Definition of Planing

Would that be a good definition for planing? A boat is planing when its waterline shortens by x% compared to static waterline. Waterline will not shorten (much) in displacement nor in semi-planing modes.

While I was windsurfing in the early 80's, a friend of mine defined planing as "when the board is in the air to the keel".

 

No, that would not work. It's easy to make a boat hold the bow down and thus, keep the WL as long as desired even though it may be running a high speed.

 

Yes it is, but would that be planing or semi-planing?

 

Interesting thought. However, Leo's opening gambit of more than 50% of the weight supported by hydrodynamic lift posses the questions how/where to measure it to be conclusive. The "x%" is not dissimilar in that regard.

But as you point out, displacement type hulls, which operate well above Fn 0.5 and semi-D hulls do not have a significant decrease in their static waterline length. So... there is some merit to that.

Only problem is, as Dom has pointed out most fully planing hulls have their running trims reduced. One could easily conceive a hull that is planing with little trim where its wetted length would not be dissimilar to that of a semi-D hull, simply owing to the trim. Raising the running waterline and positioning this on a hull and noting the differences between static and running waterlines. Since the stem and angle of entrance and forefoot dead-rise would not be significantly different in either hull form to warrant a change in wetted length at low running trim angles to say...ah ha....its planning. Using the "x%" wetted length reduction as a definition. It would be a very odd shape. (Not saying it's not possible though, just not "common").

But like the 50% argument, it does have some merit. Whether this could be used more definitively..is another matter though.

It does appear this has become a circular argument. Far too many caveats and oooh, wait wait..what about this one.

I see this in some ways a similar argument about semi-swath hull forms too. In this case such a hull form has been given the name, but 99.99% of the time is nothing like a swath, semi or otherwise, in the true measure of a swath; that being a low waterplane area relative to its displacement. Yet the term persists, mainly owing to marketing purposes. I don't see similar debates about the correct definitions of "semi-swath".

Thus, perhaps keep it simple, leave the definition as is, albeit not totally satisfactory. Let "others" more inclined (excuse the pun ) to worry or bicker about the absolutes.

 

That is not relevant either. Dynamic lift is proportional to the sine of the trim angle. Reducing the trim angle means that the speed must increase to produce the same amount of dynamic lift. Going very fast means the the needed trim angle to deliver full planing mode can be very small.

We used to make aquaplanes, before they could be store bought, from a piece of 3/4 plywood. When going very fast, particularly slinging out in a turn, the plywood would be almost horizontal. That could easily catch on a ripple, submarine and send the rider flying tail over teakettle. I hit so hard one time that my life jacket was stripped completely off. Knocked my breath out and I only made it be because of a quick pick up. Would not happen with current jackets but that one was the usual old kapok kind.

 

While using some % of lift as a definition of planing is tempting, it is actually simply arbitrary and meaningless. For a quick example of why, look at a slalom skier. Because of the narrow surface and high bottom loading, the skier isn't planing until almost 100% of the load is supported by hydrodynamic lift. Consequently it is obvious that the percentage of lift that is contributed by hydrodynamic means varies with factors like planing surface aspect ratio and bottom loading.

If you look at the mechanics of planing, the amount of hydrodynamic lift increases rapidly as the hull accelerates. This is the result of lift increasing at a square of the increasing speed, and the hull trim anlge increasing. Both of these factors contribute to rapidly increasing hydrodynamic lift percentage. As the hull attains a plane, the hydrodynamic lift percentage levels off, the trim angle starts to go down, and the hull now behaves according to planing theory. If you plotted hydrodynamic lift percentage you would see a rapidly increasing percentage of lift with increasing speed, and then there would be a distinct change of slope of the lift curve, and after a plane is attained, the amount of displacement lift is a much smaller percentage, and with further changes in speed the slope of the lift/speed curve is much lower. The point where that happens is a variable based on based on bottom loading, length to beam ratio and a number of other variables.

Whether you use the change of slope in the lift curve, or the change of trim angle with increasing speed, is moot. They both occur at the same point in time.

Sorry if I keep coming back to basics, but it's pretty clear to me that while planing is defined as the point where hydrodynamic lift predominates, the physical thing that we can observe when that occurs is the drop in tirm angle, and a flattening of the lift curve, both of which happens at essentially the same time.

Edit..

Also, in thinking about it a bit more, there is actually a transition zone (where Savitsky's analysis also breaks down), and that's were the the trim angle is actually comes off if its peak just a bit before actual proper planing is attained. However, the distinct change of slope of the lift curve is actually probably a better determinant of planing, and that occurs at a speed just above the peak trim angle.

 

Isn't it more logic to seek the definition of planing more in the characteristics of water?

 

I agree that trim is a good inicator for a typical power boat. However a water skier, sailboats or aggressive using of trimtabs can easily hide the typical changes in trim. And even more the ball example having no trim at all (although you maybe could define a trim for the wetted area) makes trim non-universal definition.

 

Is there a reason why yellowjacket's definition of a drop in trim angle would not be the valid and best definition? Assuming no change in LCG, trim tabs or so?

A ball does not plane... I've many times towed round, about football size buoys (practice rounding marks) behind a rib - first they sink, almost all the way under surface, only to pop into the air as buoyancy gets the better of the sinking force. And then the process starts again.

 

It does. I've played waterpolo for so many years and have thrown a myriad of skip shots - and I can assure you it does plane.

Check this out: http://www.youtube.com/watch?feature=player_detailpage&v=7JaOU2diM2Y#t=55s . Don't know how would you describe the motion of the ball in that fraction of the second, but I call that planing.

 

Is the objective of this thread to:

Define what planing is?

-or-

Determine a method or methods to determine if a boat is planing? This would seem to require some amount of agreement on what planing is.

-or-

Limit what boats can be considered as planing boats? This also would seem to require some amount of agreement on what planing is.

 

Sure, the ball bounces off the surface, but is that planing? Does it ever slide along the surface for a longer while?

 

A bit bouncy ride?

 

From the folks at wikipedia...

Planing - the planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag.

By definition a planing hull is one that is configured to develop hydrodynamic lift as speed increases and rise out of the water to attain a planing condition. While a water polo ball can be said to be planing, it can't get there from zero speed, it has to be shot at the water and impact it at high speed. Moreover, if it didn't have ribs to promote separation of the flow it would just bog down on the surface like the bouys that were mentioned earlier. When I was a kid we had all manner of pool balls, and the smooth ones wouldn't skip worth a darn, the'd just stick to the surface and stop almost immediately. The water polo ball skips because the speed is very high relative to the diameter of the ball, and it has ribs that promote separation, preventing it from generating a large downforce that would result from the flow staying attached. Tires hydroplane too, but that's also a case of hydrodynamic wedge action, but it isn't planing in the sense of a boat hull.

Remember folks, the term PLANING is derived from the word PLANE, which refers to a flat surface. While there are a lot of things that can be made to skip across the water, you really need some kind of surface that is relatively flat if you want to plane.

 

It works OK for typical powerboats, but not for most sailing boats and probably also not for some special power boats. Take a look at this video: http://www.youtube.com/watch?v=38D_SMWoF-Q

After jibes the Melges is definitely not planing. Then it speeds up (is it planing in the video?) without any clear change in trim. However you can clearly see that waterline is shortened.

Sailing boats hulls are more like semi-planing designs and there is typical no clear limit to planing. At what speed do you think Melges is planing? Can you feel that point?