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ContinuousWave: Whaler Performance
General Theory of Porpoising
|Author||Topic: General Theory of Porpoising|
posted 08-23-2002 02:25 PM ET (US)
I am working on what I call my GENERAL THEORY OF PORPOISING. It is still under development, but so far it goes like this:
As a boat goes up onto plane, the wetted surface decreases and moves aft.
The center of gravity (CG) of the boat remains about the same.
The point where engine thrust is applied remains relatively constant, too. Call this Point the Center of Effort (CE).
Along the fore and aft line of the boat there is some point which is in balance, where the boat would tend to rotate ("pitch") up or down equally around this point. Call this the center of pitch resistance, or CPR for short.
When the boat is in the water in displacement mode, the CPR is near amidships.
As the boat comes on plane, the CPR moves aft along with the reducing wetted surface. This distance from the CPR to the CE decreases
The boat trim is determined by the vector that the thrust is applied thru the CE. This determines the bow lift and running angle.
Pretty soon the boat is running at a speed where there is marginal wetted surface, and the CPR is practically at the transom, just inches in front of the CE.
All the weight of the boat in front of the CPR ( a long lever arm of weight) is being balanced by the very short lever arm of the thrust being applied from the CE to the CPR. It takes very little force acting on the bow--a little wave--to lift the bow up and drive the stern down. The forces at the bow have much longer lever arms around the CPR.
The result is unstable pitching or porpoising--oscillations of the bow pitching up and down from small forces being applied at the bow through the long lever arm they have on the CPR. They cannot be controlled well by the engine thrust because it is acting through a very short lever arm.
The cure to suppressing them is one of two choices:
--change the angle of the thrust vector to force the bow down. This increases wetted surface and drives the CPR forward, bring more stability to the equilibrium around the CPR.
--use trim tabs, or what I call "stern lifters", to raise the stern slightly which will bring the bow down and increase the wetted surface and move it forward.
Notice that using an engine bracket should increase the distances between the CPR and the CE, giving the prop a longer lever arm and more control over the bow. This alone should reduce pitching or porpoising.
Thats my GENERAL THEORY OF PORPOISING.
As the Montauk owners have noted, heavy engines in the stern tend to cause porpoising. This is because the wetted surface when planing is driven smaller and farther aft.
The boats that don't have porpoising problems are boats that get some stern lift going. The boats plane with more of the hull in the water measured fore and aft, but less of the hull in the water measured keel to deck.
Stern lif can also be achieved with propeller selection.
Both hulls achieve the same buoyancy by having about the same area of hull in the water, but on the boats with good stern lift the wetted surface is more forward than on boats whose sterns are heavy and dug in.
Comments are welcomed.
posted 08-23-2002 03:16 PM ET (US)
As explained to me by my father some 40+ years ago. The hull wants to "fly" as he put it and when the wetted area decreases to some point, it can no longer be held up by the engine, its cavitation plate and remaining wetted bottom and it (the hull) falls back into the water. Its not a scientific explenation but none the less practical.
posted 08-23-2002 04:04 PM ET (US)
Maybe a formula in there for you somewhere Jim.
posted 08-23-2002 04:42 PM ET (US)
Interesting, one point to note, I mounted my engine at the lowest possible point (hole) on the transom rather than the more popular 1 hole up. I do not experience any porpoising, but I barely achieve reccomended WOT. I get 4900-5000 rpm when WOT should be 5000-5500. I am confident that I would achieve this if I raised the engine up some, but do not feel I need to at this time. I like performance.
posted 08-23-2002 04:55 PM ET (US)
I have liked the porpoising to the way an aircraft wing stalls. The angle of attack goes past a critical point and loses lift.
It will be interesting to see the comparison.
posted 08-23-2002 04:58 PM ET (US)
I wonder if friction plays into the equation at all.
Would a hull with bottom paint create a little more stern lift on plane then an unpainted hull and as a result be a little less prone to porpoising.
posted 08-23-2002 07:40 PM ET (US)
Jimh - you have your work cut out for yourself. A very good project and this topic could be discussed for a long, long time.
My thoughts -- Porpoising results from the unbalance of the forces acting on the boat. Specifically those forces include the gravitational force, the prop attitude angle or thrust angle, the hydrodynamic lift from the water, drag forces on the hull and the characteristic of the water (i.e. glass-smooth, light chop, et.al). In my mind, porposing results when the hydrodynamic forces exceed the gravitational force and the boat bow rises. When the gravitational forces exceed the hydrodynamic forces, the boat bow will fall - and away we go - up and down.
The boat surface also enters into the picture - that is, a clean hull presents a lessor drag force. Some waxes may help by resulting in a different surface tension. A bottom paint will almost certainly have an adverse effect.
There may even be some translation (moving the entire boat up and down) in addition to the rotation (which we commonly refer to as porpoising). I will give this some more thought in the near future.
The porpoising of a boat is a complicated dynamic event and is not easy to evaluate. This complexity is in part seen by realizing that the thrust angle, the upward hydrodynamic force and the hydrodynamic drag forces are dynamically changing.
Many, including myself, will simply change engine tilt and trim angle to correct the porpoising problem. We also know that the water surface plays a part - because the boat may not porpoise on a glass smooth water but will porpoise on a light chop. In actuality, the water surface is the forcing function of the system.
This is a good project and could well have been used for an advanced degree research project/topic.
I will be very interested in your project and concluding thoughts. ---- Jerry/Idaho
posted 08-25-2002 02:40 PM ET (US)
I have been trying to get a handle on the the stability criteria for porpoising for some time. I have a engineering background in aircraft stability and control so I understand the language. I have made a serious search for technical papers on porpoising and it turns out there arn't many and noone has really explained what is really happening. It is generally understood that as you rise the bow far enough the boat will begin to porpoise. The water can be glass smooth and this will still happen. Look at the following link for a neat explanation of planing and the implications of cg position on maximum planing speed.
I have a copy of the paper by Savitsky referred to in the link.
My 95 Dauntlas 17 begins to porpoise in the 25 to 35 mph range as I trim out. I raised my 125 Merc as high as I could get it and I am able to trim out further before it starts. At max speed I can trim until the prop starts to break loose and it doesn't porpoise. I have hesitated to put a bracket or jackplate on because the aft movement of the cg will tend to raise the trim angle and that could be a problem. Moveing the cg aft would probably increase top speed though.
posted 08-25-2002 03:20 PM ET (US)
Dick - your comments regarding the effect of raising your engine is good and conforms the effect of the thrust angle - that is, with the thrust angle nearer the surface, the effect of changes of the thrust angle would be decreased. Effects of the thrust angle would be maximized with a lower engine position and thrust angle.
I have not yet paid any detailed attention to the porpoising problem - yes, I will occassionaly have to change trim to correct for porpoising on my 17 OR - but I haven't paid any attention to the speed. And then, I am not normally running at WOT - but somewhere around 4500 rpm (redline is 5500) - which gets me to somewhere around 25 +/- +mph.
On my previous comment - I mention that a bottom paint would have an adverse effect - forget the word adverse as I am not sure at this point if it would be beneficial or adverse. ------ Jerry/Idaho
posted 08-26-2002 11:07 AM ET (US)
Oh my goodness, you've been left alone too long. I just returned from a trip and came across your thread. I hope your realize that this is still a very "open subject" among naval designers and still being researched. If you're interested, you can look up or buy articles from my old school (Florida Institute of Technology) or consult places which have decent test tanks such as the David Taylor Tank @ Stevens Institute in NJ, or Webb Institute in Long Island, NY. These places will have very good graduate papers discussing the problem of porpoising.
Try this a primer for research:
posted 08-26-2002 12:01 PM ET (US)
Dick, I seem to recall you installed a Bob's Machine stabilizer plate on your cavitation plate.
posted 08-26-2002 03:56 PM ET (US)
DaveH - thanks a million - there is undoubtedly a world of information of interest ot many of us in that reference source. Incidently, I could not get into the second site you posted - but it is simply a side trip from the main site anyway - so I will eventually get to it.
Is there anyway that these reference can be read on-line. Also need an address from where these reports can be ordered.
Again Dave, many thanks --- Jerry/Idaho
posted 08-26-2002 10:15 PM ET (US)
I did put a plate on the engine. I later removed it since it didn't do a lot of good. The boat doesn't need it for hole shot and it wasn't very effective in preventing porpoising. Raising the engine reduced the leverage of the prop in raising the bow. This lets me get the trim up a little so the thrust line is more nearly level.
posted 08-27-2002 12:07 PM ET (US)
Dr. Z is an old professor of mine from the mid-80's. He could provide you all the papers listed in the SNAME library. All you need is to contact him with the paper # listed:
Florida Institute of Technology Department of Marine and Environmental Systems,Oceanography Program, 150 West University Boulevard, Melbourne, FL 32901-6975
He has quite an accent so I would recommend using e-mail.
posted 08-27-2002 12:16 PM ET (US)
I forgot to mention to use the US Naval Academy as a source for papers. I just saw some recent research by one of their graduate level officers 2 years ago. You can find it and many others by doing a Google search for ("planing hulls" + porpoising). Your local university may also have an exchange program which will allow you to have the document in your hands for a small fee.
posted 08-28-2002 05:31 PM ET (US)
My 1996 20' OR will porpoise anoyingly at mid range rpms, say 4000 rpms. If I accelerate to 4500 the porpoising goes away, but this is often too fast for my current sea state. So what happens is that I get caught in this narrow rpm band that minimizes porpoising and pounding. I would like to be able to plane at a low rpm such as 3000-3500, with the bow down. I could then use the bow, and a low speed, to soften my ride.
So far I have noticed that a bow full of dive gear and tanks makes my boat ride much better. Mainly because it planes with a more bow down attitude.
I don' want to fill the bow compartment with weight when I'm not going diving, and I don't really want to install trim tabs. Do you suppose a prop with more stern lift would help my problem?
This porpoising issue is the only real problem I have with my boat. Other than It has white gel coat:-)
posted 08-28-2002 05:55 PM ET (US)
I think trim tabs would cost less than a new S/S prop. They are really easy to install.
posted 08-28-2002 06:47 PM ET (US)
True, both are about the same price. I don't like drilling holes in my boat though.
posted 08-28-2002 07:36 PM ET (US)
I had no idea that the porpoising phenomena had already received such serious study, as far back as 1952!
If all those fellas with Ph.D after their names have not figured it out yet, I doubt I can shed much light on it.
posted 09-03-2002 03:52 PM ET (US)
Interesting field of inquiry. Your take on it seems sensible to me. On first reading, I thought of two other variables that appear important in the mix, namely the angle of attack of the hull relative to the water surface and the changing ratio of wetted surface fore and aft of the center of balance of the boat.
So I composed a post pointing them out, then reread your theory and seemed to find that youíd already been there. Then read it again and I wasnít sure. So what the heck, Iím posting my original musings as they were. Feel free to ignore any reinvention Iíve done of your very fine wheel.
Consider the effect of four variables (leaving all others constant) -- the center of balance (CB) of the boat, the amount of wetted surface (WS) of the hull forward of the CB (WSF), the amount of WS aft of the CB (WSA), and the angle of attack (AA) of the hull relative to the surface of the water.
Planing happens when a boat is supported by the force imparted by the surface of the water striking the hull. The force imparted is a function of the speed of the boat, the area of wetted surface affected by the surface of the water and the angle of attack of the wetted surface.
Generally, the greater the value of any of these variables, the more likely the boat is to plane.
Porposing only comes into play when planing, at which time the lift provided by the combination of speed, WS and AA are important to consider. And more specifically, we need to consider the balance of this lift on either side of the CB. An imbalance in lift on either side will result in rotation of the boat about the CB axis. When WSA exceeds WSF, the bow pitches down. In the reverse situation, the bow pitches up.
Consider the situation when the bow rises. The AA increases, which increases the total lift on the boat. As the lift increases, the boat rises from the water, decreasing the WS. This loss of WS is at the leading edge of the WS which is forward of the CB, creating less lift at the front of the boat and more toward the rear. Thus this imbalance in lift around the CB axis causes the bow to drop.
This rotation about the CB axis has vertical momentum and as the bow drops it overshoots the equilibrium point, causing too much lift forward of the CB which sends the bow up again. Further, during the bow down attitude, the AA has become too shallow to sustain the planing height at that speed, the WS has increased and drag has subsequently gone up, so the boat slows and drops lower in the water. This also contributes to again raising the bow, particularly as the AA is steeper on most boats at the forward parts of the hull.
Porpoising, as you already know, can be quelled by either trimming down(in) or increasing speed. Changing these variables rebalances the vertical force components on either side of the CB axis such that the boat is again stable (somewhat ďbow heavyĒ). If you fix porposing by trimming down(in), the engine provides an upward force aft of the CB axis, forcing the bow down, reducing the AA. Or, if you increase speed, youíve solved the problem by causing the greater WS aft of the CB to provide more lift which rotates the aft of the boat upward and the bow downward.
I suspect that a planing hull would porpoise at any planing speed where the boat is trimmed for too great an AA. In such cases too much of the planing lift occurs aft of the CB axis. Planing occurs when the combination of AA and WS is too high for the boat speed. Effectively, the boat is continually trying to drive up and out of the water, only to fall back into it again. The faster you go, the less AA you need to stay on a plane.
Actually, now that I think about it, a porposing boat reminds me of skipping small stones over ponds in my youth.
posted 09-05-2002 05:25 PM ET (US)
Another take on porpoising Ö
As already noted, there are multiple variables in the equation. This time letís focus on another theoretical culprit.
I donít know the proper reference point for discussing the trim position of a motor. Perhaps there isnít any ďneutralĒ position, perhaps in(down) and out(up) are just relative directions from the current trim position. If there is something to relate it to, it could be considered relative to the boat, relative to the water surface, or relative to anything else. For purposes here, Iíll refer to it in two ways, trim relative to the boat (TB) and trim relative to the surface of the water (TW).
Imagine a boat running through the water at planing speed when the motor suddenly stops. Ignore the fact that it is decelerating due to drag. There is some natural angle the boat will seek relative to the surface of the water. This angle is a function of the hull shape, weight distribution, hull and lower unit drag and speed relative to the water. Letís call this angle the natural trim (NT) of the hull. I suspect most hull shapes would produce a different NT for different speeds. Generally speaking, the faster the hull is moving through the water, the shallower the draft will be. Higher speeds probably also generally mean a shallower NT. The aft portions of the hull tend to be flatter and if the boat moves fast enough, this area will provide sufficient planing support at a shallow angle of attack to the water so the bow can rock back downward.
Now letís consider the boat with the motor running. Given the cantilevered attachment of an outboard motor, the thrust of the prop causes the boat to adopt a greater angle of attack to the water. This gives us a changed NT for the boat with running motor.
Now consider the effect the cavitation plate has on this NT. If we are trimmed in (TW), the cavitation plate has a positive angle of attack relative to the water and at speed will provide stern lift. Conversely, if the motor is trimmed out (TW), it tends to pull the stern down. If the cavitation plate is level with respect to the surface of the water (letís call this ďneutralĒ TW), and assuming it isnít shaped for lift, it has little effect on NT.
My guess is thatís a situation asking for trouble. At any given speed, if the NT of the boat/motor and current trim results in a nearly level cavitation plate (neutral TW), any minor change in the hull angle will be sufficient for the cavitation plate to transition between a positive angle of attack and a negative angle of attack in the water. That creates a substantial and alternate stern lifting and stern depressing force and the hull will porpoise. Even though the TB trim is not changing, the TW trim is, and is oscillating back and forth through a neutral position.
Thus trimming in a bit (TB) solves the problem by preventing any oscillation of the hull from transitioning the cavitation plate through neutral. Changing speed also fixes the problem because the hull adopts a changed angle of attack to the water, allowing the current TB trim setting to effectively change the TW trim setting to a position less neutral, and thus less likely to be transitioned through by hull oscillation.
Does this seem a reasonable hypothesis as causing or contributing with other causes to porpoising?
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