Porpoising

I have a problem with porpoising on my 12' home-built duckboat whenever I get above 3/4 throttle. Can someone explain the physics of porpoising so that I can understand the variables which I might manipulate so as to eliminate my problem. I am particularly interested in the center of gravity/center of lift relationship. Perhaps a link to a site that provides as much. Thanks, Ken.

 

HI,
at what speed does this do it at??
what HP motor are you using??
Usually this will happen when the bow is trimmed to high.
try shifting some weight forward if you cannot trim motor or Hull.
don

 

No speedometer. Probably around 22mph. 25hp Yamaha 2-cycle is trimmed in as far as it will go. Doesn't moving weight forward only lessen the frequency of the occillation in accordance with consevation of angular moment. From what I've gleaned so far, and correct me if I'm wrong, here are the dynamics of porpoising: The boat has a fixed center of mass (COM). The hull has a center of boyancy (COB) that moves aft as the wet surface of the hull decreases at higher speeds. As the COB passes aft of the COM, gravity pulls the bow down rotating on the COB point. When the bow goes down, the COB immediately and rapidly moves forward of the COM. Stability is reestablished until the COB once again moves aft of the COM.

Moving weight forward, if my thinking is correct, should only increase the likelyhood of porpoising. Though initially this might bring the bow down, and hence, move the COB forward until speed is ultimately increased enough to shift it aft of the COM again, the forward shift of the COM only appears to deminish porpoising because as the radius of arc of the ocillating COM about the COB increases, the frequency of occillation decreases to preserve the angular moment.

Trim in causes the craft to run in a more bow down attitude which serves to keep the COB forward. Porpoising is prevented until enough power is applied to move the COB aft again.

This leave me with the counterintuitive solution of moving weight aft so as to move the COM aft.

There is the possibility of my boat being overpowered (by definition of this particular instability) which moves the COB aft to the extreme, and more so than any aft movement of the COM that I would be able to achieve.

In line with my thinking, the reason that a rocker hull is thought to porpoise more is because the bow rides high, and therefore, has further to fall making porpoising more pronounced as opposed to making it more likely.

I guess the trick then is to balance weight distribution, trim attitude, and power for the most efficient use of thrust.

 

Hi,
where do you steer the boat from???
I think all that techo stuff will not be effective with that hull design and trial and error will be your only effective means of getting something to work.. as we had to do with the IVB's when we first muted improving Hickmans original design!!!
firstly you are trying to achieve much to much with that hull design , you looks as if you are way overpowered for the hull design ..
I run a 25 hp yamaha on a 3.5M IVB hull and with one up and gear for days fishing get 27MPH with no porpoising..
see boats for sale at Http://invertedvboats.bravepages.com/..
if you wish to remain with HP to hull design would look at putting trim tabs on or adding an extension to hull with a hog in it to bring bow of boat down..

don

 

I have solved that problem by building a wedge on the aft section. It gives the hull more lift and stabilizes the porpoising.