lines. If the thrust lines of the bow- and stern-mounted props intersect at the same point below the center of gravity, giving them the same moment about the c.g., the thrust line of the stern-mounted prop will be angled down, but the thrust line of the bow-mounted prop will be angled up.

    The planing dinghies I've sailed were fastest when sailed bolt upright. You're right about there being a bow-down moment from the high center of effort, which is one reason crews need to move toward the stern when planing. The dynamic lift on the hull also moves to the stern as the boat planes and the bow lifts out of the water, which is another bow-down pitching moment the crew has to counter.

    Besides the amount of rocker, I think it's important to consider where the curvature is concentrated. I found it interesting to compare the shapes of different Merlin Rocket designs when I was sailing that class. The rocker was often located forward, leaving flat buttocks toward the stern. This provided a straight contour for planing, while moving crew weight forward could reduce the wetted area of the stern in non-planing conditions. In the Merlin, the crew has to move fore-aft almost as much as side-to-side. So I don't think you can compare dinghy vs powerboat on the basis of a fixed pitch attitude. You should take the actual pitch trim angle into account.

    Another interesting planing boat, and one I used as my test subject for tank testing in college, is the Inland Lake Scow. In my case, the M-16. These scows have an absolutely flat bottom to a rounded belly near midships, followed by a flat run to the stern. Whereas a dinghy typically leaps onto the plane, and the steering becomes very sensitive after it does, with a scow the transition is smooth with finger-tip control throughout. At speeds a little below hull speed, when most boats would be experiencing a big increase in drag, the stern wave of a scow has its crest on that flat run to the stern, and the slope points the pressure forward. In effect, the scow surfs its own stern wave. The wide beam of the scow results in low drag from the dynamic lift of planing, which also contributes to low hump drag, since drag due to lift is highest at low speed. The result is an attenuated drag rise and the smooth transition from semi-planing to fast planing.

    Where heel was effective, it was at displacement speeds. When a scow is heeled approximately 15 degrees, half of the bottom is out of the water, greatly reducing wetted area. The rounded bilge of the Inland Lake Scows, along with a similar shape to the planform, results in a nearly symmetrical spindle shape for the immersed volume, with a greatly increased waterline. The static waterline length of an M-16 is 13 ft, but when heeled, nearly all of its 16 ft length is in the water. There was very little weather helm due to heeling of my scow, so the rocker, bilge, and planform shape contributed to reducing the drag due to rudder deflection as well.
     
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  8. alan craig
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    alan craig Senior Member

    I was just reading "The design of sailing yachts" by Pierre Gutelle (a few used available on Amazon) so this illustration was on my mind. Nicely illustrates T. Speer's last point.
    Edit: I don't think the boat has reverse sheer, that's just the curve in the page of the book.
     
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