Porpoising

Hi everyone

Can somebody please explain what porpoising is exactly and what causes it with a catamaran.
Maybe some good references

Thank you in advance

Kind Regards
Tobias

 

You better clarify whether it is sail or power, planing or displacement, and when the porpoising sets in.

 

It is a dynamic condition. The bow goes up and loses dynamic lift from the water. Then it comes down, where it gets too much lift. It keeps on going up and down.

 

What does mean "It is a dynamic condition":?::?: I´m not able to understand. Of course I understand that when the boat is stationary, this phenomenon does not occur.
And second question asked by TobiasJohan "what causes it with a catamaran". Also I'd like to read explanations.
Perhaps it is because the relative position of the center of buoyancy, center of gravity of the ship and center of gravity of the water plane is incorrect, the three points are too close. What do you think?
Thanks

 

Do you need an explanation on dynamics? I thought you were a naval architect. This is a good link to an explanation of it http://www.dtic.mil/dtic/tr/fulltext/u2/b240994.pdf

 

Do you mean this: http://www.youtube.com/watch?v=bKzyLIonf0c from 1:16 ?

 

Porpoising can happen in flat water. It is a cyclical condition.

 

Simply put, "porpoising" is a point of dynamic longitudinal instability. There can be several reasons for it, in power, sail, sea state, etc., you'll need to define what you're looking for a little. For example, if speaking of high speed power, porpoising occurs when you "drive over center", which simply means the CB of the contact patch under the planing hull, has moved so far aft, that the CG is "teeter tottering" over the CB and a sudden bow down trim is attained. If power is still being applied, the boat will climb back up and push it's CB aft again and the cycle repeats, which is commonly referred to as porpoising. If there's more available power on tap, you can force the boat into a very dangerous situational dynamic, if it's applied, which can cause the boat to do some nasty things, like swap stern for bow, toss skippers and crews overboard and much worse.

 

I think the phrase "dynamic condition" is a truism that does nothing to clarify the question of the OP.
You thought I was .... I do not care much what you think of me, that will not contribute much to my personal or professional category. As for the link you provided me, to correspond to your abundant kindness, I will say that what you do not understand on that text, I will gladly explain to you.
I recommend you read the post of PAR. Perhaps, in this way, you will learn something and then you can answer, based and knowing what you mean, OP's question.
What does it mean "a cyclical condition"?. I don´t know if it is or not a "condition" but of course it is not cyclical.
Cheers, Gonzo, be happy.

 

TANSL: Cyclical conditions are those that repeat over time. For example, a pendulum. Any oscillator is an example of this condition. Can you explain how porpoising is not cyclical? And if not, what is it?

 

Cars have shock absorbers to dampen the cyclical rebound of the springs. Drive a car without shocks over a bump and you will quickly have an example of a cyclical condition.

Move the CG forward and you will likewise dampen the cyclical porpoising of the boat.

Welcome, Tobias.

 

Gonzo, I see you have consulted Wikipedia. Well done !, smart.
The porpoising is not repeated periodically. This effect occurs when certain conditions, that need not be cyclical, occur.
But I think we would need to talk about certain nuances that you by lack of knowledge and me by deficiencies with the language, can not argue. So I will not continue with this.

 

Typically once you reach the point of longitudinal instability, it's cyclic in nature, though doesn't have to be, it usually is, unless the skipper backs down the throttle or conditions sufficiently change, to mitigate the situation. Of course, this is only in regard to "driving over center" types of porpoising.

 

Sorry to disagree but the porpoising is not a cyclical phenomenon, in my opinion. My poor English does not allow me to explain properly. Not even have a uniform period, given the large number of variables involved. The sequence of oscillations is not uniform, it is not repeated cyclically, I would rather say it is random in magnitude and duration.

 

Just because it's not uniform in it's oscillation, don't mean it's not cyclic in nature, it just simply means it's not a uniform cyclic result of the observation. The simple fact it repeats suggests it's cyclical. This said and having made some modifications to my race boat, I can assure it's very cyclically once this state of instability is reached.

In my case, traveling at 93 MPH, the boat began a slow, almost rhythmic oscillation, which when viewing the video, clearly showed she was porpoising. When the boat was pushed harder and at 95 MPH, she was rapidly porpoising and had to back down, in fear she'd "stuff" the bow, which as 90+ MPH is frankly deadly. There's not doubt about this longitudinal instability being cyclic, though sea state can affect how uniform this might be, it's clearly repeating and quite uniform. At 95 MPH the cycling was about every 1.5 seconds at 93 MPH this was more like every 3 seconds. Now, you may be right in that each cycle does vary to some degree, possibly 1.3 to 1.7 seconds per cycle at the higher speed, but this doesn't mean it's not cyclical, just that there can be some variability in period. Wave trains are cyclic as well, yet they don't have precise periods, just a general average with the occasional odd ball tossed in.