Porposing Four Winns

Discussion in 'Stability' started by regdunlops14, Aug 26, 2009.

  1. regdunlops14
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    regdunlops14 New Member

    This summer my family purchased an older 16ft, Four Winns 160 Freedom bowrider with a 90HP Evinrude for use at our summer camp. The engine runs great however at 3/4 throttle and above the boat porpoises severly. Also on turns you can hear the prop come out of or almost come out of the water. This happens with the motor all the way down. The people we purchased the boat from said it always has porpoised since they bought it. There is one hole left on the motor mount bracket so I could drop the engine further into the water. Are there any solutions to this problem?
     
  2. Guest625101138

    Guest625101138 Previous Member

    Porpoising usually results from having the boat trimmed bow up. It is more likely with beamy hulls. Tilting the outboard so the prop is closer to the transom should lift the stern and reduce the tendency to porpoise.

    Have you got a lot of weight in the stern?

    How much does the boat weigh?

    Where does the boat ballance on the trailer?

    Have you got a photo at rest to determine where the CoB is?

    Any chance floatation could be waterlogged particularly at the aft end?

    Does the bottom of the hull run fair to the transom or has it been bowed?

    Do you notice it more pronounced when crew sit aft?

    Rick W
     
  3. Willallison
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    Willallison Senior Member

    It would seem unlikely that a large builder like Four Winns would produce a boat with rocker in the bottom, but, without further info, that would seem the most obvious cause (as Rick eludes to in his 2nd last question). Run a straight edge along the keel (for and aft) and do the same part way up each side of the bottom (again, fore and aft) if the there is any convexity in the bottom, then that is most likely the culprit.
    If it is, the cure can be relatively simple. Adding trim tabs or wedges at the transom ought to provide you with a fix.
    Of course, all that is based on the assumption that nothing else is drstically out of whack... so a bit more info and a pic or two would indeed be useful
     
  4. Jimboat
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    Jimboat Senior Member

    regdunlops14 - What happens if you trim engine in when onset of porpoising is experienced? If you have no more trim, perhaps trying wedges is a simple and inexpensive solution. Running with less trim is usually best solution to porpoising problems.
     
    Last edited: Sep 1, 2009
  5. ABoatGuy
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    ABoatGuy Member

    Forward CG? Does it improve with weight moved aft?
     
  6. Jimboat
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    Jimboat Senior Member

  7. ABoatGuy
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    ABoatGuy Member

    A good article on dynamic instability can be found in ProBoat #31 and also the SNAME Small Craft CD. See attached. Both indicate as LCG moves forward relative to the centroid of the water plane instability is encountered. This has been my experience as well.
     

    Attached Files:

  8. Jimboat
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    Jimboat Senior Member

    When describing Porpoising specifically, the instabilities are interesting. In these cases, the dynamic instabilities are a function of trim and Lift coeff (CL), affected by deadrise angle. Savitsky showed that for a given deadrise angle, there was a specific relationship between trim angle and lift coefficient which defines the inception of porpoising.

    It is interesting to see that as the lift coefficient is decreased, indicating a lightly loaded hull and/or a high planing speed, the trim limit for stability is decreased. Further, the effect of increasing deadrise is to increase the trim angle before the inception of porpoising. This relationship is counter-intuitive at first review, but shown to be factual by the Savitsky paper.

    In any case, if a boat is porpoising at a given speed and load, the solution is shown to be reduce the trim angle to avoid porpoising. The 1ower trim angle can be acliicved in several ways - one is to move the longitudilial center of gravity forward.

    Our research has shown that any method that causes reduced operating trim angle, will have the positive result of altering the onset of porpoising regime.
     
  9. regdunlops14
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    regdunlops14 New Member

    Sorry for the slow response as I had the boat out this past weekend experimenting a little. The problem does get better with more weight in the bow however it does not go away. Also the original owner stated that the boat has porpoised since they boat it new so I do not believe it is a problem with way the boat is trailered or stored. With the engine all the way down it still porpoises as well. As originally stated the prop seems to almost come out of the water on turns and the boat loses speed. Could it be that the motor is not low enough in the water?
     
  10. JackD
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    JackD Junior Member

    My class C racing runabout porpoises at about 30 mph which goes away at much higher speeds.At 60 or so,not a trace. It is still very annoying.I believe it is a torsional function,and as such it is governed by a moment of inertia and a torsional spring rate (T) (torque per unit angle). The system rotates about the boats CG.The moment of inertia (I) is not the weight per se but the way this weight is distributed about the CG. My understanding of planing is that the center of the wetted area is directly under the CG so it seems to me movement of the CG will not significantly improve porpoising. To affect porpoising, the resonant frequency must be lowered and/or damping must be increased. To lower the frequency one must increase I or lower T. I cant see how to do the latter.Adding a concentrated weight as far forward as possible could do the trick. Since I is mr squared. while affect on CG is linear with r(r being the distance from the CG) a relatively small weight might work. Seems an easy thing to try. Next spring I will give it a try.
     
  11. Joakim
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    Joakim Senior Member

    Porpoising comes from too high trim angle (the angle between keel and water level). The maximum stable trim angle depends on speed, weight and hull form. A deeper V allows higher trim angle and is thus used in racing boats even when waves are not an issue. At higher speeds the maximum stable trim angle gets lower, but at the same time the trim angle lowers naturally as well. Depending on the boat porpoising can be a problem at higher speeds, lower speeds, always or never.

    I have also a boat that porpoises at lower speeds, but gets stable at higher speeds. This can not be cured by changing the motor trim.

    Ideally a planning boat should be on an edge of porpoising, since higher trim angle means less wetted area and thus less resistance. If a boat is well designed, you should be able to control porpoising with motor trim (both on and off!).

    The trim angle at a specific speed depends primarily on LCG (longitudal center of gravity) and secondarily on the propeller shaft location and angle, which is adjusted by trimming the motor. Also the propeller type can have an influence.

    There must be a momentum balance between all the forces. The propeller force typically tries to lift the bow. Gravity and friction typically try to lower the bow. When the trim angle gets higher the wetted surface moves back and thus the moment caused by gravity increases. Above the maximum stable trim angle this moment changes so rapidly that there is no balance. A bit like trying to stand on a ball. At just one spot there is a balance, but minimal disturbance gets you out of balance.

    Since LCG is clearly more important than motor trim, a bad LCG can not be compensated. In this case LCG is too far back. This could be due to a too heavy engine or other installation or just a design error.
     
  12. Jimboat
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    Jimboat Senior Member

    Porpoising onset is a function of planing surface design (width, deadrise, etc), angle of attack (trim), velocity and location of Dynamic CofG. Since the location of Dynamic CofG is changing throughout the velocity range, the calculation of porpoising onset is a complex one, but predictable by analysis, nonetheless.

    While the easiest adjustment to minimize porpoising is to reduce trim angle, there are indeed other design and setup actions that can be taken to change the velocity at which porpoising onset occurs, and to mitigate the effects of porpoising.
     
  13. JackD
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    JackD Junior Member

    When you guys talk about changing the velocity of porpoising onset, I hope you do not mean making it higher. For my boat, this would put the onset closer or even into racing speeds, I think a dangerous situation. Specifically, does reducing the trim angle raise or lower the onset velocity?
     
  14. Joakim
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    Joakim Senior Member

    For every boat there is a curve for the maximum stable trim angle and another curve for the trim angle that the boat actually has at different velocities. The maximum stable angle gets smaller and smaller as the velocity increases. The actual trim angle has a maximum and then decreases with velocity. Depending on these curves a boat can be porpoising at low, high, all or none planning velocities.

    When you lower the trim angle (by changing motor trim or LCG) the whole actual trim angle curve is lowered. This means that the velocity span for porpoising gets smaller or even vanishes.

    My small (3.8 m) "racing" boat porpoises quite badly from 15 to 30 knots and then is stable from 30 to 48 knots. You can not use it in the porpoising region, you just have to accelerate through it, which luckily doesn't take many seconds. With two passenger (who must sit way back) and a driver it porpoises at all planning speeds, thus it can not be used.
     

  15. baeckmo
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    baeckmo Hydrodynamics

    As this problem is a resonance phenomenon, it is a question of two systems of forces. Both are mainly linear in characteristics; the hull resistance increasing with speed and area (=low trim), and the propeller force increasing with reduced speed.

    Physically, this boils down to the following process, (which may be described mathematically as has been noted by others here):

    The boat is reaching a speed, where trim is reducing with increasing speed, causing increase in resistance. Simultaneously, the propeller thrust is going down, the speed is reduced enough to cause a trim increase (=drag reduction) and to increase thrust. Speed is now increasing again, and the process repeats.

    If you imagine a diagram showing forces (drag and thrust) over speed, the slope of the respective characteristics is critical. If both are flat, small disturbances in thrust/drag will result in big speed variations, easily setting up a swinging couple, and vice versa. Instead of increasing drag/reducing trim by moving the center of mass, you can introduce a factor with a non-linear characteristis; ie its force/speed slope is differing drastically from the others. Such a measure can be:

    1/ an increase in width of the low spray strakes up front,
    2/ a pad up front,
    3/ small transverse step fwd of LCM,
    4/ cutting out a triangular surface aft,

    A particular boat may even react if an originally sharp strake has been rounded or damaged over years of bumping and bruising; check that the strakes are intact and have sharp edges, their "horizontal" surface preferably angled down 5 to 10 degrees in transverse view.

    Typically, the propeller characteristics is comparatively flat in this situation as well, often working with some ventilation that causes a reduction in the slope of the thrust coefficient (even becoming negative if ventilating/cavitating). A change to a prop with more cup/less geometrical pitch (better "bite" in foaming water) will often restore a steeper thrust characteristics.

    Longitudinal change of mass does not always give a good result, as planing area and moment of inertia will be changed as well. A vertical shift might be more effective, but not practical from operational and safety perspectives.
     
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