Porpoising in a hull

Discussion in 'Boat Design' started by johnndeanna, Oct 16, 2013.

  1. johnndeanna
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    johnndeanna New Member

    Hello all,

    I had a question I was hoping someone could answer. I was looking at some forum threads recently on a boat that some have experienced porpoising at certain speeds and pondered the following:

    If several things in the boat remained the same, such as beam, motor size, location of crew (same percentage of overall length), would this same boat being 10% longer affect the porpoising issue? If so, what is it about the changing the length that would help alleviate the problem. Assume that there are no issues in the construction that would cause the problem and that the boats are identical except for overall length.

    I am simply trying to understand what causes the issue and does an increase in wetted surface affect this. Had nothing better to do today than ponder things I don't understand.

    John
     
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  2. Easy Rider
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    Easy Rider Senior Member

    You'll find better input later but a straight keel should minimize porpoising.

    And longer hulls porpoise less.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Hi John,
    If you do a search on this forum, you'll find a lots of threads talking about various types of boat instability in seaway.
    For a beginning, you could check this post by Mike Johns: http://www.boatdesign.net/forums/hy...s/planing-instability-48481-2.html#post655207 which contains a link to a very informative article about planing hull stability, and another link to a useful online calculator of parameters affecting the stability.
    These two links will give you a lots of valuable info from which you'll be able to come to your own conclusions.
    Then you might come back here and share your conclusions with us, so we can discuss them. :)
    Cheers
     
  4. DCockey
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    DCockey Senior Member

    The calculator from Donald L Blount Associates which MikeJohns provided a link to is not for porpoising. Rather it's for the type of dynamic instability where a boat heels abruptly.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Hi John, welcome to the forum. Longitudinal instability (porpoising) discussions can get pretty involved. How much hydrodynamic understanding do you have, so I have a good place to start, without a full dissertation on the subject?

    You beat me to it David.
     
  6. johnndeanna
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    johnndeanna New Member

    Very little Paul. I have never really read much about it and most of what I gain is from posts such as this. I really do not want a full dissertation on it since that just takes up time you don't need to spend on something I can read up on myself. I was basically after a basic premise of whether boat length and wetted surface have any effects if other things remain constant, and I know that not everything will remain constant.

    Take your Drummer for instance. At 26' long or so its a narrow, long boat meant for speeds at around 30kts I'd say, give or take. If that boat were modified and stuff adjusted to maintain the same CB, would the shorter hull have a greater tendency to porpoise, or would the altering of certain weights to maintain the same CB location keep the running attitudes the same?

    Conversely, if you kept engine size the same and locations of passengers changed as a result of the shortening, would the boat then have a more rearward CB and result in a greater tendency to porpoise?

    Again, I am going to go look for books on boat design simply because it is something I'd like to know more about, but several threads got me wondering about the issue so I thought I'd ask.

    John
     
  7. PAR
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    PAR Yacht Designer/Builder

    Generally, porpoising is when you "drive over center" far enough, causing the boat to teeter totter and flop back down on it's forward running surfaces, where the cycle starts over, as the lost speed increases again. Mostly, it's a trim issue and can usually be solved by trimming down, to keep enough of the plane patch "engaged", so you don't run with too much boat out of the water.

    I'm not sure I understand your same design except shorter or long thing, but each design has to evaluated around it's expected envelop, so the centers sort out themselves. The only time I've seen longitudinal stability issues, is when you move outside this envelop, for whatever reason. A common issue is tossing a lot more power at a specific hull form, literally driving it far enough out of the water, that she can't sustain herself without significant adjustments (moving weight, playing with running surfaces, etc.). Naturally, if a boat is shortened, but the weights inside remain in the same place, you'll trim aft, which may or may not be okay, depending on where you hacked off the length.
     
  8. gonzo
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    gonzo Senior Member

    If you have an outboard or I/O trim will fix the porpoising. Trim tabs help too.
     
  9. SukiSolo
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    SukiSolo Senior Member

    At my local sailing club we had a brand new 'Dory' which porpoised on the plane. This type of 'Dory' is the UK Dell Quay type with a triple hull shape. Not to be confused with the traditional French and Nova Scotia Dorys. Our older UK made Dell Quay 'Dorys' do NOT porpoise with similar power/weight engines. Hulls were also similar length (within 100mm).
    Cure was to put 50kg right in the bow or have a crew member lean forward over the bow to hold it steady. We sold it pronto.
    Apparently the moulds had been 'distorted' or rebuilt after transferring manufacture to an Eastern European country. Either way no one tested the product properly, it was dangerous and not suitable as a safety boat.
    I nicknamed it 'The Slapper' so you will deduce its motion!

    My own guess is that like PAR says when it rose up, it was too short on its support surfaces and there was insufficient buoyancy and/or planing area forward. Moving weight forward alleviated the problem but was not a satisfactory solution. The proper Dell Quay boats are very stable and predictable, excellent safety workhorses. We run 25hp on them and I prefer them to the RIBs on our sheltered lake. They don't blow around, low freeboard, loads of space and you don't have to re-tube them...

    I've had keelboats and dinghies pitch in a slightly strange way in a seaway and you can feel it slowing the boat down. Different problem to true porpoising but I feel it may be down to volume distribution just above the waterline. Can be just too heavy a rig acting as a pendulum and other causes.
     
  10. Barry
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    Barry Senior Member

    Understanding porpoising is not difficult though there are quite a few variables that I would doubt that there is a formula to predetermine if a hull will porpoise

    If the dynamic centre of gravity is very close to the centre of lift of a planing hull, you will probably have a porpoising hull. To make things easy, the centre of lift is the point at which all the forces acting on the boat due to hydrodynamic forces of the water could be replaced by a single force to hold the boat up. (Of course you need a couple as well but let's keep things simple)
    I use the term "dynamic centre of gravity because the centre of gravity is the single point that all the forces acting down could be replaced by a single force AND the force acting on the boat from the drives thrust

    For simplicities sake we will assume that the centre of lift is 1/3 back from the wetted surface, it changes of course with speed and attitude but we need to solidify some parameters for the discussion

    Assume a 24foot, 12 degree mono hull
    You begin to apply power, the centre of lift is near the front of the boat and vertical and the dynamic center of gravity is say 8 feet ahead of the stern and downward. because the force from the centre of lift is far from the centre of gravity it produces a couple or moment or twist or torque in the hull. (Actually the lift force is not 100% vertical but let's leave this alone)

    As the boat speeds up the centre of lift moves closer to the cg and the moment arm (the distance between the cg and the centre of lift) is smaller and the bow begins to approach a more level attitude, more level not flat

    We add more speed, and there is a point where the cg and the cl ( centre of lift ) coincide or are very close.

    Drive the boat faster and the class moves past, stern ward, of the cg and the couple, moment torque twist now is opposite from the bow up attitude so the bow pitches downward
    It pitches downward, the wetted surface moves forward, taking with it the cl, now the cl is ahead of the cg, so the bow rises, the cl moves a little further rearward than the first time due to the angular momentum and the couple gets a longer moment arm and drives the bow down even a little harder, and so it goes, porpoising

    A few remedies, move some weight to change the ci rearward, change the trim of the thrust in an outboard which changes the dynamic centre of gravity, slow down, adjust trim tabs

    This is an extremely simplified explanation as other factors are changing as the attitude and speed changes
    Skin friction due to speed, the horizontal components of the cl, and many more

    Generally if you keep the bulk of the weight in the back of the boat the cl will not reach the cg and porpoising will not occur
     
  11. Barry
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    Barry Senior Member

    Drive the boat faster and the class moves. Should have typed the cl moves forward
    Also cl is approx 1/3 back from the start of the wetted surface
    Sorry big fingers small keys
     
  12. tom28571
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    tom28571 Senior Member

    Focusing on boat length is not the way to get a handle on why a boat porpoises although it may look like it. Actually, length does play an important part but it is the relation of the CG to the wetted length that is important, not the overall length. And it is really the center of dynamic lift relative to the CG which makes it look like the wetted length is important.

    That is why adding lift fins on the anti-ventilation plate of an outboard may stop porpoising by moving the center of dynamic lift aft.

    Porpoising is a form of harmonic motion and, like all other kinds of harmonic motion, getting rid of it means upsetting the balance of forces in the system. Attacking the symptoms may mask the problem at one speed but leave the problem in place to start over again at another speed as the wetted surface changes with speed..
     

  13. philSweet
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    philSweet Senior Member

    There are two way to go after this. You can simply study dynamic stability directly if you want quantitative answers and need to be able to design stable sytems or adjust insufficiently stable ones.

    Or you can try to get a handle on the forces and general behavior. This takes considerably less complexity, but it is still math. It is normally presented as differential equations, but can be summarized graphically.

    You have, at a minimum, a boat that is pitching cyclically. That mean that mathematically, the boat in pitching is a driven harmonic oscillator. All driven harmonic oscillators have a driver that is pushing them at the same frequency as the oscillation.

    From a practical stand point, the boat is also going to be heaving up and down at the same frequency, but out of phase with the pitching. The boat in heave is also a driven harmonic oscillor. These two motions are coupled fairly strongly.

    The source of the driving force is worth exploring-

    For pitching, the driver is the sum of the moments around the pitch axis that passes through the CG of the boat. The moments include the pressure distribution on the hull bottom which doesn't not act through the cg, shear force on the hull which doesn't act through the cg, and the thrust of the motor which doesn't act on through the cg. Added together, they form a driving moment that cycles at the same rate as the porpoising.

    For heaving, the driver is the sum of the forces in the vertical direction. So basically buoyancy and dynamic lift and the part of the motor's thrust that acts in the vertical. It's worth noting that it's not just the motor's thrust angle that can change (due to pitch), the thrust itself can change with pitch.

    As PAR mentioned, porpoising vessels also tend to surge, creating yet a third driven harmonic oscillator coupled to the other two. Surge is driven by changes in drag and thrust forces.

    Each of these three motions are bound together and must all happen at the same frequency, and they interact in such a way that the driving function of each of them also cycles at that frequency.

    Sometimes, there is one item that dominates the system. If the pressure distribution is a strong and consistent function of pitch even when heave and surge may vary, it will tend to steadily drive the entire system while the effect of the minor drivers dither about until some consistency is achieved and everything settles into a constant rhythm.

    Each of the three modes, pitch, heave, and surge, have a natural response frequency to a particular disturbance. If the boat is suddenly accelerated, a boat will return to it's former speed, if heaved, it will bob up and down and come to rest again at its original waterline - same with pitching. If these natural frequencies are similar, they can effectively conspire to produce large excursions that seem to defy the expected damping forces.

    The natural frequency in pitch is proportional to the boat's moment of intertia about it's pitch axis and the way the change in the distribution of displacement changes the restoring force. The natural frequency of surge is proportional to mass and the way resistance scales with mass. The natural frequency in heave is unaffected by mass directly, but is affected by the amount of waterplane area per unit mass.

    If you change the length of the boat and leave the mass and beam alone, the heave natural frequency will be faster by the SQRT of the % length change. The pitching natural frequency changes with SQRT of % length change. The surge natural frequency is hard to predict, but shouldn't change much, and it is so highly damped it is a small matter anyway. So the coupling of the two major oscillators won't change in terms of their phase angle. The only thing that changes is how the driving forces are affected by the length change, because the hull will respond in a similar, but quicker manner. If mass is scaled with length, the hull will respond at the same frequency as before. If mass is scaled as length squared the response will be slower by a factor of SQRT of % length change.

    I'm cheating a bit here as far as how the response of coupled oscillators are affected by the natural frequency of their constituent parts, but it's a start.
     
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